1134 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 48, NO. 7, JULY 2000 Small -Shaped Antennas for MMIC Applications Dilbagh Singh, Student Member, IEEE, Christos Kalialakis, Member, IEEE, Peter Gardner, Senior Member, IEEE, and Peter S. Hall, Senior Member, IEEE Abstract—A small short-circuited -shaped GaAs mono- lithic microwave integrated circuits (MMICs) patch antenna is presented. Resonant at 5.98 GHz, it is the lowest frequency MMIC patch antenna reported that we are aware of and is intended for short-range communications (e.g., vehicular). Initial experimental and theoretical characterization of the proposed structure has been carried out on soft microstrip substrates. It has been shown that the size of an -shaped patch antenna can be reduced to as low as one tenth of that of a half wavelength patch antenna resonant at the same frequency, saving valuable substrate space. Resonance frequency, radiation patterns and gain have been investigated. Ground plane truncation effects, which are important for MMIC applications, have been examined using the finite-difference time-domain (FDTD) method. Index Terms—Electrically small antennas, monolithic mi- crowave integrated circuits (MMICs), printed antennas. I. INTRODUCTION C ONVENTIONAL microwave active antennas are built using two or more different substrates, for example gallium arsenide (GaAs) for active devices and glass loaded polytetrafluoroethylene (PTFE) for antennas. At higher mi- crowave frequencies, the parasitics of interconnections between devices become significant and degrade the antenna perfor- mance. A close integration of active devices with antennas could be achieved by integrating both the devices and the antenna on the same monolithic microwave integrated circuit (MMIC) substrate. The fabrication cost of MMICs depends directly on the substrate area occupied by the circuit. Therefore, the size of the antenna is very important if it is to be integrated with active devices on a GaAs MMIC substrate. Antennas at millimetric frequencies are small and, therefore, they can be fabricated on GaAs substrates at low cost. At microwave fre- quencies, the antenna size is relatively large and uneconomical to fabricate on a GaAs substrate. The resonance frequency of a given size antenna can be re- duced by using a higher dielectric constant substrate. How- ever, in gallium arsenide MMICs, the designer is constrained to work with dielectric constant of 12.9. Another common tech- nique to reduce the size of a patch antenna is to terminate one of the radiating edges with a short circuit [1]. The antenna is approximately quarter wavelength long and, therefore, Manuscript received October 23, 1998; revised October 26, 1999. This work was supported by an EPSRC ROPA Award, Reference GR/K63184 and by the School of Electronic and Electrical Engineering, The University of Birmingham. The authors are with the Communications Engineering Research Group, School of Electronic and Electrical Engineering, The Univer- sity of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K. (e-mail: P.Gardner@bham.ac.uk). Publisher Item Identifier S 0018-926X(00)05792-6. takes approximately half of the substrate space compared to a half-wavelength antenna. The short to ground is made with ei- ther a strip or a number of shorting pins. Sanad [2] investigated the effect of changing the number of shorting pins to ground and found the antenna resonant frequency increased by about 2% when a short strip was replaced by shorting posts. The res- onant frequency reduced as the number of shorting pins was reduced. The resonance frequency decreased by approximately 3.8% when three shorting pins to ground were used. The reso- nance frequency of an antenna can also be reduced by covering the patch with a superstrate [3]. A half wavelength -shaped antenna was investigated by Palanisamy et al. [4] for UHF appli- cations. The resonant frequency of the antenna reduced to about half of that of a half-wavelength rectangular patch antenna of the same size. In this paper, short-circuited -shaped antennas [5] suitable for MMIC active antenna applications are investigated. The an- tenna takes a fraction of the substrate area of a conventional half wavelength patch and has a convenient shape enabling the ac- tive circuits to be integrated within the antenna, taking no ad- ditional substrate area. The investigation of the performance of the antenna was carried out by building a number of antennas with varying dimensions on soft substrate (Section II). The an- tennas were fully characterized using in house finite-difference time-domain (FDTD) software (Section III) and a commercial Hewlett-Packard Momentum 1 electromagnetic simulator. The change in the resonance frequency of the antenna with its di- mension is presented in Section IV. Based on the design data obtained from the soft substrate work, an -shaped MMIC an- tenna was fabricated on GaAs substrate at 5.98 GHz (Section V). This antenna is the lowest frequency antenna in MMIC form that we have seen reported in the literature and is intended for short-range communications applications. II. ANTENNA GEOMETRY A quarter wavelength antenna at 5 GHz was built on glass loaded PTFE substrate, which has relative dielectric constant of 2.2 and substrate thickness of 0.508 mm. The length of the antenna is 9.5 mm and the width was arbitrarily chosen to 11 mm. A step in the width was introduced along the length to make the -shaped short-circuited antenna as shown in Fig. 1. Dimensions and were varied, while keeping the overall size constant; the effect on resonance frequency was also noted. has a significant effect on the resonance frequency. Therefore, a suitable value for and was chosen so that the fields are evenly distributed for an efficient radiation and for feed matching. The lengths and were chosen to be 1 Momentum is a trademark of Hewlett-Packard. 0018–926X/00$10.00 © 2000 IEEE