Hi Roelof, LF,
Roelof, why do you think that this is the case? Interesting Question! "Won't
work" means "Won't work properly" it assume. There will be a signal but perhaps
it is not optimal. And why could this be the case? If the application is in a
metallic housing that is mounted to a metallic mast, the capacity of the
"ground electrode" against the environment must be very high (can be seen as
infinite, i assume), compared to the active element (gate of the FET). Perhaps
there will also be a better coupling to qrm sources? I also have to make some
measurements. The antennas we use are comparale i think. So we could share
(all) share our experiences :-)
A special question comes up when a optic fiber cable is used, since in a coax
application there remains a high C of the ground electrode, even when a
symmetrical transformer is used (since the active element has just a few pF to
the far field). That special question is: What will happen, when the
case/ground electrode becomes small against the active element (e.g. circuit in
SMD, small battery inside)?! Will there be a change in the optimal C of the
active element to the far field? I assume, then, one has to define an optimal C
ratio of both electrodes. Will it behave as a short dipole? And what about a
short vertical (30cm) active antenna that is directly placed on the ground with
a almost ideal conducting ground plane, e.g. aluminium foil (out of the
household) in a radius of 1m (ignoring the local qrm problem, so e.g. in your
garden, apart from the city)?
On my new qth i made first steps to receive LF with the active antenna mounted
just 2m above ground but hung up on a wet tree. Results were vy bad. I thought
that SNR could be better when increasing the active element since signals were
weak (DCF39 at S7), but it wasn't. The optimal length was still 30cm. That
seems not only to be "sufficient" but rather optimal! So less and more is
worse, unaffected by the hight above gnd?
I think one has to imagine the E-Field lines that are going through the
ambience. The fieldstrength seems just to be very small in a lossy environment
(on my hill, where no trees can be found, the antenna was also 2m up and DCF39
was S9+20). So hight above ground seems to essencial, even without local qrm.
Each decrease of input signal can be compensated by more gain but SNR
decreases, of course.
What are your ideas to these thoughts? What do you think will happen if the
ground-electrode-C becomes small against the C of the active element? What will
be the optimum C for both? Will the electrodes have the same "importance", like
as a short dipole? Questions over Questions ;-)
Recently i had the idea to test that with a variable antenna of a /p UKW radio,
perhaps two for each electrode. That will be interesting to play with ;-)
73!! Stefan/DK7FC
________________________________
Von: [email protected] im Auftrag von Roelof Bakker
Gesendet: Sa 06.02.2010 13:53
An: [email protected]
Betreff: Re: LF: AW: Re: test signal wanted
Hello Stefan,
Probably a bit late, bit I have been told that an active whip won't work
well on a metalic mast.
I never tried it myself (still on the to do list) due to lack of a
suitable mast.
73,
Roelof, pa0rdt
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