Coplanar waveguide filters with floating strips

Coplanar waveguide edge-coupled filters with floating conductor strips directly below the coupled-line sections are investigated. It is demonstrated that tight coupling can easily be implemented in this way. A 50% bandpass filter with centre frequency at 3 GHz was studied. The filter used four coupled-line sections. The floating conductor strips were incorporated into the design and the authors have been able to achieve very similar even and odd mode phase velocities, so that the filter is spurious-free at twice the basic passband range. The measured passband insertion loss is <1 dB. It also has a very good stopband performance