THE ANALYSIS OF ULTRA WIDEBAND WHEEL MICROSTRIP PATCH ANTENNA USING NON-UNIFORM PHOTONIC BANDGAP SUBSTRATE STRUCTURE Yasser M. Madany 1 and Hassan Elkamchouchi 2 1 Student, MIEEE, Alex. Univ., Alexandria, Egypt, ymadany@ieee.org 2 Senior, MIEEE, Alex. Univ., Alexandria, Egypt, helkamchouchi@ieee.org Abstract The characteristics of new ultra wideband (UWB) wheel microstrip patch antenna using non-uniform photonic bandgap (PBG) substrate structure has been analyzed using a full 3D electromagnetic field inside a structure based on the finite element method. The UWB technologies have been developed to exploit a substantially new spectrum resource and to realize ultra high-speed communication, high precision radar, and other sensing systems. The antenna has UWB approximately in range of 1 MHz to 20 GHz and its performance above 20 GHz is sensitive to the gap position in the ground plane. These gap positions enhance some modes and weaken others, which have been used to control the antenna bandwidths up to 31.4 %. The proposed antenna is constructed on ground plane of thickness 0.1 mm and FR-4 epoxy substrate of thickness 1.6 mm with relative permittivity 4.4. The radiation characteristics for the designed antenna such as return loss, input impedance, near field, radiation pattern and 3D-polar radiation pattern with respect to changes on the air gap position in the ground plane are obtained using numerical simulations. I. Introduction Microstrip antennas (MSAs) have several advantages, including that they are lightweight, low cost and compact size and that they can be made conformal to the host surface. In addition, MSAs used for defense and commercial applications, are replacing many conventional antennas [1-6], which are designed to operate in relative narrow and confined range of frequencies. Accordingly, increasing the bandwidth of the MSA has been a primary goal of research in the field. Distortion less signal transmission is desirable for UWB wireless systems to accommodate many applications such as ultra high-speed communication, high precision radar, radar imaging of objects buried under the ground or behind walls, short-range, high-speed data transmissions, and other sensing systems. The antennas used in these systems should meet several requirements: directivity (either directional or omni-directional), low voltage-standing-wave-ratio (VSWR), and low dependence of gain. In the past decade, new technology has emerged which may be the key to developing UWB microstrip antennas. This technology manipulates the substrate in such a way that surface waves are completely forbidden from forming [7], resulting in improvement in antenna efficiency and bandwidth, while reducing sidelobes and electromagnetic interference levels. These substrates contain so-called PBG, which also referring to the electromagnetic bandgap (EBG). In this paper, UWB wheel microstrip patch antenna using non-uniform PBG substrate structure with g r = 4.4 and different air gap position in the ground plane is analyzed using Ansoft HFSS simulator [8]. The scattering parameters, input impedance, near field, radiation pattern and 3D-polar radiation pattern are given for various air gap positions in the ground plane of the antenna structure. II. Antenna Structure A. Ground plane The dielectric substrate is constructed on perfect conductor circular ground plane with radius 40 mm and thickness 0.1 mm, as shown in Fig.1. B. Dielectric substrate The antenna is constructed on inexpensive FR-4 epoxy circular substrate (g r = 4.4, tanf = 0.02) with radius 40 mm and thickness 1.6 mm. The dielectric substrate contains seven elements non-uniform EBG air gaps array, which are proposed in the form of non-uniform Binomial distributions. For non-uniform circular- patterned PBGs, the central element has the largest radius and the radii of the adjacent circles decrease proportional to the amplitude coefficients of the polynomial. The polynomial coefficient's amplitudes are proportional to the diameter of the PBG circle [9-10] and the distance between two adjacent air gaps is determined by a n = a 0 + 7(n-1) where a 0 = 5.5 mm and n = 1, 2, 3, respectively, as shown in Fig.1. B. Patch antenna The dimensions of the UWB wheel microstrip patch antenna using non-uniform PBG substrate structure are shown in Fig.1. Fig.1a 3D view of wheel microstrip patch antenna. G 1 G 2 G 3 G 5 G 6 G 11 G 12 G 8 G 9 a 3 a 2 r 1 r 2 r 3 Port c b (1) (2) (3) (4) Fig.1b Top view of dielectric substrate with PBGs. Fig.1c Top view of wheel microstrip patch antenna. The mechanical parameters of the proposed antenna are tabulated in Table (1). Table (1a) The mechanical parameters of dielectric PBGs. Air gap G 1 G 2,5,8,11 G 3,6,9,12 G 4,7,10,13 Radius 10 mm 7.5 mm 3 mm 0.5 mm Table (1b) The mechanical parameters of patch antenna. Parameter r 1 r 2 r 3 b c Value 30 mm 24 mm 6 mm 11 mm 2 mm III. The Design Approach The proposed antenna has UWB characteristics approximately in range of 1 MHz to 20 GHz and its performance above 20 GHz is sensitive to the gap position in the ground plane. The approach of designing involves three varieties. The first is the case 1 (main design) without any gaps in dielectric or ground plane. The second is the case 2 where the dielectric substrate contains seven elements non-uniform EBG air gaps array in x- and y-directions. Finally, the third contains the cases from 3 to 16 where the ground plane contains one gap or all gaps corresponding to the dielectric substrate air gaps G 1 to G 13 . In microwave engineering, devices are often characterized by the scattering parameters specifically the input reflection coefficient 11 S , which determines the useful frequency range or ranges [12] from; } { } { log 20 (dB) 10 11 in ref E DFT E DFT S · ? where DFT {.}means the Discrete Fourier Transform of the reflected ref E and incident in E voltages. The percentage bandwidth of the antenna is determined by [11-13]: 100 % W narrowband · Õ Õ Ö Ô Ä Ä Å Ã / ? c L H f f f B Õ Õ Ö Ô Ä Ä Å Ã ? L H f f BW broadband where c f is the center resonance frequency and L H f f , are the frequencies for a return loss less than -10 dB. The currents on the structure are used to compute the fields in the space surrounding the antenna. In the far-field region where r r | @@ , the far- field is approximately a spherical TEM wave with the equation: r H E ˆ 0 · ? j where 0 j is the intrinsic impedance of free space. IV. Simulation Results Numerical simulation is used to obtain the radiation characteristics of the three varieties of these UWB wheel microstrip patch antenna using non-uniform PBG substrate structure. The fundamental mode of the circular microstrip antenna generates a broadside radiation pattern. The second and higher order modes in a circular disk generate circularly polarized conical patterns [14-16]. These patterns provide good omni- directional coverage in the azimuthal plane and optimum sectoral coverage in the elevation plane. The conical pattern shape of the circular microstrip antenna can be obtained by exciting different modes in the antenna and /or by using different dielectric materials. Figures (2) to (4) show the 11 S in dB versus frequency in GHz, input impedance, near field E z , E-plane radiation patterns (s=0 o , 90 o ) and 3D-polar radiation patterns at points when return loss less than -10dB for the cases 1, 2 and 16 respectively, where case 16 is the case, which the ground plane contains all gaps corresponding to the dielectric substrate air gaps G 1 to G 13 . Fig.2a The return loss for case 1 Fig.2b The input impedance for case 1 a 1 . 128 ANTEM 2005 - Saint Malo - France, June 15-17, 2005.