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Multi-Band 4 Foot Magnetic Loop with Gamma Match
24 April 2013 | by
4 Foot Diameter Magnetic Loop In Operation
As my mag loop projects progress, I’m learning a lot about building them and using them. This antenna works on 17, 20, and 30 meters, with the best bandwidth on 20 meters. The bandwidth on 17 and 30 is quite small but usable. There is a 20 KHz bandwidth on 20 meters. I used a vacuum capacitor to build this particular loop for 100 watt operation. The air capacitors are good for QRP but will arc over using much power over QRP levels.
4 Foot Magnetic Loop Antenna with Gamma Match
This photo was taken before I trimmed the Gamma Match.
I didn’t know how long the Gamma Match should be, so I made it long enough to allow for trimming after the match to 50 Ohms was made.
This is the first mag loop that I made with a Gamma match. I like this match better than the usual Faraday feed loop that is featured in my previous mag loop antennas. Matching is easy and building the Gamma match is not difficult. I’m planning to build other smaller loops that will also feature the Gamma match. The smaller loops will target 10, 12, and 15 meters. Loop size determines which band its best suited too. Although it is possible to get more bands on one loop, the bandwidth is small and the tuning ‘dip’ is very sharp. I found it better to limit the number of bands in favor of wider bandwidth and antenna efficiency.
Please look up my previous posts for more details on parts and plans.
Here is a good close up of the clamp.
(Before Trimming)
Gamma Match After Trimming. This enables a 50 Ohm Match for all three bands.
Gamma Match on the left, PL 259 in the center and, Tuning Aid Stick (PVC) pipe , on the right.
The vacuum capacitor is ‘tuned’ by twisting the PVC pipe that is attacked to it with a cotter pin.
Tuning the cap to the ‘loudest noise’ on the air will put you in the ball park for operating.
You can also use an antenna analyzer if you want to know the exact SWR, and RR of the antenna.
Tuning with a radio attached is fast and easy.
Making the clamp for the Gamma Match
Soldering The Back Plate and PL 259
Removing The Oxidation Before Soldering
The main loop is 5/8 inch, soft copper tubing.
Close Up of PL 259 to Gamma Match Joint
The center of the PL 259 had a #10 solid copper wire inserted and soldered in place.
The Gamma match is 3/8 inch soft copper tubing.
This part of the job was done with a soldering iron, the rest was done with a plumbers torch.
Handy ‘third hand’. A clamp to hold the Gamma match in place while I soldered it.
My town was going to hire a ‘Mad Scientist’ but it couldn’t afford one, so they settled for a ham radio operator instead. Me!
73 de AA1IK
Ernest Gregoire
18 Responses to “Multi-Band 4 Foot Magnetic Loop with Gamma Match”
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Great job on building that antenna. Tom
the perfect portable antenna for hf tell us how it worked on the air especially 20m
all i can saw is AWESOME!
very nice job,
thank you for sharing!
73!
Hi iam interested in building this may loop as I have a postage stamp for my back yard can u tell me what size was the variable vacuum capacitor was … it looks great can not wait to build one .thank you for any info on the capacitor ..
Nice loop! Wider copper aids efficiency, but loops are anyway much more efficient than models compute; maybe 30dB more (see Sutherland, etc.)
A twisted gamma match is even easier!
Loop lovers may like to examine G3JKF’s three element array, also built by MW1CFN, both on QRZ.com
Beste Amateur Radio .com
ZOU U MIJ KUNNEN VERTELLEN.
Ik heefd eenloop voor 27.555 gemaakt ,maarik krijg er geen signaal op .
loop is 2.60 mtr dia mtr 0.82 1.2 mm koper , met gamma krijg wel 1-1 swr
maar hoord geen stations. wat is er fout ?
hoort wel 1400mhz beetje.
Ps welke condensator is nodig .
Met vriendelijke groet p.v Veldhoven
PV Veldhoven,
27.555 is not a ham radio frequency. 10 meters begins at 28.000 and goes up from there.
Looks great, I also use a similar look and it works great. My gamma match is not so tidy so I’m going to copy your idea on my loop.
Nice work and a good lot of information to follow up on! I used a Mag Loop on a field day last Saturday and was really rather surprised how well it actually worked considering it was sitting on the table next to my FT817! Thanks for putting the page(s) up. 73’s Lin. Exeter. UK
Good article. The only question I have is how long was your gamma match. I will be using aluminum pipe at about 1 1/4 inch at about 7 foot diameter. Should the gamma match be made from the same matterial. Thanks Robert
Great job doing antenna like these. i want also to build one.
Very nice! Am interested in building one for 80 meter cw (QRP). Any thoughts as to the dimensions needed?
Thank you for the very detailed close-up photo’s of the loop build. 73 de G0WXU
Great work!
I agree with you about a match instead of ring….I tried to tune it but I cutted a lot of copper wires and always had a too high swr….
So I write you because I’m looking for suggestions about magnetic loops with aluminium. I’ve seen some people using flat bar (1 mm thick X 25mm large).
Have you any experience about ?
73 de IU2MKE. Beppe.
I built a mag loop with 1/2″ copper tubing and 45 deg. Copper elbows. There is a foot between each elbow. The top is separated and has a 14 to 400 pf capacitor. I can’t get the gamma match or the feed loop to work. I have tried the shielded loop, the unshielded loop and the gamma match. I am out of ideas of what to do next. Can you tell me the dimensions of your gamma match? Length and spacing from the big loop. Thanks for any information you can assist me with. 73 de KAOGRE.
Love the article. I made all of my 40 and 20 meter magloops out of reclaimed Heliax coax cable. I too prefer the gamma match approach. Someone here mentioned about making the look with a hexangle shape. Not a fan of this construction for 2 reasons.
1. Cost of purchasing the 45 degree copper union angles to attach the 1 foot copper tubes for the hex shape.
2. Too many solder joints that increase the total resistance of the loop. We should be concerned about total resistance in the loop because each MiliOhm counts. The loop is under high abandon near the feed point.
It’s my opinion if you use 1 piece of soft rolled copper tubing of the same diameter, your reducing the total R in the loop by not having all those solder connections. You also save money not buying all those 45 degree unions.
Just my opinion.
Great photo-essay. This might be the best “mag” loop project I’ve seen yet. Now off the local amateur radio supply stores (Home Depot, Tractor Supply)
thanks & 73.