The radiating and receiving efficiencies of ALL small ferrite- rod antennas
is very, very small.

It's crudely about the same as a whip 10 times longer than the rod.

The rod has the advantage over the receiving whip because it can be fitted
inside a medium-wave pocket transistor radio and contains its own tuner and
matching transformer. In addition, the rod does not need a ground.

The MF receiving rod appears to behave very well only because MW broadcast
stations are high power, relatively local, and pocket-transistor receivers
have a higher gain than your shack transceiver which at MF is intended to be
connected to a 260-feet length of wire at a height of 60 feet.

If efficiency of a small transmitting rod should be as high even as 1
percent, regardless of whether the loss is in the wire or in the ferrite, it
is obvious 99 percent of the transmitter power will be dissipated in the
antenna and only a 20-watt transmitter would wreck it.

PS: As with a magloop the ampere-turns are high but nearly all loss occurs
in the ferrite. A serious problem is the high permeability temperature
coefficient. As the core gets hot the antenna detunes itself.

If you MUST have a dinky 160m pocket transceiver then try a dust-iron
antenna. Six rods round one rod or a number of stacked slabs will have fewer
suicidal tendencies. By far the fastest way of finding the number of turns
is to wind on some wire and if there are too many take some off again.
----
Reg, G4FGQ

------------------------------------------------------------

"Roy Lewallen" wrote
It's really hard to tell anything from a press release, and even harder
to believe most of what you can tell from it. One thing that stands out
is that the antenna being described is apparently operating at a pretty
low frequency, where all antennas are very small in terms of wavelength,
and very inefficient. Without any technical information, I can only
offer some generalities.

A small antenna (in terms of wavelength) is going to be inefficient, and
a tiny antenna is going to be very inefficient. And efficiency is
important in a transmitting antenna. (I noted an exception to this in
another posting about a terminated vee or rhombic antenna, but the
exception doesn't apply here.) Often, for a given size small antenna,
you can get lower loss by putting it on a ferrite core. But unless the
chunk of ferrite is truly awesome in size or the frequency is pretty
high, the antenna you're starting with will be tiny in terms of
wavelength. That is, you might improve a very, very, very inefficient
antenna to a very, very inefficient antenna. And that's probably the
reason you don't usually see ferrite cores in transmitting antennas --
it's better to make them a bit bigger, too big to practically use a
ferrite core.

Somebody mentioned saturation as a consideration. I'd have to work
through the numbers with an actual core size and material, frequency,
number of turns, and power level to see if it would be a problem. But
the very large air gap in the magnetic path (from one end of the rod to
the other -- which is, in fact, where the radiating field escapes)
greatly reduces the flux density, so you could probably run a surprising
amount of power without saturation. In many ferrites routinely used at
RF, losses cause objectionable heating at flux densities well below
saturation, and can often shatter the ferrite before saturation is even
approached. Among amateurs, core heating seems to be almost universally
blamed on saturation, due to a lack of understanding of the mechanism
really causing the heating.

There are certainly many applications for small, inefficient
transmitting antennas. Even a small improvement in efficiency is
beneficial, and that might be what the folks in the article have
accomplished.

Roy Lewallen, W7EL

No Spam wrote:

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