Design of planar diplexers with improved isolation using the tunable transmission zeros of a dual-mode cavity filter

A substrate integrated waveguide dual-mode single cavity filter with two inclined slots on top metal plane provides two transmission zeros. It is shown that the feed offset as well as the slot offset both can be used to tune the zeros over a wide range of frequencies. Leakage loss from the slot is negligibly small. Two bandpass filters are fabricated to validate the predicted zero variation. Further, tunability of the zeros is utilised to greatly improve the isolation characteristics of two X-band planar diplexers. Measurement results show that the isolation is better than 30dB even if the channel separation is as close as 5% of the average frequency. The effect of packaging on the filter characteristics is also presented.