Hi Stefan and Roelof, I would discount that until you have proved it for
yourself. Unfortunately there are a lot of "urban myths" about LF aerials
propagated by those who dont know how they work. There are a lot of variables
and the noise profiles are almost unique to each installation. I am sure one of
you will do it "properly" soon and advise us the outcome. Grounding the mast(or
not), connecting a ground to the feeder, where does the feeder run with respect
to the mast, what are the "standard signal" strengths (DCF39 ?) at different
heights and how does the noise level vary.......are all possible differences.
Isn't LF fun :-)) so much to experiment with.
Best Wishes
Alan G3NYK
--- On Sat, 6/2/10, Stefan Schäfer <[email protected]> wrote:
> From: Stefan Schäfer <[email protected]>
> Subject: LF: active ant on metallic mast
> To: [email protected]
> Date: Saturday, 6 February, 2010, 16:46
> 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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