Cable Parameter Measurement and Time-Domain Reflectometry.

Cable Parameter Measurement and Time-Domain Reflectometry (TDR)

With the NanoVNA, you can easily locate discontinuities in coaxial cables, measure the exact length of a feeder line, and find transmission line faults (such as open or short circuits) without the need to unroute or remove the cable.

Time-Domain Reflectometry (TDR)

This function is based on the Inverse Fast Fourier Transform (IFFT), which converts the measured vector parameters in the frequency domain into a time-domain line response. A signal pulse is sent into the cable, reflects off a discontinuity (the end of the cable, an open circuit, or a short circuit), and returns. The device calculates the exact distance based on the time delay of the reflected signal.

Schematic representation of impulse probing of a cable line (TDR).

Steps for setting up TDR on the NanoVNA screen:

1. Perform an OSL calibration on port CH0.
2. Connect the cable under test to port CH0. Leave the far end of the cable open or short-circuited (this ensures maximum signal reflection and a sharp peak on the graph).
3. Go to the menu DISPLAY → TRANSFORM → TRANSFORM ON (the menu text will become inverted to show it is active).
4. Set the pulse type: LOW PASS IMPULSE (for impulse response) or LOW PASS STEP (for step response).
5. Go to DISPLAY → TRANSFORM → VELOCITY FACTOR and enter your cable's velocity factor as a percentage (e.g., enter 66 for a standard RG-58 cable with solid PE dielectric). A decimal point is not used on the device—enter a whole integer.
6. Set the ringing smoothing via WINDOW → MAXIMUM.
7. Place the marker on the peak (spike) on the graph. The device screen will display the exact physical length of the cable up to the point of reflection (the open or short fault).

Measuring the Characteristic Impedance of an Unknown Cable

To accurately measure a cable's characteristic impedance (Z₀):

1. Connect the cable to CH0, and terminate the far end of the cable with a variable, non-inductive resistor (potentiometer).
2. Switch the device to display R (Resistance) and X (Reactance) traces.
3. Observe the traces while sweeping the frequency. Adjust the variable resistor at the far end until the wave-like ripples (oscillations) on the graphs flatten out and the Reactance (X) line settles precisely at 0 Ohms. Measure the variable resistor with a multimeter at this exact setting—this value will equal the cable's characteristic impedance.

Matching Devices: Quarter-Wave Transformers and Half-Wave Lines

In amateur radio practice, segments of coaxial cable cut to a strictly defined electrical length are widely used as impedance matching transformers, baluns, or phase-shifting lines.

Tuning a Quarter-Wave (1/4 λ) Transformer

A quarter-wave section of coaxial cable has the unique property of inverting the complex load impedance. At the resonant frequency, a cable left open at the far end will present an input impedance approaching zero. Step-by-step tuning method:

1. Preliminary calculation: Calculate the physical length of the cable stub using the formula:
   L = (300 / f) × VF × 0.25
   Where: f is the frequency in MHz, and VF is the Velocity Factor of the cable. Cut the cable leaving a technological margin of about 5–10% (a few centimeters longer).
2. Connection: Perform a calibration on port CH0. Connect the prepared cable section to CH0. Leave the far end open.
3. Finding resonance: Enable the trace display in REACTANCE (or R+jX) format. Set the sweep range around your calculated frequency. The marker will indicate the resonant point—the frequency at which Reactance (X) is exactly 0 Ohms, and Resistance (R) is at its minimum.
4. Length trimming: If the resonant frequency is lower than required, the cable is electrically too long. Snip very thin slices (2–5 mm each) from the open end of the cable, monitoring the zero-reactance point shift upwards in frequency. Keep trimming until the resonant point (X = 0 Ohms) perfectly matches your target operating frequency.

Measuring SWR and impedance of a dipole antenna at the operating frequency.

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