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Thankyou very much to those who answered my
questions.
When I have used an E-field probe to explore unwanted
radiation from circuit boards, the orientation of the probe had an effect on the
signal strength. But now I can see that on this larger scale, the plate of the
probe will generally be above the counterpoise, so vertically polarised signals
will always be favoured, whatever the orientation of the probe.
I feel I have a better understanding of how it works
now.
thanks once again.
73, Hugh M0WYE
----- Original Message -----
Sent: Wednesday, June 22, 2011 2:10
PM
Subject: Re: LF: PA0RDT
polarisation
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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