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LF: RE: Improving Earth Resistance

To: "[email protected]" <[email protected]>
Subject: LF: RE: Improving Earth Resistance
From: Rik Strobbe <[email protected]>
Date: Tue, 3 Jan 2012 22:28:04 +0100
Accept-language: nl-NL, nl-BE
Acceptlanguage: nl-NL, nl-BE
Cc: Robin Gape <[email protected]>, Chris Trayner <[email protected]>
In-reply-to: <[email protected]>
References: <[email protected]>
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Thread-index: AczKWc0lTvh2sGbHReKcV6hZrEADMAAArNy5
Thread-topic: Improving Earth Resistance
Hello John,

150 Ohm is about what I had on 137kHz.
It is a rather high value and most likely caused by either a lot of greenery 
(shrubs and trees) close to the antenna or very poor soil (low conductivity). 
The last is can also be caused by some feet of soil above solid rock.
In my case the cause is greenery and there is little you can do about it, 
except moving the antenna as far as possible away from it or removing the 
shrubs / trees.  Adding radials has little result. 
I have received the suggestion to shield the greenery by covering it with 
(grounded) metal wires but I have many trees near the antenna, so this is no 
option. And it would never have been approved by the aesthetic commission (read 
XYL).
So I had to live with it. To my relief the nefast effect of the trees is much 
less on 500kHz.
In case of poor soil radials will help and you should notice a gradual 
improvement when increasing the radial network.

73, Rik  ON7YD - OR7T

________________________________________
Van: [email protected] [[email protected]] 
namens John Rabson [[email protected]]
Verzonden: dinsdag 3 januari 2012 21:51
Aan: [email protected]
CC: Robin Gape; Chris Trayner
Onderwerp: LF: Improving Earth Resistance

LF, Robin, Chris,

I am trying to reduce the resistance of the earth connection I use with my LF 
antenna.

The antenna consists of two wires each 16 m long, connected in parallel and 
with a height of about 6 m (it was 8 m but the structure has suffered recently 
in the severe gales).

The earth system consists of three wires radiating from the shack with angular 
spacings of about 60°. They run underground through plastic conduit for a 
distance of about 10 m and the remote ends are terminated in metal rods of the 
type used here to provide safety earth connections for mains installations.  
The three wires are connected together at the transmitter to provide the earth.

The local geology is a mixture of granite and limestone with intrusions of 
calcite.

At present the resistance at 137 kHz at the feed point is about 150 ohms (plus 
of course some reactance which it is not difficult to cancel).  At a frequency 
of 980 Hz, I get a resistance of about 25 ohms, depending on whether there has 
been recent recent rain.

Reasoning that the much higher resistance at 137 kHz might result from the 
return current flowing through the ground rather than through the earth spikes, 
 I tried strapping the remote ends of the earth wires. The additional wire ran 
almost below and effectively parallel to the antenna wires.

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 kind of 
metal mesh which is used for fencing.  Unfortunately, such things are expensive 
here (I estimate the cost would run into three figures in Euros), or

2) making the mat out of hookup wire or something similar. I have plenty of 
such wire. Would I need to join the wires at each crossing, and what spacing 
should I use?

Any suggestions, please?

John F5VLF

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