"K7ITM" wrote in
oups.com:

On Feb 22, 12:23 pm, Owen Duffy wrote:
G'day all,

I am trying to work through the magical claims that are made of
shielded loops. My interest in mainly their use for field strength
measurement, but understanding them in a general sense is the place
to start.

I have not found a detailed description of operation in my text
books. The ARRL does contain information, but it is inconsistent and
IMHO sometimes just plain wrong.

I have drafted an article with a proposed explanation of the
operation of a small single turn untuned shielded receiving loop. The
article is athttp://www.vk1od.net/shieldedloop/index.htm.

Am I on the wrong tram?

Comments appreciated.

Owen

Hi Owen,

I have no time at the moment to read your draft, but I can tell you
that there is a very good qualitative explanation in King, Mimno and
Wing, "Transmission Lines, Antennas and Waveguides." I have a PDF of
the antennas chapter... I also know that in Johnson and Jasik there's
a corresponding explanation with a bit more detail. In a nutshell, in
the so-called shielded loop antenna, the antenna is the outer surface
of the outer conductor, and the feedpoint is the gap in this
conductor. The center conductor is merely a transmission line to
conduct the signal from the feedpoint to where it is used. It is
balance that yields the "magical properties," and it is possible to
build an antenna with good balance without making the loop out of
coaxial cable. The magical properties are generally taken to be
rejection of nearby electric fields (not electromagnetic fields), and
a symmetrical pattern.


Thanks Tom for the extract from King etc.

They make it very clear when they state:

"The operation of the shielded loop is explained popularly by first
stating that the desired loop current is due to the magnetic field, and
then maintaining that the metal shield cannot be penetrated by the
electric field but can be penetrated by the magnetic field. All these
arguments are incorrect in the light of fundamental electromagnetic
principles."

Thanks again for your valuable help... Owen