High selectivity X-band planar diplexer with symmetrical box-section filters

Coupled resonator filters with asymmetrical frequency response usually require couplings with opposite signs. This is the case of box-section filters, where one negative sign is needed in order to generate a phase inversion of the signal. An alternative consists of introducing the phase shift not in the couplings, but in resonators, allowing symmetrical geometries with only one kind of coupling. The resulting modified half-wavelength, parallel-coupled microstrip filters are used as building blocks for a X-band diplexer with contiguous pass bands, where the asymmetry of the frequency response is exploited in order to increase the selectivity between channels.