RADIO WAVES. PRINCIPLES OF RECEPTION AND TRANSMISSION OF RADIO SIGNALS
Here comes the end of our introductory basic course of 10 lessons. I hope you have reached the 10th lesson by honestly completing all those simple tasks and experiments that were offered in the practical work. This final lesson will be entirely devoted to oscillatory processes, studying the nature of acoustic and electromagnetic waves, and, accordingly, the basics of receiving and transmitting electromagnetic waves (radio reception - transmission). The lesson is very dense, so I advise you to treat the assimilation of information very responsibly, also because it reveals the basic fundamentals and aspects necessary for further development and understanding of the processes occurring in transceivers.
The word radio comes from the Latin radiare - to radiate or emit rays. A broadcasting station, for example, like the Sun, radiates radio waves in all directions along radii. Only certain special-purpose radio stations radiate radio waves in one specific direction. If you came to the territory of a broadcasting station, the first thing you would see is a vertical openwork metal mast or wires raised high above the ground. This is the antenna. Nearby or close by is the building housing the transmitter, which generates high-frequency electrical oscillations, which the antenna converts into radio wave energy. An underground cable — well-insulated wires in a durable sheath — goes to the transmitter from the radio studio, which can be located far from the transmitter. A microphone is installed in the studio. Not only the announcer's voice, people's conversation, and the sounds of music, but also whispers and rustles are instantly converted by the microphone into audio-frequency electrical oscillations, which travel through the cable to the transmitter. How many more transformations does the audio-frequency alternating current undergo before the receiver turns it back into sounds! A receiver will be your first practical step towards understanding radio engineering. And for this step to be confident, you need to understand the essence of the physical phenomena that underlie the technology of radio transmission and reception, and talk about the nature of sound, alternating current, and its properties.
On oscillations and waves
Oscillatory phenomena are constantly born and fade around us. A branch from which a bird flew off oscillates. Clock pendulums and swings oscillate. Trees and wires suspended on poles oscillate under the action of the wind; water in lakes and seas oscillates. Suppose you threw a stone onto the smooth surface of a lake, and waves ran out from it. What happened? The water particles at the point of impact were pressed in, displacing neighboring particles, and a ring-shaped hump formed on the water surface. Then, at the point where the stone fell, the water rose, but higher than the previous level — a second hump appeared behind the first, and a trough between them. Further, the water particles continue to move alternately up and down — they oscillate, dragging more and more neighboring water particles with them. Waves are formed, diverging from the point of their origin in concentric circles.
I emphasize: the water particles only oscillate, but do not move along with the waves.
This is easy to verify by throwing a wood chip onto the oscillating water surface. If there is no wind or water current, the chip will only rise and fall above the water level, not moving with the waves. Water waves can be large, i.e., strong, or small — weak. We call waves strong if they have a large oscillation swing, or as they say, large oscillation amplitudes. Weak waves have small humps — a small amplitude. The larger the amplitudes of the generated waves, the more energy they carry. The energy of waves generated by a thrown stone is relatively small, but it can cause reeds and grass growing in the lake to oscillate. But we know what great destruction sea waves, possessing large amplitudes and, consequently, high energy, can cause to shores. This destruction is carried out precisely by the energy that the waves continuously transfer to the shore.
Waves can be frequent or rare. The smaller the distance between the crests of running waves, the shorter each individual wave is. The greater the distance between the waves, the longer the wave. Wavelength on water is the distance between two adjacent running crests or troughs. As waves move away from their point of origin, their amplitudes gradually decrease (fade), but the wavelength remains unchanged. Water waves can also be created, for example, by plunging a stick into the water and rhythmically, in time with the water's oscillations, lowering and raising it. In this case too, the waves will be fading. But they will exist only as long as we continue to disturb the water surface.
And how do the oscillations of an ordinary swing occur? You know this well: you have to push them, and then they will swing from side to side. The stronger the push, the greater the amplitude of the oscillations. These oscillations will fade if they are not supported by additional pushes. We can see these and many other similar mechanical oscillations. But in nature, there are more invisible oscillations that we hear and feel in the form of sound. For example, you can't always see the oscillations of a musical instrument string, but you hear it sound. With gusts of wind, a sound occurs in a pipe. It is created by oscillatory movements of air in the pipe, which we do not see. A tuning fork, a glass, a spoon, a cymbal, a student's pen, a sheet of paper sound — they also oscillate. Yes, we live in a world of sounds, because many bodies around us sound while oscillating.
How are sound waves generated in the air? Air consists of particles invisible to the eyes. In the wind, they can be carried over long distances. But they can also oscillate. For example, if you make a sharp movement with a stick in the air, you will feel a slight gust of wind and at the same time hear a faint sound. This sound is the result of oscillations of air particles excited by the oscillations of the stick. Let's do an experiment. Pluck a string, for example, of a guitar, and then release it. The string will start to tremble — to oscillate around its original resting position. Sufficiently strong string oscillations are visible to the eye. Weak string oscillations can only be felt as a slight tickle if you touch it with a finger. As long as the string oscillates, we hear a sound. As soon as the string settles, the sound will die down.
The birth of sound here is the result of the compression and rarefaction of air particles. Oscillating from side to side, the string pushes, as if compressing, the air particles in front of it, forming regions of high pressure in a certain volume, and behind it, conversely, regions of low pressure. These are the sound waves. Propagating in the air at a speed of about 340 m/s, they carry a certain amount of energy. At the moment when the high-pressure region of a sound wave reaches the ear, it presses on the eardrum, slightly bending it inward. When the rarefied region of the sound wave reaches the ear, the eardrum bulges slightly outward. The eardrum oscillates continuously in time with the alternating regions of high and low air pressure. These oscillations are transmitted along the auditory nerve to the brain, and we perceive them as sound.
The greater the amplitudes of the sound waves, the more energy they carry, and the louder the sound we perceive. Sound waves, like water or electrical oscillations, are depicted by a wavy line — a sinusoid. Its humps correspond to high-pressure regions, and its troughs to low-pressure regions of air. A high-pressure region and the following low-pressure region form a sound wave. We also live in a world of electromagnetic oscillations emitted by electrical devices and all wires carrying alternating current, by a huge number of radio station antennas, atmospheric electrical discharges, the bowels of the Earth, and infinite Space. Only with the help of instruments created by humans can they be detected and recorded.
Period, frequency, amplitude of oscillations
The most important parameter characterizing mechanical, sound, electrical, electromagnetic, and all other types of oscillations is the period — the time during which one full oscillation occurs. If, for example, the pendulum of a wall clock makes two full oscillations in 1 second, the period of each oscillation is 0.5 s. The oscillation period of a large swing is about 2 s, and the oscillation period of a string can range from tenths to ten-thousandths of a second.
Another parameter characterizing oscillations is frequency — a number showing how many full oscillations per second a clock pendulum, a sounding body, a current in a conductor, etc., make.
The frequency of oscillations is measured by a unit called the Hertz (abbreviated as Hz): 1 Hz is one oscillation per second.
Larger units of frequency are the kilohertz (kHz), equal to 1,000 Hz, and the megahertz (MHz), equal to 1,000 kHz or 1,000,000 Hz.
If, for example, a sounding string makes 440 full oscillations in 1 s (thereby creating the tone A of the first octave), its oscillation frequency is said to be 440 Hz. The frequency of the alternating current in the lighting network is 50 Hz. With this current, the electrons in the network wires alternately flow 50 times in one direction and the same number of times in the opposite direction within a second, i.e., they make 50 full oscillations in 1 s.
By the frequency of oscillations of a sounding body, one can judge the tone or pitch of the sound. The higher the frequency, the higher the pitch of the sound, and conversely, the lower the frequency, the lower the pitch.
Our ear is able to react to a relatively small band (section) of sound oscillation frequencies — approximately from 20 Hz to 20 kHz.
Nevertheless, this frequency band contains the entire vast range of sounds created by the human voice and a symphony orchestra: from very low tones, similar to the buzzing of a beetle, to the barely perceptible high squeak of a mosquito. We do not hear oscillations with a frequency below 20 Hz, called infrasonic, and above 20 kHz, called ultrasonic. And if our eardrum were capable of reacting to ultrasonic oscillations, we could then hear the squeak of bats and the voice of a dolphin. Dolphins emit and hear ultrasonic oscillations with frequencies up to 180 kHz. But do not confuse the pitch, i.e., the tone of the sound, with its volume (loudness). The pitch depends not on the amplitude, but on the frequency of oscillations. A thick and long string of a musical instrument, for example, creates a low sound tone, i.e., it oscillates slower than a thin and short string creating a high sound tone. Fig. 1 will help you understand this issue.
In electrical and radio engineering, alternating currents with a frequency from a few hertz to thousands of gigahertz are used. The antennas of broadcasting radio stations, for example, are powered by currents with a frequency from approximately 150 kHz to 100 MHz. These rapidly alternating oscillations, called radio-frequency oscillations, are the means by which sound is transmitted over long distances without wires.
The entire huge range of alternating currents is usually divided into several sections - sub-bands.
- Currents with a frequency from 20 Hz to 20 kHz are called audio frequency (AF) currents.
- Currents with a frequency above 20 kHz are called ultrasonic frequency currents.
- Currents with a frequency from 100 kHz to 30 MHz are called high frequency (HF) currents.
- Currents with a frequency above 30 MHz are called ultra-high frequency (VHF/UHF) currents.
Memorize these boundaries and names of alternating current frequency sub-bands.
What are radio waves?
Suppose you pick up the telephone receiver, dial, or say the desired number. Soon you hear your friend's voice, and he hears yours. What electrical phenomena occur during your telephone conversation? The sound air oscillations you create are converted by the microphone into audio-frequency electrical oscillations, which are transmitted via wires to your interlocutor's equipment. There, at the other end of the line, they are converted back into air oscillations by the telephone's emitter, perceived by your friend as sounds. In telephony, wires are the means of communication; in broadcasting, it's radio waves.
The heart of any radio station's transmitter is an oscillator — a device that generates high-frequency oscillations that are strictly constant for a given radio station. These radio-frequency oscillations, amplified to the necessary power, enter the antenna and excite electromagnetic oscillations of exactly the same frequency in the space around it — radio waves.
The speed at which radio waves travel away from the radio station's antenna is equal to the speed of light: 300,000 km/s, which is almost a million times faster than the propagation of sound in the air.
This means that if a transmitter is turned on at a Moscow broadcasting station at a certain point in time, its radio waves will reach Vladivostok in less than 1/30 of a second, while sound during this time will only have time to spread 10-11 meters. Radio waves propagate not only in the air but also where there is none, for example, in outer space. This distinguishes them from sound waves, for which air or some other dense medium, such as water, is absolutely necessary.
When a broadcasting station starts its transmissions, the announcer sometimes announces that the given radio station operates on a wave of a certain length. We can see a wave running on the surface of water and, with some dexterity, measure its length. The wavelength of radio waves can only be measured using special instruments or calculated mathematically if, of course, the frequency of the current exciting these waves is known. The wavelength is the distance over which the energy of the electromagnetic field propagates during one oscillation period of the current in the radio station's antenna.
This should be understood as follows. During one period of current in the transmitter's antenna, one radio wave is generated in the space around it. The higher the current frequency, the more consecutive radio waves are emitted by the antenna during each second. Suppose the current frequency in the radio station's antenna is 1 MHz. This means the period of this current and the electromagnetic field excited by it is one-millionth of a second. In 1 second, a radio wave covers a distance of 300,000 km, or 300,000,000 meters. In one-millionth of a second, it will cover a distance a million times smaller, i.e., 300,000,000 / 1,000,000. Consequently, the wavelength of this radio station is 300 m.
So, the wavelength of a radio station depends on the frequency of the current in its antenna: the higher the frequency, the shorter the wave, and conversely, the lower the frequency, the longer the wave.
To convert the transmitter's current frequency in megahertz to wavelength in meters and vice versa, it is convenient to use the following formulas:
λ (m) = 300 / f (MHz)
f (MHz) = 300 / λ (m)
Where λ (the Greek letter lambda) is the wavelength; f is the oscillation frequency, and 300 is the speed of radio wave propagation, expressed in thousands of kilometers per second.
I want to warn you: do not confuse the concept of the wavelength on which a radio station operates with its range, i.e., the distance over which this station's transmissions can be received. The range of a radio station, true, depends on the wavelength, but is not identical to it. Thus, a transmission from a station operating on a wavelength of a few tens of meters can be heard at a distance of several thousand kilometers, but is not always audible at closer distances. At the same time, a transmission from a radio station operating on a wavelength of hundreds and thousands of meters is often not heard at such large distances where shortwave station transmissions are audible. Thus, each broadcasting station operates on a specific, assigned frequency, called the carrier. The wavelengths of different radio stations are different, but strictly constant for each of them. This makes it possible to receive the transmissions of each radio station separately, rather than all simultaneously.
Radio Broadcasting. Broadcasting Wave Bands
A very wide section of radio waves allocated for broadcasting stations is conditionally divided into several bands: Long Wave (abbreviated as LW), Medium Wave (abbreviated as MW), Short Wave (abbreviated as SW), and Ultra-Short Wave (VHF/UHF). In the CIS countries, the long-wave band covers radio waves from 735.3 to 2000 m, which corresponds to frequencies of 408-150 kHz; the medium-wave band — radio waves from 186.9 to 571.4 m (radio frequencies 1605 - 525 kHz); the short-wave band — radio waves from 24.8 to 75.5 m (radio frequencies 12.1 - 3.95 MHz); the ultra-short-wave band — radio waves from 4.11 to 4.56 m (radio frequencies 73 - 65.8 MHz; Note: OIRT FM band). Radio waves in the VHF band are also called metric waves; generally, ultra-short waves refer to all waves shorter than 10 m.
Television broadcasts are conducted in this band, communication radio stations installed on fire trucks, taxis, home medical services, etc., operate here. The radio frequencies of shortwave broadcasting stations are unevenly distributed across the band: most of them operate on waves around 25, 31, 41, and 50 m. Accordingly, the shortwave broadcasting band is subdivided into 25, 31, 41, and 50-meter sub-bands. According to an international agreement, the 600 m wave (500 kHz) is reserved for transmitting distress signals by ships at sea - SOS. All marine emergency radio transmitters operate on this wave, and the receivers of rescue stations and lighthouses are tuned to this wave.
Radio Transmission
If the complex technical equipment of a broadcasting station is depicted simply in the form of conventional signs and rectangles, its block diagram will look like the one shown in Fig. 2. There are five main devices and units here: a studio microphone, an audio frequency (AF) amplifier, a radio frequency (RF) oscillator, an RF power amplifier, and an antenna radiating the electromagnetic energy of radio waves.
While the studio microphone is not turned on, a high-frequency (carrier) current of strictly constant frequency and amplitude flows in the station's antenna (see the left parts of the graphs in Fig. 3). The antenna then radiates radio waves of constant length and power. But then the microphone is turned on in the studio, and people tens, hundreds, and thousands of kilometers away from the radio station hear the familiar voice of the announcer.
What is happening in the transmitter of the radio station at this time? Audio frequency oscillations created by the microphone and amplified by the studio AF amplifier are fed to a so-called modulator, which is part of the transmitter's power amplifier, and there, acting on the high-frequency current of the oscillator, they change its oscillation amplitude. Because of this, the electromagnetic energy radiated by the transmitter's antenna changes (see the right parts of the graphs in Fig. 3). The higher the frequency of the current coming from the radio studio to the transmitter, the higher the frequency at which the current amplitudes in the antenna change. Thus, the sound, converted by the microphone into audio-frequency electrical oscillations, gets a ticket to the ether.
The process of changing the amplitudes of high-frequency oscillations under the influence of an audio-frequency current is called Amplitude Modulation (AM). The high-frequency currents in the antenna whose amplitudes are changed, and the radio waves radiated by it, are called modulated radio-frequency oscillations.
In addition to amplitude modulation, there is also so-called Frequency Modulation (FM). With this type of modulation, the frequency changes, but the amplitude of the radio-frequency oscillations in the radio station's antenna remains constant.
Frequency modulation is used, for example, to transmit the sound channel in television, and in VHF (FM) broadcasting. In radio broadcasting on LW, MW, and SW, only amplitude modulation is used.
Radio waves cannot be detected by any of our sensory organs. But if they encounter a conductor in their path, they give it some of their energy. The reception of radio broadcasts is based on this phenomenon. Capturing the energy of radio waves by a receiver is carried out by the radio receiver's antenna. Giving part of the electromagnetic energy to the antenna, radio waves induce modulated radio-frequency oscillations in it. Processes reverse to those occurring in the studio and at the radio station's transmitter take place in the receiver.
If there, sound is sequentially converted first into audio-frequency electrical oscillations, and then into modulated radio-frequency oscillations, then during radio reception the reverse problem is solved: the modulated radio-frequency oscillations excited in the antenna are converted by the receiver into audio-frequency electrical oscillations, and then into sound. In the simplest receiver, operating only due to the energy captured by the antenna, the modulated radio-frequency oscillations are converted into audio-frequency oscillations by a detector, and these oscillations into sound — by headphones.
But the receiver's antenna is pierced by radio waves from many radio stations, exciting modulated oscillations of a wide variety of radio frequencies in it. And if all these radio signals were converted into sounds, we would hear hundreds of voices of people speaking in different languages. Such radio reception would hardly suit us. Of course, it is interesting to listen to the broadcasts of different stations, but only, of course, not all at once, but each separately. And for this, from the oscillations of all frequencies excited in the antenna, it is necessary to select the oscillations with the frequency of the radio station whose broadcast we want to listen to.
This task is performed by the oscillatory circuit (LC circuit), which is a mandatory part of both the simplest and the most complex broadcasting receiver. It is with the help of the oscillatory circuit that you will tune your first receiver to the signals of radio stations of different wavelengths.
Propagation of Radio Waves
Now let's consider some features of radio wave propagation. The fact is that radio waves of different bands have different properties that affect their propagation distance. Waves of one length cover large distances, while waves of another length get lost beyond the horizon. It happens that a radio signal is perfectly audible somewhere on the other side of the Earth or in Space, but cannot be detected a few tens of kilometers from the radio station. How to explain this? What affects the range of radio waves of different lengths? The Earth and the atmosphere surrounding it.
The Earth is a conductor of current, although not as good as, say, copper wires.
The Earth's atmosphere consists of three layers. The first layer, the upper boundary of which ends 10-12 km from the Earth's surface, is called the troposphere. Above it, up to 50 km from the Earth's surface, is the second layer — the stratosphere. And higher, approximately up to 400 km above the Earth, extends the third layer — the ionosphere (Fig. 4).
The ionosphere plays a decisive role in the propagation of radio waves, especially short ones.
The air in the ionosphere is highly rarefied. Under the action of solar radiation, many free electrons are released from gas atoms there, resulting in the appearance of positive ions. As they say, ionization of the upper layer of the atmosphere occurs. The ionized layer is able to absorb radio waves and bend their path. During the day, depending on the intensity of solar radiation, the number of free electrons in the ionized layer, its thickness, and height change, and therefore the electrical properties of this layer also change.
Radio station antennas radiate radio waves along the Earth's surface and upward at various angles to it. Waves traveling along the surface are called ground or surface waves, and those traveling at various angles — sky or spatial waves. When transmitting signals of LW stations, mainly the energy of surface waves is used, which curve well around the Earth's surface. But the Earth, being a conductor, absorbs the energy of radio waves. Therefore, as you move away from LW stations, the volume of receiving their broadcasts gradually decreases, and finally, reception stops completely.
Medium waves do not curve around the Earth as well and, in addition, are absorbed by it more strongly than long ones. This explains the shorter range of MW broadcasting stations compared to LW stations. To increase the range of MW stations, their power must be increased. In the evening and at night, transmissions of LW and MW radio stations can be heard at greater distances than during the day. The fact is that the upward-radiated part of the radio wave energy of these stations during the day is lost without a trace in the atmosphere. However, after sunset, the lower layer of the ionosphere bends their path so that they return to Earth at such distances where the reception of these stations by surface waves is no longer possible.
Radio waves of the shortwave band are strongly absorbed by the Earth and poorly curve around its surface. Therefore, already at a distance of a few tens of kilometers from such radio stations, their surface waves fade. But on the other hand, sky waves can be detected by receivers at a distance of several thousand kilometers from them and even at the opposite point of the Earth. The bending of the path of spatial short waves occurs in the ionosphere. Having entered the ionosphere, they can travel a very long path in it and return to Earth far from the radio station. They can travel around the world — they can be received even in the place where the transmitting station is located. This explains the secret of the good propagation of short waves over long distances even at low transmitter powers.
But during the propagation of short waves, zones can form where the broadcasts of SW radio stations are not heard at all. They are called skip zones or dead zones (see Fig. 4). The extent of the skip zone depends on the wavelength and the state of the ionosphere, which in turn depends on the intensity of solar radiation.
Ultra-short waves (VHF/UHF) are closest in their properties to light rays. They mainly propagate in a straight line and are strongly absorbed by the ground, flora, various structures, and objects. Therefore, confident reception of signals from VHF stations by a surface wave is possible mainly only when a straight line can be mentally drawn between the transmitter and receiver antennas, encountering no obstacles along its entire length in the form of mountains, hills, or forests. The ionosphere is like glass for light to VHF — transparent. Ultra-short waves pass through it almost unhindered. Therefore, this radio wave band is used for communication with artificial Earth satellites and spacecraft.
How does a radio receiver work?
In any simple broadcasting receiver, regardless of its complexity, there are absolutely three elements that ensure its performance. These elements are — the oscillatory circuit, the detector, and the headphones (or, if it's a receiver with an AF amplifier, a direct-radiating dynamic loudspeaker).
Oscillatory circuit: — the design of the simplest oscillatory circuit and its diagram are shown in Fig. 5. It, as you can see, consists of an inductor L and a capacitor C, forming a closed electrical circuit in which, under certain conditions, electrical oscillations can arise and exist. That is why it is called an oscillatory circuit.
Have you ever observed this phenomenon: at the moment the power of an electric lighting lamp is turned off, a spark appears between the opening contacts of the switch. If you accidentally connect the terminals of the poles of an electric pocket flashlight battery (which should be avoided), at the moment of their disconnection, a small spark also jumps between them. And in factories, in workshops where large currents flow through circuits broken by knife switches, sparks can be so significant that measures must be taken so that they do not harm the person switching the current. Why do these sparks occur?
You already know that around a current-carrying conductor, there is a magnetic field, which can be depicted as closed magnetic lines of force piercing the space surrounding it. You can detect this field, if it is constant, using the magnetic needle of a compass. If the conductor is disconnected from the current source, its disappearing magnetic field, dissipating in space, will induce currents in other conductors closest to it. Current is also induced in the conductor that created the EMF and the magnetic field. And since it is in the very thick of its own magnetic lines of force, a stronger current will be induced in it than in any other conductor. The direction of this current will be the same as it was at the moment the conductor was broken.
In other words, the disappearing magnetic field will maintain the current creating it until it disappears itself, i.e., until the energy contained in it is completely used up. Consequently, the current in the conductor flows even after the current source is turned off, but, of course, not for long — a negligible fraction of a second. After opening the circuit, the electric current can flow for some time through the air gap between the separated ends of the conductor, between the contacts of a switch or breaker. It is this current through the air that forms the electric spark.
This phenomenon is called self-induction, and the electrical force that maintains the current in it under the influence of its disappearing magnetic field is called the electromotive force (EMF) of self-induction. The greater the EMF of self-induction, the more significant the spark at the point of the electrical circuit break can be. The phenomenon of self-induction is observed not only when turning off, but also when turning on the current. In the space surrounding the conductor, the magnetic field appears immediately upon turning on the current; initially, it is weak, but then it rapidly strengthens. The strengthening magnetic field of the current also excites a self-induction current, but this current is directed against the main current. The self-induction current prevents the instantaneous increase of the main current and the growth of the magnetic field. However, after a short period of time, the main current in the conductor overcomes the opposing self-induction current and reaches its maximum value, the magnetic field becomes constant, and the action of self-induction ceases.
Do all conductors have the same self-induction? No. The longer the conductor, the more significant the self-induction. In a conductor rolled into a coil, the phenomenon of self-induction is stronger than in a straight conductor, since the magnetic field of each turn of the coil induces a current not only in this turn but also in adjacent turns of this coil. The longer the wire length in the coil, the longer the self-induction current will exist in it after the main current is turned off.
Remember: the property of a conductor to affect the current in the circuit and the change in its value is called inductance, and the coils in which this property is most strongly manifested are called self-induction coils or inductors. The greater the number of turns and the dimensions of the coil, the greater its inductance, the more significantly it affects the current in the electrical circuit.
Thus, the inductor prevents both the rise and fall of current in the electrical circuit. This electrical phenomenon is used in the first element of the receiver's oscillatory circuit — the inductor.
The second element of the receiver's oscillatory circuit is the electrical charge storage device — the capacitor. The simplest capacitor consists of two electrical current conductors, for example: two metal plates, called capacitor plates, separated by a dielectric, for example: air or paper. The larger the area of the capacitor plates and the closer they are to each other, the greater the electrical capacitance of this device.
If a DC source is connected to the capacitor plates (Fig. 6, a), a short-term current will arise in the formed circuit, and the capacitor will be charged to a voltage equal to the voltage of the current source. You might ask: why does a current arise in a circuit where there is a dielectric? When we connect a current source to the capacitor, the electrons in the conductors of the formed circuit begin to move towards the positive pole of the current source, forming a short-term flow of electrons throughout the circuit. As a result, the capacitor plate connected to the positive pole of the current source becomes depleted of free electrons and becomes positively charged, while the other plate becomes enriched with free electrons and, therefore, becomes negatively charged. As soon as the capacitor is charged, the short-term current in the circuit, called the capacitor charging current, will stop.
If the current source is disconnected from the capacitor, the capacitor will be charged (Fig. 6, b). The transfer of excess electrons from one plate to another is prevented by the dielectric. There will be no current between the capacitor plates, and the electrical energy accumulated by it will be concentrated in the electrical field of the dielectric. But if the plates of the charged capacitor are connected by some conductor (Fig. 6, c), the excess electrons of the negatively charged plate will pass along this conductor to the other plate, where they are lacking, and the capacitor will discharge. In this case, a short-term current, called the capacitor discharge current, also arises in the formed circuit.
The property of a capacitor to accumulate electrical charges and discharge through the conductors connected to it is used in the radio receiver's oscillatory circuit.
And now, remember ordinary swings. You can swing on them so high it takes your breath away. What needs to be done for this? First, push to move the swing from its resting position, and then apply some force, but strictly only in time with their oscillations. Without much effort, you can achieve strong swings — obtain large amplitudes of oscillations. After swinging strongly to achieve large amplitudes of oscillations, we stop pushing them. What happens next? Due to the stored energy, they swing freely for a while, the amplitude of their oscillations gradually decreases, or as they say, the oscillations fade, and finally, the swing will stop. During the free oscillations of the swing, just like a freely suspended pendulum — the stored potential energy turns into the kinetic energy of motion, which at the extreme upper point again turns into potential energy, and a fraction of a second later — back into kinetic energy. And so on until the entire energy supply is used up to overcome the friction of the ropes at the suspension points of the swing and air resistance. With an arbitrarily large energy supply, free oscillations are always fading: with each oscillation, their amplitude decreases and the oscillations gradually fade completely — the swing stops.
But the period, i.e., the time during which one oscillation occurs, and therefore the oscillation frequency, remain constant. However, if the swing is constantly pushed in time with its oscillations and thereby replenishes the energy loss spent on overcoming various braking forces, the oscillations will become undamped. These are no longer free, but forced oscillations. They will last as long as the external pushing force acts.
I remembered the swing here because the physical phenomena occurring in such a mechanical oscillatory system are very similar to the phenomena in an electrical oscillatory circuit. For electrical oscillations to arise in the circuit, it must be given energy that would push the electrons in it. This can be done by charging its capacitor, for example. Let's break the oscillatory circuit with switch S and connect a DC source to the plates of its capacitor, as shown in (Fig. 7 left). The capacitor will charge up to the voltage of the battery GB. Then we will disconnect the battery from the capacitor and close the circuit with switch S.
The phenomena that will now occur in the circuit are shown graphically in (Fig. 7 right). At the moment the circuit is closed by the switch, the upper plate of the capacitor has a positive charge, and the lower one — a negative charge (Fig. 7, a). At this time (point 0 on the graph) there is no current in the circuit, and all the energy accumulated by the capacitor is concentrated in the electrical field of its dielectric. When the capacitor is closed onto the coil, the capacitor will begin to discharge. A current appears in the coil, and a magnetic field around its turns. By the time the capacitor is fully discharged (Fig. 7, b), marked on the graph with the number 1, when the voltage on its plates drops to zero, the current in the coil and the energy of the magnetic field will reach their maximum values. It would seem that at this moment the current in the circuit should have stopped.
However, this will not happen, since due to the action of the EMF of self-induction tending to maintain the current, the movement of electrons in the circuit will continue. But only until all the energy of the magnetic field is used up. At this time, an induced current decreasing in value but of the original direction will flow in the coil. By the point in time marked on the graph with the number 2, when the magnetic field energy is used up, the capacitor will again be charged, only now there will be a positive charge on its lower plate, and a negative one on the upper plate (Fig. 7, c). Now the electrons will start their reverse movement — from the upper plate through the coil to the lower plate of the capacitor. By time 3 (Fig. 7, d), the capacitor will discharge, and the magnetic field of the coils will reach its maximum value, and again the EMF of self-induction will drive the electrons along the coil wire, thereby recharging the capacitor. At point in time 4 (Fig. 7, e), the state of the electrons in the circuit will be the same as at the initial moment — 0.
One full oscillation is complete. Naturally, the charged capacitor will again discharge into the coil, recharge, and the second, followed by the third, fourth oscillations will occur. In other words, an alternating electrical current, electrical oscillations, will arise in the circuit. But this oscillatory process in the circuit is not infinite. It continues until all the energy received by the capacitor from the battery is used up to overcome the resistance of the circuit coil wire. The circuit oscillations are free and, therefore, fading.
What is the frequency of such electron oscillations in the circuit? To understand this issue in more detail, I advise conducting this experiment with a simple pendulum. Suspend a ball molded from plasticine or another weight of 20-40 g on a 100 cm long thread (in Fig. 8, the length of the pendulum is denoted by the Latin letter L).
Move the pendulum from its equilibrium position and, using a watch with a second hand, count how many full oscillations it makes in 1 minute. Approximately 30. Consequently, the oscillation frequency of this pendulum is 0.5 Hz, and the period is 2 s. During the period, the potential energy of the pendulum twice turns into kinetic energy, and kinetic into potential. Shorten the thread by half. The frequency of the pendulum will increase by about one and a half times and the oscillation period will decrease by the same amount. This experiment allows us to conclude: as the length of the pendulum decreases, the frequency of its natural oscillations increases, and the period proportionally decreases. By changing the suspension length of the pendulum, make its oscillation frequency equal to 1 Hz. This should be with a thread length of about 25 cm. At this time, the oscillation period of the pendulum will be 1 s.
No matter how hard you try to create the initial swing of the pendulum, the frequency of its oscillations will be unchanged. But as soon as you shorten or lengthen the thread, the oscillation frequency will immediately change. With the same thread length, there will always be the same oscillation frequency. This is the natural oscillation frequency of the pendulum. You can get the desired oscillation frequency by adjusting the thread length.
The oscillations of a thread pendulum are fading. They can become undamped only if the pendulum is lightly pushed in time with its oscillations, thereby compensating for the energy it spends on overcoming air resistance, friction energy, and Earth's gravity.
The natural frequency is also characteristic of an electrical oscillatory circuit.
First, it depends on the inductance of the coil. The greater the number of turns and the diameter of the coil, the greater its inductance, the longer the duration of the period of each oscillation. The natural frequency of oscillations in the circuit will be correspondingly lower. And conversely, with a decrease in the coil's inductance, the oscillation period will decrease — the natural frequency of oscillations in the circuit will increase.
Secondly, the natural frequency of oscillations in the circuit depends on the capacitance of its capacitor. The greater the capacitance, the greater the charge the capacitor can accumulate, the more time it will take to recharge it, the lower the oscillation frequency in the circuit. With a decrease in the capacitor's capacitance, the oscillation frequency in the circuit increases.
Thus, the natural frequency of fading oscillations in the circuit can be adjusted by changing the inductance of the coil or the capacitance of the capacitor. But in an electrical circuit, as in a mechanical oscillatory system, one can obtain undamped, i.e., forced oscillations, if at each oscillation the circuit is replenished with additional portions of electrical energy from some AC source. How are undamped electrical oscillations excited and maintained in the receiver circuit?
By radio-frequency oscillations excited in the receiver antenna. These oscillations impart the initial charge to the circuit, and they also maintain the rhythmic oscillations of the electrons in the circuit. But the strongest undamped oscillations in the receiver circuit arise only at the moment of resonance of the circuit's natural frequency with the frequency of the current in the antenna. How should this be understood?
Older generation people tell a story that in St. Petersburg, the Egyptian Bridge collapsed from soldiers marching in step. And this could have happened, apparently, under such circumstances. All the soldiers were marching rhythmically on the bridge. Because of this, the bridge began to sway — to oscillate. By a random coincidence, the natural oscillation frequency of the bridge coincided with the frequency of the soldiers' steps, and the bridge, as they say, entered into resonance. The rhythm of the formation imparted more and more portions of energy to the bridge. As a result, the bridge swayed so much that it collapsed: the coherence of the military formation damaged the bridge. If there were no resonance of the bridge's natural oscillation frequency with the soldiers' step frequency, nothing would have happened to the bridge. Therefore, by the way, when soldiers pass over weak bridges, it is customary to give the command to break step.
And here is an experiment. Go to some stringed musical instrument and loudly shout "a": one of the strings will respond — it will sound. The one that turns out to be in resonance with the frequency of this sound will oscillate stronger than the other strings — it will be the one to respond to the sound. One more experiment with a pendulum. Stretch a thin rope horizontally. Tie the same thread and plasticine pendulum to it (Fig. 9).
Throw another similar pendulum over the rope, but with a longer thread. The length of this pendulum's suspension can be changed by pulling the free end of the thread with your hand. Set the pendulum in oscillatory motion. At the same time, the first pendulum will also begin to oscillate, but with a smaller amplitude. Without stopping the oscillations of the second pendulum, gradually reduce the length of its suspension — the amplitude of oscillations of the first pendulum will increase. In this experiment, which illustrates the resonance of mechanical oscillations, the first pendulum is the receiver of the oscillations excited by the second pendulum. The reason forcing the first pendulum to oscillate is the periodic oscillations of the stretch with a frequency equal to the oscillation frequency of the second pendulum.
The forced oscillations of the first pendulum will have maximum amplitude when its natural frequency coincides with the frequency of oscillations of the second one. Such or similar phenomena, only, of course, of electrical origin, are also observed in the receiver's oscillatory circuit. Under the action of waves from many radio stations, currents of various frequencies are excited in the receiving antenna. Of all the radio frequency oscillations, we need to select only the carrier frequency of the radio station whose transmissions we want to listen to.
To do this, it is necessary to select the number of turns of the coil and the capacitance of the oscillatory circuit capacitor in such a way that its natural frequency coincides with the frequency of the current created in the antenna by the radio waves of the station of interest to us. In this case, the strongest oscillations with the carrier frequency of the radio station to whose wave it is tuned will arise in the circuit. This is the tuning of the receiver circuit to resonance with the frequency of the transmitting station. At the same time, the signals of other stations are not heard at all or are heard very faintly, since the oscillations they excite in the circuit will be many times weaker. Thus, by tuning the circuit of your receiver in resonance with the carrier frequency of the radio station, you, as it were, select, isolating the frequency oscillations, only of this station.
The better the circuit isolates the necessary oscillations from the antenna, the higher the selectivity of the receiver, and the weaker the interference from other radio stations will be.
Until now, I have been telling you about a closed oscillatory circuit, i.e., a circuit whose natural frequency is determined only by the inductance of the coil and the capacitance of the capacitor forming it. However, the input circuit of a receiver also includes an antenna and a ground connection. This is no longer a closed, but an open oscillatory circuit. The fact is that the antenna wire and the ground are the plates of a capacitor that has a certain electrical capacitance. Depending on the wire length and the antenna height above the ground, this capacitance can be several hundred picofarads. But the antenna and the ground can also be viewed as an incomplete turn of a large coil. Thus, the antenna and the ground taken together also possess inductance. And capacitance together with inductance forms an oscillatory circuit (Fig. 10).
Such a circuit, which is an open oscillatory circuit, also has its own natural oscillation frequency. By connecting inductors and capacitors between the antenna and ground, we can change its natural frequency, tune it in resonance with the frequencies of various radio stations. You already know how this is done in practice. I will not be mistaken if I say that the oscillatory circuit is the heart of a radio receiver. And not only of a radio receiver. Therefore, I paid more attention to it. I proceed to the second element of the receiver — the detector.
Detector and Radio Signal Detection
A detector is a two-electrode semiconductor device (high-frequency diode) that has unilateral electrical conductivity: it conducts current well in one direction and does not conduct — or weakly conducts — current in the reverse direction.
To simplify the explanation of how a diode works as a detector, we will assume that it does not conduct reverse current at all and acts as an insulator for it. This property of a diode is illustrated by the graph shown in (Fig. 11), the diode unhinderedly passes positive half-waves of alternating current through itself and does not pass negative half-waves at all.
The diode seems to cut off the negative half-waves. As a result of this action of the diode, the alternating current is converted into a pulsating current of one direction, but changing in magnitude with the frequency of the current passed through it.
This transformation process, called AC rectification, is the basis for detecting received radio signals.
Look at the graphs shown in (Fig. 12). They illustrate the processes occurring in the detector circuit of the simplest receiver. Under the action of radio waves, modulated radio frequency oscillations are excited in the receiver circuit (Fig. 12, a). A circuit consisting of a diode and headphones is connected to the LC circuit.
For this circuit, the oscillatory circuit is a source of alternating radio frequency current. Since the diode passes current in only one direction, the modulated radio frequency oscillations entering its circuit will be rectified by it (Fig. 12, b), in other words, detected. If you draw a dashed line enveloping the peaks of the rectified current, you will get a picture of the audio frequency current with which the current entering the radio station antenna during transmission was modulated.
The current resulting from detection consists of radio frequency pulses whose amplitudes vary with audio frequency. It can be viewed as the total current and decomposed into two components: high-frequency and low-frequency. They are called, respectively, the high-frequency and the audio-frequency component of the pulsating current. In the simplest receiver, the audio frequency component goes through the headphones and is converted by them into sound.
Headphones and Their Construction
The telephone (headphone) is the third and last link of the simplest receiver, which, figuratively speaking, issues the finished product — sound.
This is one of the oldest electrical devices, which has preserved its main features almost unchanged to this day. For crystal and many simple transistor receivers, headphones are used, for example, types TON-1, TG-1, TA-4. These are two series-connected earpieces held on a headband. Let's unscrew the cap of one of the earpieces (Fig. 13, a).
Under it is a round tin plate — a membrane (diaphragm). Carefully removing the membrane, we will see two coils mounted on the pole pieces of a permanent magnet pressed into the case. The coils are connected in series, and the outer leads are soldered to the pins, to which on the outside, using clamping screws, a cord with single-pole plugs is connected.
How does a headphone work? The membrane emitting sound is located near the pole pieces of the magnet and rests on the sides of the case (Fig. 13, a). Under the action of the magnet's field, it bends slightly in the middle but does not touch the pole pieces of the magnet (in Fig. 13, b — the solid line). When current flows through the headphone coils, it creates a magnetic field around the coils, which interacts with the magnetic field of the permanent magnet. The strength of this magnetic field, and therefore the force of attraction of the membrane to the pole pieces, depends on the direction of the current in the coils. In one direction, when the directions of the magnetic lines of force of the coils and the magnet coincide and their fields add up, the membrane is attracted more strongly to the poles of the magnet (in Fig. 13, b — the lower dashed line). With the other current direction, the lines of force of the coil and the magnet are directed oppositely, and the total field becomes weaker than the magnet's field. In this case, the membrane is attracted more weakly by the pole pieces and, straightening out, moves somewhat away from them (Fig. 13, b — the upper dashed line).
If an alternating audio frequency current is passed through the headphone coils, the total magnetic field will alternately strengthen and weaken, and the membrane will either approach the pole pieces of the magnet or move away from them, i.e., oscillate with the current frequency. Vibrating, the membrane will create sound waves in the surrounding space.
At first glance, it might seem that a permanent magnet in the headphone is not needed: the coils could be put on a non-magnetized iron horseshoe. But this is not the case. And here is why. An iron horseshoe magnetized by alternating current will attract the membrane regardless of whether the current goes through the coils in one direction or the other. This means that during one period of alternating current, the membrane will be attracted during the first half-period, move away from it, and be attracted again during the second half-period, i.e., in one period of alternating current (Fig. 14, a) it will make two oscillations (Fig. 14, b).
If, for example, the current frequency is 500 Hz, then the headphone membrane will make 500 * 2 = 1000 oscillations in 1 s, and the sound tone will be distorted — it will be twice as high. Such a headphone would hardly suit us. With a permanent magnet, things are different: during one half-period, the magnetic field strengthens — the already attracted membrane will bend even more; during the other half-period, the field weakens, and the membrane, straightening out, moves further away from the magnet poles. Thus, in the presence of a permanent magnet, the membrane makes only one oscillation per period of alternating current (Fig. 14, c), and the headphone does not distort the sound. A permanent magnet also increases the volume of the headphone's sound.
Now let's analyze this question: why is a bypass capacitor (блокировочный конденсатор) connected in parallel to the headphones? What is its role? The electrical capacitance of the bypass capacitor is such that high-frequency currents pass freely through it, but it offers significant resistance to audio-frequency currents. Headphones, on the contrary, pass audio-frequency currents well and offer high resistance to high-frequency currents. In this section of the detector circuit, the high-frequency pulsating current is separated (in Fig. 15 — at point a) into components, which then go: the high-frequency one — through the bypass capacitor C_b, and the audio-frequency component — through the headphones. Then the components join (in Fig. 15 — at point b) and then again go together.
The purpose of the bypass capacitor can also be explained as follows. Due to the inertia of the membrane, the headphone cannot respond to every high-frequency current pulse in the detector circuit. Therefore, for the headphone to work, it is necessary to somehow smooth out the high-frequency pulses, to fill the current dips between them. This task is solved with the help of a bypass capacitor as follows. Individual high-frequency pulses charge the capacitor. In the moments between pulses, the capacitor discharges through the headphone, thus filling the dips between pulses. As a result, a current of one direction goes through the headphone, but changing in magnitude with the audio frequency, which is converted into sound by it.
Even more briefly about the role of the bypass capacitor, we can say this: it filters the audio frequency signal isolated by the diode, i.e., cleans it from the radio frequency component.
The quality of a headphone's performance is evaluated mainly in terms of its sensitivity — the ability to respond to weak electrical current oscillations. The weaker the oscillations the headphone responds to, the higher its sensitivity. The sensitivity of a headphone depends on the number of turns in its coils and the quality of the magnet. Two headphones with absolutely identical magnets, but with coils containing an unequal number of turns, differ in sensitivity. The one in which coils with a larger number of turns are used will have better sensitivity. The headphone sensitivity also depends on the position of the membrane relative to the pole pieces of the magnet. Its best sensitivity will be when the membrane is very close to the pole pieces, but while vibrating, does not touch them.
It is customary to divide headphones into high-impedance — with a large number of turns in the coils, and low-impedance — with a relatively small number of turns. Only high-impedance headphones are suitable for a crystal receiver. The coils of each TON-1 type headphone, for example, are wound with enameled wire 0.06 mm thick and have 4000 turns each. Their DC resistance is about 2200 Ohms. This number characterizing the headphones is stamped on their cases. Since two earpieces are connected in series, their total DC resistance is 4400 Ohms. The DC resistance of low-impedance headphones, for example, type TA-56, can be 50-60 Ohms. Low-impedance headphones can be used for some transistor receivers.
How to check the serviceability and sensitivity of headphones? Press them to your ears. Moisten the plugs at the end of the cord with saliva, and then touch them together — a faint click should be heard in the headphones. The stronger this click, the more sensitive the headphones. The clicks are produced because the moistened contact between the metal plugs is a very weak current source. You can roughly check headphones using a pocket flashlight battery. When connecting headphones to the battery and disconnecting from it, sharp clicks should be heard. If there are no clicks, it means that somewhere in the coils or the cord there is a break or a bad contact.
Practical Work
In this practical work, we will construct the simplest radio receiver (crystal receiver), without which, in my opinion, it is inconceivable to further master any radio receiving equipment. Ask any specialist in the field of radio electronics (SW-VHF radio communication) what a crystal radio is, and I think they will immediately give you a clear answer. In a word, this is a classic, the foundation of basics, where our fathers and grandfathers started. And we will try not to fall behind them.
The main advantage of this version of the simplest radio receiver is that it is easy to make any changes and additions to it, correct mistakes by reconnecting wires, since all its parts will lie laid out before you. Experiments with it will help you understand the basic principles of any broadcasting receiver and get some practical skills in radio engineering design.
For such a receiver you will need: an inductor coil, a ferrite rod brand 400NN or 600NN with a diameter of 7-8 mm and a length of 120-140 mm (such rods are used for magnetic antennas of transistor receivers), a semiconductor point-contact diode, which will be the detector in the receiver, several fixed capacitors, and headphones (Fig. 1).
Make the inductor coil yourself (from previous lessons you know how it is done). The rest of the parts are ready-made. The diode can be any of the D9, D2 series. Capacitors also of any type — mica, ceramic, or paper with a capacity from a few tens to several thousand picofarads (abbreviated: pF). High-impedance headphones, i.e., with winding resistance 1500 - 2200 Ohms, for example type TON-1 or TA-4.
A little later, when you proceed to experiments, some other parts and materials will be needed. For the coil, you will need winding wire brand PEV-1 (Wire with Enamel High-strength insulation in one layer), PEV-2 (the same, but with insulation in two layers) or PEL (Wire with Enamel Varnish-resistant insulation) with a diameter of 0.15 - 0.2 mm. Winding wires of these brands and their diameter are designated as follows: PEV-1 0.15, PEV-2 0.18, PEL 0.2. Winding wires of other brands are also suitable, for example, PBD — with insulation of two (letter D) layers of cotton yarn (letter B), or PELSHO — with enamel varnish-resistant insulation and one (letter O) layer of natural silk (letter SH). It is only important that the wire insulation is intact, otherwise a short circuit may occur between the turns of the coil, which must not be allowed.
The inner diameter of the coil former, glued from 3-4 layers of writing paper, should be such that the ferrite rod enters it with a little friction. Before winding the coil, insert the rod into the former. Do not pull the wire too tight, otherwise the former will compress and it will be difficult to pull the rod out of it. In total, 300 turns of wire must be wound on the former in one row, making taps in the form of loops every 50 turns. The result will be a single-layer six-section inductor coil with two outer leads and five taps. So that the outer turns of the wire of the finished coil do not fall off, secure them on the former with rings cut from rubber or PVC tubing, or wrap them with threads. Additionally, the turns of the coil wire can be fastened with a thin layer of "Moment" glue. Carefully trim the ends of the former with a sharp knife.
It happens that during coil winding the wire will break or one piece of wire will not be enough for the whole coil. In this case, the ends of the wire that need to be connected must be stripped of insulation, tightly twisted, soldered, and necessarily wrapped with thin insulating tape. If the connection falls near a tap, it is better not to spare a few turns of wire and make it in a loop.
Now, proceed to the assembly of your first radio receiver (Fig. 2).
Strip the ends of the leads and taps of the coil from insulation, just carefully so as not to tear the wire. Let's call one of the extreme leads the beginning of the coil and denote it with the letter (н - beginning). Connect it to the diode. Connect the second extreme lead of the coil, its end (к - end), to one of the contact pins of the headphone cord. Connect the remaining free diode lead and headphone pin together as well.
Firmly twist the antenna wire, having previously stripped it of insulation, to the conductor going from the beginning of the coil to the diode. We will call this conductor of the receiver the antenna conductor. Twist the ground wire to the conductor connecting the end of the coil to the headphones. This will be the grounded conductor. During experiments, you will have to switch it from one coil tap to another (in Fig. 2 shown by a dashed line with an arrow), without changing the ground connection to the headphones.
Let's take a walk through the circuits of the resulting receiver. From the beginning of the coil (н) via the antenna conductor we get to diode VI and through it — to the headphones B1, then through the headphones along the grounded conductor. Switch S1 and turns of the coil Ll — to the starting point. This is the detector circuit. If there is a break anywhere in this circuit, a poor contact between parts or connecting wires, for example, a loose twist, the receiver, naturally, will not work. The shortest path from the antenna to the ground is through the coil. High-frequency current excited in the antenna by radio waves will go along this path. This current will create a high-frequency voltage across the coil ends, which will cause a current of the same frequency in the entire detector circuit. The circuit consisting of the antenna, coil, and ground is called the antenna circuit or antenna tuning circuit. Pay attention: the receiver's tuning coil is included in both the antenna and detector circuits.
After such a walk through the receiver circuits, you can proceed to test it. Put the headphones on your head, press them tighter to your ears, listen closely. It is possible that you will not hear anything right away even with known good antenna and ground, previously tested diode and headphones. This is because the receiver is apparently not tuned to the carrier frequency of the broadcasting station whose signals are well heard in your area, or you hit a transmission break.
You can tune such a receiver by changing the number of coil turns included in the antenna circuit. In (Fig. 2), all 300 coil turns are included in the antenna circuit. If the grounded conductor is disconnected from the end of the coil and connected, for example, to tap 5, then not 300, but 250 turns will be included in the circuit. If this conductor is switched to tap 4, 200 turns will be included in the circuit. When switching it to tap 3, 150 turns will be included in the antenna circuit, etc. At the same time, the lower sections will not be included in the circuit and will not participate in the operation of the receiver.
Thus, by switching the grounded conductor, you can include a different number of turns in the circuit in 50-turn increments. Remember: the longer the wavelength of the broadcasting station to which the receiver can be tuned, the greater the number of coil turns must be included in the antenna circuit. Your experimental receiver can be tuned to broadcasting stations in both the medium-wave and long-wave bands. But, of course, you cannot receive the broadcasts of every station. The crystal receiver will not be able to respond to weak signals from distant stations — the sensitivity is low.
Now start tuning the receiver by connecting the grounded conductor first to tap 5, then to tap 4, and so on up to tap 1. At the same time, make sure that the coil taps and connecting wires do not touch, and the contacts in the twists are not broken. Otherwise, the receiver will not work at all, or crackles and rustles interfering with reception will be heard in the headphones. Electrical contacts will be more reliable if the connection points of wires and parts are soldered.
Having tuned the receiver to one station, remember the number of turns included in the circuit at which the station is heard with the greatest volume. Then try to find another station in the same way. I hope you have achieved some success. Try to improve the receiver's performance. Without changing the receiver tuning, connect a capacitor in parallel with the headphones (between its contact pins). The capacity of this capacitor, called the bypass capacitor in this case, can be from 1000 to 3000 pF. The volume of the headphones should increase slightly. And if broadcasting stations are more than 150-200 km away from where you live, connect the bypass capacitor at the very beginning of the experiment.
The method of tuning the receiver only by a stepwise change in the number of coil turns is very simple. But it does not always allow you to tune the receiver exactly to the carrier frequency of the station. Fine tuning can be achieved by an additional method, for example, using a nail. Try tuning the receiver to the radio station wave using the method you already know and insert a thick nail or an iron rod of a suitable diameter inside the coil former. What happened? The reception volume will slightly increase or, conversely, decrease. Pull the nail out of the coil — the volume will be the same. Now slowly insert the nail into the coil and pull it out just as slowly — the volume of the receiver will change slightly but smoothly.
Experimentally, you can find such a position of the metal object in the coil at which the volume will be the best. This experiment leads to the conclusion that a metal rod placed in a coil affects the tuning of the circuit. You will become familiar with such a method of tuning a receiver, only, of course, using a better ferromagnetic core than a nail, in the future. In the meantime, I suggest the following experiment — tuning the receiver to the signals of a broadcasting station using a variable capacitor. For convenience in conducting this and several subsequent experiments with a crystal receiver, assemble a block with plug sockets, two binding posts, a bypass capacitor on a plywood board measuring about 30 x 70 mm, connecting them under the board, as shown in Fig. 3.
Install the socket block on the board like this: drill two holes 6-8 mm in diameter with a distance of 20 mm between centers and insert the tails of the plug sockets into them. Secure the block on the board with screws or bolts and nuts. Connect the beginning of the coil and the antenna to the binding post to which the diode is connected, and connect the end of the coil and the ground to the second binding post connected to the headphone socket. The variable capacitor can have either an air or a solid dielectric. But the function of a variable capacitor can be performed by two metal plates measuring approximately 150 x 150 mm, cut, for example, from the tin of large cans. Solder 250-300 mm long wires to the plates. Using these wires, connect one plate to the antenna terminal, and the other to the ground terminal.
Place the plates on the table next to each other, but so that they do not touch, and tune the receiver to a radio station only by switching the coil sections with the grounded wire. Now bring the grounded plate closer to the plate connected to the antenna. If the volume increases, bring the plates closer and finally put one plate on top of the other, placing a sheet of dry paper between them (so there is no electrical contact). Find such a mutual arrangement of the plates at which there will be an exact tuning. If, however, when the plates are brought closer, the reception volume decreases, switch the grounded wire to the tap closer to the beginning of the coil and bring the plates closer again, achieving maximum volume.
In this experiment, tuning the receiver to the radio station's carrier frequency was carried out in two ways: rough — by changing the coil's inductance by switching its sections, fine — by changing the capacitance of the plate capacitor.
Remember: the coil's inductance and the capacitor's capacitance when tuning a receiver to a radio station are interconnected. The same radio station can be listened to when a larger number of turns are included in the receiver's antenna circuit, i.e., with greater coil inductance, but with smaller capacitor capacitance, or, conversely, with less coil inductance, but greater capacitor capacitance.
Now retune the receiver to some radio station, remember the broadcast reception volume, and then, without changing the tuning, connect a capacitor of 47-62 pF between the antenna and the antenna terminal.
What happened? The reception volume decreased somewhat. This happened because the capacitor connected to the antenna circuit changed the parameters of the entire LC circuit. Retune the circuit with the variable capacitor to the previous headphone volume. If before connecting the additional capacitor to the circuit, some other radio station close in frequency was heard during the reception of one station, now it will be heard much weaker, and possibly will not interfere at all.
The receiver began to select the signals of the station to which it is tuned more clearly, or, as they say, its selectivity has improved. Instead of a fixed capacitor, connect a variable capacitor between the antenna and the receiver. With it, you can not only change the selectivity of the receiver, but possibly tune it to different stations. The next experiment is tuning the receiver with a ferrite rod (Fig. 4).
Remove the plate capacitor, and instead of it, connect a mica or ceramic capacitor with a capacity of 120-150 pF between the antenna and ground terminals, i.e., parallel to the coil. Press the headphones tightly to your ears, concentrate, and very slowly insert the ferrite rod into the coil former. Gradually plunging the rod into the coil, you should hear broadcasts of all those broadcasting stations that can be received in your area on a crystal receiver. The longer the radio station's wave, the deeper the rod must be inserted into the coil. Find experimentally the position of the rod in the coil at which the station signals are heard loudest, and make a corresponding pencil mark on the rod.
Using it like scale divisions, you can quickly tune the receiver to the wave of this station. Continuing the experiment using a ferrite rod, connect another capacitor of 390-470 pF parallel to the coil. How did this affect the receiver's tuning? The volume remained the same, but to tune to the same station, the rod has to be inserted less into the coil. Remove the capacitor completely, leaving only the coil connected. What happened? To tune the receiver to the same station, the rod must be inserted deeper into the coil.
What conclusions can be drawn from experiments with such a crystal receiver variant? There are two main ones.
First, a ferrite rod affects the coil's inductance much more strongly than a metal object, and therefore the circuit tuning.
Second, with the help of a ferrite rod, you can smoothly and accurately tune the receiver circuit to the desired radio station.
One more experiment. Disconnect the antenna and ground from the receiver, connect a diode between them, and headphones in parallel without a bypass capacitor. That's the whole receiver. Does it work? Quietly, probably? Plus, you might hear broadcasts of two or three radio stations simultaneously. You shouldn't expect better from such a receiver. You've probably noticed that when you touch parts or connecting wires with your hand, the volume changes slightly. This is explained by the detuning of the antenna circuit introduced by the electrical capacitance of your body.
Schematic Diagram of the Crystal Receiver
To correctly connect the receiver parts, you used drawings. On them, you saw the coil, headphones, diode-detector, and other parts, devices, and connections as they look in reality. This is very convenient for a start, while you have to deal with very simple electronic designs consisting of a small number of parts. But if you try to depict the device of a modern receiver in this way, you would get a web of parts and wires that would be impossible to figure out. To avoid this, any electrical appliance or radio device is depicted schematically, i.e., using a simplified drawing — a schematic.
There are three main types of diagrams: block (structural) diagrams, schematic (circuit) diagrams, and wiring diagrams. A block diagram is a simplified drawing where groups of parts and devices that perform specific functions of a radio device are conventionally represented by rectangles or other symbols. A block diagram gives only a general idea of the operation of this device, its structure, and links between its functional groups. An example of a block diagram is (Fig. 2), from which I told you about the principle of operation of a broadcasting station.
Is it possible to depict the device of a crystal receiver in this way? Of course, it is possible. Draw four rectangles in a row and connect them with lines with arrows going from left to right. Write the word "Antenna" in the leftmost rectangle, "Oscillatory Circuit" in the next one, "Detector" in the third, and "Headphones" in the fourth. You will get a block diagram of a crystal receiver. It can be read like this: modulated radio frequency oscillations excited in the antenna enter the receiver's oscillatory circuit, and then to the detector; the detector isolates audio frequency oscillations from the received signal, which the headphones convert into sound. Previously, such drawings were called skeleton diagrams or block diagrams. Now this terminology is considered somewhat outdated.
A schematic circuit diagram is more often called a schematic or simply a circuit. On it, all parts of the radio device and the order of their connection are depicted by conventional signs symbolizing these parts, by lines. Reading a schematic diagram, like a geographic map or a drawing of some mechanism, it is easy to figure out the circuits and principle of operation of the device. But it does not give an idea of the dimensions of the device and the placement of its parts on circuit boards.
A wiring diagram, unlike a schematic, informs how the parts of the device are located in the structure and connected to each other. When assembling a receiver, amplifier, or any other radio apparatus, a radio amateur places parts and wires approximately as in the recommended wiring diagram. But the installation and all connections of parts are checked against the schematic diagram of the device. Being able to correctly draw and read radio schematics is an absolutely mandatory condition for anyone who wants to become a radio amateur.
In (Fig. 5) you see parts and devices already familiar to you and some others that you will have to deal with in the future. And next to them in circles are their symbolic graphic images on schematic diagrams.
Any inductor without a core, regardless of its design and number of turns, is depicted on a schematic diagram as a wavy line. Coil taps are shown by dashes. If a coil has a fixed ferromagnetic core (ferrite rod) that increases its inductance, it is designated by a straight line along the coil image. If such a core tunes the receiver circuit, as it was in the experimental receiver, it is indicated on the diagram by the same straight line, but crossed by an arrow together with the coil. A trimmer ferromagnetic coil core is indicated by a short, thick line intersected by a T-shaped symbol.
Any fixed capacitor is depicted by two short parallel lines, symbolizing two plates insulated from each other. If the capacitor is electrolytic, its positive plate is indicated by an additional "+" sign. Variable capacitors are depicted the same way as fixed capacitors, but crossed obliquely by an arrow, symbolizing the variability of the capacitance of this device. Sockets for connecting an antenna wire, headphones, or some other devices or parts are indicated by fork-shaped signs, and binding posts by circles.
New for you is the switch. Instead of unwinding and twisting wires when tuning the receiver, as you did during experiments with the crystal receiver, the coil leads and taps can be switched with the simplest slider, slide, or other switch design. The wires connecting the parts are indicated by straight lines. If lines converge and there is a dot at the intersection, it means the wires are connected. The absence of a dot at the intersection of the wires indicates that they are not connected.
On schematic diagrams, next to the symbolic designations of radio parts, instruments, switching and other devices, corresponding Latin letters are written. For example, all capacitors, regardless of their design features and application, are assigned the letter C, resistors — the letter R, coils — the letter L, semiconductor diodes, transistors, and many other semiconductor devices — the letters VD, V, VT, antennas — the letter W, sockets and other connecting devices — the letter X, headphones, loudspeaker heads, microphones, and other converters of electrical or sound oscillations — the letter B, galvanic cells and batteries — the letter G, batteries of galvanic cells or accumulators — the letters GB, incandescent lamps — the letter H, etc. In addition, the parts are numbered on the diagrams, i.e., next to the letter assigned to the part, a number is written, for example, C1, L1, L2, R1, V1, etc.
To simplify schematic diagrams, sometimes the antenna and headphones are not shown on them, limiting them to just the designations of sockets or terminals for connecting them, but then the corresponding letters with numbers are written next to them: W1, B1. You can find more details about the conditional alphanumeric reference designations of radio engineering elements and devices on radio equipment diagrams in reference literature or the Internet.
Now, knowing the conditional reference designations of parts, it is possible to depict the crystal receivers with which you experimented as schematic diagrams.
The schematic diagram of the first version of the experimental receiver is shown in (Fig. 6, a). You tuned it by changing the number of coil sections included in the circuit by switching the grounded wire. Therefore, switch S1 is introduced into the circuit. Remember our walk through the receiver circuits and take it again, but this time on the schematic diagram. From the beginning of coil L1, indicated on the diagram by a dot, you will get to diode V1 and through it — to headphones B1, then through the headphones along the grounded wire. Switch S1 and turns of coil L1 — back to the starting point. This is the detector circuit. For high-frequency currents, the path from the antenna to the ground goes through the coil sections and the switch. This is the antenna circuit. The receiver is tuned to the radio station by stepwise changing the number of turns included in the circuit. A bypass capacitor C1 is connected parallel to the headphones. On the diagram, capacitor Ca is shown by dashed lines. There was no such part in the receiver. But the electrical capacitance symbolizing it was present — it was formed by the antenna and the ground and was, as it were, connected to the tuned circuit.
The schematic diagram of one of the subsequent variants of the experimental receiver is shown in (Fig. 6, b). Its input tuned circuit consists of coil L1 having one tap, the variable capacitor C2 you introduced, the antenna device, and the antenna capacitor C1. The inclusion of only the upper (according to the diagram) section of the coil in the circuit corresponds to the reception of MW band radio stations, the inclusion of both sections — to the reception of LW band radio stations. Thus, in the receiver, the transition from one band to another is carried out by switch S1, and smooth tuning in each band — by the variable capacitor C2.
The last variant was a receiver tuned with a ferrite rod. You see its schematic diagram in (Fig. 6, c). The oscillatory circuit is formed by coil L1 and fixed capacitor C2. The coil has no taps, which means the receiver is single-band. To receive radio stations of another band, a coil designed to receive stations of that band must be included in the circuit. Sockets B1 are provided for connecting headphones.
Congratulations on completing the basic course! Now you are ready for practical projects!
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