> So, Tom, would it be safe to conclude from your analysis
that there is
> no real benefit to shielding the loop? If the real benefit
comes from the
> pattern, then shouldn't an unshielded loop properly
isolated from the
> feedline have the same sharp bi-directional null as a
properly balanced
> shielded loop? IOW, is the shield nothing more than a
lucky charm
> that makes people feel good or does it actually do
something beneficial?
This has been kicked around on rec.radio.amateur.antenna
dozens of times. For example:
http://groups.google.com/group/rec.radio.amateur.antenna/browse_thread/thread/a473696415048db7/12891adbb87adf50?lnk=st&q=g4fgq+shielded+loops&rnum=1#12891adbb87adf50
The original thread was caused by a common mode problem with
the floating center. As Reg G4FGQ correctly says in his
reply, no need for a "screen".
Or this one:
http://groups.google.com/group/rec.radio.amateur.antenna/browse_thread/thread/f3367e447c451a3/d48c9b9d664251b7?lnk=st&q=g4fgq+shielded+loops&rnum=2#d48c9b9d664251b7
Where Reg tells people to use only the outer conductor as
the loop radiator.
Reg also says:
"Hi Ian,
Some old wives even extol the virtues of screening the
coupling loop of a
magloop.
It may be kindly said that screening a loop at least does no
harm.
But the screen greatly increases capacitance across the loop
and thereby
restricts the tuning range of the proper capacitor. The
number of turns has
to be decreased. Or in the case of a single-turn loop its
diameter must be
reduced which also rapidly reduces receiving sensitivity.
I've a feeling it also degrades loop Q. It certainly can't
improve it."
Later copies of Reference Data for Radio Engineers agrees.
They had it wrong initially, but I wrote a letter to them
around the late 70's after which they investigated and
corrected their text to say the shield actually becomes the
antenna. I would hope it remains that way today.
It's pretty evident the shield does nothing but eventually
become the actual antenna regardless of how the antenna is
fed, so of course it only affects balance. With that in
mind, each half of the shield should be excited as evenly as
possible and be symmetrical with respect to grounding and
feed point to keep the loop balanced.
The shield screening the electric field and passing the
magnetic field through is an old wife's tale that probably
will live forever, however.
73 Tom
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