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Circuits > RF circuit > Field Strength Meter Field Strength MeterDescripton
Field Strength Meter circuit Notes:This circuit measures radio field strength by converting the signal to DC and amplifying it. This field strength meter was designed for VHF frequencies in the range 80 -110 MHz.The inductor L1 is 4 to 6 turns of 20swg wire air spaced wound on a quarter inch former or similar. Alternatively an inductor of value 0.15 - 0.35uH will suffice. Sensitivity is not as good as I would have liked, but a small 9 volt battery transmitter will deflect the meters needle from a distance of up to two feet from the FSM. Higher power transmitters give higher signal strength readings and of course from much further away.The meter used was a signal meter with FSD of 250uA. Lower FSD meters will offer greater sensitivity. The FET used in this circuit is a general purpose 2N3819. A small telescopic whip antenna is used for signal pickup. The 10k preset resistor is used to adjust bias of the FET circuit; with no transmitter present the meter reading is zero, adjust preset if not. The RF signal, whether modulated or just a plain carrier, is rectified and converted to DC by the diode,capacitor and 3.3M resistor. This small DC voltage just enough to upset the bias of the circuit and hence cause a deflection of the meter.
Title: Field Strength Meter |
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