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Author Topic: Magloop: inductive coupling lossier than capacitive coupling?  (Read 5109 times)

Posts: 313


« on: April 28, 2012, 05:16:38 AM »

I found a German web site that makes some claims I've not seen made elsewhere regarding small transmitting loop (magnetic loop) antennas. The site claims that inductive coupling is lossier (around 6 dB) than capacitive coupling due to poor coupling from a small coupling loop to the larger loop inductor. The site also claims that a large coupling loop (as large as the loop itself) improves coupling efficiency, by 11 dB! The conclusion is that for best efficiency, a coupling loop as large as the radiator or a capacitive coupling should be used.

What say you?

Machine-translated text:

Original German text:
« Last Edit: April 28, 2012, 07:58:56 AM by JAHAM2BE » Logged


Posts: 85

« Reply #1 on: April 28, 2012, 03:41:10 PM »

Hi Ham2be,

This ham has obviously spent a great deal of time and effort investigating this topic, and I greatly respect those who attempt to answer questions with actual experiments.
I am, however, puzzled by his approach and measurements.  A mag loop is a large parallel tank circuit:  The more current you can get resonating in that loop (tank), the better your transmitted signal strength will be.  So here is what is puzzling:

1.  The small loop "1/5 the size of the big loop" typically shows a fifty ohm pure resistive load at resonance.  This means there are no 'leakage inductances'.  In theory, the smaller you can make that loop, the higher the induced current in the large loop will be, and the better it will transmit.  Indeed, if you find that you have to make that loop larger to get a fifty ohm load it may be an indication that your large loop has excessive losses and needs work.

2.  The performance of the mag loop is normally dominated by things like the loop diameter and the Q of the capacitor and its connections.  The coupling network used is typically not considered a significant loss factor (it is possible to have a lossy coupler, such as some based on ferrite transformers, but simple coupling loops shouldn't have much loss).

3.  The author's measured field strength difference of 11 dB (in what I consider the wrong direction) is huge...making me think something strange is going on.  Such a measurement would cause me to think some sort of 'sanity check' of my entire system is necessary.

Glenn AC7ZN

Posts: 8911


« Reply #2 on: April 28, 2012, 04:22:44 PM »

In my opinion this kind of thing is often caused by measuring something other than what's intended.  There is no reason that I can think of why this result should happen, and the main effect of changing coupling loop size and spacing should be to change the matched impedance of the loop.  In some circumstances in a reception test, it might be that an increase in signal could be had by using a different coupling loop if the receiver input impedance is not 50 ohms, but then it won't be matched properly on transmitting.  If a small simple signal generator other than the actual transmitter is used, there's a chance that it delivers more power into an impedance other than 50 ohms without complaining about SWR like a modern ham rig would.

I don't get a clear picture of the measurement test setup from the linked articles (though I do not understand a bit of German, even after a week in Vienna, so I may miss some things ).  At this late hour I'm not really following the coupling coefficient stuff but I would mention that "poor" coupling in the sense that not much of the magnetic flux of the large loop cuts the small loop is really not a problem; it doesn't cause a loss.  It causes an impedance transformation (a generally useful one) and a leakage inductance which needs to be tuned out but this should not be a problem.

At any rate I am quite sure that there is no large improvement in actual receive performance (other than some unimportant signal level difference) or far field transmitted power when going from a small to large coupling loop.

When someone quotes a figure for antenna improvement as large as 11dB or 13dB but does not go into great detail into how they measured it, I am generally left with the impression that they were not surprised by such a huge increase.  And if they weren't surprised, I wonder if they were sufficiently skeptical to rule out other causes for their findings.  So if I'm not particularly interested in the claim they make, I will just set aside their report as not very useful to me. 

Sometimes I *AM* interested, even if I "know" that the result is "impossible."  When someone is reporting the results of their experiments in good faith, freely distributed as information to other hams, they usually have actually observed some difference.  In cases that seem too good to be true, the problem is usually just a misinterpretation of the cause of the difference.  Doing an experiment to check their claim will sometimes lead to an interesting realization about things you have to be careful of in RF measurements and the various causes for differences between antennas.

It's not very likely that 10dB+ boosts are just sitting around free and easily for the taking, and reports thereof almost always mean something is wrong in a measurement technique, the chain of reasoning about some theory, or whatever, but it can be worth putting in some kind of work on this kind of thing from time to time because the actual cause of the result will result in some good nugget of information about SOMETHING.

In other words, if I were trying to work with high performance magloops, I might take a gamble on an experiment with big coupling loops even though I'm quite sure it won't work.  It's not hard to do, it won't waste a lot of time, and it might expose some interesting aspect of magloop construction and testing that will actually improve the final product, if not by an immediate and glaring 10dB+



Monkey/silicon cyborg, beeping at rocks since 1995.

Posts: 960

« Reply #3 on: April 30, 2012, 03:54:55 AM »

The size of the coupling loop will have no effect on more RF energy coupled to the large loop.
There was an excellent article in QST a few months ago (NOV 2011), where an Amateur built magnetic loop antennas. He calculated more radiated power using 2 turns of copper pipe for the larger coil (6 feet Dia) His focus was for 80M. And he built a 3 foot dia loop (14mhz-28mhz) using a single turn for the main coil. Both magnetic loop antennas had the typical small loop to couple the RF into the big loop.
It gets really expensive to use copper pipe and silver solder all that together. The Vacuum Variable capacitor will add another $200 to the project BUT the antenna will handle legal limit, this is the hardest goal to achieve using a magnetic loop antenna.
You will have to reconsider RF exposure limits using a mag loop antenna at legal power!!!
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