QRPGuys Digital RF Probe – $25
The QRPGuys Digital RF Probe provides a handy self-contained diagnostic tool for sensing the relative strength of RF in an active circuit. With a wide dynamic range of over 80dBm it will detect RF down to -80dBm (.01nW) to a maximum of +10dBm (10mW). 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 displays a relative digital reading. We have incorporated a selectable low pass filter for adjacent rf signal overloading and increased low signal response. The AD8307 is powered by two common CR2032 lithium coin batteries (not supplied), and is active only when taking a reading. This is an SMD project that uses seventeen 1206 size components, an AD8307 SOIC IC, and LM358 SOIC Operational Amplifier. This kit is not recommended for first time SMT builders. We assume you know how to assemble SMT components. The required tools for assembly are a soldering iron with a small tip, .02” or smaller diameter rosin core solder, small side cutters, needle nose pliers, small Phillips screwdriver, and can be built in about an hour or so. On a difficulty scale of 1 to 5, 5 being the most difficult, this is rated at 5.
Many thanks to Chuck Adams (K7QO) for his advice on this project.