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Seabat wrote:
"I have a bookshelf stereo system that has a rinky-dink little loop antenna for AM and a single little wire for FM." The designer likely assumed the loop and the wire would be immersed in AM and FM fields adequate for satisfactory reception. The AM loop may not have enough signal because you`ve put it under a metal roof. The FM wire may not have enough signal because its elevation is too low at your site. You`re down in the Rio Grande valley. The FM wire is likely about 2.5 feet long, or about 1/4-wavelength. Its impedance would be about 30 ohms near resonance. The loop is likely part of the AM tuned input circuit with an impedance of about 1000 ohms. For FM, you need to get the antenna up high. A ground-plane antenna is simple and works well. Vertical and radial parts can all be about 2.5 ft. long. It can be connected to your radio with 50-ohm coax. The existing wire is your FM input. Ground to the radio is probably one of the "rinky-dinky loop" wires. If not, capacitive coupling via aluminum foil or the like will likely do. For AM, a tuned loop out from under the metal roof should capture ample signal. It can be enclosed in a non-metalic protection and does not need to be placed up high to work well. Wind a separate 2-turn coil around the loop to take out its signal. Connect this to yet another loop (which does not need to be tuned) via twisted pair. Place the untuned loop in the position next to the "rinky-dinky loop" which gives the best reception. When changing AM stations, you will need to retune the external loop for best reception. Best regards, Richard Harrison, KB5WZI |
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