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Date: Sun, 07 Mar 1999 15:16:38 +0100
To: rsgb_lf_group@blacksheep.org
From: "M. Bruno" <spin@inrete.it>
Subject: Re: LF: Care and feeding of a top-loaded vertical
In-reply-to: <199903070720.SAA28683@godzilla.zeta.org.au>
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At 18.24 07/03/99 +1100, Steve Olney wrote:

>The questions I have are:
>1.  How do you measure the capacitance to ground to be able to calculate
>the inductance of the loading coil?  I doubt you can just attach a
>capacitance meter between the base of the antenna and ground.  I am
>guessing that you just make an estimate and find the required inductance by
>trial-and-error.
>
My personal experience is that many capacitance meter are fooled by the
RF and noise pick up by the antenna. I solved the problem connecting the
vertical in parallel to a fixed C and then measuring the difference. The fixed
one was a couple of nF. 
Then I built an RF impedance bridge, and calibrated it on the bench, to be
able to measure R(loss) and C(aerial). I had the same reading as with the
capacitance meter.

>2. Is it possible to connect a smaller *variable* inductance (about 10% of
>the loading coil inductance) between the base of the antenna and the
>grounded elevated radials to effect a convenient tuning arrangement?
>
It should be possible. I have a different arrangement, having a variometer
at the base of the vertical and no inductance at the top, so no direct
experience,
but I see no particular problems to have, let say, 90% of the L on top and
the remaining at the base. The current distribution should not vary too much.

>3. Given the arrangement outlined in 2. above, is it possible to tap into
>that smaller inductance at the base to effect matching to the 50 ohm line
>or don't you get the transformer effect required because the major portion
>of the inductance is elevated and not inductively coupled?
>
Again, I see no problems. Personally I forgot SWR and coupling ... my system
has an input Z of about 35 Ohm (dry WX) to 50 Ohm (rain). The coax line is
too short in terms of lambda to worry about SWR ! I directly go to the TX
output.

On what frequency do you expect to trasmit, and what ERP ... ?


73 de IK1ODO Marco

Rivalta, ITALY JN35SA (N 45 01' 25.6", E 7 31' 09.4")