by David Skelhon, VA7SZ

As HF operators we often operate from less than ideal locations. Many of us do not have acres of land where we can erect towers or even conveniently located trees to support wires. The problem becomes particularly tricky in the lower frequency bands where half a wavelength of wire at a useful height becomes impossible due to small lot sizes and other restrictions. With these limitations, I have used magnetic loop antennas with some success over the past few years. At the moment I have a magnetic loop dedicated to the 80M band. The loop was constructed almost entirely from materials and components I had accumulated over the years and consequently cost little.

Good design information is available online so I will only cover a few essential points as this article is more about working with what you have available or what can be obtained locally.

There are several online magnetic loop calculators; they are useful for ballpark component values but final loop sizes and capacitances can be found using simple test equipment. I started with the biggest loop I could easily support, laid it on the ground and temporarily connected various capacitors until I got close to my desired frequency range. An old grid dip meter was perfect for testing resonance.

My loop is made from 24’ of 1” diameter cable TV trunk line. The shield and center conductor are aluminum separated by a foam dielectric. This creates a loop about 7.5 feet in diameter, which is stiff, light and easily supported. I was able to bend it by hand.

The ends of the loop are connected to a 200pF vacuum fixed capacitor in parallel with a 175pF air variable. Because of the high voltages involved when running 100W the capacitors are rated at several thousand Volts.

Note that running a variable capacitor in parallel with a fixed capacitor has a couple of advantages: firstly the high currents are split between two capacitors so the overall resistance is lower which is especially beneficial when using a conventional air variable and secondly, movement of the shaft of the variable capacitor has less effect on frequency. This is important because the Q of this circuit is so high that at resonance there is hardly enough room for the 3KHz bandwidth of an SSB signal. Consequently a geared system with minimal backlash has to be devised to allow fine-tuning. Even so I had to add a pulse width modulator to slow the 12VDC motor for final adjustments.

The motor is reversed using a double-pole double-throw switch and because the air variable capacitor rotates freely through 360 degrees I did not need the complication of limiting switches. A stepper motor and micro-controller would probably do the job too but I wanted to keep the design as simple and robust as possible.