Message-ID: <4F037C64.3070108@iup.uni-heidelberg.de>
Date: Tue, 03 Jan 2012 23:08:36 +0100
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: Improving Earth Resistance
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Hi John,

Am 03.01.2012 21:51, schrieb John Rabson:
> The earth system consists of three wires radiating from the shack with =
angular spacings of about 60=B0. They run underground through plastic con=
duit for a distance of about 10 m and the remote ends are terminated in m=
etal rods of the type used here to provide safety earth connections for m=
ains installations.  The three wires are connected together at the transm=
itter to provide the earth.
>   =20
That sounds not bad.
> The local geology is a mixture of granite and limestone with intrusions=
 of calcite.
>   =20
Not optimal.
What about the house mains earth? Is it connected in parallel? I would=20
do so. If you use the TX antenna as the RX antenna too, it may be an=20
idea to disconnect the mains earth automatically by a relay to prevent=20
local QRM coming in to much...

> At present the resistance at 137 kHz at the feed point is about 150 ohm=
s (plus of course some reactance which it is not difficult to cancel).
Are there many trees sourrounding the antenna? A phot would help us here.

>   At a frequency of 980 Hz, I get a resistance of about 25 ohms, depend=
ing on whether there has been recent recent rain.
>   =20
Where did you connect that frequency generator. One pole is the earth,=20
but what is the other pole?
I think it is only useful to measure the losses at the desired frequency.

> Reasoning that the much higher resistance at 137 kHz might result from =
the return current flowing through the ground rather than through the ear=
th spikes,  I tried strapping the remote ends of the earth wires. The add=
itional wire ran almost below and effectively parallel to the antenna wir=
es.
>
> This strapping made practically no difference to the 137 kHz resistance=
, so I had further thoughts:
>
> 1) laying down an earth mat or something like chicken netting, or the k=
ind of metal mesh which is used for fencing.  Unfortunately, such things =
are expensive here (I estimate the cost would run into three figures in E=
uros), or
>
> 2) making the mat out of hookup wire or something similar. I have plent=
y of such wire. Would I need to join the wires at each crossing, and what=
 spacing should I use?
>
> Any suggestions, please?
>   =20
Hmm, first i would switch the mains earth in parallel.
Then, if the goal is to optimise the antenna efficiency it appears to me=20
it is rather useful to invest in a larger pole to increase the effective=20
height of the antenna wire. There are 18m poles available, not to=20
expensive. With 2 or 3 x 3 supports there should be no problem.
The metal rods you use are excellent!
Maybe make some experiments (on 137.0 kHz, with a resonated antenna),=20
measuring the antenna current and power. First use 1 rod (exclusively),=20
then 2, then 3. You will see if it is worth to spend another 3 rods or so.

Which antenna C do you have?

73, Stefan/DK7FC