IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 11, NOVEMBER 1999 2099 Parallel-Plate Mode Reduction in Conductor-Backed Slots Using Electromagnetic Bandgap Substrates John D. Shumpert, Student Member, IEEE, William J. Chappell, Student Member, IEEE, and Linda P. B. Katehi, Fellow, IEEE Abstract— By fabricating a resonant slot over a reflecting back plate and filling the resulting parallel-plate with an ap- propriately designed artificial electromagnetic bandgap (EBG) structure, noticeable enhancements in both radiation pattern and bandwidth are achieved using a significantly lower profile than traditional designs. This design uses a two-dimensional artificial EBG substrate in conjunction with a reflecting plate to completely block radiation from the backside of the slot from propagating to the finite edges of the resulting parallel-plate cavity. Measured and simulated data for conductor-backed slots with homogeneous substrates and with EBG substrates are compared. Index Terms— Electromagnetic crystals, parallel-plate mode reduction, photonic bandgaps, slot antennas. I. INTRODUCTION E LECTROMAGNETIC bandgap (EBG) materials are found to have unique properties that are advantageous in applications involving semiconductor integrated circuits. Such materials offer the possibility of changing the physical properties of substrates used in fabricating planar circuits. A number of applications for such materials can be imagined including dielectric mirrors, resonant cavities, high filters, and frequency-selective surfaces (FSS’s). Early work by Yablonovitch [1] successfully demonstrated that light propagation could be inhibited in certain frequency gaps in special photonic bandgap (PBG) crystals. Preliminary results suggest that microwave and millimeter-wave frequencies can also be manipulated by carefully designing and fabricating EBG structures composed of regions of differing dielectric constants. The seminal work that kindled excitement in recent PBG research can be found in [1]. Excellent reviews of PBG research carried out before 1994 can be found in special issues of [2] and [3]. Recently, a book by Joannopoulos et al. outlining photonic band theory and covering a wide range of photonic crystal applications was published [4]. A number of researchers have recently begun to design elec- tromagnetic crystal structures for use in planar antenna and Manuscript received March 31, 1999. This work was supported by the De- partment of Defense Research and Engineering Multidisciplinary University Research Initiative on Low Energy Electronics Design for Mobile Platforms under the Army Research Office Grant DAAH04-96-1-0377, and by the Department of Defense Augmentation Awards for Science and Engineering Research Training under Army Research Office Grant DAAH04-96-1-0109. The authors are with the Radiation Laboratory, Department of Electrical Engineering and Computer Science, The University of Michigan at Ann Arbor, Ann Arbor, MI 48109-2122 USA. Publisher Item Identifier S 0018-9480(99)08786-4. circuit applications, particularly for use as reflectors in planar dipole antenna structures [5]–[10]. Recent numerical work in determining theoretical bandgaps for planar microwave antenna applications has been done by Yang et al. [11], while Leung et al. [12] measured the radiation patterns of a slot antenna placed on a layer-by-layer PBG crystal. For planar antennas operating at a frequency in the bandgap of the three-dimensional PBG crystal, energy which would have been radiated into the substrate is reflected. However, at the interface between the PBG and the air, the periodicity of the PBG is broken and a parasitic mode (surface state) can exist. These surface states decrease the efficiency by stripping power away from the radiating element. By fabricating the slot over a reflecting back plane and filling the resulting waveguide with an appropriately designed EBG structure, noticeable enhancements in both radiation pattern and bandwidth can be achieved while maintaining the low profile. Traditional designs typically place some type of reflecting surface or cavity behind the slot in order to reduce backside radiation and increase the gain. Unfortunately, increasing the profile of the slot negates one advantage of the planar radiating element. In addition, placing reflecting surfaces behind the slot reduces the efficiency by creating para- sitic modes and using a cavity-backed design often necessitates narrowing the bandwidth. If the slot antenna is backed by a metal plate to increase the front-to-back ratio, parallel-plate waveguide (PPW) modes will be excited, both decreasing the efficiency and distorting the pattern. The theory and design of the EBG structure used in our application to minimize these problems is developed in Section II. The folded-slot antenna design and fabrication is outlined in Section III. Observations concerning the performance of the EBG structure (crystal) and its use for backing slot antennas is detailed in Section IV. This paper concludes with general observations about the usefulness and possible applications for these special materials presented in Section V. II. EBG THEORY AND DESIGN In order to determine the frequencies of interest for a complex structure, a full-wave integral equation (IE)/method of moments (MoM) code has been developed [13] to determine the band structure (propagating modes) of a periodic two- dimensional inhomogeneous dielectric region. If the reflecting plate shown in Fig. 1 is located near enough to the slot such that the operating frequency is below the cutoff of the first TE/TM mode, only the dominant TEM mode ( ) with zero 0018–9480/99$10.00  1999 IEEE