Antenna radials

Andre' Kesteloot
Fri, 23 Jul 1999 07:31:25 -0400

Rik Strobbe wrote:

> 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
> Villadreef 14  B-3128 Baal  BELGIUM   (JO20IX)