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Author Topic: Faraday Loop  (Read 4845 times)
WE5I
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Posts: 117




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« on: March 10, 2009, 07:45:10 AM »

I have found several illustrations of the Faraday Loop, but none of them are detailed to the extent that it is clear how the connections are made at the
"T".

I took one of the illustrations and duplicated it with a cut-away view of what I assume is inside the "T" portion of the loop. Is my assumption correct?

Here is a link to that illustration:
http://members.cox.net/we5i/Faraday.JPG

Thanks and 73

Graham Welch - WE5I
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WA3SKN
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Posts: 5436




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« Reply #1 on: March 10, 2009, 11:29:07 AM »

The center conductor goes half-way around the circle where it is cut and connected to the shield.  It is not connected at the T end.  At the T, the shield is connected to the other shield.  The center conductor is connected to the shield at the half-way point.
Now this antenna is a VERY narrow bandwidth antenna!  I DO NOT recommend it if you can install ANYTHING else.
Small transmitting loops are not practical for most hams.  Enjoy experimenting with it.
73s.

-Mike.
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KZ1X
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Posts: 3227




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« Reply #2 on: March 10, 2009, 11:52:32 AM »

Are you planning to use the Faraday loop as an exciter for a large magnetic loop radiator?  If so, I can endorse this as I have done it many times.  In fact, right now I have a 10' diameter 40 meter mag loop (made of 1 5/8" Heliax) that works just this way.
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WB6BYU
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Posts: 13017




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« Reply #3 on: March 10, 2009, 01:09:51 PM »

Here is W8JI's web page on the subject, with references:

http://www.w8ji.com/magnetic_receiving_loops.htm


I use a similar loop in my hand-held DF antennas for 80m.
In the simplest case I simply wind the coax into a loop
and connect the center conductor at the end to the shield
where it meets the coax, with the ground braid left floating.
This isn't ideal, as the break in the shield is at the
bottom of the antenna rather than centered at the top,
but it works well enough when it is inconvenient to
break the shield.

Otherwise I'd connect the center conductor AND shield
of the end of the coax to the shield of the rest at
the bottom of the loop and cut a break in the shield
at the top of the loop.  That isn't quite as good as
Tom's recommended balanced loop (where the loop is fed
with a balanced signal with respect to the shield).

In the case you've drawn where the center of the coax
from one side of the loop is connected to the shield
of the other side,  you don't have to make any connection
to the center conductor of the green side of the
drawing since the current is flowing on the outside of
the braid at that point, not on the center conductor.
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VK1OD
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Posts: 1697




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« Reply #4 on: March 10, 2009, 02:12:03 PM »


This article probably answers your questions, and clarifies some of the misunderstandings of shielded loops.

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

Owen
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W8JI
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WWW

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« Reply #5 on: March 10, 2009, 09:08:54 PM »

Graham,

There is no such thing as a "Faraday shield loop". All radiation and reception comes from the shield. Always. Just like it is a single turn conductor.

The idea you can shield the loop is as silly as the idea you can shield a ground lead.

Once you understand that you will also understand that the only way to not have common mode on the feedline is to make the shield perfectly symmetrical about the feedpoint and have both sides of the "shield" symmetrical and identical with the grounding and feedline entry.

My website shows why high frequency fields cannot penetrate the shield, and why the radiation and reception all comes from the outside of the shield.

A shielded loop is electrically no different than a equally balanced unshielded loop.

73 Tom
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WA3SKN
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Posts: 5436




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« Reply #6 on: March 11, 2009, 04:37:52 AM »

Graham...
FYI... The "shielding" reference in the handbook is for "magneteic shielding", and the handbook explains why you can use copper or aluminum, but not steel in the construction of the loop. This is for the small receive direction finding loops, not the small transmitting loops. (No reason to shield the transmit antenna... that is a bad plan!)
73s.

-Mike.
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W8JI
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Posts: 9304


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« Reply #7 on: March 11, 2009, 04:53:47 AM »

Mike,

It is absolutely foolish to think copper or aluminum is any better than steel at letting a time-varying magnetic field pass through a shield wall.

If the Handbook claims what you said they should be taken to task for such a terrible statement.

No field can pass through a conductive wall several skin depths thick or thicker, be the material copper, aluminum, or steel. Also the magnetic field is NOT what radiates in any antenna and allows communications or reception at a distance.

What page of the Handbook is that gross misinformation on?

73 Tom

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VK1OD
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Posts: 1697




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« Reply #8 on: March 11, 2009, 09:31:50 PM »


Tom said "If the Handbook claims what you said they should be taken to task for such a terrible statement."

I have pointed out in various articles on my website, flaws in ARRL publication content. Before doing so, I used once write to the ARRL advising them of the issue.

I have NEVER received a response, and don't waste the time nowdays.

In my experience, the ARRL does not consider possible errors in their publications.

Good and all as they are, they are neither infallible, nor in the class of reputable textbooks.

From R. W. P. King, H. R. Mimno and A. H. Wing, Transmission Lines, Antennas and Waveguides (New York: Dover Publications, Inc, 1965), footnote 1 on p 235 is relevant:
"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."

Owen
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VK1OD
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Posts: 1697




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« Reply #9 on: March 12, 2009, 12:56:33 AM »


I should also have commented on the illustration Graham gave at http://members.cox.net/we5i/Faraday.JPG .

This would be a balanced loop IF the feedline was routed away symmetrically from the loop. Balance / symmetry is vital to minimising the common mode current that flows on the feedline, or in other words, ensuring that only the loop picks up EM energy and not the feedline.

The diagram shows asymetric feedline routing, which suggests that the author doesn't really understand how a 'shielded loop' works.

Owen

PS: The diagram shows a different detail at the loop gap than most commonly shown. It impacts on the loop feed impedance, but does not affect its balance, pattern, gain (apart from impact on line loss). In some cases it is a better connection for performance, but less convenient to arrange.
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W8JI
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« Reply #10 on: March 12, 2009, 03:51:16 AM »

Owen,

I'm always amazed that very simple basic things, like operation of a loop antenna or shield, are so misunderstood.

The ARRL does need to be more responsive when they make an error. Hopefully you (and others) will continue to point out problems. Giving up only allows myths like the shield somehow magically "passing a magnetic field while blocking the electric field" to continue.

73 Tom


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WE5I
Member

Posts: 117




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« Reply #11 on: March 12, 2009, 08:40:30 AM »

Dear Tom and Owen,

I have read where both of you have said it is very important to correctly route the coax away from the feed loop of a "Magnetic Loop" antenna.

I wonder if you would each elaborate in layman's terms about exactly how to route the coax.

Thanks and 73.

Graham Welch WE5I
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VK1OD
Member

Posts: 1697




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« Reply #12 on: March 12, 2009, 12:19:05 PM »


Graham,

One objective of the balance / symmetry is that if common mode current (ie current on the outer surface of the coax) flows on the feed line, that at the tee at the loop, it divides exactly equally into each side of the loop. The currents reaching the gap in the loop will be equal in magnitude and opposite in phase, and will cancel.

If it does that, the common mode current will not cause any current to flow on the inside of the feedline.

The loop symmetry requirement includes not just the feedline, but its environment, including nearby structures and ground.

I think I explained that in my referenced article, but it seems you may not have read it.

Owen
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WA3SKN
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Posts: 5436




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« Reply #13 on: March 15, 2009, 04:02:21 PM »

Tom, sorry for the delay... computer problems!
The reference I quoted was the ARRL Antenna Handbook 20th edition, page 5-5 under the shielded loop paragraph.  It was the first reference to this I had seen, not being mentioned in my 1970 and 1974 Antenna Handbooks.  I quoted it, but have never tested it... now I might have to!
In my "spare time"... maybe!
73s.

-Mike.
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W8JI
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« Reply #14 on: March 16, 2009, 04:03:02 AM »

The ARRL should have had more technical resources than to repeat something so silly.

It is impossible for any time-varying field, magnetic or electric, to pass through the walls of a shield once that shield is several skin depths thick. After all, that is what skin depth is all about!!!

It does not matter if the shield is iron, copper, or aluminum. The magnetic field, once the frequency is high enough that the wall is many skin depths thick, simply doe not go through the wall.

The shield on ANY small loop is the actual radiator. It is the current on the outside of the shield, just like happens with coax, that allows the loop to radiate. The outside of the shield is the antenna, not what is inside.

How that shield is constructed is critical to both balance and feedline radiation. To prevent common mode on the feedline and keep the loop balanced, the shield has to symmetrical in distance and angles from the shield gap to ground. The ground and feedline exit point has to exactly opposite the gap in the loop "shield". This is because the voltage across that gap is what actually excites the shield.

Why the ARRL or anyone else would think a shield in a loop magically behaves differently than a shield on coax or a shield in a radio chassis or cabinet is really strange. The slightest amount of common sense would tell us the shield in one thing is based on the same physics as the shield in something else. If the high frequency magnetic and electric fields can't pass directly through the copper or aluminum walls of a coaxial cable, screen room, or a radio cabinet they surely can't pass through the walls of the shield in a loop antenna.  

It is always the gap or break in the shield that lets it radiate.....always.

73 Tom
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