Instructions for an AM Loop Antenna
By Finn McCuhil
You may have accepted static and poor reception on your AM radio as a fact of life. The fault may not be with your signal or even your location in relation to the transmitting tower. It could be the fault of your radio’s antenna. The ferrite coil antenna in most AM radios is a small rectangular bar that is just large enough to allow the radio to receive local stations. You can improve your reception by improving your antenna.
A loop antenna, as the name implies consists of a loop or number of loops made of copper wire wound on a nonconductive frame. The frame is generally made by joining two pieces of wood together to form an X shape. On a spiral loop antenna, the copper winding begins on the outside edge of the X and is wound in a spiral shape toward the center.
An edge-wound loop antenna uses the X-shaped frame constructed from 1-by-6 inch boards. The ends of the board are notched at half-inch intervals and, as the name suggests, the copper wire is wound around the edges of the frame.
In both cases, the two ends of the wire are connected to a variable 365-picofarad capacitor that is used to tune the antenna to the correct frequency for the radio station.
Building an Edge Wound Loop Antenna
Take two 18-inch 1-by-6 inch boards and place a mark on both at the mid-point on one edge. Place two more marks on both boards 3/8 inch on either side of the mid-point. This will give you a 3/4-inch measurement in the center of the boards. Use a square to mark a slot beginning at these marks running to the middle of the flat side of each board. A 1-by-6-inch piece of finished lumber actually measures three-quarters of an inch by five-and-a-half inches, so the middle of the flat side will actually be two-and-three-quarters inch. Use a small handsaw to cut the marked slots out of each board. Put the boards together by aligning the slots and pushing the outside edges of the boards until they reach the bottom of the slot. The result should be an X-shaped frame.
Use the small handsaw to make eight, half-inch deep slots, one-half an inch apart on all four ends of the frame.
Wrap 22-gauge copper wire around the outside of the frame through the slots. Mount the variable capacitor anywhere on the inside surface of the frame and attach both ends of the copper wire to the capacitor’s terminals.
Tune your AM radio to a station with weak reception. Place the antenna on or next to the radio and slowly adjust the dial on the capacitor up and down. Loop antennas are directional, after the reception improves, try rotating the antenna from left to right.
Finn McCuhil is a freelance writer based in Northern Michigan. He worked as a reporter and columnist in South Florida before becoming fascinated with computers. After studying programming at University of South Florida, he spent more than 20 years heading up IT departments at three tier-one automotive suppliers. He now builds wooden boats in the north woods.