W5ALT Simple Field Strength Meter

W5ALT Simple Field Strength Meter

I was at an electronics surplus store (EPO near Clear Lake City, TX) and happened across a 3 1/2 digital panel meter at a decent price. Since I couldn't pass it up, I plunked down my $3 and bought one to play with. Later, when playing with my antennas, I needed a cheap field strength meter to check the performance. The panel meter looked like a good place to start, so I pulled it out and built a very simple field strength meter around it.

The circuit is actually very simple. The panel meter reads voltage directly with a full scale reading of 200 mV. The rest of the circuit consists of a 9 volt battery and switch to provide power for the panel meter, a small telescoping whip antenna from Radio Shack to pick up the RF, a couple of back-to-back germanium diodes (I used 1N34's that I had laying around) to rectify the signal, and a 10 kohm volume control potentiometer to adjust the sensitivity. None of the parts are critical. I mounted everything in a 5 x 2 1/2 x 1 1/2 inch project box from Radio Shack.

Parts List

  • 3 1/3 digit, 200 mV LCD Panel Meter
  • 9 v battery
  • Battery connector
  • Single pole ON/OFF switch
  • 12 inch telescoping whip antenna
  • 2 germanium diodes (1N34 or similar)
  • 10 kohm potentiometer
  • Plastic project box to hold everything

Construction consisted mainly in cutting holes in the box for the panel meter, the antenna, switch and potentiometer. I don't have a nice workshop, so everything was done using my Dremel tool. Once everything was mounted, the battery was connected to the on/off switch and the meter power connections to test. Then the diodes, potentiometer, and antenna were wired and connected to the voltage input of the meter. It worked the first try. Here's what the finished product looks like.

Operation is simple. With power applied, the rectified voltage is directly read on the meter. For strong signals, the reading will go off scale, so the sensitivity can be adjusted with the potentiometer. That's all there is to it.

All in all it was amazing how simple the field strength meter really is. It's a perfect first project for any ham who wants to check his signal strength.

73,
Walt, W5ALT