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Message-ID: <3.0.1.16.19990723102121.2f1f9324@mail.cc.kuleuven.ac.be>
Date: Fri, 23 Jul 1999 10:21:21
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: 22. july
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At 07:26 23/07/99 +0200, DL1SAN wrote:
>Yesterday I installed abt. 20 radials (10-35m) to my antenna system. But
>there was no significant change of the antenna current. The current is abt
>.6 A with and without the redials. Can anyone explain this?

I encountered a similar problem when starting on 136kHz (april 1998).
Maybe my 'story' will be of some help to others.

Whith my first antenna (a 10m high 'umbrella-antenna') and 6 radials each
20 to 30 meter long (iolated, on the ground) I got a 'groundloss' of about
250 Ohms. Adding an 1 meter eathstake at the antenna feedingpoint reduced
the loss to 220 Ohms.
First I though that the soil was very bad (for LF) but when I lowered my
antenna from 10 to 5 meter the losses reduced from 220 to 150 Ohm, with a 2
meter high antenna even to 110 Ohm. So I came to the conclusion that the
main loss was not in the ground but in the many trees that were surrounding
the antenna. 
For your information, 3 sides of the 'lot' my house is on are surrounded by
forrest. I once read that you should avoid to have any 'greenery' in a zone
around the antenna that is equal to the height of the antenna, in my case I
had about 35 trees (up to 20 meter high) in this zone.
As a next step I decided to re-arrange the complete antenna and to try to
get it as far as possible from the trees. I constructed a 13 meter high
inverted-L with a 26 meter long topsection, consisting of 4 parallel wires
each sparated 80cm. The number of threes in the 'critical zone' was reduced
to about 20, the loss was now 150 Ohm (instead of 220 Ohm). Next I replaced
the existing 'ground-system' (1 earthstake of 1 meter and the 6 isolated
radials) by 6 earthstakes of each 2.5 meter (and all separated about 10 to
15 meter) and a radial system that interconnects all this stakes (total 100
meter of wire). The radials are blank galvanized iron (2mm) and dug about
15cm deep in the soil. The earthstakes are also galvanized iron (2cm
tubes). The loss was now reduced to about 120 Ohm. During winter I added
several very long radials (up to 50 meter) but no further decrease of the
loss was found. When I had to remove the extra radials in spring (mowing !)
also no increase of loss was found, so this extra radials seemed to be
useless.
So I am afraid that the main part of my 120 Ohm loss in not in the earth
but in the trees and that I will have to live with it.

Some final remarks :

1. Lowering the antenna decreased the loss, but decreased the radiation
resistance even more, so dispite higher antennacurrent I had lower ERP.

2. Do not try to get a 2.5 meter long and 2 cm wide iron tube into the
earth with a hammer, it will bend or break. Just connect the upper end of
the tube to the watersupply (using the garden hose) and you can gently push
the tube into the ground with one hand. But stop the waterflow when the end
of the tube is still about 50cm above ground and wait about 10 minutes (it
will sink some 10 to 30 cm deeper) and then sink it to its final position
with very low waterpressure. If you stop the waterflow too late the
tubemight dissapear completely into the earth. Also the soils has to
'fixate' the tube (can take days to some weeks), during the time the
'groundconnection' will further improve.

73, Rik


Rik Strobbe  ON7YD
rik.strobbe@fys.kuleuven.ac.be
Villadreef 14  B-3128 Baal  BELGIUM   (JO20IX)