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From: Hans-Joachim Brandt <hajo.brandt.dj1zb@t-online.de>
To: rsgb_lf_group@blacksheep.org
Date: 19 Jan 2005 17:04 GMT
Message-ID: <1CrJPv-1ZFoRO0@fwd00.sul.t-online.com>
Subject: Re: LF: Antenna Tuning
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Dear Walter,

I have seen your contribution rather late, because I have been travelling.

My opinion is that circuit A allows tuning the whip to resonance and matching it to the amplifier, by changing the number of link turns. Therefore tuning is very sharp.

In circuit B (which I have seen for the first time) the coupling between the amplifier resonant circuit and the aerial coil will be  much tighter. Tuning the plate capacitor will even influence the aerial resonance, therefore only few taps are needed for the whip. On the other hand I do not see any means to match the whip (plus aerial coil resistance) to the amplifier. Obviously this has been omitted to simplify the circuit. I guess the load resistance of the amplifier will be lower than optimum, hence the broad tuning characteristic.

HW?

Happy New Year!

73 Ha-Jo, DJ1ZB 


"Walter Blanchard" <blanch@pncl.co.uk> schrieb:
> It's a bit late for Christmas puzzles but try this one:
> 
> Circuit A is the usual antenna tuning cct for a short highly reactive whip. It has the advantage that the antenna tuning inductor can be a long way away from the PA.
> Circuit B has been widely used for HF walkie-talkies using a short whip. Only one coil is needed because the end of the antenna can be brought right onto the PA board.
> 
> Puzzle:
> The tuning coil in A tunes very sharply; you really need a continuously-variable inductor.
> That in B tunes quite flatly and you only need a few fixed taps.
> Why?
> 
> Incidentally, are they really the same circuit?
> 
> Happy New Year!!!
> Walter G3JKV.
>