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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