A sensitive and reliable rf field strength meter is an invaluable instrument in amateur radio and in radio controlled model area. A field strength meter is used to align an antenna to get the best possible gain, to determine the transmitting range of radio controllers …
Designing the rf field strength meter to be sensitive it is a requirement because there must be as many wavelengths as possible between the meter and the transmitter, the measurement can be done without using a stronger carrier signal and most of the radio controllers are of low power type.
Considering these factors, a dual MOSFET is used as an RF amplifier in this circuit. The amplification can be adjusted by P1 and with S1 switch we can select three frequency ranges:
L1 – 480 kHz to 2.4 MHz
L2 – 2.4 MHz to 12 MHz
L3 – 12 MHz to 40 MHz
The antenna is made of 30 cm long metal rod.
The rf field strength meter circuit schematic
Active components: 3N140, 3SK35, 3SK37