HIGH PASS FILTERS

A high pass filter is designed to pass frequencies above its design "cutoff" frequency while attenuating frequencies below its cutoff. There are different ways to design a highpass 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 below of the cutoff frequency.

An example of using a highpass filter might be the need to allow your 432 MHz signal to pass through the filter with minimum attenuation while reducing the Local Oscillator signal that the IF signal is mixed with. If you are mixing (for example) and 50 MHz SSB signal with a 382 MHz local oscillator signal then all you want to reach the antenna is the 432 MHz signal, not the 382 MHz or the 50 MHz signal. A highpass filter would allow this to be easily handled.

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.