Compact metamaterial zeroth-order resonator bandpass filter for a UHF band and its stopband improvement by transmission zeros

A microstrip bandpass filter for a UHF 900 MHz band is significantly miniaturised and its performance is enhanced using metamaterial (MTM) zeroth order resonator (ZOR) characteristics. Firstly, the authors propose that each of the resonators for a bandpass filter is implemented by a one-cell ZOR, and the coupling between neighbouring different ZORs is controlled to meet the primary goal on the bandpass filtering in the form of a sequential coupling mechanism. Secondly, this MTM ZOR filter is suggested to have transmission zeros (TZs) owing to source load coupling to improve the isolation from other wireless communication bands. The first and second proposed filters are less than one-sixth of the conventional filters in terms of size, and they present lower insertion loss and return loss in the passband and higher isolation in the stopband owing to the TZs, as verified by simulation and measurement. In addition, the MTM properties are verified with a dispersion diagram and the no-phase variation electric field distribution over the ZOR by a 3D EM simulation.