This circuit is from an old Popular Electronics, but it can be easily implemented nowadays, even adapted to act as a shield. The circuit has the frequency given by C1 and adjusted in R1.
This line protector circuit was found in a 2002 documentation, but it is totally current and...
This antenna for the frequencies 146 or 440 MHz was found in a magazine 73 for Amateur Radio of...
With this circuit it is possible to increase the output voltage of a 7812 regulator. The circuit...