[Lf] Improving LF Antennas

[Andre Kesteloot]

James Moritz wrote:

> 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