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
[email protected]
|
