[Lf] [Fwd: LF: RE. Counterpoise Expt]

[Andre Kesteloot]

Alan Melia wrote:

> Hi Jim, that was an interesting test. In a way you confirmed some ideas I
> have had from my tests, and also that Laurie has proposed. These are that
> the most important bit of 'ground' is directly under the wire.
>
> After comments in the discussion started by Alex and John, I went back to
> re-read the article in the Appendix of the new Handbook. One thing I found
> that had eluded me at the first read was that the maximum 'earth' current
> was under the remote end of the aerial. Although the test aerial was a sort
> of flat-top 'T' , the effect should be the same for an inverted 'L'. This
> surprised me initially until I realised that the remote end had the highest
> RF voltage. Meissner does not describe a counterpoise but multiple earth
> stakes, but I think the conclusions are still valid. It does suggest that
> the extent of the 'mat' at the far end is the most important , then getting
> the current back to the feed point. One 'worry' is that if only a small
> portion of the current flows in your counterpoise, where is the rest going?
> Simplistically if you consider the counterpoise and the natural ground in
> parallel, it suggests that the counterpoise 'resistance' is about 4 times
> the natural resistance. That doesnt seem sensible, so the only other thing
> can be that it is only intercepting 25% of the the 'lines of force'
> (Meissner's terminology). Another suggestion is that maybe you should
> connect the remote end of the counterpoise to earth stakes, to collect the
> ground current and conduct it efficiently back to the feed-point. The other
> worry is, " does putting a counterpoise at that height reduce the effective
> aerial height by 2 metres?". If so at low top heights it could be
> counter-productive.
>
> I suppose from another angle...you recorded a field strength of 6dB below
> the calculated value averaged over many readings. Maybe this means that half
> the current is diverted into environmental losses anyway.
>
> I am not sure whether I have all the measurements but I know that at one
> stage Finbar experimented by rolling out and removing an insulated wire
> counterpoise wire (on the ground) under his inverted 'L' (at that time) and
> it made little difference to the loss resistance measured. He does have
> quite a low value anyway anout 20 ohms. He found that increasing his aerial
> capacitance to 1050pF (from 550pF)  dropped the loss resistance down to
> about 11ohms. I am just trying to sort through this set of measurements to
> add to my web site.
>
> Cheers de Alan G3NYK
> alan.melia at btinternet.com