IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 43, NO. 2, MAY 2001 219 Short Papers_______________________________________________________________________________ Compensation of Superstrate Effects on the Resonant Behavior of the Microstrip Ring Structure by Using an Air-Gap Control Çi˘ gdem Seçkin Gürel and Erdem Yazgan Abstract—In this paper, spectral-domain analysis is utilized to inves- tigate the possibility of compensation of superstrate shielding effects on the resonant behavior of the microstrip ring structure by means of an adjustable air gap inserted between the substrate and the ground plane. In order to provide the desired amount of tuning, the adjustment of the proper gap thickness is discussed for various structural parameter cases. It is shown that resonant frequency shift due to superstrate shielding can be compensated, resonant bandwidth and, hence, the electromagnetic interference can be directly controlled resulting of the proposed tuning scheme. Index Terms—Microstrip, resonance, shielding, superstrate. I. INTRODUCTION Additional superstrate layer on top of the microstrip patch may occur as a result of physical conditions such as snow and ice or may be directly introduced as a radome in the manufacturing stage for the purpose of protection from the environmental hazards. The main per- formance characteristics of the structure may be adversely affected if permittivity and thickness of the superstrate are not chosen prop- erly. In the millimeter- and submillimeter-wave printed circuit appli- cations of the microstrip structures due to the usage of the high-per- mittivity substrates such as GaAs and Si, efficiency is very low. Alex- opoulos and Jackson [1] have shown that when very thin substrate is used , nearly 100% efficiency can be achieved due to the elimination of the surface waves. However, if substrate is not suffi- ciently thin and has high-permittivity value efficiency tends to decrease zero. In that study, addition of superstrate layer with properly adjusted thickness is suggested in order to overcome this problem and addition- ally enhance the gain and the radiation resistance [1]. However, it is known that input-impedance matching becomes poor as a result of the superstrate shielding [2]. Hence, this fact must also be considered in the design. In the medical and the geophysical applications of the mi- crostrip structures in order to provide efficient coupling of electromag- netic energy to the external object with minimum leakage, investiga- tion of the superstrate shielding effects is important. In the study of Lind and Iskander [3], it has been shown that amount of coupling can be controlled by adjusting the superstrate thickness. It is also investi- gated in a following study that if the thickness and the permittivity of the superstrate are not carefully chosen, electromagnetic interference effect increases in a microstrip circuit [4]. It can be concluded based on the results of all those studies that in order to make the superstrate as the desirable part of the microstrip structure, its physical parameters must be optimally chosen. Manuscript received June 23, 1999; revised June 12, 2000. This work was supported by the Turkish Prime Ministry State Planning Organization (DPT) and by the Hacettepe University Research Foundation under Grant 98K 121710. The authors are with the Department of Electrical and Electronics En- gineering, Hacettepe University, Beytepe 06532, Ankara, Turkey (e-mail: cigdem@hacettepe.edu.tr; yazgan@hacettepe.edu.tr). Publisher Item Identifier S 0018-9375(01)02785-5. Fig. 1. The geometry of superstrate-loaded microstrip ring structure with air gap. It is well known that performance of a microstrip structure can be improved by operating it around the resonant frequency. Resonant fre- quency of the microstrip structure is shifted to a lower value as a re- sult of superstrate shielding or increasing medium temperature. In such cases, this shift may cause unexpected behavior of the structure and, hence, the operation of the supporting electronic circuitry is also af- fected. In such a case, resonant frequency shift must be compensated without disturbing the original configuration and degrading its perfor- mance. In the present study, it will be shown that unwanted resonant frequency shift resulting of the superstrate or increasing temperature can be compensated by including an adjustable air gap between the sub- strate and the ground plane. The analysis is applied to a lossy microstrip ring structure that finds extensive applications especially in the mi- crowave integrated circuits as filters, circulators, and resonators in the biomedical and the printed antenna systems as radiators due to its var- ious advantages [5]–[9]. Microstrip ring antenna structure with air gap and without superstrate has been analyzed by using the cavity model in the previous studies [7], [9]. The results of those studies show that in- clusion of an air gap lowers the equivalent permittivity of the structure and increases the resonant frequency. This effect is more significant when the substrate is thin and has high-permittivity value. In a later study, superstrate-loaded microstrip ring antenna has been analyzed in spectral domain without considering the air gap [8]. It has been inves- tigated that superstrate layer on top of the patch shifts the resonant fre- quency to a lower value while considerably increasing the bandwidth. However, the possibility of tuning of superstrate or increasing tempera- ture effects on the resonant frequency by applying an adjustable air-gap control have not been investigated. In the present study, it is shown that this can be fully or partially satisfied by means of an air-gap control. In order to verify this effect and to give an insight for the adjustment 0018–9375/01$10.00 © 2001 IEEE