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 at
http://www.vk1od.net/shieldedloop/index.htm .

Am I on the wrong tram?

Comments appreciated.

Owen

On Thu, 22 Feb 2007 20:23:49 GMT, Owen Duffy wrote:

Comments appreciated.

Reading:
the note "capacitance from shield to conductor is uniform around
circumference. The capacitance from the inner of the outer
conductor to the outer of the inner conductor acts entirely inside
the transmission line and has no effect on the external balance of
the loop. The conditions on the inside of the loop outer conductor
are isolated from the outside of the conductor due to skin effect
as described above.

I would add a precautionary that the isolation is not complete. There
are many instances of this in small signal designs that fail for this
very reason. I am not sure what you are saying and what you are
repeating because there is no close quote in your commentary.

73's
Richard Clark, KB7QHC

Richard Clark wrote in
news
On Thu, 22 Feb 2007 20:23:49 GMT, Owen Duffy wrote:

Comments appreciated.

Reading:
the note "capacitance from shield to conductor is uniform around
circumference. The capacitance from the inner of the outer
conductor to the outer of the inner conductor acts entirely inside
the transmission line and has no effect on the external balance of
the loop. The conditions on the inside of the loop outer conductor
are isolated from the outside of the conductor due to skin effect
as described above.

I would add a precautionary that the isolation is not complete. There
are many instances of this in small signal designs that fail for this
very reason. I am not sure what you are saying and what you are
repeating because there is no close quote in your commentary.

Sorry about the lost closing quote. Here is the offending paragraph with
the quote fixed and a fuller reference:

====
Another figure in the ARRL Antenna Handbook 18th Edition, p5-6 Fig*8
contains the note "capacitance from shield to conductor is uniform around
circumference". The capacitance from the inner of the outer conductor to
the outer of the inner conductor acts entirely inside the transmission
line and has no effect on the external balance of the loop. The
conditions on the inside of the loop outer conductor are isolated from
the outside of the conductor due to skin effect as described above.
====

Does that clarify it. It is not the greatest bit of prose, but is it
clear enough?

Re the effectiveness of skin effect in isolating the inner and outer, I
agree that it isn't perfect, and less perfect in the case of braid than
for a solid tube. However, even through braid leakage is small relative
to the coupling through the gap as I describe, so the error in
simplification seems small to me.

Thanks Richard.

Owen

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.

Cheers,
Tom

On Feb 22, 10:16 pm, "K7ITM" wrote:

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.

Cheers,
Tom

I agree. There are no magic properties to a shielded loop. All the
design
does is to ensure balance. I've done careful tests comparing a
shielded
loop, vs a good unshielded loop. I could not tell a lick of
difference.
It's all in the balance. An unshielded loop can be just as good, if
the balance is good. And thats not hard to achieve. If I really
thought
shielded loops were better, I would use them. But I don't, so I
don't.
I have a 44 per inch side unshielded loop right next to me. "5 turns"
It's my best loop of the bunch I have tried, and it's the one I stuck
with. BTW, I have also tested using shielded loops as the "coupling"
loop. Again, not a lick of difference being both versions of mine were
well balanced.
MK

"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