IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL 47, NO. 11, NOVEMBER 1999 High-Impedance Electromagnetic Surfaces with a Forbidden Frequency Band Dan Sievenpiper, Member, IEEE, Lijun Zhang, Romulo F Jimenez Broas, Nicholas G. Alexopolous, Fellow, IEEE, and Eli Yablonovitch, Fellow, IEEE Abstract-A new type of metallic electromagnetic structure has been developed that is characterized by having high sur- ■匚匚匚匚匚E face impedance. Although it is made of continuous metal, and conducts de currents, it does not conduct ac currents within a forbidden frequency band. Unlike normal conductors, this new surface does not support propagating surface waves, and its image currents are not phase reversed. The geometry is analogous to a corrugated metal surface in which the corrugations have been folded up into lumped-circuit elements, and distributed in a two-dimensional lattice. The surface can be described using solid-state band theory concepts, even though the periodicity is much less than the free-space wavelength. This unique material is applicable to a variety of electromagnetic problems, including new kinds of low-profile antennas Index Terms- Antennas, corrugated surfaces, photonic bandgap, surface impedance, surface waves, textured surfaces. L. INTRODUCTION A. Electric Con A FLAT METAL sheet is used in many reflector or ground plane [1]. The presence of a ground Fig. I. (a) cross section of a high-impedance surface, fabricated asar plane redirects one-half of the radiation into the opposite to a solid metal sheet by vertical conducting vias. (b)Top view direction, improving the antenna gain by 3 dB, and partially high-impedance surface, showing a triangular lattice of hexagonal metal shielding objects on the other side. If the antenna is too close to the conductive surface, the image currents cancel the currents vertically if scattered by bends, discontinuities, or surface the antenna, resulting in poor radiation efficiency. This texture problem is often addressed by including a quarter-wavelength Surface waves appear in many situations involving antennas space between the radiating element and the ground plane, but On a finite ground plane, surface waves propagate until they such a structure then requires a minimum thickness of A/4. reach an edge or corner, where they can radiate into free space Another property of metals is that they support surface The result is a kind of multipath interference or"speckle, waves [21, [3]. These are propagating electromagnetic waves which can be seen as ripples in the radiation pattern. Moreover, that are bound to the interface between metal and free space. if multiple antennas share the same ground plane, surface They are called surface plasmons at optical frequencies [4], currents can cause unwanted mutual coupling but at microwave frequencies, they are nothing more than the normal currents that occur on any electric conductor. If the B. High-Impedance Surfaces metal surface is smooth and flat, the surface waves will not By incorporating a special texture on a conducting surface couple to external plane waves. However, they will radiate it is possible to alter its radio-frequency electromagnetic Manuscript received March 8, 1999, revised July 9, 1999. This work was properties [5],[6). In the limit where the period of the surface ported by the Army Research Office under Grant DAAHo4-96-1-0389, by texture is much smaller than the wavelength, the structure IRL Laboratories under Subcontract SI-602680-1, and by the Office of Naval can be described using an effective medium model, and its Research under Grant No0014-99-1-013 qualities can be summarized into a single parameter: the L ZhangrF.J. Broas, and E. Yablonovitch are with the Electrical En- surface impedance. A smooth conducting sheet has low surface ineering Department, University of California at Los Angeles, Los Angeles, impedance, but with a specially designed geometry, a textured surface can have high surface im Engineering, University of Califomia at Irvine, Irvine, CA 92697 USA An example of a high-impedance surface, shown in Fig. I Publisher Item Identifier S 0018-9480(99)08780-3 consists of an array of metal protrusions on a flat metal sheet 001894809910.00@1999IEEEIEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 11, NOVEMBER 1999 2059 High-Impedance Electromagnetic Surfaces with a Forbidden Frequency Band Dan Sievenpiper, Member, IEEE, Lijun Zhang, Romulo F. Jimenez Broas, Nicholas G. Alexopolous, ´ Fellow, IEEE, and Eli Yablonovitch, Fellow, IEEE Abstract— A new type of metallic electromagnetic structure has been developed that is characterized by having high sur￾face impedance. Although it is made of continuous metal, and conducts dc currents, it does not conduct ac currents within a forbidden frequency band. Unlike normal conductors, this new surface does not support propagating surface waves, and its image currents are not phase reversed. The geometry is analogous to a corrugated metal surface in which the corrugations have been folded up into lumped-circuit elements, and distributed in a two-dimensional lattice. The surface can be described using solid-state band theory concepts, even though the periodicity is much less than the free-space wavelength. This unique material is applicable to a variety of electromagnetic problems, including new kinds of low-profile antennas. Index Terms— Antennas, corrugated surfaces, photonic bandgap, surface impedance, surface waves, textured surfaces. I. INTRODUCTION A. Electric Conductors AFLAT METAL sheet is used in many antennas as a reflector or ground plane [1]. The presence of a ground plane redirects one-half of the radiation into the opposite direction, improving the antenna gain by 3 dB, and partially shielding objects on the other side. If the antenna is too close to the conductive surface, the image currents cancel the currents in the antenna, resulting in poor radiation efficiency. This problem is often addressed by including a quarter-wavelength space between the radiating element and the ground plane, but such a structure then requires a minimum thickness of . Another property of metals is that they support surface waves [2], [3]. These are propagating electromagnetic waves that are bound to the interface between metal and free space. They are called surface plasmons at optical frequencies [4], but at microwave frequencies, they are nothing more than the normal currents that occur on any electric conductor. If the metal surface is smooth and flat, the surface waves will not couple to external plane waves. However, they will radiate Manuscript received March 8, 1999; revised July 9, 1999. This work was supported by the Army Research Office under Grant DAAH04-96-1-0389, by HRL Laboratories under Subcontract S1-602680-1, and by the Office of Naval Research under Grant N00014-99-1-0136. D. Sievenpiper is with HRL Laboratories, Malibu, CA 90265 USA. L. Zhang, R. F. J. Broas, and E. Yablonovitch are with the Electrical En￾gineering Department, University of California at Los Angeles, Los Angeles, CA 90095 USA. N. G. Alexopolous is with the Department of Electrical and Computer ´ Engineering, University of California at Irvine, Irvine, CA 92697 USA. Publisher Item Identifier S 0018-9480(99)08780-3. (a) (b) Fig. 1. (a) Cross section of a high-impedance surface, fabricated as a printed circuit board. The structure consists of a lattice of metal plates, connected to a solid metal sheet by vertical conducting vias. (b) Top view of the high-impedance surface, showing a triangular lattice of hexagonal metal plates. vertically if scattered by bends, discontinuities, or surface texture. Surface waves appear in many situations involving antennas. On a finite ground plane, surface waves propagate until they reach an edge or corner, where they can radiate into free space. The result is a kind of multipath interference or “speckle,” which can be seen as ripples in the radiation pattern. Moreover, if multiple antennas share the same ground plane, surface currents can cause unwanted mutual coupling. B. High-Impedance Surfaces By incorporating a special texture on a conducting surface, it is possible to alter its radio-frequency electromagnetic properties [5], [6]. In the limit where the period of the surface texture is much smaller than the wavelength, the structure can be described using an effective medium model, and its qualities can be summarized into a single parameter: the surface impedance. A smooth conducting sheet has low surface impedance, but with a specially designed geometry, a textured surface can have high surface impedance. An example of a high-impedance surface, shown in Fig. 1, consists of an array of metal protrusions on a flat metal sheet. 0018–9480/99$10.00  1999 IEEE