the temperature even among the three transistors (to minimize thermal drift), it protects
the components from physical damage, and it holds the battery holder on the board.
As I used this probe last nigh to determine if a 384 MHz oscillator was really working or not, I remembered email I received a while ago, asking how to make a field strength indicator without the microcontroller. Thus this page. If you want the auto zero version, which is this circuit with an auto-zero integrated circuit, use the circuit shown on this page for details.
This broad band probe has a small antenna (about a 15 cm length of insulated wire). Radio Frequency energy coupled to the antenna is detected and made available to drive millivolt level signals to the input of a DVM (Digital Volt Meter). Its battery powered for convenience with very low current drain and automatic shutdown for long battery life.
The MPSH34 has two things going for it: low input
capacitance, and I have a lot of them on hand.