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365 pf Capacitors
How to make a loop resonant
to cover the entire AM band

Question: I recently constructed an AM loop antenna using a 29 inch square wooden frame with 8 turns of coated 22 guage wire spaced one-quarter inch apart and a variable capacitor I salvaged from a table model radio I acquired at a local thirft store. It works great. Only problem is the antenna is resonant from approx. 1100 kHz to 2000 kHz. What can I do to lower the resonant freqencies of the antenna to cover the entire AM broadcast band?

To make the loop antenna tune lower, you need either more inductance or more capacitance.

Adding turns will do the trick but with the size of your frame, I'd try only two or three at first, more only if needed and then only one turn at a time. My air-core loop is 1 metre on the side, having 9 turns, the centre turn wrapped with a pickup turn.

Looking through old papers, I see the Sanserino loop was 10 turns, with 1/2" between turns, with a tap at two turns to feed the receiver. I always had a FET preamp connected instead of a low impedance tap on mine.

The typical variable cap used in old radios was 365 pF per section but not always. If you have a two gang cap and the plates of one section are larger than the other, use the bigger section. Note that the connections are between the frame and the tab of one section, not between the two tabs of the two sections. Most caps would have a built-in trimmer cap-a copper plate separated from the frame by a piece of mica, and an adjustment screw. This can be removed to get the maximum spread between min and max C.

Since you are tuning from 1000 kHz to 2000 kHz, the proper proceedure would be to add turns to bring the upper limit down closer to 1600. Since you are getting a 2:1 tuning range, you won't get 540 to 1600 directly, about a 3:1 range. Now add a switch to connect an extra fixed capacitor in parallel with the tuning cap, 270 or 300 pF should work well. You'll end up with two bands, perhaps 540-1000 with the switch on and 800-1700 with the switch off. adjust the number of turns and the size of the extra cap to set the freq coverage you need. In some cases, people had three band loops.

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