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Date: Sat, 6 Feb 2010 21:57:20 +0000 (GMT)
From: ALAN MELIA <alan.melia@btinternet.com>
To: rsgb_lf_group@blacksheep.org
In-Reply-To: <38A51B74B884D74083D7950AD0DD85E82A1AEC@File-Server-HST.hst.e-technik.tu-darmstadt.de>
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Subject: Re: LF: active ant on metallic mast
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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 varia=
bles 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. Gro=
unding 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" stren=
gths (DCF39 ?) at different heights and how does the noise level vary.....=
..are all possible differences. Isn't LF fun :-)) so much to experiment wi=
th.

Best Wishes
Alan G3NYK

--- On Sat, 6/2/10, Stefan Sch=E4fer <schaefer@hst.tu-darmstadt.de> wrote:

> From: Stefan Sch=E4fer <schaefer@hst.tu-darmstadt.de>
> Subject: LF: active ant on metallic mast
> To: rsgb_lf_group@blacksheep.org
> Date: Saturday, 6 February, 2010, 16:46
> Hi Roelof, LF,
> =20
> 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 :-)
> =20
> 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)?
> =20
> 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?
> =20
> 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.
> =20
> 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 ;-)=20
> =20
> 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 ;-)
> =20
> 73!! Stefan/DK7FC
>=20
> ________________________________
>=20
> Von: owner-rsgb_lf_group@blacksheep.org
> im Auftrag von Roelof Bakker
> Gesendet: Sa 06.02.2010 13:53
> An: rsgb_lf_group@blacksheep.org
> Betreff: Re: LF: AW: Re: test signal wanted
>=20
>=20
>=20
> Hello Stefan,
>=20
> Probably a bit late, bit I have been told that an active
> whip won't work=20
> well on a metalic mast.
> I never tried it myself (still on the to do list) due to
> lack of a=20
> suitable mast.
>=20
> 73,
> Roelof, pa0rdt
>=20
>=20
>=20
>=20