A Miniature Dual-Mode Bandpass Filter Using Al$_{2}{\hbox{O}}_{3}$ Substrate

A dual-mode bandpass filter using the degenerate modes of a modified square ring structure printed on the high-permittivity substrate is proposed. The dual-mode resonator is based on a square ring loaded with a cross-strip. The filter is printed on the Al
₂
O
₃
substrate with a permittivity of 9.8 and could be easily integrated with radio frequency circuits. Besides its simple structure, the filter shows low insertion loss and small size, owing to the high-permittivity substrate used. The proposed filter shows a size reduction of 44% compared with the case using a simple cross-strip, and it also provides transmission zeros on both sides of the bandwidth. Experimental results are in good agreement with simulated values.