Topband: "Magnetic" receiving loop.
w8ji at contesting.com
Tue Dec 20 09:35:42 EST 2005
> I was going to build a "magnetic" receiving loop as
> described in the second edition of Low Band DXing.
The name "magnetic" can be misleading. I have a tutorial on
First, the shield really does nothing except modify the
balance. If you build and connect the shield wrong, it can
make the loop worse. If you build an unshielded loop
correctly, the shield won't make any difference at all in
how it works.
Second, noise is not electric field dominant. It is like any
other radiated signal, and can be either electric or
magnetic field dominant in the induction field area close to
the antenna or noise source. That's why you can take a tiny
little whip (electric field antenna) and phase it with a
small loop (magnetic field antenna) and make a
unidirectional antenna for all signals. That's why a
broadcast FS meter is calibrated in uV per meter (electric
field strength) when it uses a small loop antenna!!
What you are left with is directivity, and a small loop has
poor directivity. Now there are cases where a small loop
will help just becuase it is differnet..... so they are
worth trying as an experiment. In general or as an overall
average they are no better than anything else you might try
that has a poor directional pattern.
On the same topic, K9AY's, Flags, EWE's, and Pennant
antennas all act as two short verticals that are phased. The
purest and most directional form would actually be two short
verticals, so they are slightly less directional than a two
element vertical array of similar dimensions. They are
easier to build, but will be outperformed by two properly
It's amazing how, when we look at this, there is no free
lunch or magic. There is a bit of random luck involved, but
> I notice that there is no reference to magnetic loops
> in the most recent edition.
> My loop would have been installed vertically and be
> 12m in circumference, tuning capacitor at the top. The
> TXing antenna is an Inverted L.
> What are your thoughts on "Magnetic" loops before I
> build/buy my variable capacitors. Should I continue
> with this project?
> Thanks Robert F5VHN ex-G0HGW.
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