Re: Passive reradiating antenna


"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.

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. 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.

Capture area depends on the volume of the region of the e.m. field
surrounding the core which is distorted by the core. See iron filings
sprinkled round a bar magnet. When the reluctance of the rod itself
is zero compared with that of the air path the limit of permeability
has already been reached.

And high values of rod permeability are always associated with high
core loss. The moral is obvious.

The sensitivity of the coil itself to e.m. waves is calculable. I'll
stick out my neck and say the best a ferrite core can do is to
increase the effective diameter of the coil by crudely 5 or 6 times.

Which means that the gain of your tiny pocket MF transistor radio is
much better than your super-dooper transceiver which needs a 260-feet
long dipole on the 160 meter band.

Just for the sake of argument!
----
Reg.


.

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