Schematics > RF circuit > Field Strength Meter
Field Strength MeterDescripton
This is a wide band signal strength meter circuit which responds to small
changes in RF energy, designed to be used for the VHF spectrum and will respond
to AM or FM modulation or just a plain carrier wave.
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
Source: unknown
Published on: 2006-01-15
Reads: 486
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