Hi Pieter-Tjerk,
I agree. Now one could think that there must be a difference regarding
the way the feed line goes to ground (vertically down or a few meters
horizontally and then down or even 20 cm hor. right then 40 cm up then
40 cm left and then down to ground). The E field distortion due to the
cable will be quite small i think, since its just a wire and not a
plate. It depends on the distance between grounded wire and active plate.
And that will give an idea what happens when a isolated (really
isolated, i.e. fiber optic link, not an "isolating" transformer) short
active antenne is used. The whip electrodes must be larger or the signal
voltages will be lower...
Now back the sun, outside :-)
73, Stefan/DK7FC
Am 07.07.2013 12:15, schrieb Pieter-Tjerk de Boer:
On Sat, Jul 06, 2013 at 06:05:01PM +0200, Stefan Schäfer wrote:
Am 06.07.2013 15:16, schrieb Pieter-Tjerk de Boer:
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.
Actually, I meant the real height above ground, multiplied by the
free-space field strength.
But of course, you are right that what the device responds to, is the
field between the probe and the nearby ground. However, the field
strength there will be much larger, due to the fact that the ground
has been "brought up".
This much larger field strength mostly compensates for the much smaller
distance, so the output voltage is still roughly the same as when
one just multiplies the real height above ground by the free-space field
strength.
See the attached sketch: the equi-potential lines in the area near the
probe are much closer together because the pole is at ground potential.
B.t.w., a member of this mailinglist contacted me off-list and pointed
out that there is an article in VHF Communications 96/2 in which this is
discussed in more detail. A German version of the same article has
appeared in UKW-Berichte in 1994/1995 (but those years unfortunately are
missing in our radio club's collection).
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...
Indeed.
73, Pieter-Tjerk, PA3FWM
|
