An RF field strength meter is possibly the easiest piece of test equipment one can build. It can be used as a building block to other more versatile forms of instrumentation. Add an amplifier for increased sensitivity. Include a tuned circuit and it will detect signals in a particular band. Convert it into an RF watt meter and use it as a dummy load to check out your rig's operation. This simple circuit has few parts and needs no PC board. Solder components together and house in a plastic box with a toy walkie-talkie aerial.
Detecting the Presence of an RF Field
This field strength meter is not made to make precise measurements. It is used to check transmitters, experiment with antennas and test RF oscillators. The meter is like a crystal set and needs no external power. But unlike a crystal radio, it is not tuned. The circuit works by converting any RF signal at the antenna to a DC voltage. This voltage then is indicated on the uA meter. A sensitivity control is included and becomes necessary at close range to the RF source.
The Homemade FS Meter Build Out and Testing
A meter from an old radio or tape system can be used directly from your spare parts box. The diodes can be any geranium type rectifiers. Do not worry about component values. Even a 50 percent variation will have a small effect on the operation. Dig deep into the parts box and save a few dollars on a valuable tool for one's instrumentation arsenal. The schematic indicates two diodes, three capacitors, a uA meter and a potentiometer.
Place the field strength meter's antenna close to a handheld VHF transmitter. Your meter should indicate the presence of an RF signal. If it does not workl, check diode directions and make sure they are not silicon diodes. They may work but at reduced sensitivity. The earth wire clip is optional.
Other circuit modifications mentioned here may be found in standard handbooks like ARRL and others. Related Home brew equipment is listed in QSL Magazine (See Resource).
- multimeter image by Aleksey Bakaleev from Fotolia.com