Hi all,
Am 06.07.2013 15:16, schrieb Pieter-Tjerk de Boer:
Hello all,
I tend to think about the mini-whip on LF and MF in terms of an (almost)
static electric field.
Then it essentially measures the electric field's _potential difference_
between a point somewhere up in the air, namely where the mini-whip's
metal plate is located, and ground. The ground reference is brought up
to the mini-whip's electronics either via the metal pole on which it is
mounted, or (the outside of) the coaxial cable.
I can agree with this model as well as the model describing two
capacitor plates.
This explains Roelof's observation (see below) that it doesn't matter
whether he mounts the mini-whip on a vertical pole, or on a horizontal
pole out of a window (but in the same position).
In both cases, it measures the same potential difference, although in
the latter case the ground connection is longer, namely taking the detour
via the horizontal pole and whatever is inside the house. Presumably,
Roelof's house is small compared to the 399.5 kHz wavelength, so this
detour shouldn't matter.
This view also at least approximately matches VK1OD's NEC4 calculation,
in the sense that he finds an output voltage which is of the order of
the field strength times the antenna height.
Not the antenna height but the distance between the probe and the
grounded shield of the coax, or the grounded part of the circuit, which
is the closest grounded part to the probe. The potential difference, the
intagral E . ds... Of course the E field is not homogen in the electrode
arrangement, not a simple idealized plate capacitor...
73, Stefan/DK7FC
A weak point in this reasoning is the fact that since the entire pole
(or outside of the coaxial cable) is at ground potential, it distorts
the electric field around it. So the mini-whip's plate is not at the
same potential as it would be without the ground connection reaching out
to it. I still intend to try to calculate how much this distortion is.
Of course, this whole reasoning breaks down at higher frequencies, where
the height of the pole is not small compared to the wavelength; then one
cannot simply assume anymore that the entire pole is at ground potential.
73, Pieter-Tjerk, PA3FWM
On Fri, Jul 05, 2013 at 07:47:33PM +0200, Roelof Bakker wrote:
Hello Jay,
I don't think so.
I have carried out a test with a vertical feed line and a horizontal
feed line on a pole pushed out an upstairs windows. In both cases
the antenna was in the same position and showed equal signal levels
from the groundwave of ONO-399.5 at 59 km.
73,
Roelof, pa0rdt
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