Simplified calculation of the P-loop. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Amateur radio calculations

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The following describes a method for calculating the elements of the P-loop circuit, which shortwavers often use in the final stages of radio transmitters (see figure).

End stage radio transmitter

Dashed lines indicate parasitic capacitances C01 and C02, as well as the load resistance of the circuit Ra (input resistance of the antenna-feeder system). When calculating the capacitance of Ca and C01, let's combine CA and C02, denoting: C1=Ca+C01;

С2=СA+С02 (we assume that the capacitance of the capacitor Ср is large and we neglect its influence).

Initial data for calculation:

Frequency fo.

Lamp anode load resistance Ra; it is determined during the calculation of the lamp mode or taken from tables for the typical mode of the selected lamp (see, for example, [1], p. 236).

Loop load resistance RA; it depends on the type and design of the antenna-feeder system and, with good matching of the feeder with the antenna (SWR close to unity), is equal to the wave impedance of the feeder.

Quality factor of the loaded circuit Qe; most often, Qe = 10-20 is assumed (for higher frequencies, a smaller value is taken).

The efficiency of the circuit, which is given from general energy considerations, it is advisable to take n = 0,8.

Calculation of contour parameters

For the convenience of calculation, we introduce an auxiliary coefficient:

Let's calculate the capacitance values:

Then the equivalent capacity of the circuit is determined:

and coil inductance

For all calculations, the following values ​​are taken: C in pF, fо in MHz, R in kOhm, L in μH.

According to known formulas (see, for example, [2], p. 75), we determine the number of turns of the coil, given its dimensions.

In conclusion, we determine the values ​​​​of parasitic capacitances:

C01=Cvx+Cm1+Cdr+Scat+Cabeginning and C02=Cm2+Sper+CAbegin,

where: Сout is the output capacitance of the lamp,
Cm1 - mounting capacity of the anode part of the circuit,
Cm2 - mounting capacity of the antenna part of the circuit,
Cdr - own capacity of the throttle,
Skat - own capacity of the coil,
Sper - parasitic capacitance of the switch,
SA start and SA start - the initial capacitances of the capacitors.

The value of Cout is found in reference books. The intrinsic capacitances of coils and chokes are determined by calculation (see 12], pp. 81-82) or by measurements (on a Q-meter or by the resonance method); the initial capacitances of the capacitors and the parasitic capacitance of the switch are also best measured.
Mounting capacitance (Cm1 and Cm2) is usually set within 5-20 pF, depending on its design.

Evaluation of calculation results

It is necessary that the values ​​of C1 and C2 be greater than the values ​​of C01 and C02, respectively. Sometimes, especially at high frequencies, it happens that C1

Capacitance C1 (for a given frequency fo) can be increased by increasing the quality factor of the P-loop or reducing Ra. Increasing the quality factor in amateur conditions is difficult to implement. A decrease in Yes can be achieved by using another lamp, which in a typical mode requires a smaller Yes, or by changing the mode of the previously selected lamp, reducing Ua, and slightly increasing the excitation.

Reducing CO01 is possible through the use of parts with smaller parasitic capacitances and improved installation. True, the improvement of installation, as a rule, requires an increase in the dimensions of the final stage.

After clarification of certain parameters, the calculation is again carried out in the specified sequence.

The accuracy of the calculation essentially depends on how close to the real value the initial data for the calculation are taken.

With correctly selected parameters of the P-loop, the operation mode of the cascade will be optimal. This can be judged by a decrease in the anode current of the lamp by 20-25% when the circuit is tuned to resonance.

Literature

1. S. Bunimovich, L. Yaylenko. "Amateur Single-Sideband Radio Technique", ed. DOSAAF, 1964
2. "Handbook of a radio amateur" under the general editorship of V. V. Melnikov. Sverdlovsk book publishing house, 1961

Author: L. Chernov (UA3SR), Ryazan; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Amateur radio calculations.

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