james moritz wrote:
I have been doing some investigation into the design of tapped loading
coils for LF antennas � specifically, where to put the tap for the TX
connection to achieve matching to 50 ohms. As far as I can see, there
is no simple formula that gives a realistic result, and generally the
tapping point is selected by trial and error. But it would be nice to
know at least approximately where the taps should be, to take the
guesswork out of making a coil.
Experimental work in 1996 to 1998 on the 73kHz band was concerned mainly
with how to transfer RF power from the amplifier to the antenna. Most of
the amplifiers used at the time were high power audio amplifiers with an
output impedance of 4ohms so the 50ohms system was a bit irrelevant.
The tapping point (loading) was affected by the resonance adjustment
(tuning). In fact I used a 8 turn multi-tapped coupling link on a 7mH
coil, normally tapped at 4 turns (73kHz).
The method of tuning was to note the PA FET current while tuning through
resonance, while at the same time noting the antenna RF current. The
objective was to obtain the greatest RF current while at the same time
ensuring the PA FET power dissipation was not exceeded.
Regards
Peter, G3LDO
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