A Common-Resonator Filtering Hybrid With a Wide Stopband and Compact Size

In this brief, a compact filtering microstrip hybrid with a wide stopband is proposed. The filtering hybrid is designed with 2nd order filtering performance with only three resonators in total. The sum and difference channels share a common centre-shorted open-loop resonator (CSOLR). The even and odd mode resonances are adjusted to be the same by deliberately designed short vias and open-loop structure. By splitting the even and odd modes of the CSOLR, the shared CSOLR provides two inherently isolated channels for the hybrid. This characteristic not only makes sure the high isolation between the sum and difference channels but also leads to a very compact structure. Besides, because different modes are involved for both channels, a wide non-radiative stopband up to
$5.45f_{0}$
can be observed for the proposed hybrid. To verify the concept, a second-order filtering hybrid is designed, fabricated and measured. The detailed synthesis procedure is provided in this brief. The measured result demonstrates two well-isolated channels and a stopband up to
$5.45f_{0}$
with a minimum rejection level of 20.7 dB.