Parallel Plate Mode Suppression in Low-Frequency Microwave Circuit Packages Using Lid of 3-D Cross by a 3-D Printing Technique

A novel lid of three-dimensional (3-D) cross is proposed for parallel plate mode suppression in low-frequency microstrip circuit packages. The dispersion analysis of the unit cell of the periodic 3-D crosses between parallel metal planes shows that its stopband fractional bandwidth is increased to 75.4% and shifts toward lower frequencies. The electric size of the unit cell is also reduced to 0.21λ0 × 0.21λ0 × 0.18λ0. Moreover, a two-dimensional equivalent circuit model is proposed to give a physical insight. Finally, a 3-D printing technique is used to print the lid of a 3-D cross prototype and a simple microstrip line is fabricated for testing its parallel plate mode suppression performance. The measured results show that the parallel plate mode is suppressed in the stopband, and the insertion loss of the microstrip line is similar to that of the unpackaged one.