414 P. KOVÁCS, T. URBANEC, EBG STRUCTURES: PRACTICAL TIPS AND ADVICE FOR ANTENNA ENGINEERS Electromagnetic Band Gap Structures: Practical Tips and Advice for Antenna Engineers Peter KOVÁCS, Tomáš URBANEC Dept. of Radio Electronics, Brno University of Technology, Purkyňova 118, 612 00, Brno, Czech Republic kovacsp@feec.vutbr.cz, urbanec@feec.vutbr.cz Abstract. In this paper we discuss the use of electromag- netic band gap (EBG) structures in antenna engineering from a practical point of view. Our aim is to point out the most common mistakes and myths related to design, analy- sis and application of EBGs in the field of antennas. The paper could be helpful for beginners giving a short course on designing EBGs but also will bring novel findings for experts, investigating the effect of different number of unit cells on radiation characteristics of a planar antenna. An important part of the paper is the experiments showing the surface wave distribution over an EBG board and over the fabricated antennas with and without the periodic structure. Keywords Electromagnetic band gap (EBG), surface wave distribution, patch antenna, radiation properties. 1. Introduction The major problem associated with planar antennas originates in the guiding of plane waves by a plane interface between two different media: conductor- dielectrics or dielectrics-dielectrics. The electromagnetic energy trapped between the interfaces, and forming into surface waves, is substantial: an elementary dipole, placed on a uniform substrate with no losses and represented by the relative dielectric constant ε, radiates ε 3/2 times more power into the substrate than into the air; a second problem is that the electromagnetic waves radiated into substrate and reaching the dielectric-air interface at angles greater than θ c = sin -1 ε -1/2 are totally reflected [1], [2]. The power transferred into the surface waves does not contribute to the main radiation of the antenna, but it is scattered off the edges of the finite ground plane and leads to deep nulls and ripples in radiation patterns, increased back radiation, gain deterioration, lower polarization purity, etc. In general, the higher the permittivity of dielectrics and thicker the substrate, the stronger the influence of the surface waves. During last decades, many techniques were developed to reduce surface waves excited by printed antennas. To name only a few: drilling an air cavity below the patch [3], placing an additional dielectric layer over the patch [4] or optimizing the patch shape so that the surface waves are not excited [5]. In the late 80's, a novel structure called photonic band gap (PBG) has been introduced. The struc- ture was able to permit electromagnetic wave propagation in certain frequency bands called band gaps [6], [7]. After successful implementation of PBGs in photonics, they became widely used also in microwave and antenna engi- neering as electromagnetic band gap (EBG) surfaces [8]. EBGs make possible to intensively suppress surface waves in printed antenna boards. The most widely used type of EBGs are metallo-dielectric structures, consisting of peri- odic array of patches, connected (mushroom type) or not (planar type) to the ground plane (Fig. 1). The mushroom and planar EBG differs in many aspects and which to choose always depends on application. For successful implementation in practice, also particular care should be paid to their correct design and computer modeling. a) b) Fig. 1. Mushroom (a) and planar (b) metallo-dielectric EBG. The parameters are: period D, size of the square patch P, via diameter d, dielectric slab thickness h and dielectric slab permittivity ε. The main goal of this paper is to clarify some com- mon mistakes related to use of electromagnetic band gap structures in antenna engineering and to give the reader a brief and clear overview about their basic theory and nu- merical simulations. On the other hand, we also demon- strate the practical exploitation of EBGs on measurement examples. The most original contribution of the paper is the investigation of different number of EBG unit cells on performance of a probe-fed planar antenna. The presented experiments could be useful for designers to achieve the desired radiation characteristics of a planar antenna and to keep its size as small as possible.