Elevated Magnetic Loop for 160 & 75 Meters

3" aluminum tubing 15' transmitting magnetic loop.


Loading up this antenna was different. On 160 meters the bandwidth from resonance to 2:1 SWR was 800 cycles. Feed impedance dropped by half as the antenna was hoisted up from five to 25 feet. Apparently there are considerable induced ground currents at these long wavelengths. Resonance, however, barely changed with height. Rotating the elevated loop revealed surprising directivity and deep nulls, with 20-dB-over-S9 local noises reduced to s-4, but for only a few a few degrees. With its turning radius of only 7', a small "clearing" under the tree branches was easily found to get 360 degree null coverage. 

Transmit signal reports were found to be comparable to stations using full sized antennas. This was surprising, coming from an urban lot with 13kV power lines on two sides and the property line within 25' of the house. The first night it was up conditions were good enough to contact two UK stations. Receive performance lived up to the loop's reputation as well, as one of the UK stations worked was unreadable by the other stateside stations on frequency. Local high angle performance was tested by working an upstate New York station with one watt each way. 

Rotation of the antenna turned out to be a big operating bonus, as the null was as effective in killing distant SSB stations intruding in the AM window as it was with local line noise. A local AM broadcast station's flock of birdies in my Icom 706's porous front end were also effectively snuffed by the loop's narrow bandwidth 

Voice transmissions with such a narrow bandwidth were reported as clean, despite the SWR bouncing well above 2:1, as the sideband energy was out well beyond the 800 cycles. SWR was minimized by tuning the antenna 1500 cycles below the carrier for LSB. On AM transmission voice peaks were about 4 to1, but audio reports were consistently good. Even FM, with its 15 kc deviation and SWR well over 5 was reported to be cleanly readable. 

The bad news is cost. The antenna's 60 lbs of 3 inch aluminum tubing go for $500 at typical 2007 jobber rates. Decent power on 160 and 75 is a kilowatt, which generates 30 Kv across the tuning capacitor. The tiny .016 ohm radiation resistance on 160 demands extreme low loss high Q construction, eliminating all but air or vacuum tuning capacitors. I found a football-sized broadcast type vacuum variable that jobbers quoted at well over $1000. Finally, five 120 degree elbows were fabricated at $50 each. Managing the 30,000 volts and 100 amp currents found on this antenna requires a high level of engineering practice: long insulators, strap connectors with rounded corners, tight clamping and bolting.

Close-up of tuning capacitor layout.

Work with the ARRL's engineer in charge of public RF exposure revealed that this particular magnetic loop's danger zone is 30 times larger than a dipole's, or about 30 feet at legal limit power. A wrought iron table 60 feet from the antenna will pick up enough energy to make a visible arc to ground; At 200 feet, field strength spontaneously starts and runs our vacuum cleaner at a speed proportional to the carrier power. You don't want to even think about what it would do to a pacemaker. 

Mechanically, hoisting and lowering the antenna's 75 pound total weight was made easy using two blocks to make a 3:1 come-along. Scrupulously keeping the feed and gearmotor DC lines along the antenna's centerline keeps the SWR below 1.2 to 1 (at resonance.) Anything touching the side of the loop will get rapidly burned. Early experiments with a 10 Kv capacitor provided a flash so impressive my son thought the tree was on fire. 

Corrosion of the aluminum will rapidly degrade performance by increasing the resistance in the joints, and on the loop's surface, where skin effect forces the majority of the current. Galvanic DC potential is kept negative, and corrosion minimized by the marine practice of using zinc, in this case by tying it to a short length of galvanized (zinc coated) pipe, hammered into the ground. Grounds tied to copper rods show a crucial half volt positive DC potential vs the zinc ground and should be avoided like the plague.

Half size proof of concept. Note coupling loop and tuning capacitor layout.

W1LYQ's 160 meter magnetic loop, written up in the June 1993 QST, had logged transmit performance as good as a full sized wire antenna. Modeling software showed his square antenna 20 feet on a side had a radiation resistance of .07 ohm, while the copper pipe conductor he used had a resistance of .12 ohm, for an efficiency of 40%. 

The near field of this type of antenna is magnetic rather than electric, which theoretically reduces the effect of ground current losses. Antenna modeling software shows a 160 meter dipole up 30' suffers 8 dB of ground absorption loss, 3dB more than W1LYQ's loop at ground level. An elevated loop at 25 feet gets back another 2 dB. This "gain" shows up principally in the loop's fuller low angle polar pattern, as demonstrated by W1LYQ's loop outperforming his 65 foot high inverted "V" on contacts across the Atlantic. 

On hand was a generous amount of scrap 3 inch aluminum tubing, which would make a practical 15 foot diameter, 60 pound hexagonal loop. Unlike W1LYQ's copper tubing loop, this one would be rigid enough to hang in a tree like a giant earring. This would give it two advantages, rotation to null out noise, and a 25 foot height above ground to reduce ground current loss. 

Compared to a square of the same perimeter and weight, the hexagon required an 8 foot rather than a 12 foot shipping container, and got 20% more area, giving 44% greater radiation resistance. (radiation resistance of a small loop goes as the fourth power of diameter) The aluminum conductor resistance of .05 ohm was about three times the radiation resistance of .016 ohm, for an efficiency of 25%, just neutralizing the gain from reduced ground losses and giving it dipole-equivalent performance. 

Five 120-degree elbows were fabricated, slotted lengthwise, and pried open about an inch to fit over the tubing lengths' ends. Radiator hose clamps were used for the low loss connections, making a rigid hexagon resembling a giant benzene ring with a gap at its bottom. 

The electrical equivalent to a high Q tank circuit is completed by attaching the tuning capacitor across the gap, using 6" wide soft aluminum bands bolted in place. About 30 kV and 550 pF was needed to tune this size loop to 160 meters and handle a kilowatt of transmitter power. Because of the criticality of the tuning and high voltage, a DC gearmotor drive with a three inch long plastic drive coupling was required. 

Feed consists of a coax directly attached to the ends of a 2 foot copper conduit coupling loop inside the top angle, opposite the gap and capacitor. This gives low SWR across the entire 160 or 75 meter bands. As exposed metal loads and erodes the zinc galvanic protection, the antenna was painted. For discretion a dark color was chosen, and for ease, an aerosol can was used. By attaching the coax shield end of the coupling loop directly to the antenna, the coax shield was used as the DC ground conduit to the galvanized pipe. This required isolating the antenna from powerline ground, which was done by using an MFJ artificial ground, 240 volt amplifier supply, and a car battery exciter supply. The MFJ ground was tied to a copper rod. An alternative DC isolated low loss RF ground would be a length of coax lying on the ground.

Four turn mini receiving loop. Note relative size of the capacitor.

Receiving, even AM broadcasters, had been unsuccessful inside my metal hangar office using an assortment of wire antennas, due to the neon lights, compressors, wifi modems, computers, etc all also inside. To see how a small receiving loop antenna might work a four turn 18 inch mini loop was tried. It proved effective, picking up WWV, aviation weather, shortwave broadcasters, hams, etc. with sharp nulls that killed the intense hash from the nearest WIFI box.

3 footer, transceiver, and battery next to 3' by 5' window.

Next, experiments were conducted with a 100 Watt transmitter and a single turn 3 foot conduit loop near a window inside the metal hangar to test the idea of a mobile loop on the higher frequencies On 20 meters a 15 minute test yielded a contact with a French station near Paris. A contact was even made on 75 meters, where efficiency was of the order of 1% before taking into account the metal building's shielding effect.