Those inexpensive portable Public Service Radios do a fine job of receiving police and fire calls. But if you are not tuned precisely to the center channel of the base station, it’s more than likely you don’t hear the much weaker signals from the mobiles. With an FM Tuning Meter hooked onto your public service inhaler, you’ll be able to copy any signal that can fight it’s way into the antenna terminals. The schematic shows a detector circuit common to most public service portables. Resistor Rx and Capacitor Cx form the de-emphasis network. Connect one end of sensitivity control R1 to the junction of Rx and Cx as shown. Meter M1 is the zero center miniature type; one rated either 50 or 100 microamperes will be satisfactory. Adjust R1 so the meter pointer is not driven offscale when the signal is detuned to the extreme of the sidebands. Proper tuning is then indicated when the meter shows center scale.
Parts List For FM Tuning Meter
M1 – 50 or 100 microAmpere meter, zero center
R1 – 1 megohm miniature potentiometer
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