Monday, November 15, 2010
A 6-Inch Tunable Loopstick
The homemade tunable loopstick device, similar to the Q-Stick by DXTools continues to be a popular article on RADIO-TIMETRAVELLER. Let's revisit this simple device and build another one, this time a 6-inch version.
Old transistor or table radios can be had for almost nothing at your local yard sale or flea market. I have reported on yard sales and flea markets before. Great radio values can be had from uninformed sellers.
Don't Laugh at Yard Sales
Another Deal, The Panasonic RF-565
Viva Flea Markets
I always keep my eye out for both working and non-working varieties, as the non-working ones can be stripped for their parts. Most important for building a tuning device: the variable capacitor and the ferrite loopstick.
The better buys are the radios having longer loopsticks or serviceable tuning capacitors. Many pocket radios are configured with the tuning knob directly connected to the capacitor, though the capacitor itself is often small in this case. Larger radio varieties often have a slide rule type dial mechanism, usually driving a more substantial capacitor. Surprisingly, most tuning capacitors in old radios, even the small ones, have 1/4 inch shafts, though short. So if the main tuning knob is intact and is one which is connected directly to the capacitor shaft, all the better. Save the knob.
This summer's flea market take was four old transistor radios. One was pocket sized, and the others were the type you would set on a desk, about the size of a small book. Three of the four had seen better days and did not even work. The other actually worked decently enough to keep and experiment with. Each cost two dollars or less.
The best loopstick of the bunch was a nice six inch one, a rod instead of a bar, wound very nicely with evenly-spaced turns of Litz wire to about 80 percent the full length of the stick. A short IF coil was at one end. I carefully removed the IF coil, as it is not needed.
I selected a small tuning capacitor which had a nice dial knob. At the end of the loopstick where there was room, I taped the capacitor to it by slotting the tape with a razor so it would fit over the shaft. The capacitor could also be hot glued to the rod if desired.
The coil wires were then carefully soldered to the center and edge terminals of the capacitor, forming the parallel tuned circuit. Be sure to use the edge terminal having the greatest capacitance, as the other one is the IF side of the capacitor with lesser capacitance. If you want to get fancy, you could enclose the loopstick device in a short length of PVC pipe, capping the coil end, and fastening the tuning capacitor on the other end of the pipe.
To use the loopstick, simply hold it parallel and near to the internal loop in your radio, then tune it to peak the signal your radio is tuned to. Rotate the two together, radio and loopstick, for maximum signal. The loopstick can also be rotated independently to null an offending co-channel station. A short length of wire or even a longwire could also be attached to one of the capacitor terminals, providing more signal, though it will lessen the directivity of the loopstick.