Magnetic Loop Antennas - Why You Should Have One.

Magnetic loops are quite well known in the ham world.  It's just that they aren't all that well-used.

This is very odd, because over the past couple of winters, I've been playing with my own homebrew version, and the results are spectacular.  I would also refer you to the WSPR results of my colleague, Ken Franklin G3JKF, who has some of the most carefully-made loops producing amongst the best signals in the EU on several bands.

Thanks to this nice image by M0UKD, you can see how simple, beautiful and practical a magloop really is. Once you start trying to make one, you will realise they are not as demanding or difficult to tune as many make out.

By 'spectacular', I don't mean 'it will beat the pants off a yagi at 120 feet'.  That's just a silly comparison.

But, if you have ever put up a dipole and been a bit disappointed with the performance and noise levels, then the magnetic loop should be next on your 'to do' list.  Note: not on your 'to buy' list!

The reasons I say a magloop is worthwhile are as follows:

• Physically small - about 1.25m per side for a 20m version, which you can also tune to the higher bands with a suitably rated capacitor.  

• Does not need height for proper operation.  A metre or so is enough.

•  Because of the size, is a (perhaps the only) solution in very high wind areas (like mine!)

• Won't be recognised as an antenna by most, giving you far less likelihood of neighbour and planning problems (loops for 20m are generally within planning rules, if you live in such an intolerant area.)

• Magloops can be used very successfully indoors.   Mine gets across the world on 5W WSPR beacon mode, at 0.5m off the kitchen floor.  The difference in signal between a quarter wave vertical with elevated radials out in the wild weather and the loop indoors can be undetectable (WSPR tests.)

• Magloops are essentially immune from household mains and local noise, allowing effective use in proximity to such sources. 

• Built from 22mm copper tube, are very cheap to build for the 20m and higher bands.  Even at 80m, with a perimeter of 15m and made of wide 42mm copper tube, the price is hundreds of dollars lower than a commercial loop.

• Capacitors can be found second hand, homebrewed, or bought.  If you are good with heavy duty soldering and electronics, a comb-type capacitor can be made and the loop tuned by opening and closing the two halves of the loop (which does, admittedly, demand high insulation and construction standards.)  Vacuum capacitors are to be preferred, as they are very reliable and predictable in tuning.

• Performance is often said to be similar to a dipole, but without the height.  I think this is a fair comparison, though allow for much lower noise and QRM levels from the loop.  I find a horizontal dipole is rubbish next to a magloop (or, in fact, much of any other antenna type.)

There's a lot of stuff written about loops that tends to put people off.  Chief amongst these is talk of very high voltages across the tuning capacitor.  This is really silly, because, whilst it is true there is a high voltage present, this is not something that should scare off those comfortable with basic radio and electronics.

The main thing to contend with is very sharp tuning.  So you need good quality, stable and weatherproof means of installing and adjusting your capacitor.

A loop is, purely and simply, a fairly short perimeter of copper tube - make it as wide as resources and the cost-benefit analysis allows- with either a direct feed using various matching legs, or via a Faraday loop, which is an even shorter bit of wire made into a loop and connected to the coax feed.

Somewhere in the loop, usually the top, you must install a capacitor - either a high voltage butterfly air spaced type, or a vacuum type.  Vacuums are available on e-bay from eastern European or Russian sellers regularly, at prices well below $100.  I've never yet had a problem with sellers from those regions.

The only care you need to take is in making very good connections to the loop and capacitor.  A rush job won't do.  Use wide copper braid or heavy duty cable, and don't mix metals by using stainless steel clamps bonded to copper, which can lead to much lowered efficiency.

Having used all sorts of wire antennas, the magloop is the one that stands out as being a very good performer, immune from heavy winds, can be used indoors with little performance reduction, and can be practically scaled with only modest cost impacts to 80m (or lower!)