Back to Basics: Global DX from the kitchen.

It's time this blog got back to its roots and promoted cheap, uncomplicated amateur radio.

Not for the first time, I'm going to extol the virtues of a magnetic loop antenna.  These are, to put it mildly, superb antennas that anyone can make for pocket money.

What do you need?  A basic loop covering at least the 30 and 20m bands, the mainstay of DX operations you might say, is just 1.2m on a side.  With increasingly sharp tuning and lower efficiency, it is here, and ought to be elsewhere, tunable down to 80m.  On a single wooden prop to lift the loop about 0.8m off the floor, this is still small enough to comfortably fit through a doorway, and can be stored behind a wardrobe.

So, 15mm or, better, 22mm copper pipe.  You need 4 x 1.2m altogether, plus four 90 degree 'elbow' bends, either presoldered or unsoldered, and some plumber's solder and flux; make sure you clean the joints with wire wool to immaculate dry shines, or they won't solder properly!  You can even use compression fittings, especially if going portable, such that the loop can be disassembled.  The primary (small) loop to which you connect the coax can be made from thick electrical wire or microbore copper tube.

A trick with the primary loop is to make it an oval, rather than a circle.  It also helps matching considerably if you allow some of the loop to hang across and slightly below the lower part of the secondary loop.  A lot of loop experts have noted this.

The only other thing you need to scavenge for is a decent wide spaced capacitor with a value of 0-200pf as a starting point.  You can use vacuum caps, but these are a little expensive and very heavy, really only usable for permanent loops.  They are also much harder to tune remotely, needing relatively high-torque motors.  An air-spaced cap is easily rotated with a cheap low-torque, low speed motor, or just by hand and ear; a magloop has very sharp tuning, so it's pretty obvious to the ear when you've hit the sweet spot.

An hour or so of 20m WSPRing at 5W into a kitchen-based magloop.  Note the three VKs hearing me!

But does such an unlikely - and very small - antenna actually work?  It absolutely does!  At 14MHz, the magloop just described is about 86% efficient, or about 1dB down on a 100% efficient antenna.  In other words, it's bloody efficient!  This is confirmed when running on the WSPR beacon mode, which shows a 5W input easily traversing all corners of the globe.  Switch to JT65 or similar, and you're QSOing with the best of them!

So, if you live in an apartment, or have no garden, or are wondering which antenna to buy, think seriously about a magloop.  So long as you get the right capacitor (check regularly on ebay), then building one is easy.  You can sometimes get a cap for a few pennies, but they are often much sought-after!  Soldering doesn't need to be perfect, nor does the whole thing need to look pretty.  

So, go build a magloop today! This site will help with the numbers.