Calculation of the transmitter P - contour

P-contour Calculator for Power Amplifiers

Pi-network calculation based on the methodology of I. Goncharenko (DL2KQ).

To start the calculation, set your values for frequency F, source impedance R1, load impedance R2, unloaded coil Q-factor Qxx (assuming the unloaded Q of capacitors C1 and C2 is much higher than that of L, which is true for most capacitors), and your desired loaded Q-factor Qh. Then, click "Calculate".

For a tube amplifier operating in class AB with a small idle current, the source impedance R1 can be roughly estimated using the following formulas:
For the anode circuit, R1 ≈ Ea / (1.8 * Ia) (where Ea is the DC anode voltage and Ia is the maximum DC anode current) or R1 ≈ Ea² / (2.5 * P) (where P is the maximum effective output power).
For the input Pi-network of a grounded-grid PA, R2 ≈ 1/S (where S is the tube's transconductance).
The approximation of R1 is related to the conduction angle (i.e., idle current) and the type of tube, or more precisely, its minimum allowable instantaneous anode voltage.

Increasing the loaded Q-factor (Qh) of the Pi-network improves harmonic suppression and increases capacitor values (important for tubes with high output capacitance), but it lowers efficiency and increases reactive currents, imposing stricter requirements on component quality.

To evaluate the structural requirements for the Pi-network components, the calculator provides the effective current in the coil (to help select the wire cross-section) and in the capacitors (to help choose the contact quality, plate thickness, and dielectric). It also calculates the peak voltage (which capacitors must withstand with a safety margin) and the reactive power (in kVAr) for both capacitors.

For capacitors other than vacuum and air-variable types, the rule of thumb is: if the datasheet does not specify reactive power or allowable current at the operating frequency, the capacitor is likely not designed for high-power RF circuits and will burn out. Keep in mind that the voltage on the output capacitor can increase by 1.5 to 2 times depending on the antenna SWR and tuning accuracy. Leave a sufficient gap between the plates of C2 to prevent spark breakdown and plate welding when tuning the Pi-network at full power.

Input Parameters

Frequency (kHz):

Output Power (W):

Source Z - R1 (Ω):

Load Z - R2 (Ω):

Unloaded Q (Qxx):

Loaded Q (Qh):

Calculation Results

Actual Output Power (P)	-
Power Losses in Coil	-
Transmitter Efficiency	-
Coil Inductance (L)	-
Current in Coil	-
Hot Capacitor (C1) Capacitance	-
Cold Capacitor (C2) Capacitance	-
Reactive Power in C1	-
Reactive Power in C2	-
Peak Voltage across C1	-
Peak Voltage across C2	-
Current through C1	-
Current through C2	-

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