The sensivity of most moderately-priced AM receivers is poor in the mediumwave band. worse yet, selectivity also drops otf sharply within the upper part
or tn" band, making ine receiv-er highly susceptible to images, noise and
annoying heterodYnes.
Even the best antennand a good booster amplifier can't make up for these
deficiencies in a common receive/s front end. But, using an RF preselector
b;trd; the antennand the receiver's antenna inputerminals will give you
real increase in the reception quality.
This circuit matches the antenna impedance with the receive/s antenna input
according the frequency of the tuned station'
Figure I shows schematic diagram of the Medium-wave Preselector'
Figure2 shows an exploded view of the circuit, which can be housed in a
small plastic box.
Ll is formed by turns of No.28 enameled wire, wound on a 5/16-inch ferrite
form. Taps are made each 15 or 20 turns and its number is not critical' The
number of taPs can be altered.
Attach your antenna and ground to the input terminals of the preselector'
Tune to the desired station on your receiver, and, by choosing the best.position of the alligator clip on the itrip anO by adjusting CV find the best signal'
The required antenna will depend upon the station you want to tune' A random length of wiie will give iairly good performance through the A{ band'
Antennas with lengths b-etween tsano 150 feet are recommended for best
performanc
Parts
L1
CV
X1
- Medium-Wave Coil - see text
- 365 pF miniature variable capacitor
- Alligator clip
- 2-position barrier striP
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