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Date: Thu, 11 Mar 2010 15:42:38 +0100
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: Re: AW: 9kHz loop/preamp for soundcard receiver
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Dear Jim, group,

OK so far. And what do you think will be the most suitable TX antenna 
type? The standard marconi as i will try it, with all its disadvantages 
(high coil and earth losses, high voltages, high height needed...) or a 
earth pole antenna that should (theoretically) form something like a 
loop antenna? I am not very positive with these earth pole antennas, if 
we want to reach more than 0,1...5km. If >100km is the goal, then i 
expect the standard monopole antenna will be the best choice. A TX loop 
antenna, out of a copper tube (say a tube >10mm diameter with 1mm 
thickness) might be an interesting alternative since the skin effect is 
not such a problem. But one can almost forget the radation resistance, 
clear. It will be problematic but this is the challenge :-)

Someone of the group could start to check what is available with those 
earth antennas. Not checking the web for literature, no transmission of 
broad band audio signals but i mean rather trying narrow band modes with 
an optimised antenna for one QRG. Those who have the place to reach a 
distance of say >2km wire (hidden) and use a thick cable may reach more 
since the "loop area" increases exponentially with the wire length. It 
would be interesting if an improvement of the efficiency can be reached 
when placing coils at both ends and so on. A quite interesting 
alternative field...


73, DK7FC


Am 11.03.2010 00:37, schrieb James Moritz:
> Dear Stefan, LF Group,
>
> So far, I have not had time to make any detailed measurements and 
> comparisons - but the "quick look" screen shot attached was taken at 
> my home QTH on Sunday when both the QRN and the local mains QRM were 
> exceptionally low for a short period. The frequency range is 5kHz - 
> 20kHz, and the FFT resolution is about 1.5Hz. The 3D waterfall shows 
> the alpha beacon "spikes" rather well. The continuous line just above 
> the highest alpha frequency appears to be a 15.625kHz TV line 
> timebase. Normally, frequencies below about 10kHz are filled with 50Hz 
> harmonics well above the QRN level, although the level varies greatly. 
> I did briefly try the RX at a /P location this afternoon - mains noise 
> was much lower than at home, but the QRN level was much higher today 
> than in this screen shot. However, since the 50Hz harmonics mainly 
> produce narrow-band spectral lines in the waterfall, these should not 
> be a great problem for receiving very narrow band QRSS-type modes, 
> providing the frequencies do not coincide exactly.
>
> I'm not sure what to do about the TX side of 9kHz yet - I want to look 
> in a bit more detail at the technical problems that need to be 
> overcome before making a proposal for a NoV to Ofcom.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>