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.

It follows that the tiny bandwidth makes this a good antenna for a noisy, crowded band as on receive it acts like a front-end preselector.

The connections were made using copper foil (from a metals recycling depot). This was soldered to the aluminum shield of the loop using a special solder and flux (local tool store). The driving loop is also made from the same hard line and the shield is soldered directly to the coax transmission line - again using the special solder and flux. Note that copper braid seems to have higher losses than solid copper – hence the copper foil connections. Because of the high circulating currents all connections need to offer the lowest possible resistance and should be welded or soldered whenever possible.

The capacitors and drive mechanism were mounted on a repurposed plastic chopping board bolted into the waterproof case.

The loop remotely tunes from 3.4 to 4.2MHz. I can tune anywhere on the band simply by watching the noise peak move on my IC-7300’s spectrum analyser. A low power carrier is applied for final tuning while using the motor in slow mode to get a 1:1 SWR.

The driving loop is one-fifth the diameter of the main loop. With the driving and transmitting loops in the same plane there was a 2:1 SWR but an offset of 30 degrees gave me an SWR of 1:1 and this is good for the whole 80M band.

The capacitors and gear drive are enclosed in a waterproof case and the whole assembly is supported on glass-fiber poles, which are held vertical on a portable steel workshop table, weighted with bricks for stability in strong winds.

The gear drive assembly, pulse width modulator, case and glass-fiber poles all came from Princess Auto Surplus. The steel base originally came from Canadian Tire.

Ferrite beads are used on the coax and control cables to prevent RF getting back into the shack.

Performance

The loop had been in use for over year and works well for such a small antenna. It is very quiet and can be turned to null-out noise from local sources. I have made SSB contacts in Quebec and JS8 Call contacts in Texas. I have little problem checking into provincial nets – either hearing or being heard.

The downside is that it takes time to move around the band and I do have to briefly apply a low power carrier to confirm a match – especially at the lower frequencies. Bandwidth increases with frequency and at the top end of the band I can usually “eyeball” a good match on the scope. The geared motor refuses to work at temperatures below about -15C, probably due to the viscosity of the lubricants in the mechanism.

In Conclusion

I hope I might have encouraged those of you operating from small lots to experiment with magnetic loop antennas. They work well on all bands and can be multi-banded although tuning can be tricky if covering too many bands. They also lend themselves well to portable operation – especially at QRP power. Copper or aluminum tube, or Heliax can be used for the loop. Vacuum variable capacitors are a good choice for higher powers because of their high voltage rating and they already contain internal gearing to aid tuning. However, they can be expensive and a little delicate.

Although a loop of this small size has low overall radiation efficiency compared to a dipole it makes up for it by providing gain in the plane of the loop and a good spread of radiation at lower angles. It’s definitely a keeper!