The signal booster,
built around a few transistors and support components, offers an RF gain of about 12 to
18dB (from about 100kHz to over 30 MHz).
The RF signal is direct-coupled from Q1's source
terminal to the base of Q2, which is configured as a voltage amplifier. The output of Q2
is then direct-coupled to the base of Q3 (configured as an emitter- follower amplifier).
Transistor Q3 is used to match and isolate the gain stage from receiver's RF-input
circuitry.
Inductor L1 is used to keep any power source noise
from reaching the FET(Q1) and any value of RF choke 0.5 to 2.5 mH will do. The value of R2
sets the Q2 bias at about 2V. If the voltage is less than 2V, increase the value of R2 to
1.5k ohm. To go below 100kHz (to bottom of the RF spectrum), increase the value of C1 to
0.002uF. The antenna is short pull-up type (42" to 86" long). (Reprinted with
permission from Popular Electronics, 11/90, p. 85, Copyright Gernsback Publications, Inc.)
From figure FD1-1 of Encyclopedia of Electronic Circuits on CD-ROM Vol. 1.
