From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
CC: Robin Gape <robingape@ukfsn.org>, Chris Trayner <C.Trayner@leeds.ac.uk>
Date: Tue, 3 Jan 2012 22:28:04 +0100
Thread-Topic: Improving Earth Resistance
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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 greener=
y (shrubs and trees) close to the antenna or very poor soil (low conductivi=
ty). 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, e=
xcept moving the antenna as far as possible away from it or removing the sh=
rubs / trees.  Adding radials has little result.=20
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 mu=
ch less on 500kHz.
In case of poor soil radials will help and you should notice a gradual impr=
ovement when increasing the radial network.

73, Rik  ON7YD - OR7T

________________________________________
Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org=
] namens John Rabson [john.rabson@numeo.fr]
Verzonden: dinsdag 3 januari 2012 21:51
Aan: rsgb_lf_group@blacksheep.org
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 rece=
ntly in the severe gales).

The earth system consists of three wires radiating from the shack with angu=
lar spacings of about 60=B0. They run underground through plastic conduit f=
or a distance of about 10 m and the remote ends are terminated in metal rod=
s of the type used here to provide safety earth connections for mains insta=
llations.  The three wires are connected together at the transmitter to pro=
vide 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 (p=
lus of course some reactance which it is not difficult to cancel).  At a fr=
equency of 980 Hz, I get a resistance of about 25 ohms, depending on whethe=
r 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 spi=
kes,  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 ex=
pensive here (I estimate the cost would run into three figures in Euros), o=
r

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 spaci=
ng should I use?

Any suggestions, please?

John F5VLF=