> I just received the rag here. Very interesting this
> article, in light of
> the many discussions on this reflector and antennas
> reflector,
> concluding by our best experts that a shield contributes
> to NO USEFUL
> function whatsoever in such a receiving loop! It just
> essentially
> becomes electrically the loop itself; the shield's outer
> skin IS the
> antenna. No S/N ratio advantage, no anything except adding
> to a very
> critical balancing result. No noise discrimination, no
> magic. So what
> gives with this mythological article?
Roy and all,
It appears QST isn't as careful as they used to be years
ago.
It is a very well known physical property of a "shield" more
than several skin depths thick that essentially nothing goes
through that shield. It's a Faraday cage, and when the
time-varying electric field goes to zero so does the
magnetic field. This is explained in nearly every handbook
(even ARRL publications) and is the reason coaxial cables
have that "third path" on the outside of the shield for
common mode currents. This is why a bead balun or a coil of
coax works to stop common mode on the outside of the shield
and does not affect the stuff inside the cable.
Obviously any technical explanation that requires fields to
go through the shield isn't accurate.
It is also a very well known fact that radio waves are
electromagnetic, and the interference (unless in the
induction field area of less than a half wave or so) is a
radio wave, and like all radio waves it is neither magnetic
or electric. It has a fixed ratio or electric to magnetic
fields. It is also established that a very short distance
from a small loop the predominant field is electric, not
magnetic. Clearly a shield can't "filter" noise, since noise
is not field sensitive.
So what does the shield in a loop do? The shield is actually
the antenna element. The shield is actually what receives
(and/or transmits) the EM wave. NOT the wire inside the
shield. The wire inside the shield simply couples whatever
device is connected to the antenna (the loop's shield) to
the receiver.
If we have a poor coupling system to the loop, how we
configure the shield can certainly affect the balance of the
loop and the common mode current. Say we use an unbalanced
line or we use an unbalanced amplifier directly attached to
a loop. Now we have created a problem, the feedline feeding
the loop acts just like part of the antenna system. We have
an unbalanced feedline or amplifier tied to a balanced
antenna, and so every conductor going to that point acts
like part of the antenna system. We can couple noise and
signals picked up by the feedline shield and everything in
the house to the input of the receiver.
I go through this problem in detail at:
http://www.w8ji.com/magnetic_receiving_loops.htm
If you read carefully you'll see no has ever said the shield
and how the shield is connected won't change the system when
the system is not designed correctly. When the feedpoint is
done correctly, the presence or absence of a shield has no
effect at all on the system. All the stuff about the shield
"filtering" the fields and blocking the electric field is
nonsense. But some construction methods do result in better
loop balance.
The characteristics of an improperly done feed system are
affected by the construction of the loop, but that isn't
because the shield is necessary or that a solid shield
behaves differently than braided shield. It's because
something was more wrong with the construction in one system
compared to the other, not because of the shield quality.
73 Tom
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