OBSERVATIONS FROM SCOTLAND                            28 May 2008                                GM1SXX

MORON Mag-Loops.

There's one really good thing about mag-loops I had not really anticipated... their rejection of any signals outside their very sharp passband. This is a huge advantage in noisy locations, like my shack!
The computers kick up a lot of QRM (and I have 4 PC's plus two LCD panels) so they wreak havoc on reception with wire aerials, but not with the mag-loop. It rejects everything outside its very sharp passband so my receivers don't see all these interfering signals.

Another advantage is that being small, a mag-loop can easily be rotated. This can be used to good effect to null out unwanted signals. Only a cheap light-duty rotator is needed if the loop is placed in your attic or pole-mounted.

The mag-loop is readily detuned by nearby objects. Just walking close to it shifts the resonance a fair bit. This is to be expected given it's high Q. Why do mag-loops use remote control of the tuning capacitor? Now you know. If you build a mag-loop, use a motor/gearbox with the biggest reduction ratio you can find. I used a MAXON motor/gearbox. It was expensive but it is very controllable. The gearing is 148:1 and even that is too coarse on 12V, so for fine tuning, I feed it on 3V to slow it down. By varying the supply voltage, it's easy to vary the tuning rate. The motor is a DC permanent-magnet type with brushes. I connected a 100N ceramic across the motor contacts and wound the leads through a ferrite sleeve. This kills much of the QRM generated when tuning. Remember too that the motor is at RF ground potential (or should be) while the capacitor (both sides) is hot-hot-hot. Use a good quality insulating coupler... good for 3KV at 5W and 10KV at 100W.

One area I need to look at more is the matching transformer. Not many people use this method of coupling into the loop but it's small and convenient unlike a secondary feed loop.

A T200 of suitable core type for the band(s) in use is ideal but the turns ratio is open to debate.
Where people have used transformers, the photos I see have only a couple of turns. According to to Reg G4FGQ's, software, this is too few and I tend to agree.
As the frequency goes down, you need more turns and the converse is also true. I think the correct ratios are somewhere in the middle. Experiment!

For a permanent mag-loop, one should use the biggest pipe diameter feasible. This is because losses decrease in proportion to the active surface area. Bigger is better. There's quite a difference in efficiency between using 15 & 22mm pipe.

I saw photos of one guy's setup in Japan where he put a mag-loop on the roof of his car. It looked like a big copper lifebelt... about 4 inches diameter wrapped in bonded copper tape and using a vacuum variable. This would have very good efficiency on 80 & 40 compared to one made from copper pipe.
I did model such a 'fat' loop and the efficiency was superb. Of course you'd need to partner it with a 10KV high quality vacuum capacitor to tune it.

I've also discovered that braided connections are NOT best for the lowest losses. This is because although braid has a low ohmic resistance, each single strand has a small cross-sectional 'skin' area to carry the RF, so it's not so good at RF frequencies. A heavy solid copper strap or tube is best because it maximises the RF carrying surface area. A silver plated one is better still, because all the RF current travels in the surface 'skin'.
Because of the large circulating currents, the connections need to be as good as possible. They carry many amps, even at QRP levels, and they need to stay that way (good weather protection).

Butterfly type or vacuum tuning caps are best (no wipers). Wiping contacts are a 'no-no' on the mag-loop. They are just too lossy.

If you are only building a small loop for experiments, by all means use whatever sort of tuning capacitors you have lying around, but for a serious installation, a home-made wide-spaced butterfly type or a vacuum variable is the best choice. For anyone with access to light engineering facilities, there are designs around for DIY butterfly capacitors that can be made at home. Why would you want to make a tuning capacitor? Well, do a web search for vacuum variable capacitor and take a look at the prices. Then you might want to try making a DIY Butterfly.

Now, back to making a portable loop for use at work.

73 Al.

GM1SXX

Back to OBSERVATIONS