Stopband optimised E-plane filters with multiple metal inserts of variable number per coupling element

Improved stopband attenuation of E-plane filters is achieved by replacing merely a few of the conventional single insert coupling elements by triplets or quintuplets of metal inserts. The method of field expansion into suitable eigenmodes used considers the effects of finite insert thickness and the mutual higher order mode interaction of all inserts. Calculated stopband attenuations of more than 50 dB up to about 30 GHz, or 60 GHz respectively, have been attained, although the passbands of the corresponding filter examples are chosen to operate at the upper waveguide band limit of the mount (at 18 GHz for Ku-band and at 40 GHz for Ka-band respectively). The results demonstrate that high stopband conditions can be satisfied by designs with only a modified number of multiple inserts. This helps to maintain the low-loss and low-cost qualities of E-plane filters and also provides for improved stopband demands. The thickness of all inserts of t = 190 µm makes possible accurate metal-etching fabrication.