Alternative Solutions for Reducing the Undesired Coupling Effect in Stub Loaded Microstrip Filters for Ka-band Applications

This work characterizes the undesired coupling effect that appears in stub loaded microstrip Ka-band filters due to the propagation of surface waves. This effect limits the achievable isolation level in the stopband of this type of filters. To overcome this limitation, we propose two solutions: alternate the direction of the resonator stubs and implement the filter following a bent configuration. To validate these solutions in practical microstrip devices, four stopband filters in Ka-band are designed, manufactured and measured. The measurements demonstrate that these two strategies allow to improve the selectivity in the stopband, therefore improving the isolation level in stub loaded microstrip filters.