Magnetic Loop Antenna Updates

I finally got around to finishing a few details on the Magnetic Loop Antenna. I installed the 120pF silver mica capacitor with a SPST toggle switch to enable use on 40m. After measuring the actual diameter of my loop I noticed it is a little larger than what calculations would expect. This is due to the added 4″ width of the capacitor enclosure as well as some variation in the length of the loop coax. As a result of this the original coax coupling loop was a bit small making it closer to 1/3 vs 1/5 loop diameters. I made a new one with the larger size and decided to build a Faraday or shielded loop vs the old one which was an unshielded loop.

The loop makes a big difference in SWR across all bands serviced by the larger diameter loop (i.e. 40m-15m). SWR is now <1.2 on all bands. The matching is much improved but I have not rationalized the effect it has on bandwidth which if I use 2:1 SWR points is much greater than what the design spreadsheet would indicate. This would mean losses are greater…except that with lower SWR, more of the SWR curve is below 2:1 making the spread larger i.e. greater bandwidth. Need to investigate this more but one reference recommends measurements using a wattmeter and a field strength meter rather than measuring VSWR.

I did build a constant current source circuit using an LM317 adjustable voltage regulator to measure the milliOhm resistance loss in the loop and the connections to the capacitor. It worked out to a total loss of 25 milliOhms. This is including the loop itself.

I plan to publish my various measurements here in the near future…meanwhile how did it all work? I tuned up 20m this afternoon and heard XE1B in Mexico City calling CQ. I answered the call on 5W with the loop indoors about four feet off the floor in a vertical position. He came right back with a 58 report!

Something is working right….


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