BAND REJECT FILTERS

A band reject filter is designed to reject a narrow range of frequencies while allowing higher and lower frequencies to pass through with minimum attenuation. There are different ways to design a band reject filter which can affect the bandwidth (generally defined at the -3 dB power points), insertion loss at the desired frequencies, and of course the attenuation level of signals falling inside the designed rejection frequency.

A band reject filter might be used for 144 MHz overload of a local TV set at the neighbors. Here you would certainly want to attenuate the 144 MHz signal but not affect the TV reception any more then you need to. Since the Amateur two meter band (144-148 MHz) falls in between TV channels 6 & 7, a band reject filter could be used. In this way, TV channels 2 through 6 and 7 and above would pass through with minimum attenuation while the 144 MHz signal would be subjected to a high level off attenuation.

The ARRL Handbook is a great source of information on bandpass filters. You can also refer to many other ARRL and RSGB publications for additional information.