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Magnetic Loop Test Number 2
September 7th, 2009 by kmack

The antenna restricted condo dweller (me) continues to look for a viable alternative to get on the air without having to go outside (think rain) and setup and tear down a portable antenna. I built a new frame for the magnetic loop and tried a few different styles of feed and resonating.

Ended the day with a 1.12 swr on 30m with this setup. I tried (fingers crossed) a dual section polyvaricon cap that I had on hand. It is mounted on a lexan square just below the gap on the mag loop at the top. After some time spent with an exciter loop, I decided to try the gamma match approach and regrouped. Using two stainless steel hose clamps to clamp the 12 guage solid copper wire I used for the gamma match. (actually the same wire I used for the exciter and 1/5 of theĀ  10 ft. length of the tubing used) I cleaned up the copper tubing with a bit of fine grit sandpaper and tightened the braid to the main loop. Then I connected and soldered the center connector to the 24 in 12 guage wire for the gamma match. I drug the end of the gamma match and watched my antenna analyzer for a dip. Once I found the sweet spot, I cleaned up the copper tubing with sandpaper and clamped the wire while adjusting for best match. I then adjusted the distance of the gamma match wire from the loop and fine tuned for the lowest swr on the analyzer. After a good match, I put my Wilderness SST on the antenna and though the band was not loaded and had lots of QSB, I tried to answer a CQ from WB3YIP and getting no answer called CQ myself just to see if anyone would pounce on me! No contact yet, but at least the capacitor did not blow up yet! Sort of fun seeing what it will handle. More later as I get further along in the project. Thanks to CW-Code Warriors Yahoo Group for the help and encouragement they have offered.

Here is the 2nd iteration of the mag loop in my living room

Here is the 2nd iteration of the mag loop in my living room

The gamma match in test state

The gamma match in test state

The polyvaricon capacitor-- will it last or burn up from high voltage?

The polyvaricon capacitor-- will it last or burn up from high voltage?


4 Responses  
  • Scott writes:
    April 3rd, 201011:42 amat

    kmack, how did this loop perform for you? Did you do any transmitting, and if so, how did that go? I live in an apartment and have been trying various antennas, all homemade, and am considering trying to build something similar to your loop as my next project. So far my biggest struggle has been with being heard on the air. I don’t know if a magnetic loop would make a difference, but I figure that at the very least I’d have some fun putting one together and tinkering with it. I’m really interested in reading your thoughts on how yours performed.

  • kmack writes:
    April 3rd, 201012:35 pmat

    I have done some transmitting but I do not think I have the right tuning yet. Partly it is the need for a better capacitor (less lossy) and finding the right feed method. As weather improved I have not played with it as much and have concentrated on portable ops more.

    Bottom line, don’t have a good answer. I think it is a workable antenna if you set it up properly. It probably won’t compare to any decent outdoor antenna, but it is great for listening and getting cw practice as well as some qso’s. Since I run qrp, it makes a tough way to make qso’s even more difficult. Hope that helps… good luck and I’d say go for it. There’s a good bit of info on the internet if you google Magnetic loops and dig down a bit.

    73,
    Kelly K4UPG

  • Scott writes:
    April 5th, 20109:23 amat

    Thanks for the response. :) Yeah, I probably will try constructing a mag loop. The only problem I can see, apart from the tuning and solving potential performance issues, is what to use for the capacitor. I would love to be able to use something from Radio Shack since it’s the only place around here that carries components. What drew my attention to your antenna was that you aren’t using an air capacitor, so that was why I commented. Maybe I just need to find some air caps and then I can have some fun experimenting with those. It’s probably a worthwhile investment. Most of what I’ve read seems to indicate that mag loops work well as compromise antennas, which is right up my alley thanks to my apartment situation. Anyway, thanks again! I may stop by and comment again if I have any success.

  • kmack writes:
    April 5th, 20109:55 amat

    Actually, I am going to try building a

  • trombone
  • style capacitor. Google that a bit and check some ARRL handbooks for some references. They offer a bit simpler control and seem easier to build than trying to make an air variable out of pieces of sheet metal! There are several guys that show details out there. When I am not tied up at work and meetings I’ll try to send you some URL’s but you should find them with a google or two. Good luck and don’t quit. There’s a way to get ‘er done. I’m mostly using end fed wire off my porch and held up by a ladder and Jackite pole. I also use my Buddistick bungeed to a ladder in the backyard. It’s not as neat as a mag loop, but until I perfect the loop it works. My mag loop will tune to a decent swr and hears fair, but it is EXTREMELY sharp tuning and the current cap is way too touchy with hand/body capacitance, etc to make it a workable setup. Still fun to try though! Take a look at the Rock Loop too… a bit simpler.
    73,
    Kelly K4UPG


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