Hi Ford,
> broadside to the loop. Why do I get no (or very little) response held
> vertically? These experiments are done at 4' to 7' height above loamy
> ground. Can anyone explain the polarization differences?
This is an excellent demonstration of the false myth that this an
other small loops are "magnetic radiators" and are not influenced
by electric fields.
You are holding the loop so the magnetic flux lines are vertical.
That means the electric field lines are horizontal.
Electric field lines are "short circuited" by the earth. If you don't
allow time-varying electric fields, you must have no time-varying
magnetic fields. Because you "shorted the electric field out" by
parallelling it with the conductive earth, the response goes to near
zero.
> 2) The permeability of the ferrite is listed in the specs as "minimum
> 1000 and maximum 3000" All the formulas and information I can gather
> from experimentation indicates that with the loop geometries the way
> they are, a permeability of 90 to 100 is obtained, which is consistant
> with the article in the antenna book and suggested by the formulas
> presented therein.
The "advertised" permeability is given at dc, and it varies with
frequency. You should always find the effective permeability at the
operating frequency when selecting a core. At some higher
frequency it will drop like a rock, go high enough in frequency...and
it acts like a shorted turn and cancels inductance. You also have
an open core, and so air is part of the path. Geometry is important.
> (e.g. a pair of them) using principles similar to those used in
> traditional verticals (e.g. 130' in air for 1/4 wavelength spacing) or
> will some other principle apply?
You could, but sensitivity will be poor and it will be no better than a
bigger conventional loop. The small loop trades size for efficiency,
and since the directivity is about the same you see no
improvement in S/N..
You also run into another problem, the loop antenna has narrow
bandwidth when resonant. That means phase and sensitivity level
will shift like crazy with frequency and temperature. It will not make
a good element for a phased array.
> (<15kHz). E.g. Wind three coils on a rod. Couple the antenna to the
> end coil, the receiver to the other end, and resonate the middle core.
> I was shocked at the depth and sharpness of the filter--and
> incredibly simple! My question is, why is this so? Why can't I wind
> three, five, or 15 coils in parallel and resonate them together?
> Instead, they act as attenuators. What's up with that?
Think of what each one is. A series tuned circuit that is shorted in
a closed loop with little resistance. Each winding reflects an
opposing field back into the core, and that opposes any flux
passing through the core.
If you connected a load resistance (like a receiver) across each
winding, they would pass flux because they would not look like a
resonant shorted turn. They would look like resistances, and
absorb some of the power and let the rest pass.
I hate to be a wet blanket, but unless you intend to carry the
antenna around or have very limited room for a larger loop...ferrites
are going to be a waste of time and money.
73, Tom
(W8JI@akorn.net)
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