Influence of Angular Stability of EBG Structures on Low Profile Dipole Antenna Performance

In this paper the broadband diamond dipole antenna performance above an Electromagnetic Bandgap (EBG) structure composed of an array of square metallic patches and Jerusalem crosses without any vias over a PEC backed dielectric substrate is investigated and compared to each other. The result shows that angular stability of the EBG structures does not significantly affect on the performance of the antenna. INTRODUCTION EBG structures are used to mimic magnetic conductors over a limited frequency range for use as a ground plane in low-profile antenna configurations in addition other microwave applications. Previous research shows that EBG surfaces can act as AMC ground planes by utilizing a periodic patch with or without vias [1,2]. Both methods are capable of creating the AMC condition indicative of a reflection coefficient with magnitude of 1 (in the ideal lossless case) and phase of 0°. But it has been demonstrated that for the EBG structure (with or without vias) with square patches, the surface impedance strongly depends on the incidence angle and polarization type (TM or TE). As a consequence, these artificial surfaces tend to represent a magnetic wall only for a relatively small part of the angular spectrum of the antenna radiation [3,4]. So it is of interest to investigate the influence of the angular stability of the EBG structure on low profile antenna performance. In this paper, a wideband dipole antenna has been placed on top of the two different EBG structures without any vias. One of the shapes is square, which has poor angular stability and another one is based on Jerusalem cross which has been reported as more stable [4]. Characteristics of the wide band dipole antenna such as S11, radiation pattern and gain are investigated in details on the mentioned EBG structures. ANTENNA DESIGN A dipole antenna was used as a broadband linearly polarized antenna element. Fig. 1 shows the geometry of the dipole antenna made on 0.5 mm thick FR4 substrate ( 4 . 4 = r ε ), 2 mm above the EBG structures which consist of the element array on top of a 3 mm PEC backed substrate with dielectric constant of 2.2. Thus, the overall height of the dipole antenna from the bottom ground plane of the EBG structure is 5.5 mm. (0.09 GHz 5 λ ). Each side of the square forming the dipole is 17 mm. The balun, which is used to connect the coax to the dipole, is a wideband microstrip tapered balun . To achieve the same resonance frequency for the square and Jerusalem cross, the thickness and permittivity of the substrate has been assumed to be the same. But due to the effect of element shape on the resonance frequency, their cell sizes are slightly different. In Fig. 2 the unit cell of the square EBG and Jerusalem cross EBG with dimensions and the related reflection phases, which have been obtained by HFSS simulations, are shown. 253 0-7803-9444-5/06/$20.00 © 2006 IEEE.