AM (Medium Wave)
Resonant Loop Antenna
Ironically, one of the easiest to make, this loop seems to be the most efficient ... so far. It's 2 feet on a side. The cross pieces are 24" x 1.414 = (you do the math).
Details of the intersection, a "tee" drilled through or a "cross" will do. I used 1/2" PVC pipe. The 4 ends are "tees" with elbows for feet. Use a small piece of pipe to join the "tees" and elbows.
Details of the windings. There are 7 or 8 turns which resonate with the 15 - 365 pF air variable capacitor. Note that I've added a triple section cap for lowfer (LF) reception. This loop covers 2 MHz down into the beacon band and LF band. Note the pickup winding and "F" connector
This circuit is as basic as it gets. I won't put a diagram here, because there are a number of good sites on how to build a box loop for AM broadcast.
Simply put: It's a resonant loop of spaced windings (spaced to keep stray capacitance low). I'm guessing, but I figure there is 56 feet of wire. The coil is wired in parallel with a variable capacitor (usually a 15 - 365 pF). This parallel circuit should resonate somewhere from 0.5 to 1.7 Mhz (500 - 1700 KHz).
Depending on the components, you may have to add or remove wire to put the resonant limits within the band of your choice. I had to add more cap to cover the loop's intended purpose. But, experimentation is easy and educational. The nice thing about PVC over the more popular wood frame is the spring it has, thus keeping the windings fairly tight.
Use an ordinary portable AM radio placed near the loop to fine tune the windings. Check the peaking at the low and high ends of the b'cast band ... it's quite amazing actually. The antenna can be turned and tilted to peak a station or null and interfering one.
The pick-up loop is soldered to an "F" fitting. make this a one-turn loop and space it as shown or experiment, before you make it permanant. With the pick-up loop, you can feed the low impedance input of a communications receiver. With a portable radio, just orient it, near the antenna, for best receiption of your selected station and don't use the pick-up winding connection.
Happy listening,
Bob, N1KPR

Feel free to copy info, but please give credit as necessary. July 31, 2005
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