Magnetic Loop Construction

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The key to effective magnetic loop operation is keeping the resistance of the loop and electrical connections between the loop and the capacitor to a minimum. Loops are usually constructed from aluminum tubing or copper tubing. Metal strips and even the braid of a large size coaxial cable (RG/8 or RG/213) can be used.

The voltage generated across the capacitor can be substantial, so large capacitors with wide spacing are required to prevent arcing. The capacitors included with the MagicLoop are motor driven by a DC motor with a vernier planetary reduction drive. The 160-80 capacitor is about 2 feet (60cm) long.

Some spreadsheets for loop design from the j•Com archive are shown below. These might be useful for designing the appropriate loop for a given frequency and understanding the tradeoffs between loop size, number of turns, capacitance required, and plate spacing to avoid arcing.

An article on the design of low band loops by I1ARZ appeared in the February, 1989 issue of RadCom. For reference, the article is made available here: [Page 1] [Page 2] [Page 3] [Page 4]. The article includes a BASIC program for calculating dimensions and detailed construction suggestions.

Tuning the loop can be made easier with a phasometer for determining whether it is necessary to tune up or down. A phasometer is a kind of SWR meter that also indicates whether the antenna is resonant above or below the operating frequency. G4FM's design, Tuning the I1ARZ Loop, appeared in Radio Communication, August, 1989. Another Phasometer was designed by K9MLD using a zero center meter and appeared in the last issue of Ham Radio Magazine, June 1990. Images of the article: [Page 1] [Page 2].

The easiest way to tune the loop to the correct frequency is to listen to the receiver noise. Noise increases dramatically as the loop resonant frequency is adjusted. The DC motors do a good job of creating wide band noise to facilitate tuning but are silent once tuning is accomplished.

Performance

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The performance of these small antennas can be magical. I constructed a 1 meter diameter loop out of copper tubing in Europe around 1990 and set it on my desktop, indoors, in our apartment. I made a scheduled contact on 20 meters with a ham in New Zealand who was also using a magnetic loop antenna. We were able to maintain communications down to 1 watt at each end.

For use in apartments or locations with limited space for antennas the magnetic loop antenna can be an excellent solution provided it is carefully designed to keep the resistance of all the connections low. It is very easy to tell how successful you have been with your construction. The better the antenna, the narrower the bandwidth. A well built magnetic loop antenna may only have a usable bandwidth of a few kHz. That is why motor driven tuning capacitors are necessary.

A A & A sold the same antennas to Her Majesty's Navy. The loops for the antennas were embedded in the fibreglass hulls of some vessels and the capacitors could be remotely tuned from the radio room.