Found in a publication by Gernsback from 1960, this circuit uses germanium transistors of the time and commercial coils. It can be changed to operate with modern transistors and other wavebands.
The figure shows how to implement an alarm buzzer in an LM3914 to warn about crossing the scale...
This circuit can act as a shield being controlled by a microcontroller by pin 1 or 2. The...
This circuit was obtained from a 1980's QST magazine. The circuit can generate 1 to 15 MHz signals...