A modified circle dual-mode bandpass filter with tuning slot

One circle-type patch and two circle-type patches with square slot on Al/sub 2/O/sub 3/ substrate are proposed, simulated, fabricated, and measured for planar dual-mode bandpass filters with microwave frequency design. The proposed structures consist of 50 /spl Omega/ microstrip lines, one for input and another for output. The two 50 /spl Omega/ microstrip lines are connected to the central of adjacent sides of the patch with two notches, and the patch will act as the elliptic-function bandpass filters. Our measured results show that the Al/sub 2/O/sub 3/ substrate presents a number of attractive features for practical application.