On Sat, 30 Sep 2006 09:56:11 +0300, Paul Keinanen wrote:

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

Woha. A lot of info to assimilate. Thank you.

A few quick questions; the efficiency at higher frequencies does only
increse logarithmically, correct? Around where is the 'knee' where
increment of frequency no longer provideas a significant increase in
efficiency? - Ballpark numbers and guesstimates are perfectly OK for me,
since I'm only curious of the general idea.
Finally, what exactly do you mean by 'near field'? Is it an arbitrary line
in the sand that separates near fields from normal fields, or a completely
different physical phenomena?

--
Nos