Total posts : 45366
Actually, it acts as a very sharply peaked tuning circuit. A pi-section acts as a low pass filter when the input and output capacitances are equal and the output load is purely resisitve.
In this transmitter, the output load is purely capacitive reactance. The output trimmer capacitor is in parallel with the antenna for tuning. The input capacitor sets the load presented to the RF output transistor and the series inductor(s) are selected to resonate with the antenna/trimmer capacitance.
This is not a classic pi-network tank or pi-section low pass filter because the output load is not purely resistive.
When using the base loaded vertical antenna. The internal inductors are bypassed out of the circuit and the trimmer is connected in paralled with a fixed cap to ground. The result is an L-net formed by the capacitors and part of the external loading coil. The trimmer then serves as a loading adjustment and the antenna is tuned to resonance by selecting a coil tap and adjusting the length of the antenna. This is a vastly more efficient tuning method resulting in much longer range compared to the internal tuning circuit which is good for shorter range and convenient tuning over the entire band.