Earth losses

[Andre Kesteloot]

-------- Original Message --------
Subject: Re: LF: Earth losses
Date: Wed, 09 Jul 2003 17:29:23 +0100
From: James Moritz <j.r.moritz at herts.ac.uk>
Reply-To: rsgb_lf_group at blacksheep.org
To: rsgb_lf_group at blacksheep.org
References: <002001c34622$caa13220$80c828c3 at captbrian>



Dear Bryan, LF Group,

The ground here is a heavy clay, that remains pretty wet all year round, so 
I imagine fairly high conductivity. The usual ground system I use with my 
antenna (40m long single inverted L wire abt 10m high) is 6 x 1m long 
ground rods, distributed over a few m radius around the antenna feed point. 
This arrangement has a loss resistance of about 36 ohms. Using just a 
single ground rod increased this to 56 ohms, so I estimate the resistance 
of each rod is about 20 ohms. I have tried several larger arrays of ground 
rods (it is then necessary to make arrangements to equalise the currents in 
the ground rods) - but the best result I have achieved so far was to reduce 
the loss resistance to 33 ohms - so a point of diminishing returns is 
quickly reached.

I also tried a counterpoise system; covering the whole available area 
(about 50m x 12m) in counterpoise wires about 2m off the ground and 1.5m 
apart. The best result with this system was to reduce the loss resistance 
also by about 10%. I think a counterpoise works in two ways - firstly, it 
acts as a large area, capacitive ground connection. but as the ground rod 
experiments showed, this does not seem to make a lot of difference. Also, 
it acts as a screen between the antenna and the actual ground, which 
prevents the electric field of the antenna reaching the lossy dielectric of 
the soil, and provides a metallic return circuit for the ground return 
currents. But for me, and I imagine most other amateurs, the available area 
for a counterpoise is much too small to intercept a large proportion of the 
field of the antenna. so it has a limited effect. The same kind of argument 
would apply to buried radials.

It seems to me that the majority of loss in small amateur LF antennas is 
caused by dielectric losses, partly in objects around the antenna like 
buildings and trees, and partly in the ground itself under the antenna. 
This is bourne out by measurements which show that loss resistance falls 
with increasing frequency - as the frequency gets higher, the antenna 
reactance decreases, therefore the antenna voltage for a given current 
decreases, and so the electric field and the dielectric losses. This would 
also explain why increasing the amount of wire in the air reduces the loss 
- also increasing the height has a similar effect.

The weather conditions have a substantial effect on loss resistance - a 
period of rain gives about 25% increase in loss resistance, icy weather 
about 15% reduction.

 From a practical point of view, beyond some minimum amount of ground 
system, there is little to be gained by extending the ground system further 
- the best I could get was about 0.5dB increase in radiated power. In my 
circumstances, there is little to be gained by having more than the 
original 6 rods - allthough I suspect this would be different in areas of 
low ground conductivity. Any benefits are swamped by day-to-day weather 
changes. On the other hand, the height of the antenna has much more impact. 
putting a 13m high pole under the middle of the span of wire, making it 
into an inverted V shape gives an immediate 3-4dB improvement in ERP. I 
reckon the best improvement to be had from all the ground system mods is 
worth about the same as a 0.5m height increase of the antenna wire. So the 
first priority must be to get the antenna as high as possible, even if this 
means less wire in the air. The trouble is the neighbors don't like it...

Cheers, Jim Moritz
73 de M0BMU




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