Microwave filters are distributed, periodic in frequency response and subject to large parasitic effects. Design can be tricky
with a good dosage of black magic.
A series of quarterwave parallel coupled lines (often rods or bars) arranged to construct a bandpass filter. The lines are shorted at alternative ends
and capacitivly loaded at the other.
Parallel Coupled Line Filters
Commonly used on microstrip or stripline to produce bandpass filters. Cheap, easy to produce, subject to spurious responses.