On Fri, 29 Sep 2006 16:05:21 +0300, Ceriel Nosforit
wrote:

Hey all,

I read about people using their sound card to catch transmissions
from dedicated senders such as SAQ by just hooking a roll of wire to the
mic-in and apparently finding some success in this. This got my wondering,
what aside from laws and fines is stopping me from hooking my 80W
stereo amp up to some sort of antenna for global transmission fun?
Somebody must have thought of this before...

At least previously, the frequency tables started at 9 kHz, so
anything below that would not cause any interference to any other
service.

However, the problem with VLF is that any practical antenna is going
to be very short compared to wavelength and since the radiation
resistance is proportional to the square of frequency for antennas
well below 1/4 wavelength, most of the power injected into an antenna
is going to be dissipated in resistive losses.

At the LF aeronautical beacon band with 90 m antennas, the antenna
efficiency based on measurements flown around these beacons seems to
be about 1 %. In Europe, the maximum _radiated_ power limit on the 135
kHz amateur radio band is 1 W, but generating that kind of radiated
power with reasonable sized antennas (30 m) would require at least 1
kW of transmitter power, indicating that the practical antenna
efficiency is about 0.1 %. At 13 kHz, the efficiency would be about
0.001 %.

The near field distance for a simple antenna extends to about 1/6
wavelength, so at VLF, the practical communication range for amateur
communication systems would be well within the near field.

Since you are apparently from Finland and since the Finnish
telecommunication law only grants the jurisdiction to the
telecommunication authorities for "freely propagating" electromagnetic
radiation, my interpretation of the law is that it does not cover any
near field i.e. magnetic or electrostatic communication systems, in
which the near field communication systems work.

Of course, if you are able to generate huge magnetic or electric
fields that cause interference to other systems, this may cause
problems to you.

But otherwise, go ahead with your experiments, but unfortunately the
laws of physics will hit you sooner or later :-).

Paul OH3LWR