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Author Topic: My magnetic loops have the wrong resistance and impedance. Thoughts?  (Read 15015 times)
VE7TIT
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« on: May 20, 2012, 03:46:17 PM »

I'm testing a big square magnetic loop. It's 5 feet on each of its four sides. When I test it with my noise bridge with the "standard" 1/5 copper feed loop, I get about 5 ohms and inductive. Really, it's as if I'm just measuring the feed loop (though I am getting good reception and it peaks, so it's working). It's just press fit 1/2" copper pipe at this point, not soldered. So could that be it? I'm testing in my living room (I don't have a yard), so there's house wiring and a big radiator nearby etc. Could that be the problem?

I tried making a small (1 m diameter) circular loop out of 16 gauge wire just to take another measurement and even though it, too, peaks with tuning and seems to receive OK, I'm getting the same low resistance with a big inductive component.

This isn't what the books, websites, and calculators say is supposed to be happening. Help! Wink
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K4SAV
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« Reply #1 on: May 20, 2012, 06:42:54 PM »

Without any details on what you built, any answers would be a wild guess.  At what frequency are you making the measurement?  It could be something as simple as locating the coupling loop in the center of the larger loop.  If you have a round coupling loop inside a square one, the diameter of the smaller loop is probably a little too small.

Jerry, K4SAV
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VE7TIT
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« Reply #2 on: May 20, 2012, 09:08:31 PM »

Without any details on what you built, any answers would be a wild guess.  At what frequency are you making the measurement?  It could be something as simple as locating the coupling loop in the center of the larger loop.  If you have a round coupling loop inside a square one, the diameter of the smaller loop is probably a little too small.

  • I've made measurements at 7, 10, and 20 meters.
    It's 1/2 copper pipe, square, 5' on a side, and has copper elbows.
    Tuning (at this early, experimental stage) is with a sheet of aluminum foil wrapped in a tube around mylar sliding along the top of the antenna. The sound peaks up very nicely at just the capacitance levels expected from calculation, so at least that's working right.
    The coupling loop is bare #14 wire so I can clip on at different lengths, and I've tried everything from about 1/10th to 1/4 the loop diameter. The major difference is that the inductance gets higher but the resistance reading stays low--about 5 ohms or so.

I have built a lot of antennas and thought I understood small loops after lots of reading, but this has me scratching my head. Thanks for any suggestions.
« Last Edit: May 20, 2012, 09:10:25 PM by VE7TIT » Logged
JAHAM2BE
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« Reply #3 on: May 20, 2012, 09:29:46 PM »

It's 1/2 copper pipe, square, 5' on a side, and has copper elbows.
Tuning (at this early, experimental stage) is with a sheet of aluminum foil wrapped in a tube around mylar sliding along the top of the antenna. The sound peaks up very nicely at just the capacitance levels expected from calculation, so at least that's working right.
I'm afraid I can't offer any specific advice about your problem (other than to suggest trying to measure the antenna characteristics in a clear and open area), but I am intrigued by your capacitor design, as I've also tried similar capacitor designs (strip loop with overlapping tails, sliding one plate over two other fixed plates, etc.).

How long is the foil that you are using for the capacitor? I think capacitor losses will go up with increasing capacitor length, but with a loop as large as you have, perhaps the capacitor is relatively short for your frequencies of interest. Will your final capacitor design be similar, but perhaps with a more solid sleeve material?
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VE7TIT
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« Reply #4 on: May 20, 2012, 09:53:30 PM »

> suggest trying to measure the antenna characteristics in a clear and open area
    Yeah, I think that's the next step. The plumber's delight setup should amuse the neighbors. (Do you know the Tom Waits song, "What's he building in there?" Wink) This living room is a very electrically/magnetically cluttered environment with tons of pipes and wires (stove, scuba tanks, etc. etc) -- NOTHING like "free space."

> I am intrigued by your capacitor design... How long is the foil that you are using for the capacitor?

The top pipe is five feet long, so the capacitor is 2 1/2 feet long (the max it can be). Mylar has good dielectric and insulating properties, and the setup is for QRP at this point, so one layer works fine, with the aluminum foil over that. It slides right along. Eventually I'm thinking of a motor and trombone-slide arrangement, but for now, I just need to get the impedance to match...
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AC7ZN
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« Reply #5 on: May 21, 2012, 07:57:40 AM »

Hi,

Although I have found the size of the coupling loop is just a rule of thumb and often needs adjusting, I'm afraid that an impedance that low is a strong sign that your main loop losses are too high. 

Think of the loop as a parallel tuned circuit you are coupling to with another winding.  Losses in the loop are a resistor in series with the L and C and lowers the resonant impedance of the loop (Part of the loss is radiation and since you have a fairly large loop some of the impedance drop may be due to that).  This lower impedance will tend to also lower the impedance of the coupler.

73,
Glenn AC7ZN
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G8HQP
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« Reply #6 on: May 21, 2012, 12:25:04 PM »

If the joints in the main loop are not yet soldered, then maybe all you are seeing is the joint resistance. Small loops can only work if all sources of loss are reduced as far as possible, as the radiation resistance is so small. A bit of joint resistance won't disturb the inductance, so it will tune up OK but will have very low efficiency.
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VE7TIT
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« Reply #7 on: May 21, 2012, 01:02:09 PM »

If the joints in the main loop are not yet soldered, then maybe all you are seeing is the joint resistance. Small loops can only work if all sources of loss are reduced as far as possible, as the radiation resistance is so small. A bit of joint resistance won't disturb the inductance, so it will tune up OK but will have very low efficiency.

The recurring theme here seems to be that my mysterious problems are probably arising from trying to test a half-finished project in my living room instead of a finished antenna outdoors. Looks like it's time to fire up the torch!
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K4SAV
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« Reply #8 on: May 21, 2012, 01:26:27 PM »

AC7ZN  is correct that the problem could be excessive loss in the large loop.  Added resistance in the large loop will cause the size of the coupling loop to be too small to match the large loop's impedance.  In that case you should measure an impedance that has a very low real part and a high reactance.  Since that is exactly what you measured, this is likely the cause.

Jerry, K4SAV
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AA5TB
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« Reply #9 on: May 22, 2012, 07:29:42 AM »

It usually isn't hard to get a relatively good match.  Make sure that you are at resonance of course and you may want to try rotating the small loop in and out of the plane of the larger loop to adjusted the impedance.  The small loop doesn't need to touch the larger loop and if it is a little larger than it needs to be and you can make it free to rotate obtaining a perfect match is a piece of cake.

I'd go ahead and solder the larger loop together first to remove that as a variable.

73,
Steve - AA5TB
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VE7TIT
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« Reply #10 on: May 22, 2012, 11:43:54 AM »

Thanks for the thoughts and suggestions, guys. I can't wait to get back to it this weekend.
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VE7TIT
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« Reply #11 on: May 24, 2012, 10:55:13 AM »

Another question: If the big loop is off resonance, is the reactance obviously present in smaller coupling loop? That is, is the input impedance at the small loop affected significantly by the tuning of the big loop? Thanks for sharing your experience and expertise.
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VE7TIT
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« Reply #12 on: May 24, 2012, 04:03:14 PM »

Another question: If the big loop is off resonance, is the reactance obviously present in smaller coupling loop? That is, is the input impedance at the small loop affected significantly by the tuning of the big loop? Or to state it yet another way, if I'm seeing 50 ohms +j0 with the loop tuned to resonance, will I see a significant change in impedance as I move away from the resonance point? From all of my reading it sounds like you can get a pretty good match across a broad range of frequencies--even entire sets of bands--but is that good match just at the resonance point?

Thanks for sharing your experience and expertise.
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K4SAV
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« Reply #13 on: May 24, 2012, 05:41:45 PM »

Yes the small loop will reflect a change of reactance in the larger loop.  If you measure a low SWR across a wide frequency range (without retuning) then you have successfully matched the impedance of a very lossy loop.  If you get this loop adjusted for a good match and have eliminated most of the loss problems, on 40 meters you should expect something close to about 5 kHz bandwidth (2 to 1 SWR points).

Jerry, K4SAV
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VE7TIT
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« Reply #14 on: May 24, 2012, 07:39:11 PM »

Yes the small loop will reflect a change of reactance in the larger loop.  If you measure a low SWR across a wide frequency range (without retuning) then you have successfully matched the impedance of a very lossy loop.  If you get this loop adjusted for a good match and have eliminated most of the loss problems, on 40 meters you should expect something close to about 5 kHz bandwidth (2 to 1 SWR points).

Jerry, K4SAV

Thanks Jerry. Great answer. I think I might know now what the problem is. I'll post when I get it figured out. (Don't hold your breath though!)
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