LF: Re: Wilkinson combiner, Phase Shift

[MarkusVester@aol.com]

Dear LF Group,

one thing worth considering is the phase shift between the transistors and the antenna input, including all filters and matching circuitry. I have always tried to keep that at 0° or 180°, as this lets the FET's see a series resonant load. This becomes a high impedance in case of antenna detuning or arcing, a safe condition for a class-D voltage source. Actually I have not lost a single FET in spite of several antenna faults (... knocking on wood as I write this ;-).

Feeding a tap on the loading coil will introduce a phase shift close to 90°, showing impedance inversion. A Wilkinson combiner also adds in a quarterwave. But of course it is the total electrical length of the output chain at 137kHz that counts.

73
Markus, DF6NM