Size reduction and spurious suppression in microstrip coupled line bandpass filters by means of capacitive electromagnetic bandgaps

This paper is focused on the application of electromagnetic bandgaps based on capacitive loading to the implementation of microstrip coupled line bandpass filters with reduced size and spurious suppression. Size reduction is due to the slow-wave effect caused by the loading capacitances of the different coupled lines, whereas spurious suppression is related to periodicity. By properly designing the capacitively-loaded coupled line sections of the filter, implemented by means of square patches in practice, it is possible to significantly reduce filter size and simultaneously achieve spurious cancellation. As an example, an order-3 Chebyshev bandpass filter with 30% length reduction (as compared to the conventional counterpart) and spurious rejection up to the third harmonic is reported.