Characteristics of Bilaterally Metal-Loaded Tri-Plate Strip Line Filter at Millimeter-Wavelength

Although dielectric-supported tri-plate strip lines are usually used at centimeter frequencies, they suffer from a lot of transmission loss due to their rough under-surfaces of metal strips. And moreover, undesired radiation occurs in circuit elements such as a band-pass filter due to their asymmetrical structures. In this paper, we solved this problem by making center conductors on both surfaces of the dielectric substrate and designed a three-pole 0.1 dB ripple Chebyshev band-pass filter by using such guiding structure, termed bilaterally metal-loaded tri-plate strip line. It was obvious that the transmission characteristics of the bilaterally metal-loaded tri-plate strip line was unaffected by the skin effect of the under surface of the metal strips, because the two center conductors were the same voltage, and hence, the low-loss nature. Insertion loss of the designed band-pass filter was small due to its low-loss nature as well as no undesired radiation.