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

On Wed, 29 Dec 2004 09:46:57 -0600, (Richard
Harrison) wrote:


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.

So, if I took the 'rinky-dink' plastic loop antenna outside and placed
it above the roof, it would work? Do I have to have a pair of wires
for a lead-in or would just one do?

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.

I suppose an old car antenna would work for the FM if I placed it up
high, also?

Ground to the radio is probably one of
the "rinky-dinky loop" wires.

I don't see any place to connect a ground.

If not, capacitive coupling via aluminum
foil or the like will likely do.

This I don't understand.

--
The Seabat

Seabat wrote:
"So, if I took the "rinky-dink" plastic loop antenna outside and placed
it abive the roof, it would work?"

Most likely. Seabat said his car radio worked on the site. An a-c radio
should normally be sensitive enough to operate on the same site.

An AM radio antenna does not need to be st high altitude as medium waves
follow the earth`s curvature as they propagate in most cases. The
existing metal roof in combination with the earth itself form a sandwich
with the loop antenna in the middle. This has proved unsatisfactory.
Seabat has "The Tin Riof Blues". It`s only necessary to get the AM
antenna in effect out from under the metal roof.

I suspect that just extending loop wires would be fraught with problems.
A variable capacitance in the radio likely tunes the loop. This loop in
parallel with the capacitance makes a high impedance parallel-resonant
circuit. Its impedance rises to about Q times the inductive reactance.
This circuit is susceptible to upset due to stray capacitance from any
long high-Z feeders. That`s why I proposed two additional loops to
capture the signal and to transfer it to the radio. A tuned loop in the
clear would capture in the signal, and the untuned loop would transfer
the signal to the radio`s "rinky-dinky" plastic antenna loop on the
radio shelf. These two loops would be interconnected with a twisted-pair
of wires.

Seabat also wrote:
"I suppose an pld car antenna would work for the FM if placed up high,
also?"

Yes. It`s exactly what I`ve long used as a 2-meter antenna. It is
collapsed telescopically to about 19 inches. I added (4) equally spaced,
drooping by 45-degrees, radials under the vertical radiator. They are
also 19 inches long. With full forward power into the antenna, the
reflected power is very low. The rig talks and listens very well.

For the FM broadcast band the elements would need to be lengthened to
almost 30 inches.

Construction details are given in books such as the "ARRL Antenna Book"

Best regards, Richard Harrison, KB5WZI