Snap-on choke hurts shielded loop
 

I performed an interesting shielded loop experiment today with a snap-on
ferrite rfi-choke:

1) When the choke is placed anywhere on the coax feedline the choke has
no affect whatsoever.

2) When the choke is snapped anywhere around the loop-element coax, it
really attenuates the loop signal, and also detunes it. I had to
readjust my tuner's capacitor quite a bit. It also seemed to flatten
out the nice sharp Q. It did not affect the directivity however.
Placement of the choke anywhere on the loop element seemed to produce
the same amount of attenuation - from right over the gap to anywhere
else on the loop.

Unless I misunderstand how ferrite-chokes work, this does seem to
physically point out that the shield is the actual antenna. (This is
what I've been told all along here.) Nobody in their right mind would
place an RFI choke on their transmission line in an effort to reduce
common-mode radiation if it also severely attenuated the wanted signal
right?

Am I on the right track? The correct theory is nice to know, but it is
also great to see it proven physically....

73
Brian

How can a shunt fed tower be an antenna when it's connected to RF
ground? How about the elements of a "plumber's delight" Yagi? Or a
folded monopole?

Roy Lewallen, W7EL

donut wrote:
Loopfan wrote in
rthlink.net:


Unless I misunderstand how ferrite-chokes work, this does seem to
physically point out that the shield is the actual antenna.



How can the shield be the antenna when it's connected to RF ground?

donut wrote in message ...
Loopfan wrote in
rthlink.net:

Unless I misunderstand how ferrite-chokes work, this does seem to
physically point out that the shield is the actual antenna.


How can the shield be the antenna when it's connected to RF ground?

A time-varying (RF) magnetic field generates an EMF in any loop
surrounding it.

(Faraday's Law of Magnetic Induction: there is an EMF proprotional to
the rate of change of the magnetic field enclosed by any closed line.
Ohm's law: the voltage drop in a resistance is proprotional to the
resistance and to the current. Kirchoff's Voltage Law: the sum of
voltage drops around a closed loop is equal to the sum of EMFs in the
same loop. Edwards' Law Quotation Law: Anyone quoting laws is not to
be taken too seriously. Or something like that. So by making the
loop a conductor with a gap, the EMF given by Faraday's law is
delivered almost completely to the gap. Doesn't matter if the loop
conductor is grounded at some point or not. Note also that in the
presence of a time-varying magnetic field, the voltage you measure
between two points depends on the path you take.)

Cheers,
Tom