Fundamental Problem Building Magnetic Loop?

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Nicholas McKinney:
I'm in the middle of trying to build a magnetic loop antenna, modeled after the AA5TB antenna.  My main loop is 1m in diameter with a 100pF air variable capacitor and using a gamma matching stub.

In trying to tune the gamma match, my impedance doesn't vary beyond 2 ohms.  I know there's a problem somewhere in the design, but I'm at a loss where to troubleshoot.  I've made every adjustment I can think of to the stub, without seeing any effect.  What else should I be looking at?


Nicholas McKinney:
Some measurements:

14 MHz:  1.8 + j 15.9 (SWR 31.27:1)
18 MHz:  13.3 + j 147.7 (SWR 36.85:1)
21 MHz:  6.3 - j 136.1 (SWR 67.29:1)
28 MHz:  1.9 + j 15.9 (SWR 29.12:1)

Still no idea what's going on.  Thought it was how the capacitor was wired up, but I double checked it.  Still not super confident on that.

Glenn Dixon:
Hi Nicholas,

Your gamma match impedance will be very low except at the exact antenna resonant frequency, which is hard to determine with a transmitter/swr meter or antenna impedance meter.  The antenna is very narrow band, so it is easy to skip right by the resonant point if you are watching a meter while transmitting and tuning the capacitor.  If you are using something like an MFJ antenna analyzer you may never be able to get the frequency stable and correct enough to get a valid measurement.

Your antenna may be working...try hooking it up to your transceiver, tuning the antenna capacitor for best reception, and seeing what SWR your transmitter says you have (start at low power;->).

Glenn AC7ZN

Peter Chadwick:
This may be a case for a grid dip meter. As Glenn said, the gamma match is very difficult to adjust with a non resonant antenna: it is possible to get a match, but it is very narrow band. I believe that a more usual feed system for a magnetic loop is a coupling coil.


Why don't you try to use a couple loop.
That is a small loop abt 1/5 of the loop diameter.
I build a few in the past with such a small loop to couple and they worked FB and I got no troubles in reaching SWR of 1 or 1.1 on 17 , 20, 30 and 40 mtrs with this 1 meter diameter magloop.

I bend the small couple loop out of ordenary solid insulated copperwire for house electric instalations about 13 gauge or 2.5 mm square. 

The biggest chalenge in building magnetic loops are resistive losses in the loop.
capacitor conections must be big and bulky and the capacitor must be able to handle high currents and high Voltages.
though the haigh voltages may lead to the conclusion that the curents are not so big at this point, there are hugh amounts of charges swept around from one plate to the other.
I constructed my own remote motordriven controled 5KV+ air capacitors and that was a lot of work to build these.

If you want a picture of this highly experimental first loop i only have a few pics of the first one I build, that might give you enough idea to make your own couple loop.
Just send an E-mail to my QRZ e-mail adres.

I later made these loops a bit neater and gave them away at friends whom couldnt put up an outdoor antenna for HF.

I didnt like these loops at all, so small in bandwith and not very effective in compare with all my other antennas i have designed and made in the past 20 yrs.

I made my loops out of a single piece of red copper pipe (abt 1/2 inch diameter)  bended into a circle.

73 Jos


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