{"id":49,"date":"2024-01-15T00:22:03","date_gmt":"2024-01-15T00:22:03","guid":{"rendered":"https:\/\/ai7sg.com\/?p=49"},"modified":"2024-01-28T16:15:31","modified_gmt":"2024-01-28T16:15:31","slug":"my-second-antenna-project-the-mag-loop","status":"publish","type":"post","link":"https:\/\/ai7sg.com\/index.php\/2024\/01\/15\/my-second-antenna-project-the-mag-loop\/","title":{"rendered":"My Second Antenna Project: The Mag Loop"},"content":{"rendered":"\n

For most of my career, the goal has been isolating from magnetic fields coupling where they shouldn’t, so building something to intentionally radiate was a refreshing change of pace.<\/p>\n\n\n\n

Design<\/h3>\n\n\n\n

I already had some 4 inch aluminum tape and 1 inch copper tape. My first thought was to cover some PVC pipes, but I didn’t have them and didn’t feel like going into town. I also wasn’t sure what I’d do for a tuning capacitor.<\/p>\n\n\n\n

I’ve always thought corrugated plastic (your typical yard sign) had some potential so I decided to see how it worked structurally for a magnetic transmitting loop. As a bonus, 4 millimeters of corrugated polyethylene would make a great high voltage capacitor!<\/p>\n\n\n\n

I ordered my yard signs and did some research and calculations. I wanted as much bandwidth as possible on 20 meters. I had been disappointed in the performance of my attic full-wave loop on 20m, and hoped that the mag loop on my balcony would perform better (spoiler: it did). <\/p>\n\n\n\n

I didn’t really care about sharp nulls, so I settled on a single loop with circumference \/ perimeter of about 1\/3 \u03bb, probably in the shape of a square. Wikipedia had the equation for how much radiation resistance I could expect, which worked out to about 1.6 ohms.<\/p>\n\n\n\n

https:\/\/en.wikipedia.org\/wiki\/Loop_antenna<\/a><\/p>\n\n\n\n

<\/p>\n\n\n\n

I saved myself the effort of estimating how much inductance I’d have and just used this calculator:<\/p>\n\n\n\n

https:\/\/www.allaboutcircuits.com\/tools\/rectangle-loop-inductance-calculator\/<\/a><\/p>\n\n\n\n

Then using F=1\/(2\u03c0<\/em> sqrt(LC)) I had a good starting point for my tuning capacitance. I would build a square, a little over 5 feet on a side, in two sections, with 4 inch aluminum tape on the outside. The sections would overlap at the top by a couple of inches for my tuning capacitance, and at the bottom I would use a shunt capacitor to match to 50 ohms (plus some reactance, but who cares? We already need a tuning cap at the top!).<\/p>\n\n\n\n

I used this tool: <\/p>\n\n\n\n

https:\/\/www.analog.com\/en\/design-center\/interactive-design-tools\/rf-impedance-matching-calculator.html<\/a><\/p>\n\n\n\n

to determine that I’d need a little over 1200 pF for my shunt capacitor. I threw it all in LT Spice to see if it looked reasonable. It looked good enough to proceed to building it!<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

From the model, it was clear that the tuning capacitor C1 would see some pretty high voltages, but the matching capacitor, C2, would not. <\/p>\n\n\n\n

I cut some strips of yard sign material with my Dremel Ultra saw (I’m not really a power tool guy, but that was perfect for the job), duct taped together the two sides of the square, and added the 4 inch aluminum tape to the outside, leaving 1\/4″ margin at the ends so the tuning capacitor couldn’t arc through the air alone (it’d have to go through 4 mm corrugated plastic). It looked like my tuning capacitor would need about 3-4″ of overlap from the calculations, but in practice it turned out to be a fair bit less than that.<\/p>\n\n\n\n

For the 1200 pF cap, I used some 8 mil x 4 inch mat tape I had left over from my Jiu Jitsu days, with 4″ aluminum tape on both sides (avoiding the edges), taped to corrugated plastic for structure. Update: I guess mat tape is PVC, which will HEAT UP when exposed to RF<\/em>, so probably not a good choice… Something store-bought might have worked just as well or better, but I didn’t have anything on hand. The online capacitor calculator predicted about 60 pF per square inch, which turned out to be pretty close. I added 1 inch strips of copper tape (easier to solder to, and the thin oxide layer on the aluminum tape formed a capacitor with the copper too large to affect anything), and put it all together.<\/p>\n\n\n\n

I tried tuning it in my garage, but something was way off. So I took it up to my balcony and tried there. Much better! Everything but the tuning capacitor almost exactly matched expectations. Surprisingly, my IC-7300 antenna tuner was able to match it across about 200 kHz of bandwidth!<\/p>\n\n\n\n

With the mag loop, my noise floor was more than a full S unit lower (roughly 6 dB) than the attic antenna, and the signals were coming in strong. So I had to try transmitting. After a couple of easy contacts, one on each coast of the U.S., I decided the project was a success.<\/p>\n\n\n\n

And all for about $50 in parts. My wife saw it and said, “Oh, you built a portal!” And in a number of ways, it really is!

Note: If I could do it again, I would probably use the PVC pipes. The corrugated plastic is not that great in the wind…<\/p>\n\n\n\n

Next Steps<\/h3>\n\n\n\n

I actually built two of the rectangles. I’m thinking about putting the two together as a 2-turn loop, with a larger capacitor of course, and tuning for 80 meters. I still have zero contacts logged on 80 meters.<\/p>\n\n\n\n

<\/p>\n","protected":false},"excerpt":{"rendered":"

For most of my career, the goal has been isolating from magnetic fields coupling where they shouldn’t, so building something to intentionally radiate was a refreshing change of pace. Design I already had some 4 inch aluminum tape and 1 inch copper tape. My first thought was to cover some PVC pipes, but I didn’t […]<\/p>\n","protected":false},"author":1,"featured_media":50,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-49","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-blog"],"_links":{"self":[{"href":"https:\/\/ai7sg.com\/index.php\/wp-json\/wp\/v2\/posts\/49","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ai7sg.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ai7sg.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ai7sg.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ai7sg.com\/index.php\/wp-json\/wp\/v2\/comments?post=49"}],"version-history":[{"count":8,"href":"https:\/\/ai7sg.com\/index.php\/wp-json\/wp\/v2\/posts\/49\/revisions"}],"predecessor-version":[{"id":109,"href":"https:\/\/ai7sg.com\/index.php\/wp-json\/wp\/v2\/posts\/49\/revisions\/109"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/ai7sg.com\/index.php\/wp-json\/wp\/v2\/media\/50"}],"wp:attachment":[{"href":"https:\/\/ai7sg.com\/index.php\/wp-json\/wp\/v2\/media?parent=49"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ai7sg.com\/index.php\/wp-json\/wp\/v2\/categories?post=49"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ai7sg.com\/index.php\/wp-json\/wp\/v2\/tags?post=49"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}