On Fri, 6 Oct 2006 22:27:47 +0200, "Henry Kiefer"
wrote:

Sure Paul. It is as a transmitting antenna for short-range only.

So we are talking about a near field system.

In the far field the ratio between the electric (E) and magnetic (H)
field is constant and is 120 pi or 377 ohms. This field is inversely
proportional to the square of distance since both the electric and
magnetic field is inversely proportional to the distance.

However, very close to the transmitting antenna (near field) the E/H
ratio is no longer 377 ohms. The magnetic field is inversely
proportional to the square of distance and the electric field
inversely proportional to the cube of distance.

There is much debate were the near field ends and were the far field
begins. For simple antennas, the distance is less than one wavelength,
the value lambda / (2*pi) often appears in literature. For large
parabolic reflectors or lasers, the far field begins at hundreds or
thousands of wavelengths.

Near field communication systems have been used for decades e.g. in
induction loop systems for the hearing impaired, in which a magnetic
loop is surrounding the room and audio frequencies are fed into the
loop. The headset will pick up the field, amplify the signal and feed
it to the ear of the person with reduced hearing. Of course, such
systems are now more or less useless due to the stray magnetic fields
caused by various inverters.

Do you know
about:
http://www.ancom.no/
products? They claim their ferrite rod styled antenna works as good
transmitter. See their patent application.

These seem to be near field devices even at MF, where they seem to
operate.

Paul OH3LWR