Active Antenna With Gain
Posted on Apr 10, 2012 3208
The signal booster, built around a few transistors and support components, offers an RF gain of about 12 to 18 dB
(from about 100 kHz to over 30 MHz). The RF signal is direct-coupled from Ql"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 the receiver"s RF-input circuitry. Inductor LI is used to keep any power source noise from reaching the FET (Ql) and any value of RF choke from 0.5 to 2.5 mH will do. The value of R2 sets the Q2 bias at about 2 V. If the voltage is less than 2 V, increase the value of R2 to 1.5 KOhmhm. To go below 100 kHz (to the bottom of the RF spectrum), increase the value of CI to 0.002uf. The antenna is a short pull-up type (42" to 86" long).