Design of three-dimensional filtering balun with inherent complete ground and enhanced stopband rejection
A novel design approach of three-dimensional filtering balun with inherent complete ground and enhanced stopband rejection is proposed. The whole structure consists of a horizontal feeding plane, a vertically mounted plane, and three ports. By arranging the double-sided parallel-strip line filter symmetrically on the two sides of the vertical plane, the frequency selectivity and complete ground are effectively synthesised. By inserting a rectangular conductor plane with two crossed slots into the middle of the vertical plane, the balanced-to-unbalanced conversion and stopband rejection are significantly enhanced. For demonstration, an experimental filtering balun is fabricated. The measurement agrees well with the simulation, achieving a wide upper stopband under −15 dB from 3.77 to 9.74 GHz.
