LOWPASS FILTERS

A low pass filter is designed to allow a certain frequency to pass through with minimum attenuation while providing substantial attenuation ot frequencies abovie its design frequency. There are different ways to design a lowpass filter which can affect the bandwidth (generally defined at the -3 dB power points), insertion loss at the desired frequency, and of course the attenuation level of signals falling above the passband frequency.

An example of using a lowpass filter might be for an 50 MHz transmitter. You want to be able to get as much of your 6 meter signal to the antenna yet not transmit a strong second harmonic at 100 MHz. The lowpass filter would then be deisgned to pass the 50 MHz signal with minimum attenuation while providing a substantial amount of attenuation to the harmonics.

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.