I've always wondered about dipoles / loops at 136k?
A 'dipole' just lying on rocky ground where the water table is many metres
below the surface could be quite effective. The angle of radiation would
be poor
but the efficiency may not be as bad as first suppossed. One advantage of
a
big antenna on the ground is that it can't fall down and, for the local
planning committe, it's less than 9 feet high.
Some time ago I tried to simulate this by making a dipole for 21MHz and
had a
number of QSOs through it while gradually lowering it's height and
retrimming
as I went. I managed to work Romania were I sent "Ant dipole 5cm agl". I
have no idea what the Romanian though of this madness and he didn't stay
around
long to find out.
Incidentally, at about 2 inches off the ground the antenna was resonant at
about 20% less than it's normal 'free space length'. Never did find a big
field
to try it on 136k.
David G0MRF
I did some work on low horizontal dipoles when in GW, though always with
some centre inductive loading. I was on top of a mountain for two reasons:
there was plenty of space, and I assumed that the soil conductivity was
poor. I ran several hundred metres of wire along a fence line, slightly
sloping as the fence ran uphill. These experiments were on 73kHz, and for a
short time I held the world one-way distance record (8km!) using this
arrangement.
When trying for the first GW-GI on 73kHz, I was forced to repeat the test
because there was absolutely no wind and my kite antenna would not fly, even
on top of the mountain. The dipole was not successful and the test failed.
The Q seemed very low and I think absorption by the ground was too great. It
worked OK on receive, though.
In 1999, the late John Taylor, G0AKN, did some tests from Cornwall using
very long horizontal wires terminated in earth rods. The results were poor
(see http://www.lf.thersgb.net/gallery/g0akn.htm).
I suspect the critical factor is height. The antenna would have to be as
high as some of our Marconis are, but also very long.
Mike, G3XDV
http://www.lf.thersgb.net
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