RF field detector electronic project

An RF field detector that can be used in measurement and verification stage power antenna and transmitters.
This detector is made with a radio frequency transistor type MOS-FET with two gates. Gain factor is set by the potentiometer P1. S2 switch allows selecting one of three ranges: 480 kHz ... 2.4 MHz (L1), 2.4 ... 12 MHz (L2) and 12 ... 40 MHz (L3).
A telescopic antenna with a length of 30 cm is sufficient for the antenna.