Microwave Filters
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.
Interdigital Filters
A series of quarter wave 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.
Capacitive Gap Filters
Suitable for narrowband filters, capacitive gaps are used as resonator-invertor elements.