The Octoloop

William E. Payne, N4YWK

Of several shielded loops I recently made, one in particular, the "octoloop" stands out in performance and ease of construction.

The octoloop is a length of 25 pair telephone wire inside an octagonal loop shield of 3/4 hard copper pipe. Mine is about 7 ft dia and wired for 50 turns, and gimballed for noise rejection. The pipe shield is a very effective electrostatic shield at MF, and also reduces VLF/ULF microphonics, which for obvious reasons plague most loops. The self resonant freq has not yet been measured, but is probably near 12 kHz.

I took the octoloop to the backyard and leaned it against a tree, oriented to null the noise. For the first time ever, I was able to tune my Rycom 6040 to 0 Hz and hear something besides BUZZZ! The loop has good sensitivity with an almost perfect noise null, a real pleasure to operate! I clearly heard FIVE of the eight OMEGA stations, a NiCd battery charger 450 ft away, and some tweeks and pops. Even inside the house, I hear OMEGA, WWVB and the stronger sferics.

The octoloop is easy to make for about $20. You need some 3/4 in hard copper pipe, eight 45 deg elbows, and two male threaded adapters. You also need a 3/4 inch sealed conduit body tee, a 3/4 CPVC coupling, enough 25 pair cable to go around the loop, and your favorite connector.

Cut eight pieces of pipe to length "L" to make your loop the desired size. The finished diameter will be 2.41*L and the circumference will be 8*L. I used 36" for L, giving a finished size which barely passes thru my basement door. Now cut two pieces in half. Sweat two of the L/2 pieces into the male adapters, thread them tightly into the conduit body, and trim off both ends to reduce the total length back down to L.

At this point you should have eight pieces of length L, with one cut in half, and one with the conduit body inserted into it. Sweat the ends of the pieces and the elbows with solder. Lay out the octagon on a flat surface with the L/2 pieces opposite the conduit tee. Sweat together. Allow to cool, then clean.

Now to pull the cable through. Tie a thread to a small cloth rat and blow it through one half of the loop using a shop vac or hair dryer. Next, pull a strong cord through. Use wire pulling lube or liquid soap to help pull the wire through one half of the loop, starting in the conduit body. Doing each half separately makes a hard pull easier. Clamp the work in a vise or conscript a friend or family member if necessary.

IMPORTANT NOTE: slip the 3/4 inch CPVC coupling over the wire before pulling into the second half! Continue in the same manner to complete the loop. When the wire pulls tight, it should pull the two L/2 pieces snugly into the CPVC coupling. This insulator is important to prevent the shield from becoming a shorted turn.

If you used 25 pair cable, you have 50 turns to splice together. I used solder and shrink tubing to splice together ten 5 turn windings which can be put in parallel for 5 turns or in series for 50 turns. Splicing took me less than two hours. Mount your favorite balanced connector in the third port of the conduit tee, or use a bushing to pass a balanced cable. I use XLR microphone connectors and mic cable for balanced loops. A small preamp may be put inside the conduit body. My octoloop is mounted via cable clamps on a 2x4 with a gimbal in the center.

This construction technique is my favorite so far. It is easily extended to 24 ft dia, and further with modifications. There are also several ways to get more turns. I hope those who make octoloops will report their results and the improvements which they will undoubtedly make!

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