Magnetic California

It’s not going to win any beauty contests, but with my slapped-together Small Magnetic Loop Antenna (small MLA), I just worked two stations in the San Francisco Bay area with 20 Watts on FT8! (11pm CDT, 20-meters, antenna in living room.) By comparison, it takes me over 40 Watts to do the same on my 20m dipoles hanging outside. Huh.

Also, I heard a station in Hawaii and another further out in the Pacific, though they couldn’t hear me to log a contact. That’s a first for me.

Small Magnetic Loop Antenna, totally slapped together. Tripod, PVC pipe, non-conductive broom handle, pipe clamps, Velcro straps, and zip-ties. Primary (gamma match) loop: 1ft diameter, squished for SWR adjustment; RG57/U. Secondary (resonant) loop: 4ft diameter, RG8.
20~145pF air variable capacitor at top. Knob tunes the secondary loop for the target frequency (sweep knob until static on radio sharply increases in volume). Braid and center conductor of RG6 cable soldered together and clamped.
Primary loop (gamma match); RG-59/U, 1ft diameter. Braid and center conductor soldered together, then soldered across SO-239.
Close-up of bottom construction of gamma match.

The antenna matches with low-enough SWR on 20m (still have to use a tuner), and the capacitor isn’t high enough in value to get a good match on the low end of 40m (my band of interest), but it works better than expected. It’s a difficult, complex thing to fidget with, and is very fickle and easy to disturb because everything’s floppy. But…it works.

A nice feature of small MLAs (“small” meaning the outer loop is less than 1/10 wavelength of your intended frequency) is that they are selective and have strong nulls perpendicular to the loop plane. This allows you to rotate it to find and cancel nearby sources of radio noise. I’ve identified a few noises around the apartment complex, and want to take the rig outside (after some rework) to see if I can do better location of the noises.

I’m glad I put so little work into it; the cost of failure is lower this way. I was frustrated with it Sunday night when I built it, but tonight, even after sunset (with California in gray-zone), I managed to log a few contacts.

Research and more reworks are in the future. Potential improvements:

  • better construction with SO-239/PL259 connectors on weatherized capacitor box
  • use RG8/U (50ohm) for primary instead of RG6 (75ohm)
  • make primary closer to 1/5 size of secondary as recommended
  • use higher value capacitor (200pF?) with free 360° motion
  • design a remote tuning gearbox.

So let me say this: MLAs are interesting.

Published by Shawn

He's just this guy, you know?