Sensitive Field Strength Meter

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.