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Date: Fri, 28 Jul 2006 00:03:04 +0100
From: Peter Dodd <g3ldo@ukonline.co.uk>
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Subject: Re: LF: Tapped loading coil design spreadsheet
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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