DIY Mag Loop Continued - Post 3

After my FDIM presentation on portable magnetic loop antennas (reprinted in QRP Quarterly, October 2017) I decided to make one final (maybe) version based on what I had learned through my experiments. Below is a list of the “improvements” I made.

· Earlier I used LMR 400 coax for the large loop. This time I substituted superflex hardline. Hardline makes for a more rigid loop, yet allows it to collapse into a manageable coil for carrying. Also, the smooth surface of the outer layer is said to be a better radiator than the rougher braid.

· Instead of PVC pipe for the support, I used wooden screw-together paint poles with plastic joints (it took two packages at about $5 each). This is more rigid than the PVC was, and screws together easily.

· For the yoke that supports the top of the coil, I modified the plastic handle of a paint roller, removing the metal shaft.

· I decided to use thin copper tubing instead of coax for the smaller coil, again for its rigidity in an outside environment.

· I had done much of my prior testing during the winter in a second floor bedroom. When I moved the loop outside, I found that I needed more options for adjusting the position of the coil. I made a carrier for the small loop out of a fiberglass rod from the junk box, using two small cable straps at right angles to connect the copper loop to the fiberglass rod at the top. At the bottom, the rod is mounted to a wooden block with cable straps. The block has a hole through it for the paint pole support, with a nylon set screw. I can thus move the coil up and down and also rotate it by moving this block.

· Although my earlier tuning unit would have worked, I made a new one with a 2-gang capacitor (365 or 395mm per section), connecting the two sections in series. This avoids reliance on a lossy wiper contact for connection. Also, I managed to find one on Etsy that had the rotor plates welded to the shaft rather than pressure fitted, reducing potential loss. It had some built-in gearing but as in my original version, I added a planetary gear to make for finer tuning.

· The first step in tuning the loop to frequency is by ear. In the October article, I suggested various ways to fine tune from there. A small SWR meter is one easy way. To make this more compact and self-contained, I decided to add the SWR Indicator/Bridge Kit from Pacific Antenna (http://www.qrpkits.com/swrind_case.html). For more nuanced tuning, I replaced the LED indicator with a small meter I had in the junk box.

All of this helped to make the loop more rigid and less finicky in actual use. Perhaps it improved efficiency, too. At any rate, I’ve made contacts with it, and that’s what matters.

[This is the third of three articles published in QRP Quarterly. The original was published in the April 2016 issue.]