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Author Topic: Planar, wide-strip magnetic loop?  (Read 2826 times)
JAHAM2BE
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« on: April 18, 2012, 04:49:23 PM »

I've got a question about possible side-effects of flattening a magnetic loop antenna into a planar configuration.

I've built a prototype magnetic loop (small transmitting loop) antenna from 45-cm-wide aluminum foil formed into a loop with overlapping ends forming the capacitor. Foil orientation is currently such that the loop forms a cylinder with radius equal to the loop radius and depth equal to the foil width. Coupling is with a 1/5-size loop. Planned use is for 5W transmitting from 40m-15m.

Current appearance (non-planar) is something like this N7VE design:
http://www.azscqrpions.org/ScQRPions_Williams_Program_Report_files/image040.jpg

For ease of mounting I am considering flattening the entire loop assembly into a plane. This effectively makes the loop inductor "edge wound". The idea is illustrated in the below, similar design, the K3MT receiving loop design.

Planned, new apperance (planar):
http://web.archive.org/web/20110522154120im_/http://users.erols.com/k3mt/hla/hla9.gif

Description:
http://web.archive.org/web/20110522154120/http://users.erols.com/k3mt/hla/hla.htm

I suppose that VHF/UHF loop antennas on printed circuit boards, where the loop is formed by a flat and planar copper trace, are also similar.
 
My question: I was wondering if there are any conceivable ill effects to flattening a strip-based transmitting loop antenna into a plane as in the above diagram. I seem to recall there was some discussion on eham about this a while back, regarding current flow in the planar edge-wound case, but I've been unable to find the thread.

Thanks for any advice.
« Last Edit: April 18, 2012, 09:26:42 PM by JAHAM2BE » Logged

K1TWH
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« Reply #1 on: April 18, 2012, 05:06:38 PM »

I think you are referring to "current bunching" when turns of a multi-turn loop are spaced at less than 10 times the diameter of conductor.   While there was much stated about the MFJ loops versus the AEA loop, I have never read a credible source state there was a difference unless the loop was multi-turn in nature.
       73,  Tom Howey  WB1FPA
                = = = = = = = =
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K5BJS
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« Reply #2 on: April 18, 2012, 08:05:32 PM »

Quote
I've built a prototype magnetic loop (small transmitting loop) antenna from 45-cm-wide aluminum foil

Aluminum foil is too thin for this application (skin depth in aluminum at 7 MHz is about 31 microns or 1.2 mills).  Suggest you look at aluminum flashing instead.  It comes in widths up to 28" (71 cm).
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WX7G
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« Reply #3 on: April 19, 2012, 04:41:34 AM »

This is about your present loop. I assume it has a diameter of 1 meter and the aluminum foil is heavy duty, 20 um thick.

At 7 MHz the radiation resistance is about 10 milliohms and the loss resistance is 10 millioihms giving a maximum potential radiation efficiency of 50%.

At 14 MHz the radiation resistance is about 200 milliohms and the loss resistance is 10 milliohms giving a maximum potential radiation efficiency of 95%.
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JAHAM2BE
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« Reply #4 on: April 19, 2012, 05:04:32 AM »

This is about your present loop. I assume it has a diameter of 1 meter and the aluminum foil is heavy duty, 20 um thick.
[... useful numbers omittted ...]
Thanks for the calculations. The foil is 40 microns thick and 45 cm wide, and 1m is the approximate initial diameter I am trying.

Are the efficiency calculations still approximately valid even if the loop conductor is squashed into a flat plane as in the planar image linked to above? (http://web.archive.org/web/20110522154120im_/http://users.erols.com/k3mt/hla/hla9.gif)

As a note, I am using thin polyethylene packing foam as a sleeve around the foil for strength and to serve as the capacitor dielectric.
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AC7ZN
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« Reply #5 on: April 19, 2012, 05:33:57 AM »

I did some modeling of this configuration a while ago, using a 1 meter dia loop 12" wide.  At the center of the width, RF current is 1/2 that at the edges.  This is not skin effect, but I think is called 'edge crowding'.  I have yet to model a 'pipe' loop but my sense is they will be fairly close in terms of efficiency.  The nice thing the cylinder buys you is the inexpensive and efficient capacitor.  I tend to think many loops have efficiency losses through the capacitor joints even for the expensive vacuum variables and especially for air variables.  No joints in your loop!

When I built one with aluminum roof flashing and hula hoops I found you had to be careful with mechanical stability at the capacitor.  The loop is inherently flimsy and will detune easily without something to stabilize it mechanically.  I used some braces backing the capacitor plates but am still looking for something simpler/better.  Definitely not an outdoor antenna!

73,
Glenn AC7ZN
 
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K5BJS
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« Reply #6 on: April 19, 2012, 11:07:17 AM »

Quote
At 7 MHz ... the loss resistance is 10 millioihms ...

At 14 MHz ... the loss resistance is 10 milliohms ...

What method did you use to calculate the loss resistance?  I'm surprised it's the same at both frequencies.
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JAHAM2BE
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« Reply #7 on: April 20, 2012, 05:01:49 AM »

When I built one with aluminum roof flashing and hula hoops I found you had to be careful with mechanical stability at the capacitor.  The loop is inherently flimsy and will detune easily without something to stabilize it mechanically.  I used some braces backing the capacitor plates but am still looking for something simpler/better.  Definitely not an outdoor antenna!
In my very preliminary experiments I've found that altering the loop shape and thus its inductance (and probably its distributed capacitance as well), it is possible to tune the antenna by a few kHz at 21 MHz. Using a fixed capacitor and a squashable loop might be mechanically more feasible.

I'm still wondering if anyone knows the answer to the original query about the effects of flattening the loop into a plane.  Grin
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N3OX
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« Reply #8 on: April 20, 2012, 07:32:58 AM »

I'm still wondering if anyone knows the answer to the original query about the effects of flattening the loop into a plane.  Grin

You might be able to find some formulas in the professional literature on PCB antennas but in general you would need to actually numerically calculate the fields around the loop and get the current density in the foil to be able to calculate the loop's inductance, loss resistance, etc.  Not many people have access to what they need for that.   And actually because you've folded the foil over and not bonded it completely, that's probably un-simulatable unlike a continuous PCB strip.

Won't effect the radiation... a small loop is a small loop, though I suppose the question of what the radiation resistance is is made harder by wondering what the "size" of the loop is...  but this is an issue of prediction not of actual performance. 

In most cases  people find it easier to experiment than to dig in what's known for applicable predictions, and the kind of software you need to model this kind of thing directly is something that even particularly technically-oriented hams will find either excessively expensive or excessively hard to use for hobby purposes.  For proximity effect stuff about coils, for example I'd love to have Ansys HFSS, but it's expensive for a legit license and would take me a long time to understand and get confident in the results. 

And I'm only interested in that because I'm interested in what we can do to have good completely reliable formulas for coil Q, a prediction that very rarely matters to most ham antennas.  The exact resistance and inductance of your loop are similar: it would be nice to have a prediction but much of the time it's easier and more reliable to just skip the prediction and experimentally determine how a thing works.

It's important anyway: there are a lot of predictions out there that simply haven't been tested and things that you might leave out of the prediction (like the effect of the overlapped aluminum foil) even if you had sophisticated simulation tools.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
JAHAM2BE
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« Reply #9 on: April 20, 2012, 10:01:28 AM »

in general you would need to actually numerically calculate the fields around the loop and get the current density in the foil to be able to calculate the loop's inductance, loss resistance, etc.  Not many people have access to what they need for that.   And actually because you've folded the foil over and not bonded it completely, that's probably un-simulatable unlike a continuous PCB strip.

Right, thanks for the detailed response. I have the entire length of foil coated in a 1mm-thick polyethylene foam, so folding at the corners will create an overlapping foil area with slight separation. This will conceivably affect current flow and introduce additional stray capacitance, which I vaguely recall being a bad thing (e.g. inter-turn capacitance in a multi-turn loop is a bad thing by introducing circulating currents, reducing loop Q, and increading I^2 R losses - ref. http://forums.qrz.com/showthread.php?241400-Magnetic-Loops&p=1961567#post1961567).

As for EM field simulation, now you have piqued my interest to see of there is free software available for this kind of simulation. A quick search found this: http://www.sourcefiles.org/Scientific/Physics/emap3.c.shtml. I'd really love to be able to simulate this stuff because I don't have the equipment to measure circulating currents and the like. I'd at least like to be able to prove to myself, on the computer, the known phenomena about small loops.

Edit: This electromagnetic simulator looks promising: http://www.openems.de/index.php/Main_Page. There are many examples of antenna simulations, e.g. http://www.openems.de/index.php/Tutorial:_Helical_Antenna.
« Last Edit: April 20, 2012, 10:24:19 AM by JAHAM2BE » Logged

N3OX
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« Reply #10 on: April 20, 2012, 02:13:52 PM »

Edit: This electromagnetic simulator looks promising: http://www.openems.de/index.php/Main_Page. There are many examples of antenna simulations, e.g. http://www.openems.de/index.php/Tutorial:_Helical_Antenna.

Hm that's pretty interesting.  Will have to take a look. 
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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