Transistor Q1-configured as a source-follower buffer stage, offering a bit under unity voltage gain--gives the unit a high-impedance input of about 1 M ohm shunted by about 10 pF, which keeps only minimal loading on the equipment being tested. C1 serves as input dc blocking capacitor.

The Circuit Diagram of RF Probe

The Q1 output is coupled by C2 to a simple AM detector circuit made up of D1, D2, R3 and C3. Capacitor C4 provides output dc blocking. Total current consumption should be somewhere around 1 mA. The circuit responds to frequencies from 100 kHz to well over 50 MHz.