Tuesday, September 15, 2009
The 42-Inch Passive Loop
The 12-Inch Passive Loop, constructed earlier, is great for its portability. It sits very nicely on a small garden table. Unfortunately, its signal gain is not much better than a tuned ferrite stick of moderate length. For the next project I considered building an 18-inch or 24-inch, but I rejected the idea outright - not a big enough improvement for the cost and effort to construct them. For real signal grabbing power, the loop size must be increased to 36 inches or more. 42 inches seemed like a good compromise between a 3 and 4 foot loop.
I prefer working with wood here, as I have a lot of scrap available and a nice table saw for cutting whatever size I want. How long to make the cross members, you ask? The formula is quite simple.
Multiply the side length of any loop by the square root of 2 (1.414). So, taking the side length of 42 x 1.414 we get a cross member length of 59.388 inches. I located a nice piece of 1/4 inch pine that was two inches wide, and cut two pieces 59 inches long. It is lightweight and strong enough to support the wires without bending. Each piece was notched halfway through at the middle and fitted together. Next I cut an 8 x 8 inch piece of 1/8 inch veneer and screwed it to the center to sturdy-up the cross and hold the members at right angles. No glue was used on this project at all.
A 42-inch loop is a fair-sized piece of hardware. How do we rotate it? It certainly won't sit on a garden table. I had a 36 inch piece of 1/4 inch steel rod lying about and the idea occurred to me that it might somehow be attached to the loop and pushed into the ground to serve as a pivot. I cut a 27 inch length of 1 x 2 furring strip and screwed it to the center support plate so that it bisected one of the sides. It extends about one inch past the coil edge. Carefully, I then drilled a 1/4 inch hole into the end of this furring strip, about three inches deep to accept my steel rod. As shown, the loop can now be pushed into soft soil and it will rotate freely.
Now to construct a mount for the tuning mechanism. On the 12-inch loop it hardly mattered and I mounted the variable capacitor through the center support plate. On a loop of this size, the tuning mechanism needs to be out at the end of one of the members where the loop ends terminate. I had some scrap 1/8 inch plexiglass, so I cut a small rectangular piece and mounted a 365 pf variable capacitor and an SO-239 coax receptacle through it. The SO-239 coax connector will be left unwired, to be used at a future time for the termination of a one-turn coupling coil. I then screwed the plexiglass to the edge of one of the two cross members nearest the rotating arm.
The 42-inch loop requires 9 turns of wire, 126 feet total. I had 100 feet of white, 20 gauge solid insulated wire, so to that I soldered a scrap of 20 gauge black wire that was lying around. This is why you see two different colors of wire in the photographs. Small wire nails hammered into the ends of the cross members serve as starting and ending pins for the coil. At the end of each of the four cross members I filed notches 3/16 inch apart to properly space the loop turns. When the coil was wound and secured, I then soldered the ends to the variable capacitor.
Signal strength on this loop is phenomenal, and exponentially better than the 12-inch loop. Nulling and peaking are both fairly sharp. The base tuning range of the loop is approximately 520 KHz to 1500 KHz, too narrow to cover the entire mediumwave band. A short jumper wire with clips at each end was fashioned to short-circuit one turn of the loop. This changes the tuning range to approximately 650 KHz to 1750 KHz. The loop couples well to my ultralights and the Tecsun PL-600, and often a radio only needs to be one or two feet away for improvement. Pushed into the ground and ready for rotation, the tuning panel is perfectly accessible from lawn chair height.
All-in-all, this has been a very worthwhile project. Though this sized loop would be cumbersome to carry in a small car, it performs very nicely in a backyard environment and can be stored flatly on a wall in the garage. Sometime in your DXing life try building one of these larger loops. You will be amazed at their performance.