Reg Edwards wrote:
"Roy Lewallen" wrote

A ferrite loop
antenna simply has better efficiency than a standard loop of the

same

physical size. Hence it has better gain or capture area.

=========================================

It's only a minor point, but when a ferrite core is placed inside a
loop the efficiency remains the same. It's the same wire, the same
coil dimensions, and hence the same loss in the resistance.

If anything happens to efficiency it is reduced due to a loss in the
ferrite core material.


Sorry, that's not true. When the ferrite core is inserted, the loss
resistance stays the same, as you say. But the radiation resistance
increases, resulting in increased efficiency.

What happens is that the effective cross-sectional area of the loop
increases approximately in proportion to the permeability of the core.
For small permeabilities the capture area is much increased.

This is a good illustration of why I don't like using "capture area" for
antennas of small dimensions -- even Reg gets the mistaken idea that
physical or effective physical area is directly related to capture area.
For some antennas, like horns and ones with a parabolic reflector, it
is. But for simple, small antennas, it isn't. The effective aperture
(capture area) of a small loop is 3 * lambda^2 / (8 * pi) where lambda
is the wavelength, for a loop of any size as long as it's electrically
small enough to have essentially uniform current. Notice that the
capture area doesn't increase as the loop size increases as long as the
current is essentially uniform. This is, in fact, exactly the same thing
that happens for a short dipole. Making a loop larger (or short dipole
longer) decreases the radiation resistance which in the presence of
inevitable loss, increases the efficiency. But if you could make a
lossless loop or dipole, you couldn't get any more power out of it by
increasing its size, again with the restriction that the current remains
essentially uniform, or for a dipole that the length is very short
compared to a wavelength. (The above discussion assumes that effective
aperture and capture area don't include the effect of loss. This is the
assumption commonly used in texts.)

But for
larger permeabilities, say above 100, the effect diminishes and the
effective core permeability settles down to the order of 20 or 30. It
depends on the ratio of length to diameter of the core rather than of
the coil.

To visualise, it should be remembered most of the magnetic circuit
lies in the air between and near the ends of a ferrite rod. There is
no point in increasing permeability of rod material beyond a certain
amount in an attempt to increase capture area.

Again, adding the ferrite improves efficiency by reducing radiation
resistance, not by increasing capture area.

. . .

Roy Lewallen, W7EL