Topband: QST Jun 06 RX Loop

Michael Tope W4EF at
Wed May 17 21:03:00 EDT 2006

----- Original Message ----- 
From: <k3bu at>

>> Could it be that my co-ax loop is rejecting something in its very near
>> field,(the TV of my neighbour that I suspect is about 10 meters
>> away from my
>> RX antennas!)or is it a case of unbalance in the Flag
>> antennas,(that would
>> make sense)I do not have to rotate the loop to get rid of the
>> offendingnoise, it simply is not there!I also tried K9AY (various
>> ones) with the same
>> problem as the flags.
>> 73
>> Raoul Coetzee
> YES Raoul,
> the shielded or coax loop is providing electrostatic field shielding from 
> the near by interference by its virtue of capacitance of the shield to the 
> ground.  Shield is a shield and NOT an >antenna as W8JI claims, otherwise 
> you would not see the effect (signal is a signal he claims).


I am not very good at field theory (too few brain cells), but I do know
how to invoke Kirchoff's current law. If you first assume that the current
in the closed loop formed by the inner conductor of the coaxial
shielded loop is approximately uniform (good approximation for a loop
that is < 0.1 lambda in circumference) and that the excitation of the inner
loop is balanced with respect to the shield, then you have to conclude
that there is a current flowing on the inside of the shield that is equal in
magnitude and opposite in phase to the center conductor current.
When this equal and opposite current which is flowing on the inside of
the shield reaches the gap, however, it has to flow somewhere. The only
place it can go is to flow on the outside of the shield and back around
to the other side. This is what causes the radiation. If you close the gap,
the current will stay on the inside of the shield, and the loop won't 
If you take away the shield, you still have the same amount of current
flowing around the same enclosed area (e.g. same radiation). The
shielded loop may have a different near-field response than the
unshielded loop, however (this could in principle make it better for
rejecting E-field dominated nearfield noise sources). I'll leave that to the
guys with more brain cells to analyze (time to walk the dog before it gets
dark here).

73, de Mike W4EF...............................................

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