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Magnetic Loop Antenna Experiment


Can you find the antenna? 7 foot 60 meter magnetic loop.

After selling the airplane, I knew I would be suddenly spending a whole lot more time driving instead of flying. A decent mobile signal became a priority. With the low bands being the only game in town from sundown to sunup, they were essential for the long nights of distance driving. Unfortunately, a cacophony of man made roadside noises made receiving with the whip difficult. A possible solution was the mobile magnetic loop, as the Russian and French military had discovered, because of its reputation for quiet receive. 

A quick and dirty first try for home testing was so successful that it is described here. A 50' coil of 3/8" copper conduit yielded an antenna small enough to be discreetly hung from a tree like a giant earring, yet capable of putting out a decent signal on 75 meters, meaning use with an amplifier and efficiency of 50%. Because it was situated in a neighborhood with 30' house spacing and lots of power, cable, and telephone wires, it would be a good measure of receive noise and amplifier harmonic suppression.


The 16 foot by 3/8" loop. Note coax feed going to top, and tuning cap at bottom.

On receive it clearly heard a 5 watt ham in the Bronx, a mobile in California, and aircraft crossing the Atlantic and their controllers on 5 MHz. that were unreadable on the mobile whip in the driveway. It regularly received the high seas forecast far more clearly than my maritime mobile using the same Icom 706. On transmit it averaged two s-units better than the efficient bugcatcher style mobile whip with 500 Watts. One ham even heard it in the next state with his antenna disconnected. No neighbors have complained in two months of daily operation, much of it during "prime" evening hours. 

Computer modeling shows a 75 meter 16' by 3/8" copper antenna has radiation and ohmic loss resistances of about 2/10 of an ohm each, for 50% efficiency. Conceptually I think of it as a dipole with such extreme sag that the element ends don't just point down, but come together. Bridging them with a capacitor brings the resonant frequency way down, from around 10 to less than 4 MHz. As a result the current distribution around the loop mimics the middle 50 feet of a 75 meter dipole, becoming fairly uniform around the entire circumference. A horizontal loop can be thought of as an "extreme" halo.

With the two opposite polarity high voltage points next to each other, the voltage field's contribution to propagation and reception is much reduced. The magnetic field thus becomes the dominant means by which this antenna radiates and receives signals. Like a one turn coil, it has sharp nulls where a quad loop would have its maximum signal, and maximum signal in its plane, where the five times bigger quad would be weakest.

Thought of another way, the broadside dipole field gets cancelled when the ends droop around and create this nearly uniform circular current, and a new field is created from the out of phase currents in opposite sides of the loop, similar to that of extremely close spaced (1/20 wavelength) dipole elements fed 180 degrees out of phase. 

Fed with a kilowatt, the loop's nearly 500Watt magnetic field can be dangerous, as can the ends of the loop at a calculated to 15,000 volts. With its 4/10 ohm total resistance, ohm's law current at this power level is 50 amperes, resulting in a total of three quarters of a million volt-amperes. Electrically, this mimics a tank circuit with big out-of-phase currents, voltages, and magnetic flux induced by a modest power input. Mechanically, it means rigorous high voltage as well as high current design and construction. 

Assembly of the "earring" pictured here used an assortment of parts. An 8 foot insulated large gauge (#10 or heavier) gamma wire connected to the center conductor of the coax, a 100 pf 15 kV vacuum variable capacitor, 50 foot roll of 3/8" copper air conditioning conduit, surf casting rod with 40# test line and a 5 oz sinker, tie wraps, black spray paint, plastic container big enough to hold the capacitor with generous air space, a cheapie yachting block, and 100' of black or dark _ inch (stealth) rope, some clean aluminum straps to wrap around the capacitor, a pair of 3/8" terminal lugs to solder onto the conduit, and assorted fasteners. Three 6' fiberglass rods were used, one to stiffen the top of the loop, another to attach to the capacitor tuning shaft. The loop is floppy, so more stiffeners would have been better. I used a 13' quad spreader attached to the third 6' fiberglass rod to span the loop vertically and support the capacitor's weight at its bottom.


Detail of capacitor showing tuning rod, connector clamps, and weather shield.

Pick a tree branch with an eye to having a clear 30 feet or so of space for the antenna to turn 180 degrees and be up about 15 feet to minimize ground losses. Cast the line and sinker with the surf rod over the tree branch, pull up the black (stealth) _ inch line and cheap yachtie block, to which you've ALREADY strung the antenna hoist line. The conduit is unrolled, formed into a loop, the lugs soldered on (vertically), the coax and gamma wire assembled, with an inch of coax shield exposed and tie-wrapped tightly for a good electrical connection at the top center, where the hoist line and support are also attached. The gamma wire is tie-wrapped loosely at intervals to keep it within an inch or two of the conduit, with its bared end tie-wrapped tightly to the conduit (no capacitor necessary).

Any reasonable combination of fishing line and stiffeners will work. One way is to attach a pair of fishing line support bridles to the hoist line 4 feet or so above the antenna and to the ends of the 6' fiberglass pole stiffener, which is tie-wrapped across the top of the loop. Attach another pair of support bridles to the top center of the loop and tie them to either side of the bottom about 5 or 6 feet out from the capacitor. Leave generous tails, as these will be useful as rotators for nulling when tied to rocks placed on the ground.

Once the loop itself is complete, it is hoisted in stages and a coat of black spray paint is applied to keep the copper smooth and low resistance, and the appearance low profile. When the bottom is at working height, the metal straps and capacitor can be assembled as shown in the picture, with a strap wrapped around each of the capacitor's contact bands, bolted (not too tight) and pointed in opposite directions, drilled and to bolted to the 3/8" lugs soldered to the conduit ends, so the cap is hanging vertically, tuning shaft down. After hoisting head high, the 6' fiberglass tuning rod is attached with a little hose clamp and the antenna's resonance adjusted to the desired frequency. Very little change should be apparent when hoisted to its final height, as the magnetic fields seem to have less interaction with surroundings. The sharpness of the tuning determines the antenna's Q, and hence efficiency. Typically, 2:1 SWR points should be about 4 KHz or 1 part in 1000 apart, for a Q of 1000. If it tunes broad, look for loose hardware or poor contacts. With radiation resistance of only 2/10 of an ohm any sloppiness will be fatal to performance. The impedance can be adjusted by lengthening or shortening the gamma in the usual way. 

An antenna tuner in the shack gives a 70 kHz bandwidth. Keep the coax away from the loop, or it will burn. My 15 KV cap arced over at 900 watts so spectacularly it looked like it would set fire to the tree. The tails from the bridles on the sides are handy for turning to null thunderstorm and neighborhood TV set noises as well as the occasional "nut" or "jammer" that 75 meters attracts. A plastic quart bottle with the neck cut off and slit 2/3 of the way down the sides has proved to be an effective weather shield when slid down over the capacitor, allowing full power operation in pouring rain.

A1500 Watt aluminum 36 footer for 40, 80 and 160 meter bands. A 1500 pf 15 kV vacuum variable capacitor gives continuous coverage from 1.6 to 8.5 MHz, with 90% transmit efficiency on 40, 55% on 80 and 7% on 160 meters. With 2' instead of 6' leg lengths coverage is 3 to 25MHz.

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