Capacitive iris bandpass filters with spurious harmonic modes suppression

A new design of capacitively coupled bandpass filters, that provides suppression of spurious passbands for the dominant and second waveguide modes without any extension of the filter dimensions, keeping the same return loss within the passband, is presented. Spurious passband suppression is provided by cutting an additional pair of slots into the iris strips . Cutting the additional slots leads to the appearance of attenuation poles, which are placed in the spurious passbands. The k-inverter of the corresponding two-side double-slot iris changes slightly and may be easily fine-tuned. Numerical examples are presented for the three-section 5% filter in a WR90 waveguide.