[AMRadio] Question about ferrite rods


Bob Bruhns bbruhns at erols.com
Fri Jun 16 12:49:48 EDT 2006


Hi Conor,

A ferrite rod antenna should be perpendicular to
the oncoming signal wavefront, and also
perpendicular to its electrical field (or parallel
to its magnetic field).  Because you would
ordinarily be close to the ground with a loopstick
radio, signals will tend to be vertically
polarized at the loopstick - so for most signals
the ferrite rod would be horizontal, and
perpendicular to the incoming signal.

If you "null" a signal by rotating the rod to
point toward the signal, you may find that the
best null is obtained with the rod tilted slightly
upward toward the transmitter.  This is because
the signal wavefront tilts slightly toward the
earth.

Loops and loopsticks can be used to null
interfering stations to hear weaker signnals on or
close to the strong station's frequency.  Skywave
peaks and nulls are less precise than groundwave -
nulls and peaks vary as skywave paths vary and
fade, especially at shorter wavelengths, I think.
But with groundwave, the behavior of a loopstick
is fairly precise.

There is something about paramagnetic materials
that attracts magnetic flux and concentrates it in
the material, and such magnetic material will also
increases the inductance of windings around it, so
the coils do not have to be as large as they would
be in air.  This is why ferrite loopsticks are
used - a very small antenna can be effective.
Ferrite is useful up to the megahertz range, and
some ferrites are even useful to UHF and beyond.

   Bacon, WA3WDR




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