A simplified technique is used to design multiband bandpass filters using a compact ultrawideband (UWB) microstrip bandpass filter with spiral‐shaped λg/2 resonators, spaced apart by λg/4.Pairs of transmission zeros are generated on predefined locations in the UWB to create dual‐, triple‐, or quad‐band bandpass filters, as desired. The bandwidth and location of the sub‐bands can be easily controlled by the locations of the transmission zeros. These are generated by inserting additional resonators with predicted lengths. Highly selective quasielliptic multiband controllable responses can be achieved. Equations for fast calculations of filter dimensions are derived. Measurements show excellent agreement with the simulations. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1075–1079, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26742

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tors have also been presented. A convenient chart for filter design has been constructed using this model and the equivalent relations. A simple systematic design procedure has been established for filter designers. Finally, an example of a three-order compact filter has been designed. The new filter is much smaller than the conventional filter. This technique can also be applied to other planar structures, such as coplanar waveguide. This compact filter is very useful for MMIC design.

A novel defected ground structure, with a predominantly magnetic coupling, is designed, fabricated, and used to suppress the out‐of‐band harmonics of a class of multiband filter, characterized by very abrupt spurious response. A one‐pole lowpass filter is realized solely by making slots on the ground plane and keeping the original filter structure unchanged. The proposed structure has very minor effect on the performance of the original multiband filter, for example, on fractional bandwidths or level of S11. Measurements show the achieved suppression level of the filter spurious response to vary from 13 to 37 dB over a frequency range of about 1.5 GHz. Excellent agreements are observed between measurements and simulations within the filter bands. The filter size is maintained compact as the original. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2726–2731, 2014

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Abstract-A new class of microstrip filter structures are designed, optimized, simulated and measured for ultra-narrowband performance essential to the wireless industry applications. More accurate model of the coupling coefficient is outlined and tested for narrowband filter design. Two sample filters are fabricated and measured to verify the simulations and prove the concept. The idea behind the new designs is based on minimizing the parasitic couplings within the resonators and the inter-resonator coupling of adjacent resonators. A reduction of the overall coupling coefficient is achieved even with less resonator separation which is a major issue for compactness of such filters. The best new designs showed a simulated fractional bandwidth (F BW ) of 0.05% and 0.02% with separations of S = 0.63 mm and S = 0.45 mm, respectively. The measured filters tend to have even narrower F BW than the simulated, though its insertion loss deteriorates, possibly due to mismatch at the interface with external circuitry and poor shielding effect of the test platform. The investigated 2-pole filters are accommodated on a compact area of a nearly 0.6 cm 2 . An improvement of tens of times of order in narrowband performance is achieved compared to reported similar configuration filters and materials. A sharp selectivity and quasi-elliptic response are also demonstrated with good agreement in both simulations and measurements. In all filters, however, the study shows that the narrower the F BW , the larger the insertion loss (IL) and the worse the return loss (RL). This is confirmed by measurements.