At present, it is very important for transceiver subsystems to be in a planar format with a maximum number of circuits functions on a single substrate. There are two general approaches for this problem: hybrid and monolitic. The hybrid represents the "now" approach, whereas the monolitic must be considered the approach of the future. Discribing rejection filter is a component of hybrid technology, it has two Impor- tant advantages, namely higher attenuation level and a simple construc- tion as comiered with previosly known one .This has been achived by usi a comoination of two transmission lines: an insulated image guide (IIG)ng and microstrip.
S. A. Pogarsky, S. A. Pogarsky,
"Rejection filter", Proc. SPIE 2250, International Conference on Millimeter and Submillimeter Waves and Applications 1994, 22508E (30 November 2017); doi: 10.1117/12.2303319; https://doi.org/10.1117/12.2303319