This is a simple HF-probe. The guts of it are two condensators, and two diodes forming together a simple voltage doubler. This was build already some time ago, I guess somewhere in the middle 1990's I used the circuit as published in the now gone Elex magazine. Two diodes of the type AA112 and two condensators of 1nF. Cutting the PCB-board and fitting it into the enclosures did probably cost me most of the time.
Lately, I made a small change; the two wires going to the DVM (digital volt meter) were replaced with shielded cable so it didn't pick up any hum.
As you might guess sensitivity at lower voltages is limited but once the input voltage passes the diode threshold voltage (~200mV for Germanium), it's OK.