Simple Field Strength Meter

By Andy Collinson

Simple Field Strength Meter circuit

Parts List

1 OA91 Germanium Diode
1 3.3M Resistor
1 100p Capacitor
L1 7 turns on a 1/4 inch former (suitable for around 100MHz)

Notes

The multimeter should be set to the lowest dc volts range for maximum sensitivity. This is normally 200mV DC for most meters. The circuit works well at VHF (around 100MHz) and was quite pleased with the results. L1 was 7 turns on a quarter inch former with ferrite slug. This covered the UK FM band. A digital multimeter, as opposed to an analogue signal meter offers several advantages in this circuit. First, the impedance of a digital meter is very high, around 10Meg/Volt on most meters. This does not shunt the tank circuit unduly. Second, compared to an analogue meter, very slight differences in signal strength can be more easily observed. Thirdly,a digital meter will have better linearity, responding well to both weak and stronger signals.

1841 19 September 2009

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