From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Mon, 23 Jul 2001 17:58:53 +0000
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Subject: LF: Re: Amateur VLF DX
Message-ID: <E15Oj10-0002FM-00@gemini.herts.ac.uk>
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<?color><?param 0100,0100,0100>Dear Stewart, LF group, Lowfers,<br><br>KK7KA wrote:<br>&gt;<?/color>Although the recent amateur VLF contacts were fairly short range,  &gt;I believe that DX operation is possible, perhaps even across the  &gt;pond.<?smaller><?smaller><?smaller><br><br><?color><?param 0100,0100,0100><?bigger><?bigger><?bigger>It's an interesting idea, certainly.<br><br>About the ERP requirement - The ERP of SAQ has been measured  to be around 10-20kW. 50dB down on this is 100 - 200mW. This is  a pretty respectable ERP to be acheived on the 73kHz UK LF  band, so is probably rather optimistic for 9kHz, without an   exceptionally large TX antenna. I think 60 or 70dB down would be  more realistic, unless you can get hold of a 100m high antenna.<br><br>The "earth loop" antennas have their adherents, and seem  appealing because of the potentially large size possible. Although  they are good for cave radio applications, I'm not so sure their  effectiveness compared to 
the tuned vertical types has ever been  demonstrated for "above ground" signals. The simple radiation  resistance formulae suggest that loops should always lose out  compared to verticals as the frequency decreases (Rrad  proportional to f^2 for a vertical, f^4 for a loop), although any real  situation is likely to be more complex than this.  Do any  measurements of efficiency for this type of antenna exist?<br><br>Audio amplifiers can be persuaded to work reasonably well for  136kHz, so 9kHz shouldn't be a problem. The tricky part is likely to  be matching the amplifier output to the antenna reasonably  efficiently. Rather big inductors and/or capacitors will be required  to cope with the many kVA circulating in a resonant antenna at  9kHz. Voltage or current breakdown may well be the limiting factor  on power for even quite large antennas - it is with moderate-sized  ones at LF.<br><br>It is interesting to listen to the noise on 9kHz - compared to LF it is  much more "crack
le and pop" than "hiss", almost all the noise  seems to be lightning transients. I once estimated it to be about  30uV/m, although it is hard to get a sensible measurement, due to  it's impulsive nature. This was during winter time, when the noise  level decreases considerably. I don't know if this might be  important for designing a suitable modulation method.<br><br>Cheers, Jim Moritz<br>73 de M0BMU<br><br><br><br><br><br><br>