Using an operational amplifier, this circuit from a 90s Popular Electronics, is characterized by its high sensitivity. R5 can be added to control the gain.
This traditional doubler configuration with the 555 IC can be found in other articles on the...
This circuit is from an 80's component manual. The components are from the time, but the circuit...
This circuit for the 3.5 MHz band (80 meters) was found in an old radio amateur documentation. The circuit...