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