Tuesday, September 10, 2019

Magnetic loops for 144 MHz

Magnetic loops are normally thought of as the type of HF antenna you'd use if you've got not space for something that's full size. However they can also be used on 2 metres. As I write this I'm testing one indoors. So far stations have detected me at 50km distance with many dB to spare. I'll do a video on it eventually. 

My loop is pretty close to a copy of this one by VK5BR. My material was stiff copper wire about 3mm diameter instead of the original 1/4 inch copper tubing. But my length is about the same. As is said in the article if you go any longer you'll have difficulties resonating with the variable capacitor. If you want to see the original Amateur Radio magazine article you'll find it here

Here's a loop by IW5EDI. There's few words but the pictures should give an idea. I really like its robust construction. 

Finally, this is a dainty little loop by M0UKD. It should only take a few minutes to make - it's just soldering some pieces of stiff wire together. It could make a good attic antenna. But I'd use a better trimmer for more than QRP power levels. Scroll down for other loop ideas. 

A well-built loop should give close to a dipole's performance. However its bandwidth will be narrow. It won't be so good for repeater operation due to the 600 kHz offset (you'd need to optimise the loop on transmit and your receive sensitivity will be down). But for SSB or digital modes it will work well. Note though that it will be horizontally polarised so you'd need to tilt it 90 degrees for the vertical you'd need on FM. 

PS: Want more small antenna ideas? Many are in Hand-carried QRP antennas. Thousands sold. Favourably reviewed. Available in electronic or paperback. Find out more here

1 comment:

  1. Having read your article along with the others referenced in your links, it seems everyone is overlooking significant beneficial features of this antenna: "intermod" (mixing of strong nearby transmitters) or "desense" (signal overload of the receiver front end) could be substantially reduced both by the high Q (narrow bandwidth) of the antenna and it's ability to null or reject signals off the side of the loop. For Radio Amateurs in a very RF-dense environment this would be worth a try before going the more complex route of notch, band pass and band reject filters by incorporating the filtering into the antenna itself.


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