Message-ID: <3888D0A3.85ED8E7A@alg.demon.co.uk>
Date: Fri, 21 Jan 2000 21:33:23 +0000
From: "Steve Rawlings" <steve@alg.demon.co.uk>
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Re.Ground Loss
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Laurie G3AQC wrote:

> I have just measured my TX ant impedance(R term) and it is 120 ohm!
> much higher than I originally thought.  It is of course all ground
> loss . . . . 

With the simple verticals which I use on 136 kHz, I have always tried to
minimise the proximity between the radiator itself and earthy objects
such as our house; the garden fence; and nearby trees.  From earlier
contributions to this Reflector (thanks are due to Peter Martinez G3PLX,
and others), I learned that the parallel loss caused by the proximity
between the radiator and lossy ground translates into a
series-equivalent resistance - which, in turn, contributes to the [120
ohm] "ant impedance".  [I hope I have learned my lessons well, Peter!] 

So improvements to your ground system may help, but it might also be
worth taking another look at your antenna to see whether there are
opportunities for increasing the distance between your antenna system
and nearby (earthy) structures.

> . . . .  I wonder whether anyone can offer me comparitive
> figs for impedance and details of ground systems?

Before pruning trees in my neighbour's garden:
**********************************************
ANTENNA CONFIGURATION      WET DAY     DRY DAY
12 m vertical              160 ohm     130 ohm
20 m vertical              130 ohm     100 ohm

After pruning trees in my neighbour's garden:
**********************************************
ANTENNA CONFIGURATION      WET DAY     DRY DAY
12 m vertical              130 ohm     100 ohm
20 m vertical              100 ohm     75 ohm

Ground system
*************
Shack earth is isolated from the antenna ground connection using a
matching transformer.  'Cold' end of transformer secondary winding is
connected to outside water tap.  

For more information, see:
http://www.alg.demon.co.uk/radio/136/experi_vert.htm

The trees (prior to pruning) can be seen just behind (apple tree), and
to the right (flowering cherry tree), of my vertical antenna at:
http://www.alg.demon.co.uk/radio/136/pictures/expvert1.jpg


Regards to all,
Steve GW4ALG