A variety of quasi-planar low-pass, bandpass, and bandstop filters suitable for millimeter-wave applications is reviewed. The emphasis is on ladder-shaped E-plane bandpass filters to highlight their advantages as well as limitations in terms of design, performance, and manufacturing. To extend their range of application it is suggested that E-plane filters be cascaded for better passband separation. A modified finline filter is presented to improve the performance and manufacturing of filters in waveguides below cutoff. It is shown that plated through-holes can simplify filter housing fabrication and that surface-metallized composite housings are a lightweight and low-cost alternative to metal housings.
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