Field Strength Meter (FSM)

 

Suitable for AM/FM from 2Mhz - 1Ghz

Peak reception: 3-200Mhz

 

 

 

Ever needed to know just how much radiated power you've got from your CB/Ham radio antenna?

 

Well here's a simple rf detector circuit than you can easily modify and calibrate to your own needs!

This circuit has proven itself to receive a very wide band and be fairly sensitive. It has been used to tune various antennas (some home made) on a Ham radio (of 3Mhz), detect celular phone signals, receive local fm radio and test serial radio modem ic's!

 

The design is partly based upon crystal radio receivers and partly on experimental design (seeing what works!).

 

 

Parts:

 

- LM358 (or others) Voltage Comparator

 

- OA91 (the heart of the circuit), You can also use any other germanium diode here. I've also found very good ones inside television/video recorder rf demodulators. Open the metal case, look near the antenna socket and find a small glass encapsulated diode. You'll know it's the right one, when you look inside the glass case you can see a very thin metal wire that is bent in a zig-zag shape.

 

- Speaker (if you want to hear the sound being received

 

- An analogue meter with a full swing of 200mV (with numbers on the dial if you want to calibrate it). You can add a nice backlight behind the display too if you glue a white led to it.

 

- A switch (to change between speaker and meter

 

 

- A case (metal or plastic) and a PP3 9V battery

 

- An extendable antenna (if required, wire could be used), it should be a metal telescopic type (the ones seen on portable radios) of about 30cm.

 

- A couple of capacitors/resistors/potentiometers (see schematic)

 

 

Schematic:

 

 

Calibration:

Place the finished meter near a source of rf and monitor pin 5 of the LM358 Op-Amp on an oscilloscope and watch the peak-peak voltage that is being received. Zero the Signal meter when the required sensitivity is observed.
Next adjust the gain control to a desired position. Be aware that the gain control adjustments will effect the meter zero point. The gain potentiometer is very sensitive to adjustments and will require a “delicate” touch.

 

Please do not reproduce anything contained within my website, as it maybe hazardous to your health unless you fully understand what you are doing.

I cannot be held responsible. This website is copyright.

© Oliver Hunt 2006-2011