QRPGuys Digital Field Strength Meter – $30
The field strength meter is one of ham radio’s earliest diagnostic tools that can be used in a variety of tasks inside the shack and out in the field, from detecting rf leaks in coax cables, testing antenna improvements, and aid in determining antenna emission patterns. The QRPGuys Digital Field Strength Meter can detect RF energy over the VLF-500MHz range with sensitivities from -80dBm to +10dBm. It uses the popular Analog Devices AD8307 logarithmic detector/amplifier, used in many popular VNA’s, coupled with an on-board digital voltmeter that has been modified to compare small signal changes of about 0.7dB, and display a relative digital reading. We also see use of this device as an RF probe for troubleshooting signal flow thru circuits. The sensitivity is equal to our retired RF Probe, and the BNC input jack is the input point to connect a scope probe for this use. It is powered by a 9 volt battery (not supplied) attached directly to the pcb, with a continuous “on” switch, as well as a “momentary” pushbutton switch for spot checking the presence of RF. There is a potentiometer to adjust the amplitude of the RF sampled, an SMA connector with antenna, and a female pcb mounted BNC for connection to longer antennas, probes or other test inputs. It does contain one SMT IC component (AD8307), but all the other components are through hole. The pcb is small, 2.50” x 2.75” (63.5mm x 69.9mm) and can be used without a chassis. On a difficulty scale of 1 to 5, 5 being the most difficult, this is rated at 3 to 4. Allow an hour or so to build, depending on your experience.