Tdonaly wrote:



Yes, but does your small, inefficient, shielded loop improve the
signal-to-noise
ratio in the directions of its maximum gain over say, a non shielded loop?

Many claim this, but I didn't see it when I compared them. I found a
shielded coax loop just as susceptible to local, and not so local noise,
as a non shielded loop assuming both are balanced. This is not counting
the feedline, or any common mode currents unbalancing the loops. Both
are capable of very sharp nulls. No difference really, and both are good
at nulling a noise source. But a shielded coax loop quieter than a
regular loop? I don't see it. It's not the loop itself, or having a
shield. It's the keeping of good balance. The shielded loop design and
method of feeding forces a good balance. But if you have a regular loop
that is also just as balanced, I maintain it's just as "quiet". To me,
this "shielded loop being quieter" theory is an old wives tail of sorts.
My two favorite MW loops are both unshielded. Ones a 16 inch dia circle
with 12 turns, and my big one is a diamond with 44 inches per side. "5
turns". Both are on floor stands indoors, and rotate.
I've tried using shielded coax loops, and I saw no reduction of noise.
I've also compared using both shielded and non shielded coupling loops
to feed the loops. Again, no difference in noise levels. MK

--
http://web.wt.net/~nm5k

Mark Keith wrote,

Tdonaly wrote:



Yes, but does your small, inefficient, shielded loop improve the
signal-to-noise
ratio in the directions of its maximum gain over say, a non shielded loop?

Many claim this, but I didn't see it when I compared them. I found a
shielded coax loop just as susceptible to local, and not so local noise,
as a non shielded loop assuming both are balanced. This is not counting
the feedline, or any common mode currents unbalancing the loops. Both
are capable of very sharp nulls. No difference really, and both are good
at nulling a noise source. But a shielded coax loop quieter than a
regular loop? I don't see it. It's not the loop itself, or having a
shield. It's the keeping of good balance. The shielded loop design and
method of feeding forces a good balance. But if you have a regular loop
that is also just as balanced, I maintain it's just as "quiet". To me,
this "shielded loop being quieter" theory is an old wives tail of sorts.
My two favorite MW loops are both unshielded. Ones a 16 inch dia circle
with 12 turns, and my big one is a diamond with 44 inches per side. "5
turns". Both are on floor stands indoors, and rotate.
I've tried using shielded coax loops, and I saw no reduction of noise.
I've also compared using both shielded and non shielded coupling loops
to feed the loops. Again, no difference in noise levels. MK

--
http://web.wt.net/~nm5k



This pretty much squares with an article on shielded loops written by
Glenn S. Smith of the Georgia Institue of Technology in _The Antenna
Engineering Handbook_. He says the shield enforces symmetry so that
the pattern doesn't suffer, and that's what it's supposed to do. No mention
of noise at all.
73,
Tom Donaly, KA6RUH

"Tdonaly" wrote in message
...
Mark Keith wrote,

Tdonaly wrote:



Yes, but does your small, inefficient, shielded loop improve the
signal-to-noise
ratio in the directions of its maximum gain over say, a non shielded
loop?

Many claim this, but I didn't see it when I compared them. I found a
shielded coax loop just as susceptible to local, and not so local noise,
as a non shielded loop assuming both are balanced. This is not counting
the feedline, or any common mode currents unbalancing the loops. Both
are capable of very sharp nulls. No difference really, and both are good
at nulling a noise source. But a shielded coax loop quieter than a
regular loop? I don't see it. It's not the loop itself, or having a
shield. It's the keeping of good balance. The shielded loop design and
method of feeding forces a good balance. But if you have a regular loop
that is also just as balanced, I maintain it's just as "quiet". To me,
this "shielded loop being quieter" theory is an old wives tail of sorts.
My two favorite MW loops are both unshielded. Ones a 16 inch dia circle
with 12 turns, and my big one is a diamond with 44 inches per side. "5
turns". Both are on floor stands indoors, and rotate.
I've tried using shielded coax loops, and I saw no reduction of noise.
I've also compared using both shielded and non shielded coupling loops
to feed the loops. Again, no difference in noise levels. MK

--
http://web.wt.net/~nm5k



This pretty much squares with an article on shielded loops written by
Glenn S. Smith of the Georgia Institue of Technology in _The Antenna
Engineering Handbook_. He says the shield enforces symmetry so that
the pattern doesn't suffer, and that's what it's supposed to do. No
mention
of noise at all.
73,
Tom Donaly, KA6RUH


That's what the ARRL antenna book also claims. They talk about shielded
loops in the context of direction finding antennas. The shield is supposed
to make the antenna balanced with respect to ground, and retain
directionality. Also, and I don't recall if anybody mentioned this, but the
shield can not be closed around the circumference, and the maximum wire
length is on the order of lambda/10. The loop is tuned to resonance with a
parallel capacitor.

Tam/WB2TT

Mark, I fully agree.

The amount of unscreened signal picked up by an unscreened multi-turn loop
is negligible compared with what is picked up by the action of the loop
itself and then magnified by the Q of the loop. The unwanted signal cannot
possibly be more than that which would be picked up by a very short vertical
of height equal to the loop diameter and would probably be less.

If there's a problem it is more likely to be picked up on the feedline which
is not influenced by the presence or absence of a screen around the loop.
If something must be screened then screen the feedline.

Loop screening is needed only when precision direction-finding bearings are
being taken with an in-the-clear, precision-constructed, large loop.
----
Reg, G4FGQ

Reg Edwards wrote:

If there's a problem it is more likely to be picked up on the feedline
which is not influenced by the presence or absence of a screen around
the loop.

Agreed

If something must be screened then screen the feedline.

It generally is screened already (coax) but it does need some kind of
balun.

It's amazing how many loop designs are paranoid about balancing and
screening the loop itself, but then connect the coax in a totally
unbalanced way. The result is a beautifully balanced loop in parallel
with a vertically polarized random wire.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Hi Ian,

Some old wives even extoll 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.
----
Reg, G4FGQ

========================================

"Ian White, G3SEK" wrote in message
...
Reg Edwards wrote:

If there's a problem it is more likely to be picked up on the feedline
which is not influenced by the presence or absence of a screen around
the loop.

Agreed

If something must be screened then screen the feedline.

It generally is screened already (coax) but it does need some kind of
balun.

It's amazing how many loop designs are paranoid about balancing and
screening the loop itself, but then connect the coax in a totally
unbalanced way. The result is a beautifully balanced loop in parallel
with a vertically polarized random wire.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek