Simple Field Strength Meter

Circuit : Andy Collinson
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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.

simple fsm Parts List
1 OA91 Germanium Diode
1 3.3M Resistor
C1 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.

Modification for other Frequencies
To use the circuit at other frequencies, it is only necessary to change the values of L1 and C!. In fact if C1 is made a variable capacitor then the circuit can be varied to operate over a particular frequency range. For convenience the turns calculator and resonant frequency are shown below. Other calculators on my Tuned Circuit page.

Resonant Frequency Calculator
Capacitance:
Inductance:
Resonant Frequency:


Turns Calculator
Diameter :
Length :
Inductance (µH) :  Turns 
External Diameter

For use on 80 Meters.
Change C1 to a 47pF variaable capacitor and add a parallel capacitor of 15pF
Change L1 to 40uH
To make L1 wind 63 turns of 24swg wire around a 2 cm plastic pipe and spread the turns over 3 cm. This covers approximately 3.1 to 5.6MHz.

For use on 40 Meters.
Change C1 to a 47pF variaable capacitor and add a parallel capacitor of 33pF
Change L1 to 8uH
To make L1 wind 45 turns of 30swg wire around a 8mm plastic former such as a pen spaced about 1cm. This covers approximately 6.9 to 9.1MHz.

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