My MagLoop so far

Posted on Sunday, April 7th, 2013 at 11:07 am in

Magnetic Loop

Magnetic Loop

Well, if you have been following my other posts you will know that I live next to a monster of a power line servitude, less than 40 meters away from my shack. And 40 meters high. On my favourite HF band the noise levels almost hurt your ears. Something had to be done.

I tried a few things such as ferrite beads and a folded dipole but no real improvement. Scanning the internet the magnetic loop (small transmitting loop) antenna often came back as a lower-noise antenna, and after visiting Eddie (previous article) I decided to try my hand at building an experimental prototype. There was a lot of information available on the internet and I will add my plans and drawings to the bottom of the article as downloads. The achilles heel of these antennas is the high voltage capacitor, and after searching through two flea markets unsuccessfully I decided to make my own.

Essentially I wanted a copper water pipe loop that can be resonated around 7.060 MHz, as that would allow me to compare the results with my other radios and antennas. So I purchased a length of 28mm copper pipe and 8x 45 degree elbows. I also purchased a length of 50mm white PVC with which to hold the whole construction in an upright position. Initially I cut 7 pieces of 620mm each and soldered them together in a loose octagon with what seems to be a diameter in the 1550mm region. The 8th leg was to be decided later, and two loose pieces of around 200mm and 400mm respectively was soldered in temporily.

I need to say, right up front, that I am not a purist. While I will do strange and wonderful things to reach the optimum, my favourite motto is “MOL” (More Or Less). Get the basics right and the rest will take care of itself. So the few compromises I was willing to make are:

  • Not a round shape, but an octagon (8 sides)
  • A trombone capacitor
  • Feeding scheme to be decided later

Initially just to see if the loop will resonate at all I tried feeding it with a piece of circular coax in a faraday loop configuration but that didn’t really give strong results, but I think it was my own fault because I was standing too close to the loop. As soon as I soldered in a tab for a gamma match, things changed drastically.

First I tested with an old valve receiver spaced-plate capacitor (no power transmitting yet) and my antenna tester to see MOL (more or less) what frequency I’m getting. Wow, it was all over the show and I couldn’t get any useful readings. A cup of coffee and lots of staring into the distance later and I realised that the adjustments I’m making on the capacitor were WAY too much. In fact, lesson 1: To tune across the ENTIRE 40m band (100KHz) you only need from about 118pF to 121pF !! (yes, around 3Pf). Lesson learned: Make very small changes. Reference: LOOP CALCULATOR

After that it was plain sailing to the first QSO, I didn’t even bother to make proper solder connections for the gamma match and literally clamped a piece of 1.5mm electrical earth cable to where it resonated around a SWR of 1.5 and I fired up my set. Bingo, very nice contact with ZS6TY (Johan) and the concept is proven. At this stage, remember, I was using an in-line monoband trombone capacitor, and 1mm linear adjustment would give me a 25kHz shift in band so it was very sensitive, but it worked. In comparison to my G5RV when the noise was around S9 on the dipole, the loop would be S6 and the audio quality more or less the same. Definitely an improvement.

To see how I made the capacitor download the diagram at the bottom of the article.

Now for version 2 I wanted to make a remote-control capacitor. I don’t want to spend money on making it automatic yet, but I wanted to at least be able to change the capacitance from inside the shack. So out comes the soldering kit, out comes the two short pipes and they are replaced by two 300mm pieces. The old trombone capacitor is now turned through 90 degrees and the inside element is soldered to the one 300mm piece with a T-piece (look at the diagrams below). The outer element gets a 5mm brass nut soldered onto an end-cap with a 500mm long 5mm threaded link. A plastic coke bottle top becomes my adjuster on top and voila! the new antenna goes up.

In measuring I find the loop has a natural resonance at 24MHz (no additional capacitance inserted). As soon as the capacitor gets connected it can be tuned to 7.060MHz very easily, and the gamma match can be set equally easily. Unfortunately I offset the feed point a little to make room for the supporting pole, and I will have to go back and clean up the construction.

The next few weeks will be spent doing fine tuning and documenting the antenna. At the moment I’m pointing slightly west of north to get past the power pylon and hopefully catch some north-west ZS amateurs, but I was happily chatting to people north-east of me as well so I’m not too sure how deep the side lobe nulls are, yet. As it stands, it is a cloud warmer because I have no chance to get out horizontally or even at 60 degrees elevation.

Magnetic Loop in back yard

Magnetic Loop in back yard

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