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Date: Mon, 26 Nov 2001 14:02:44 +0000
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Subject: Re: LF: Re: g4ftc
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At 18:52 25/11/2001 +0000, you wrote:
>The one thing which I've never seen in any amateur text books is what are
>the value of the ground and system losses normally achievable? I know this
>is akin to asking how long is a piece of string, but there must be some
>typical examples?
>
>73s
>
>David G4FTC

Dear David, LF Group,

 From my own investigations and those of others,  amateur 136k antenna loss 
resistance can vary between 10-20 ohms at minimum to as much as a few 
hundred ohms, so yours appears to be at the upper end of the spectrum. Much 
bigger, commercial antennas have loss resistances of a few ohms.

The resistance introduced by the actual ground system itself appears not to 
be the dominant factor in most cases - several people have found that once 
a certain point has been reached, improving the ground system further makes 
little difference. The most likely explanation for this seems at the moment 
to be that most of the antenna resistance is due to dielectric losses in 
objects in the field close to the antenna, such as buildings, trees, and 
the ground itself. There seem to be 3 practical approaches to reducing losses:

Increasing antenna height
Increasing the separation from surrounding "lossy" objects
Increasing the amount of top loading.

These three things are obviously interdependent - it is hard to increase 
the length of top loading wires and at the same time increase separation 
from other objects, so experimentation to find the best compromise is 
needed. The most effective measure is to increase height, since this also 
increases the radiated power for a given antenna current, but there are 
usually practical restrictions on this.

For example, I was able to reduce losses in my antenna substantially by 
eliminating the parts of wires closest to the ground, moving the remaining 
wire away from trees and building, and as a side effect increasing the 
height slightly because of the reduced sagging in the wire. On the other 
hand, G3AQC was able to obtain a substantial improvement by increasing the 
area covered by the top loading wires of his antenna. It seems each 
individual location will have a unique solution to what is the best antenna 
to minimise losses.

Good luck with your antenna, and hope to see you on the air soon,

Cheers, Jim Moritz
73 de M0BMU