Extending the Frequency Limit of Microstrip-Coupled CSRR Using Asymmetry

This article explains the frequency limitation in designing microstrip circuits based on a complementary split-ring resonator (CSRR) and reports a novel technique for increasing its operating frequency, which makes the CSRR suitable for high-frequency applications. This study helps in synthesizing the dimensions of symmetric CSRR (SCSRR) and asymmetric CSRR (ACSRR) circuits, which shows the applicability of the proposed technique in designing mm-wave circuits. It is observed that the upper frequency limit of the microstrip-coupled SCSRR is mainly due to weak electric coupling. The weak coupling implicates radiation loss in the structure, and the frequency at which the SCSRR has maximum loss due to radiation is found to be much less than the cutoff frequency of the higher order mode of the microstrip line, thus effectively limiting the operating frequency range of the circuit. In this article, the type of coupling and associated electrical parameters, e.g., reflected power, transmitted power, radiated power, losses, quality factor, voltage standing wave ratio, and gain, are investigated in detail. The allowable bandwidth of the fundamental mode of the microstrip line can be utilized to design CSRR with improved coupling. It is found that the ACSRR enhances the coupling coefficient using the cross-coupling effect. At higher frequencies, where the electric coupling is becoming too weak for the SCSRR, it is possible to design a magnetically coupled ACSRR of the same dimensions as the SCSRR. Finally, the design profile for both the SCSRR and ACSRR is studied in detail, and the radiating condition is discussed as a function of the height and dielectric constant of the substrate. For practical-verification purposes, two circuit prototypes are fabricated on a 1.5-mm-thick Rogers RT5880 substrate, and the measured results are found to be in good agreement with the proposition. It is found that, for a given microwave substrate, the maximum allowable resonating frequency of the ACSRR structure is nearly twice that of the SCSRR structure.