This circuit was found in an ancient American documentation. With it is possible to double the frequency of a transmitter For the range of 6 meters obtaining 8 W. The circuit makes use of varicap.
This configuration using the four transconductance operational amplifiers of an LM3900 was found...
This alarm, from a 1996 magazine, can be fitted with two BC548s in the original configuration...
This circuit was found in a 1973 magazine but can still be assembled using modern equivalent...