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Author Topic: Small multiturn wire loops for transmitting  (Read 50167 times)
JAHAM2BE
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« on: June 15, 2012, 07:22:46 PM »

I've been doing some research into multi-turn wire loops for transmitting and am wondering if it is completely inadvisable and inefficient as many seem to say, or if there are some situations where it can make sense and might achieve, say, around 50% efficiency at 7 MHz.

The reason I am investigating wire loops is that I have a stock of 15m of 2mm-diameter copper wire. I was thinking that if I made a 1m-diameter, 2-turn or more-turn loop, while keeping turn spacing large at say 5cm or even more (to reduce proximity effect), I could increase the surface area which might improve radiation efficiency.

I understand that for a given surface area it is better to use a single-turn, smooth conductor for the loop radiator, but the point is that it is much easier and cheaper to increase surface area by winding a few more turns of 2mm wire than it is to buy, bend, and/or solder thicker copper pipe.

The downsides to multi-turn loops, as I understand it, are threefold: (1) self-capacitance increasing circulating currents within the loop which increases I^2 R losses; (2) the requirement of empty space between turns to prevent arcing, leading to inefficient use of available antenna volume; and (3) proximity effect between turns forcing current to flow in more restricted patterns which increases loss.

If we ignore item (2), assuming adequate volume is available to insert space between the turns, then it seems that both (1) and (3) can be mitigated simply by having a large enough spacing between turns.

AA8C reports success on 75m with a multi-turn wire loop, and also quotes a detailed study performed by the US Navy about optimizing multi-turn wire transmitting loops:
http://members.verizon.net/~vze24qhw/loop.html
http://members.verizon.net/~vze24qhw/smith_1971.pdf

I plan to run some simulations in 4nec2 but I think that NEC2 does not model proximity effects so the efficiency results may be optimistic.

So again, my question: is it possible to around 50% efficiency at 7 MHz with a nominal 1m-diameter loop and some to-be-determined number of turns of 2mm-diameter copper wire?

edit: spelling fixes; adding keyword "magnetic loop" for search engines
« Last Edit: June 15, 2012, 07:33:58 PM by JAHAM2BE » Logged

W5DXP
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« Reply #1 on: June 15, 2012, 08:57:59 PM »

is it possible to around 50% efficiency at 7 MHz with a nominal 1m-diameter loop and some to-be-determined number of turns of 2mm-diameter copper wire?

IMO, if it were possible, someone would have already done it. Vacuum caps are very low loss and that makes single turn loops more efficient, i.e. most of the losses are in the inductor so why increase the inductor losses with multiple turns?
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
N3OX
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« Reply #2 on: June 15, 2012, 10:00:09 PM »

Quote
AA8C reports success on 75m with a multiturn wire loop, and also quotes a detailed study performed by the US Navy about optimizing multiturn wire transmitting loops:
http://members.verizon.net/~vze24qhw/loop.html
http://members.verizon.net/~vze24qhw/smith_1971.pdf

The latter .PDF has some good predictions Re: multiturn loops and if you work through that it might be a worthwhile thing.
But even that study doesn't seem actually check the finished efficiency of some predicted multiturn loops.  They check the proximity effect but this is not the only issue.  I recommend sticking to sources that actually quantify the relative performance of finished antennas if your intent is to trust the information implicitly.  The Navy study is good guidance for what to expect for proximity effect (at least for a couple turns at low enough frequencies... of course I did not read the whole thing but I noticed at the beginning that they were neglecting displacement currents in some of the calculations... maybe not throughout?)

I've not seen people trying 1, 2, 3, 4, and 5 turns with any kind of signal comparison between them.  Maybe you've run across this?  

I feel like there's room for experimentation but I have't seen it or tried it.

Quote
I plan to run some simulations in 4nec2 but I think that NEC2 does not model proximity effects so the efficiency results may be optimistic.

Right, no proximity effect in NEC-2, very likely to cause errors.  Maybe not so important with multiturn loops of the sort you describe, but I am not so sure.

Quote
So again, my question: is it possible to around 50% efficiency at 7 MHz with a nominal 1m-diameter loop and some to-be-determined number of turns of 2mm-diameter copper wire?

No, I don't think so, even setting aside proximity effect.  I'm getting 8.5% efficiency (-10.7dB) out of a 5 turn 3 foot loop with 2 inch spacing (actually darn close to self resonant, FYI) which is 7dB better than a single turn of that size but not even close to what you're looking for.  More than 5 turns will be impossible... and actually even 4-5 turns are both probably out because they need such tiny capacitance.  4 turns is giving -11dB, FYI.  Not much worse than the 5 turn (don't know if that is as would be predicted by simple analytical arguments).  I am not going to run the 2 and 3 turn models but if you want a starting point:

http://n3ox.net/files/eznec/multiturn_loop.ez

I expect with ~1m loops of 2mm wire they are between -17dB and -10dB for everything smaller than self-resonant.  (Plus these were run over perfect ground which gives a radiation resistance boost; subtract approx. 3dB for free space numbers)  

I wouldn't be too surprised if some proximity effect and other wire imperfections tip the scales so that multiple turns are uniformly worse.  That seems to be the LORE anyway,  that multiturn loops suck; I  haven't seen a direct report where it was tested quantitatively with reasonable construction though.  I think it's plausible that two or three turns would outperform a single turn of the same conductor by quite a bit but you're not going to get 50% efficiency out of a 0.025 wavelength diameter loop with wire.

Quote from: W5DXP
most of the losses are in the inductor so why increase the inductor losses with multiple turns?

The radiation resistance goes up like the square of the number of turns.  I've seen it assumed that the loss resistance goes up like the number of turns, this is a big advantage; the efficiency goes up linearly with the number of turns.  I think the assumption of linear increase of loss resistance is often (usually?) a bad assumption, plus it's cheaper to go to a bigger conductor if you have fewer turns, so I don't think it's a good idea, but in principle you're greatly enhancing the radiation resistance.
« Last Edit: June 15, 2012, 10:03:28 PM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #3 on: June 15, 2012, 10:07:00 PM »

The assumption of N^2 radiation resistance is probably generally bad too.. certainly is for that 5 turn loop I posted; it has less than half the current flowing in the two outermost turns compared to the middle turn.  Not even close to five turns carrying a uniform current.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
JAHAM2BE
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« Reply #4 on: June 16, 2012, 12:15:14 AM »

I've not seen people trying 1, 2, 3, 4, and 5 turns with any kind of signal comparison between them.  Maybe you've run across this?  

I feel like there's room for experimentation but I have't seen it or tried it.

I haven't found much other than the AA8C site and the referenced US Navy paper. There is a 1974 IEEE journal article that looks relevant but, in accordance with the best traditions of academic publishing, it's behind a paywall. Anyone with a membership feel like having a look and summarizing the results?

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1140814&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D1140814

From that page:
Quote
On thin-wire multiturn loop antennas

A general theory of thin-wire multiturn loop antennas is presented. For convenience the windings of the loop are considered to form a circular helix. An integral equation is derived and solved numerically for the current distribution on the antenna. The antenna impedance, efficiency, and pattern are obtained.

However that article, too, looks like it might not actually measure the radiated signals. I agree that there is room for amateur experimentation here, if only because almost no one in the amateur community seems to be investigating multi-turn transmitting wire loops.
« Last Edit: June 16, 2012, 12:20:55 AM by JAHAM2BE » Logged

KC4MOP
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« Reply #5 on: June 16, 2012, 04:38:19 AM »

A few QST's ago (NOV 2011), someone did build a two-turn magnetic loop antenna. The cost of materials and meticulous construction is the reward of a great antenna. Should be better than an elevated vertical,according to the author/builder.
He used a nice Russian made vacuum variable and an interesting tuning motor to tune his loop. And the antenna was able to handle full legal limit. This is the difficult part of building a loop antenna.
You will have to re-calculate the RFE as the field from a loop antenna is much stronger than a wire antenna or vertical.
At 3.5 mhz he calculated about 20% efficiency. He figured that a larger diameter loop would improve efficiency. And operating 40M might get close to the 50% the OP mentioned.
The nice thing about magnetic loop antennas is the huge reduction of noise. Electrical noise from MaNature, your neighbors............
My retirement level of finances prevents me from building one.
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WX7G
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« Reply #6 on: June 16, 2012, 06:16:02 AM »

Let's do some calculations for a small multiturn loop. R = (177NS/lambda^2)^2, S = loop area

At 7 MHz and for a 1 meter diameter loop R = [(177 X 2 X 0.785)/43^2]^2 = 0.023 ohms radiation resistance

Skin depth in copper at 7 MHz is 0.03 mm. The resistance per square is 560 u ohms. The 6.3 X 6300 mm strip of copper is 1000 squares and the resistance is 560 u X 1000 = 560 milliohms.

The radiation efficiency of your antenna is 0.023/0.56 X 100% = 4%.

How would one get 50% efficiency? Using an MFJ loop tuner and a 1/4 wavelength loop (note it's no longer a 'small' loop) made of 1/2" copper tubing one can approach that efficiency. For a circular loop the diameter is 3.4 meters. The RF resistance at 7 MHz for 1/2" copper tubing is 560 u ohms per square or 560 u ohms per 1/2" (12.7 mm). For the 3.4 meter loop the RF resistance is 0.15 ohms. The radiation resistance of the loop (using NEC and a square loop) is about 0.3 ohms. The efficiency is 67%.

« Last Edit: June 16, 2012, 06:25:34 AM by WX7G » Logged
JAHAM2BE
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« Reply #7 on: June 16, 2012, 07:02:39 AM »

I've not seen people trying 1, 2, 3, 4, and 5 turns with any kind of signal comparison between them.  Maybe you've run across this?

By the way, here in Japan there is a company selling a commercial multi-turn small loop antenna. It uses aluminum tubing, not wire, but the uncommon multi-turn design is I think relevant to this thread. See the "MK-4A" at the bottom of this page:

http://www.rocket-co.jp/ham/loop-ant.html

I had a look at that model in the store some time ago and they used a screw (or maybe a bolt; I can't recall), a lug, and a bit of wire to connect the loop element to the sealed tuning box. Either the multiple turns have raised the radiation resistance to such a point that mechanical connections are acceptable, or the antenna is poorly designed. I'll try to see if I can find any reviews or measurements of the performance of the antenna.

Edit: taking a closer look at the antenna specs makes me wonder if it's really a small loop antenna or not. It claims it can tune from 3.7 to 50 MHz and that it specifically uses an "ATU" for easy tuning. This starts to make it sound like it may be relying on feedline radiation.
« Last Edit: June 16, 2012, 07:29:48 AM by JAHAM2BE » Logged

W5DXP
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« Reply #8 on: June 16, 2012, 09:52:16 AM »

The radiation resistance goes up like the square of the number of turns.
According to The ARRL Antenna Book, when a loop is longer than ~0.1WL, it no longer meets the definition of a "small loop" because the in-phase current assumption is not valid everywhere on the loop. A one-turn, 1m diameter loop used on 40m meets the ARRL's definition of a small loop (0.1-0.085WL max length) but a two-turn, 1m diameter is about 0.157WL long on 40m and the adjacent turn currents may be ~20+ degrees out of phase causing transmission line effects and reduced radiation efficiency.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KC4MOP
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« Reply #9 on: June 16, 2012, 04:26:55 PM »

In the QST article of NOV 2011, WV6N used a 6.6 foot diameter loop using 1.5 inch dia. copper pipe. And he was using  4NEC2 as a calculator. He also claimed to be using a free spreadsheet formula from AA5TB for loop antennas. What he built was above and beyond an MFJ type loop antenna.
Members of ARRL can see the QST in depth articles and you will see the healthy construction of this Ham's loop antennas. QS1111paun.ZIP
He built a 6.6 foot dia for 160-40M and a 3 foot dia for 20-10M.
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N3OX
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« Reply #10 on: June 17, 2012, 06:23:06 AM »

There is a 1974 IEEE journal article that looks relevant but, in accordance with the best traditions of academic publishing, it's behind a paywall. Anyone with a membership feel like having a look and summarizing the results?

Much of the article is dedicated to deriving integral equations which are then used in numerical calculations of the antenna properties.  They're using a thin wire approximation

There is a reference to some measurements by Flaig which should be here:

http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA951890

But doesn't seem to be coming up.

Flaig, T.L., "The Impedance and Efficiency of Multiturn Loop Antennas," Technical Report 2235-3, The Ohio State University Electroscience Laboratory, 3 April 1968.

There's another one that may be relevant too.

Munk, B.A. and Flaig, T.L., "Radiation Resistance and Efficiency of Multiturn Loop Antennas," Technical Report 2235-4, The Ohio State University Electroscience Laboratory, 1968.

The measurements by Flaig are in the same ballpark as the numerical calculations in this paper around 50MHz and a 5 turn loop 0.2m in diameter with 1cm spacing and 0.8mm wire has an efficiency less than 10% at 50MHz.  But this particular loop is evidently well above self resonance when operated at 50MHz.  All of the measurements are above the first self resonance.

I hope I haven't misrepresented anything by going through the paper too quickly... maybe that Flaig paper will eventually be accessible Sad 

Quote
However that article, too, looks like it might not actually measure the radiated signals. I agree that there is room for amateur experimentation here, if only because almost no one in the amateur community seems to be investigating multi-turn transmitting wire loops.

Yeah I'd forgotten about WV6N but it's one of the few I'd seen.  I'm sure people are out there playing with multi-turn loops but not publishing..  Maybe we will attract some more of them.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
JAHAM2BE
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« Reply #11 on: June 17, 2012, 04:43:02 PM »

Quote
So again, my question: is it possible to around 50% efficiency at 7 MHz with a nominal 1m-diameter loop and some to-be-determined number of turns of 2mm-diameter copper wire?

No, I don't think so, even setting aside proximity effect.  I'm getting 8.5% efficiency (-10.7dB) out of a 5 turn 3 foot loop with 2 inch spacing (actually darn close to self resonant, FYI) which is 7dB better than a single turn of that size but not even close to what you're looking for.  More than 5 turns will be impossible... and actually even 4-5 turns are both probably out because they need such tiny capacitance.

OK, then how about 5 or even more thin-wire conductors in parallel, which should maintain constant current in each conductor and increase the surface area without increasing the inductance?

This is exactly what AA8C does in his aluminum-tubing loop, and he reports that adding more conductors does not increase the loop inductance.

http://members.verizon.net/~vze24qhw/loop2.html

Though using turns in parallel, not multiple turns, he justifies his use of parallel turns by citing the same US Navy study done on multi-turn loops.

This raises two interesting questions in my mind:

1) What is the difference between a multi-turn loop and a loop made of parallel, spaced conductors?

2) Since adding more loop conductors does not increase the loop inductance, it seems there might be no limit to the number of parallel conductors that could be used in this manner. Is this right, and if not, what is the limiting factor?
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KC4MOP
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« Reply #12 on: June 17, 2012, 05:24:26 PM »

The effectiveness of the transmitting loop is the use of pipe. Wires are not going to cut it. There's a lot of math involved to calculate distributed capacitance, etc, etc and you begin on using at least 1.5 inch diameter copper pipe. Aluminum is acceptable. And whatever material you use needs to be welded or silver soldered. No mechanical connections allowed.
You can squeak by using wires as a receiving loop.

Fred
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W0BTU
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« Reply #13 on: June 17, 2012, 08:25:20 PM »

The effectiveness of the transmitting loop is the use of pipe. Wires are not going to cut it. ... You can squeak by using wires as a receiving loop.

Yes! Unless your only interest is investigating the proximity effects of multiturn loops, a single turn loop made of large copper or aluminum pipe with low-resistance connections is the way to go for an efficient STL (small transmitting loop).

Take a look at the links at http://www.w0btu.com/magnetic_loops.html. Lots of great information by a lot of people who have actually built and tested STLs.
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N3OX
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« Reply #14 on: June 18, 2012, 07:09:37 AM »

1) What is the difference between a multi-turn loop and a loop made of parallel, spaced conductors?

No enhancement to the radiation resistance.  Somewhat reduced  inductance (but not hugely).

Quote
Though using turns in parallel, not multiple turns, he justifies his use of parallel turns by citing the same US Navy study done on multi-turn loops.

That seems fine regarding the proximity effect formulas.. how to parallel some wires to make it better than one wire: what are the limits in a given volume of space?

This will help tell you what to do with a given size round conductor if you decide this should be an absolute design constraint.  If you are going to stick with your wire, for sure, I think you'll get good relative guidance from that Navy report. 

Quote
2) Since adding more loop conductors does not increase the loop inductance, it seems there might be no limit to the number of parallel conductors that could be used in this manner. Is this right, and if not, what is the limiting factor?

I would guess that it never gets quite as good as a single conductor with similar surface area.  But I don't know that for sure.  You still have proximity effect concerns, but you also have current bunching concerns on flat strips and things like that too... without doing a quantitative analysis from the relevant theory I can't even guess the trade-off point between, say, a bunch of wires in a six inch wide array and a piece of aluminum flashing... and then you have to ask if you can make a more compact loop by buying a piece of copper tubing, unless you have a special purpose in mind like a foldable loop or something.

I also think that at some point if you try to add wires without getting them too bunched together you're going to have a geometrical problem that leads to wires of slightly different lengths and bends that may cause significantly different currents in the different wires. 

I actually checked a model of a 1m magloop on 7MHz consisting of 5 parallel wires separated out 3 inches and connected in parallel with straight wires, and this has the opposite problem as the 5 wire multiturn: in this case, the middle loop has 30% less current than the two outer edges: seems plausible that this is the discrete-wire equivalent of currents bunching on the edges of a flat strip.  It's not a huge effect but it will become a limiting factor eventually.  The big question with wires is whether you ever approach the efficiency of a different possible, practical conductor configuration  that you could use?  Something that is quite clear is that if you are absolutely, positively constrained to use ~2mm wire to make a 1m transmitting loop for 40m, you are better off if you use multiple wires... in a multi-turn configuration or parallel, both give a large improvement over a single wire. 

But the 5 parallel wires spread out over a foot of width for a 1m diameter magloop on 40m is still in the -10dB class, similar to the 5 turn multi-turn loop.  It actually seems the 5 parallel wires is a bit worse.  Perhaps packing them more densely will lead to a substantial improvement over that.

I guess the nice thing about wire loops is that they are very easy to solder which could allow you to try several different loops on the same capacitor.. and it's easy to add more loops to see how the efficiency improves compared to whatever predictions.. whether analytical or from 4nec2 or whatever.

====================

I should say one more thing which is that I think 50% efficiency out of a 1m loop on 7MHz is probably a pipe dream.  The single loop radiation resistance is maybe 5-6 milliohms? 

So we should really assess various wire configurations in the context of a practical soldered/welded, fat copper pipe loop which seems more like 30% efficiency for 2 inch copper pipe, less once you include cap losses. 

Five wires either multiturn or parallel spread a couple inches between them seems to clock in at the 10% level (neglecting cap losses) and that seems about equivalent to half inch copper pipe.  (EZNEC results)  So I think the discussion may be skewed a little bit if we don't leave the idea of 50% efficiency from a 1m 40m magloop behind (unless 4 or 5 inch diameter round copper conductor is a possibility... I did see a Japanese ham who made some giant doughnuts out of copper foil, but not sure the actual performance of that technique... )



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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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