In this paper, the design of a planar ultra-broadband sum-and-difference network is pre-sented. This network employs a novel power divider and anti-phase balun as a building block. An equivalent 180o coupler with a bandwidth of 6.2～14GHz is achieved by back-connecting the power divider and balun together. Four such couplers are connected to form an ultra-broadband sum-and-difference network which has a bandwidth of 91%. This network, with insertion loss less than 1.8dB in sum port and nulls less than -20dB in all delta ports, is verified to be excellent, resulting in the advantages of being compact, easy manufacturing and low cost.
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