Monday, July 20, 2009

Alex goes professional

As an enthusiast of magnetic loop antennas, one of my favourite websites has long been that of Alex Grimberg, PY1AHD. I decided to visit it again today whilst doing a bit of research to find out whether, if making a magnetic loop out of coaxial cable, the inner should be connected to the outer shield or left disconnected. And I found that since I was last there Alex's site has had a professional makeover.

What was even more interesting was that Alex has decided to release a portable magnetic loop commercially. Called the AlexLoop SML 7-30, it is a small portable magnetic loop that covers 40m to 10m and is intended for QRP use. Supplied as a kit, you get a hand-held mast with a rubber holding grip, near which is mounted a box containing the tuning capacitor. The loop itself is formed from a length of flexible thin wall copper tubing which must be soldered to the variable capacitor and then secured to the mast with cable ties.

Apart from the fact that it is hand-held rather than table standing, doesn't have a built-in SWR indicator, uses a fixed size loop and is not apparently easily dismantled for packing and travel, the AlexLoop looks pretty much like what I am trying to create with my Wonder Loop. I would guess that the bits to make one could all easily be purchased for less than $30, which is about what I've spent on my antenna. So I was dumbfounded when I clicked the Buy Now button to find that the AlexLoop has a regular price of $219, though if you act now you can get it at an introductory price of $179, with postage to Europe adding a further $72!

Spurred on by the thought that my Wonder Loop could be the idea that will earn me that long-deserved fortune, I continued my researches and decided to do some more experiments. I never did find out whether it is better to connect the inner of the coax or not when using it to make the main element of a magnetic loop antenna. Some constructors do, others don't, and many more don't specify. I did find a couple of sites that said a thicker element improves efficiency, so I decided to re-make the 40cm diameter loop using an odd length of RG-213 coax. However, it didn't seem to perform any better on transmit than the one I made using 10A PVC covered wire. It seems that I'll have to make the loop at least twice the diameter to have a chance of radiating a decent signal. Hopefully I might manage to try that tomorrow.


Anonymous said...

Have you seen the G4TPH portable magnetic loops? They look interesting, and the price doesn't seem too bad either. I suppose the only problem it is rather too big for a table top! :-)
G4TPH Magloop

I like your idea as an alternative to the Miracle Whip ...

73 de Andre', M0JEK

g4ilo said...

Yes, the G4TPH loop was reviewed in RadCom a while ago. As far as I know it is not self supporting, it needs to be suspended from something. The 20m version is just over 1m in diameter. However my tests have shown that a much smaller loop just has too low efficiency. I am currently working with a bigger loop than my first attempt.