Re: Questions on 160/80 metre AM transmitters.
My preference [and what I am using in my homebrew 5763 transmitter] is to use a parallel-tuned-circuit in the PA anode circuit, with a link coupling winding around the 'cold' end as the output.
This then feeds a symmetrical band-specific low-pass-filter [3 toroids, 4 capacitors] designed for 50 Ohms source and terminating impedance, and then via coax into the antenna-matching unit.
I have always felt that the 'classic' pi-tank-with-two-variable-capacitors is a bit of a compromise, intended to allow the PA to be conveniently matched into antennas that are reactive rather than purely resistive, but this means that you rarely arrive at the optimum values of L and C to perform the low-pass-filtering function. Separating the impedance-transformation and the low-pass-filter functions allows them to be separately optimised.
My approach also means you only have one variable capacitor.
__________________
I'm the Operator of my Pocket Calculator. -Kraftwerk.
|