Topband: QST Jun 06 RX Loop

Tom Rauch w8ji at
Sat May 13 13:35:44 EDT 2006

> 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 
> 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 

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:

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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