<?color><?param0100,0100,0100>Dear Stewart, LF group, Lowfers,<br><br>KK7KA wrote:<br>><?/color>Although the recent amateur VLF contacts were fairly short range, >I believe that DX operation is possible, perhaps even across the >pond.<?smaller><?smaller><?smaller><br><br><?color><?param0100,0100,0100><?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>
|
