Saturday, August 22, 2009
The 12-Inch Passive Loop
Now on to some projects I've been working on here this summer.
Haven't built a passive loop in many years, probably since way back in the 1970s, so thought I'd get back into it. The first is a 12-inch loop (12 inches per side). Now where do I find 100+ feet of wire? Who wants to buy wire? Not me.
I had a 20 foot length of 6-conductor, solid, twisted-pair telephone wire in a box in the cellar. The cover can be stripped with a razor blade or Xacto-Knife if you work carefully. In 15 minutes I had it complete. Take each pair of wires, tighten them up in an electric drill motor chuck, tie the other end to a door knob, and unwind them. Now you have 120 feet of nice, 24 gauge insulated wire of different colors.
I then spliced and soldered the lengths together. The wooden form consists of two 1/2 x 4 x 17 inch hardwood pieces formed into a cross by notching and gluing. The 4-inch width makes it sit up nicely all by itself. I then cut a small, 4 x 4 inch piece of thin wood veneer and nailed it to the center of the cross for support and a mount for the variable capacitor.
I didn't bother filing spacing notches for the wires, choosing to wind the coil in a close-wound fashion. Small wire nails in appropriate places make nice anchors for the wire. 26 turns of wire were wound around the form. My variable capacitor is a 250 pf unit, so a few extra turns were called for. Normally you wouldn't need 26 turns for a 365 pf unit. This loop tunes from about 510 KHz to 1500 KHz or so. A small jumper wire, about 4 inches in length with alligator clips on either end, is used to clip off a couple of turns of the loop to get the tuning range up to 1710 KHz.
Sensitivity of the loop is good, better than a 4-inch tuned ferrite rod placed next to your ULR. Nulling is sharp. Best thing about this loop is that it is extremely portable and can be set on a small garden table next to your chair. New York City, about 265 miles from here, is not generally receivable under daytime circumstances. Using this loop, stations like WFAN-660, WABC-770, and WCBS-880, all 50KW, are adequately above the noise level and are comfortably received using headphones.