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Date: Sat, 06 Jul 2013 18:05:01 +0200
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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References: <CAKvcm2dpuftdtK6OdQv8fOfiw+4yQrPBQBHJ-FN2gDbT4P_oMQ@mail.gmail.com> <51D498BD.3050007@iup.uni-heidelberg.de> <op.wzq2tgvnyzqh0k@pc-roelof> <005b01ce798a$b0476600$6d01a8c0@DELL4> <op.wzrfpjyyyzqh0k@pc-roelof> <20130706131640.GG5246@cs.utwente.nl>
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Subject: Re: LF: VK1OD's analysis of the MiniWhip antenna
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Status: O

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
>>
>>