hairpin microstrip filter

Impedance and admittance inverters are a nice way to make math work for you in filter design. Sometimes you come up with a filter structure with shunt elements where you want a series element, or vice-versa. Using J and K inverters let you transform one type of element (series/shunt) into the other, so you can use the device that's easy to implement in your filter's construction.

Start on page 3 for an example of how an inverter can make a series circuit look like a parallel circuit.
whites.sdsmt.edu/classes/ee322/class_notes/322Lecture13.pdf

See page 6 for some additional inverter examples.
home.sandiego.edu/~ekim/e194rfs01/filterek.pdf

Examples of using inverters in a filter design
http://www.ifmicrowavescouldtalk.com...design_web.htm