This powerful transmitter for the 27 MHz band was found in a Japanese publication from 1961. The circuit is powered by battery and has the details of the coils given next to the diagram.
This circuit for frequencies from 1Hz to 100 kHz was found in the June 972 Radio Electronics...
This circuit generates short pulses. The transistor can be a 2N2222 or BC548 and the frequency is given by...
We found this filter in a Popular Electronics from 1993. The operational amplifier can be the 741....