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Date: Sun, 07 Jul 2013 19:06:53 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: VK1OD's analysis of the MiniWhip antenna
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Status: O

Hi Pieter-Tjerk,

I agree. Now one could think that there must be a difference regarding=20
the way the feed line goes to ground (vertically down or a few meters=20
horizontally and then down or even 20 cm hor. right then 40 cm up then=20
40 cm left and then down to ground). The E field distortion due to the=20
cable will be quite small i think, since its just a wire and not a=20
plate. It depends on the distance between grounded wire and active plate.

And that will give an idea what happens when a isolated (really=20
isolated, i.e. fiber optic link, not an "isolating" transformer) short=20
active antenne is used. The whip electrodes must be larger or the signal=20
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 Sch=E4fer wrote:
>
>   =20
>> Am 06.07.2013 15:16, schrieb Pieter-Tjerk de Boer:
>>     =20
>>> This view also at least approximately matches VK1OD's NEC4 calculatio=
n,
>>> in the sense that he finds an output voltage which is of the order of
>>> the field strength times the antenna height.
>>>       =20
>> 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.
>>     =20
> 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 fiel=
d
> 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 i=
s
> discussed in more detail. A German version of the same article has
> appeared in UKW-Berichte in 1994/1995 (but those years unfortunately ar=
e
> missing in our radio club's collection).
>
>   =20
>> 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...
>>     =20
> Indeed.
>
> 73, Pieter-Tjerk, PA3FWM
>
>   =20