Hello Hugh,
The model i have in my mind is a capacitive divider that is within an E
field (TX signals and noise). There are 2 plates, the probe and the
counterpoise. The counterpoise has a C against ground that is several
orders higher as the probe, normally. If the antenna is in a fixed
position, you may vary the way the feed cable runs, and change its
capacity against ground from say 1 nF to 2 nF. But the plate has just
about 3 pF and so the voltage across the gate and source of the amp
input does not change significantly.
However, if you place the probe near a tree or a high building, the
signal usually drops significantly, since the E fields becomes much
lower near lossy and conductive things. So this directly affects the
input voltage and thus mostly the SNR.
Rising the antenna much above ground is usually useful. But if you would
be outside in a flat landscape without trees and local noise, is would
be totally appropriate to put it on the roof of a car.
Good luck with your works. What do you plan? :-)
73, Stefan/DK7FC
Am 21.06.2011 17:23, schrieb Roelof Bakker:
Hello Hugh,
Though it may not make sense, mounting orientation is irrelevant.
And the feed line does not need to run vertical either.
I have tested the antenna at the same "position in space" with two
ways of mounting.
The first one was the normal way with a vertical mast in the garden.
The second way was using the same mast and cable length, but this time
extended horizontal from an upstairs window.
The receiver was a battery operated selective level meter, which could
be read to 0.5 dB.
The test was carried out receiving the vertical polarized signal of
NDB ONO on 399.5 kHz, 59 km away.
The measured levels agreed within 1 dB!
So the good news is when the need arises, the antenna can be mounted
on a pole and pushed out of the window!
Best regards,
Roelof Bakker, pa0rdt
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