Roy Lewallen wrote:

[...]

As an antenna gets smaller, the impedance does rise, causing a
requirement for more voltage for a given power. The rise, however, is
due to increasing (series) reactance, not radiation resistance. If you
make the antenna long enough to reach resonance, then continue making it
longer, the impedance again rises until you hit "anti-resonance"
(parallel resonance). In that region it's due to both an increasing
reactance and an increasing radiation resistance.

A real consequence of the low radiation resistance of a small antenna is
that the conductor current is very high for a given applied power. This
results in increased I^2 * R loss in the conductors. The loss can be
very substantial in small antennas.

Roy Lewallen, W7EL

Thanks for the excellent description. It's not clear if the device works like a
regular antenna - apparently it is encased in ferrite the size of a grapefruit.
Here is the url again:

http://www.aftenposten.no/english/lo...ticleID=609108

Now we all know ferrite antennas work great in AM radios, especially where a
strong interfering signal can be nulled out. But it's not clear how well they
work as transmitting antennas.

Wouldn't they have the same inefficiency problems as a small loop antenna? In
other words, very low radiation resistance? If so, it doesn't seem possible it
would have long range.

Perhaps it is meant for local communication over short distances, which can be
useful for secure links. There is an interesting article on magnetic induction,
where the field strength falls off as 1/r^6 instead of 1/r^2. This group talks
about using very small antennas for distances up to 2 meters:

http://www.auracomm.com/Downloads/webwireless.pdf