Microstrip filter with slot resonators in ground plane

V.G. Krizhanovski

Donetsk National University

The development of contemporary SHF systems calls for a broad nomenclature of frequency-selective devices, band-pass filters in particular. One of the possible directions of filter design is the usage of electromagnetic-bandgap (EBG) structures, which can be implemented in the form of certain slots in a microstrip line’s ground plane. It was previously proposed to use these structures as parts of output matching networks of high-efficiency SHF amplifiers built according to harmonic-manipulation approach. The use of the mentioned structures would be more effective if they themselves could perform as matching network.
By utilizing the results of theoretical and experimental research of harmonic filters built on microstrip line’s ground plane slots, let’s consider design of networks that are capable of performing other relevant function in addition to that of band-reject filters. The goal of the current work is to investigate the possibility of application of the microstrip line’s ground plane slot resonators (band-reject filters) to the design of band-pass filters. This research can also contribute to the design of such output matching networks of SHF amplifiers that must simultaneously perform the tasks of harmonics filtering and impedance matching on the fundamental frequency of amplifier’s operation. In order to achieve this task a band-pass filter was proposed and studied theoretically and experimentally. The filter includes two sections of band-reject filters, each section consisting of two slot resonator located in microstrip line’s ground plane, while the line section with slots at the same time works as a part of three-section band-pass filter. Each slot resonator can be represented as a parallel resonant circuit connected serially into the transmission line. Parameters of the parallel resonant circuit are calculated from the known resonant frequency of the slot resonator and wave impedance of the slot line.
The characteristics of the filter were calculated with the aid of ABCD matrices. The matrices represented every slot, line sections between the slots and variation of the line’s characteristic impedance (the width of microstrip line). The results of calculation were compared with results of electromagnetic simulation and experimental study of the filter test sample. The considered filter was found to have characteristics similar to that of a filter built on half-wave resonators, but the signal suppression in the upper pass-band was much higher compared with situation when only a step-like variation of characteristic impedance was used in the filter. Herewith the overall length of the filter with slot resonators was not higher than that of the prototype.
Using a test sample of the three-section filter manufactured on Rogers 3010 material with thickness 0.635 mm and nominal relative permittivity epsilon=10.2  the signal suppression in 2.0–3.6 GHz band was measured to be within the range of 0.1 to 3 dB, while in 4.6–6 GHz band the signal suppression was found to be more than 20 dB.