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
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