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Shielded-Loop AM Antenna
Oct 11, 2001 12:00 PM
AM reception can be very difficult because of the inherent
succeptibility AM has to electrical noise and other sources of
interference. The built-in antenna in most receivers - a coil and a
ferrite rod - is not sufficient for clear reception. An external
antenna can usually greatly improve reception.
The loop antenna is a proven design. There are commercially produced
models available, but, depending on the use, it is sometimes hard to
justify the cost of the antenna.
What follows is a copy of a schematic I originally discovered around
1994, and I am told that it is from the Motorola C-Quam AM Stereo
bulletin, issue 3, from 1989.
No guarantee as to the suitability or accuracy of this schematic is
made or implied for its use or any undesired consequences resulting
from its use.
Note: I have also made an antenna like this using three individual runs
of a single-pair, twisted, shielded cable, such as Belden 9451 in place
of the 8777.
Chriss Scherer
editor, Radio magazine
The original text follows.
AM Loop Antenna for AM Reception
We've been asked to reprint this passive loop antenna design,
offered by a C-QUAM station engineer in one of our previous bulletins.
The loop self resonates approximately in the middle of the AM band, and
by adding or subtracting turns it can be optimized higher or lower in
the band.
Instructions for assembly:
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1. Cut a seven-foot length of Belden 8777 or similar three-pair foil
shielded audio cable.
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2. Strip back two inches of outer jacket and foil from both ends.
Cut the shield wire on the right-hand end of the cable, and tape or
heat shrink the end to keep the cut-off ends from touching
anything.
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3. Wire the cable as shown above. We used six feet of RG-59/U to
connect to the receiver.
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4. Heat shrink or tape all connections.
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5. The plane of the loop should be pointed toward your station. It
isn't necessary to support it as a geometrical figure, although a
circle would represent the greatest area.
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