HF Magnetic Loop - In An Afternoon.

It's been a very long winter and, with the sun finally out and the weather warm, all that pent-up antenna building energy had to come out!

So, on the bank holiday, I decided I would have a go at a magloop for HF.

Magloops seem to have a bit of an air of mystery about them.  This seems to be down to two things - the need for a tuning capacitor of high voltage handling, and the very sharp tuning range.  Neither, in practice, are anything to worry about.

I was sorely tempted to scavenge a capacitor from my ATU, which these days tends to lie rather redundant owing to my improved antenna-building skills.  But I opted first for the drinks-can capacitor system, as it's cheap and is interesting to make.  You can see a fine image of the capacitor courtesy of Alex, PY1AHD below:

Looks unlikely, the cans are a bit weak and unstable, but it does work.

The loop was made to a square profile using 15mm copper tube; shape doesn't seem to make a huge difference, but a square is, some say, somewhat less efficient than a circular profile.  In the International Antenna Collection book, a professor says shape makes very little difference.  A square is infinitely easier to construct, so you may want to weigh up the costs and benefits when wondering whether to roll some copper!  I'm mostly interested in the 20m band, so I built the loop to be about 5m all the way round.  Remember to clean those joints and use flux for a good fix!

Talk of capacitors and high voltages make magloops sound daunting.  They're really not.

 
The drinks cans, being very thin aluminium, have an annoying tendency to distort once the ends are cut off, and they then tend to make contact with one another when they should slide without touching.  I suppose you could use steel, though there may be some reduced performance; I don't know.  That said, never knock something that allows you to make a start, however flaky!  After a few goes, I could manage to get an SWR of 1.3:1 by ear - and no ATU!

The headache with mag loops is the very sharp tuning - slide that capacitor just a wee bit and you are past the sweet spot by a mile.  With the drinks can, although you can rig up the syringe and airline tubing system (which I did), it quickly becomes tiresome to tune the loop.  You could fix the capacitor in place if, for example, you are only interested in WSPR or one of the ROS frequencies.  But that's a pretty inflexible antenna.

The best option is to forget the drinks can capacitor, even though it does work and costs next-to-nothing, and go for a proper air-spaced capacitor or, heaven forbid, buy a decent used vacuum capacitor.  Russian ones, which have a good reputation, are available on e-bay all the time, but you have to be careful who you buy them from.  MFJ also make a capacitor you can buy.  All these things will tend to set you back about £100, possibly a bit more.  You will then need to get a slow-motion DC motor to remotely tune.  In all, even buying a capacitor and motor, you will probably save about a half on buying an MFJ magloop (about £430 RRP), and a lot more on the much better-built Baby Loop by Wimo (about Euro 1100 list at time of writing.)  

The Wimo Baby Loop.  Euro 1100, but look at the quality - and the capacitor!!

Even with tuning of the cans being a bit of a hit-and-miss affair due to their flexure, the magloop showed itself to be a very good antenna.  It compares very favourably indeed with my full-wave delta loop, giving the same or only slightly lower signal strengths on SSB.  SSTV signals were also stonking in in glorious 595 from across Europe.  I haven't had time to mount in the open and try WSPR tests yet, but that will have to wait for a proper, more stable capacitor.  My first morning of operation brought in UN7LDZ (Kazakhstan) on about 20W actual output on RTTY.  That's not bad going for a loop indoors, a foot off the kitchen floor!

As a first test, my magloop confirms the oft-quoted view that it's the best small HF antenna you can get, giving full, 30-foot high dipole-like performance when mounted only a few feet off the ground.  With a proper, remotely-tunable capacitor, I expect this will be a very interesting antenna to experiment with, especially when it is, as so often, blowing a hurricane up here.  Heck, I can even use it indoors, or have it sheltered in the kids' tree house!  

UPDATE:

I've now ditched the drinks can capacitor.  It worked fine, but it just isn't controllable enough for external use.  I've now rigged-up a trombone or piston type capacitor using 1/2" copper tube, insulated with PVC tape, running inside 3/4" copper tube.  This is much more stable, though I had to wind three thicker parts of PVC tape to the inner plates to make them run centrally.  To push the plates in and out, I used threaded rod on a couple of old bent mouse traps that act as brackets, and Araldite-glued two nuts onto these to act as smooth runners.  Not only is this fine tuning by hand, it allows a pretty bog-standard motor to be used to tune as well; the threaded rod acts like a reducing gear of sorts. It's still not easy to tune without a remotely-operated motor, but it is certainly better than cans!

The new, improved 'trombone' or 'piston' capacitor after soldering.  Keep the pipes parallel when making these!

 

Tuning with this system is restricted to the 20m, 17m and 15m bands, with 15m being a bit tricky as the capacitor is running out of legs by then!  A butterfly capacitor would be a huge improvement, but I haven't got one at the moment!

One curious thing I noticed just by indoors operation was that the antenna seemed to work better end-on, rather than broadside.  When I checked the pattern outside, with the base of the antenna at about 2m, the vertical radiation off the ends was seen to be considerably stronger than the horizontal radiation off the face of the loop.  Not sure if that would still be the case if the antenna was placed at a reasonable height.  I guess it would even out then.

A beautiful loop.  The secondary loop is made of heavy-gauge domestic earth wire.  I found the SWR drops significantly (by half an unit or more) if the loop is kept higher, rather than lower on the support.  More playing is needed to see if the shape and position can lower it further.

I've managed to have two hours on WSPR now, and during last evening, managed to get across to Wake Island - a military bit of US sand in the middle of the Pacific, Japan, Taiwan,  all of Europe, Morocco, Venezuela and the far west coast of the US.  Not bad at all for a small loop running 5W!  Luckily, G3JKF was running his twin loop at the same time, an antenna at a much more advanced stage of development, and signal reports into Taiwan, for example, were identical.

Just under 2 hours' worth of WSPRing with the magloop outside, base at 2m.  Pretty impressive!

Certainly an antenna worth having.  I would even buy one, to be honest.  A heavy-duty Baby Loop from Wimo is very expensive, but stick that on top of a decent tower, and you get really quite respectable low angle radiation and gain figures.  But if you are any good at electronics and motors, you really could have a very good homebrew, remotely tunable antenna for peanut money.

In fact, coming to think of it, the magloop is by far the most interesting and fun antenna I have worked on so far.  I would go so far as to say every ham should have one; certainly those starting out.  This is not least because a magloop is a very capable, relatively affordable, also relatively easy to build antenna that has the huge advantage of almost certainly not needing planning permission in the vast majority of cases (private restrictions are another matter, but even then, magloops work really well inside non-steel buildings.)  From my perspective, it's a hurricane-resistant, small, portable antenna.  Quite remarkable, really.