Physically a cavity filter is a resonator inside a conducting "box" with coupling loops at the input and output, as follows:
Cavities are usually constructed as cylinders, with an axial tuning capacitor
In this diagram a tuned circuit is formed by L1 and C1 which are the cavity body and a "central" tuning capacitor and a tunable series LC coupling circuit using L2 and C2.
What does a cavity filter do?
A cavity filter has two functions:
- to pass the desired frequency
- to reject as much of the undesired frequencies as possible
Cavities are often grouped in series with each other to increase filter effectiveness by making the pass band "deeper" with respect to surrounding frequencies. This can be very useful when ham repeaters are situated vey close to other spectrum users such as pager whose nwanted signals can interfere with the ham equipment. Cavity filter are a very effective way to create a "notch" at thr repeater frequencies.
Why use a cavity when other tuned circuits can do the job?
Other tuned circuits can do the job but not as effectively as a cavity. The "Q" of a tuned circuit is a measure of how sharply it attenuates unwanted signals. Simple, component based tuned circuits do not have a high enough Q for repeater use, particularly in shared positions as described above
What is a duplexer?
A duplexer is a group of filters linked together so that the two antenna connections on a repeater can be linked to the same antenna, at different frequencies,
Can other filters be used for duplexers?
Yes, the Hybrid Ring filter is used for this purpose, but rarely in amateur installations.