Short Takes #28: Mastering the Waves -- A Primer on the Mag Loop Antenna

I enjoyed Bill's article this month (W9WRP) about Magnetic Loop antennas. Have you been considering either building or buying an antenna of this design? There are a variety of them on the market . . . tell us here which choice you made, and how you like it. If you can, compare it to other antennas at your shack to help out those who are new to the Mag Loop. Thanks for reading! Dave Jensen, W7DGJ

 

I have a homebrew 8ft diameter Small Transmitting Loop (STL / Mag Loop), made of 3/4" copper pipe. With my vacuum variable capacitor it handles 100w, no problem. It works quite well on 40m and 80m ! My first contact was with Tasmania on 40m. It is definitely some dB's down from a full-size antenna, based on A/B tests, but it's quite capable for the size! It's pictured on my QRZ page.

Some resources for those who want to build their own.
From @AK5B:
https://forums.qrz.com/index.php?threads/if-you-really-want-to-build-an-optimized-stl.801779/

I started a website about it, but haven't touched it in a while:
stl.moxonantennaproject.net

73 and good luck!
James
W7JMP

 

Thanks James! Great addition to the column, Dave

 

Magloops are enduringly popular. There's quite a few solid and informative articles about them in ARRL QEX magazine over the years. They are a compromise, but it's a pretty good compromise.

-Michelle

 

Hi Michelle - Agree with you on the compromise aspects of that antenna design. You are compromising a high-Q and resonance limited to a very tiny section of a frequency, requiring retuning frequently. There are so many "compromise" antenna solutions . . . it's almost as if you have to choose between those compromises when buying/building. Dave, W7DGJ

 

Mag loops are not very popular in my area, and I wonder why that is so. HOA restrictions abound and one could pretty easily put up a loop and not be chastised for a ghastly antenna appearance. Let's face it. Get on the air with just a little bandwidth is better than not getting on the air. W9WRP

 

I agree Michelle. I believe those with Technician licenses should give loops a try. It's a good construction project too, as the commercial versions can be pricey.

 

I don’t understand the high price on some of these, most notably the Italian ones . A mystery to me …. Dave W7DGJ

 

I think the main downside to the mag loops is the bandwidth. But that is not the kiss of death. One can work around the limitations. All antennas are compromises in some form, but Hams are ingenious. I liked my MFJ mag loop which was my first antenna when I returned to the US from overseas. It served me well until I could put a more capable antenna in the air. Don't know how the MFJ current situation will affect the mag loop market.

 

Has anyone ever made one (or seen one for sale from an antenna mfgr) that handles full legal limit? Dave, W7DGJ

 

Precise RF makes a QRO Mag Loop that can go up to full legal limit PEB at a 75% duty cycle for SSB (800 W at 100%), but it costs over $3,000.

HG3 QRO Stepper Mag Loop Antenna – PreciseRF

 

The Ciro Mazzoni product line does look pretty robust. The Baby Loop (covers 10 Meter to 40 Meter bands) is one meter in diameter and uses 50mm aluminum tubing for the main loop. The large blade capacitor can handle 400W (don't feed more than 200W into the tuning unit). Keep in mind that the price (about €1,600 at WiMo in Germany) also includes the tuning unit (sells separately for €750 at WiMo).

 

Bill did a great job

 

I worked an op who was on a cruise ship off Hispaniola, running five watts into an Alexloop (no, this isn’t a pirate story), and I decided to ask for the antenna for Christmas. I think about selling it, because I have a few antennas and it doesn’t get used that much. But then I set it up and try it and I decide to keep it. I took it for a POTA activation in a local park, with a Mountaintopper MTR-4B running five watts, after it had rained, and set it up in a sheltered pavilion. 23 contacts in 40 minutes and I decided to keep it.

Yes, the bandwidth is narrow, especially on 40, about 8 kHz with SWR<2.0, expanding to 25kHz on 20. I’ve been using a NanoVNA to find the sweet spot and sticking within the range. But the loop works and I keep it. I took it out on my deck the other night, with five watts, and worked a POTA station in WI from here in PA and I1MMR in Genoa, Italy (339 report). Maybe not a good choice for your only antenna, but the mag loop works and is great for limited locations, such as an HOA.

 

Thanks RIch. I'm a QRP operator frequently, and I'll consider that nice option. Appreciate you being here for a comment, Dave