wrote in message
...
On Jan 21, 12:12 pm, "christofire" wrote:


I understand it becomes increasingly difficult to create a purely-magnetic
antenna as the frequency rises, and ferrite with the required properties
becomes progressively more expensive! Some VHF pagers used ferrite rods,
and one or two-turn coils. Screened one-turn loops are used in the
short-wave bands, by some amateurs as well as by the military (e.g.
British
Royal Navy).

Chris

The way I look at it, there is no such thing as a "magnetic" antenna.
As an example, some call shielded single turn loops "magnetic"
antennas. They claim special properties such as lower noise reception.
But this is not the case. They receive the same s/n ratio as any other
single turn loop.
The only advantage the shield provides is inherently good balance.
Good balance improves the depth of the nulls.
But you can construct plain wire single turn loops to have just as
good balance if you use good construction.
I've side by side compared the two, and came to the conclusion
most of the theories about shielded or so called magnetic loops
to basically be a myth.


I believe the issue is that if an open loop isn't perfectly balanced then it
will respond to an electric field, acting as a monopole. Then, when the
loop is oriented so the magnetic field of a signal should be in one of its
nulls, the cancellation may be incomplete because of sensivity to the
electric field component of that signal. Screening the loop overcomes this
effect so it could be claimed that screening improves the balance, although
this isn't what's really happening.

With a pair of screened loops and a whip it is possible to receive
separately the magnetic and electric fields associated with a radio signal
and to record their strengths at different locations. This can reveal
significant differences on account of building and electrical clutter, but
only if the loop is adequately screened. No myth!

Chris