Simple Field Strength Meter
Circuit : Andy Collinson
Email: anc@mitedu.freeserve.co.uk
Description:
This Field Strength Meter is simple and also quite sensitive. It uses an ordinary digital voltmeter to measure RF
signal strength up to a few hundred MHz.
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.
