IMPROVED PATCH ANTENNA PERFORMANCE BY USING PHOTONIC BANDGAP SUBSTRATES Ramon Gonzalo, 1 Beatriz Martınez, 1, 2 Peter de Maagt, 2 and ´ ´ Mario Sorolla 1 1 Microwave and Millimeter Wave Group Universidad Publica de Navarra ´ 31006 Pamplona, Spain 2 ESTEC European Space Agency 2200 AG Noordwijk, The Netherlands Recei  ed 31 August 1999 ABSTRACT: In this letter, a patch antenna on a photonic bandgap substrate is presented. A reduction in the le el of surface-wa  e mode excitation has been obtained. This leads to impro  ed efficiency, gain, and far-field radiation pattern. Furthermore, impro  ements in the input return loss ha e been reported.  2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 213215, 2000. Key words: patch antennas; photonic bandgap structures; surface-wa  e modes I. INTRODUCTION Ž . Photonic bandgap PBG structures have recently emerged as   a new multidisciplinary field of study 1  3 . In analogy with electronic bandgaps in periodic semiconductor crystal lat- tices, a periodic lattice of dielectric ‘‘atoms’’ can yield a gap of forbidden electromagnetic energies. Within these gaps, electromagnetic waves cannot propagate as they experience exponential attenuation. The key to the operation of these PBG crystals is their periodicity, both the pattern and the repetitive spacing between ‘‘atoms,’’ and the dielectric con- trast between the constituent materials. The promise of new applications using these structures has spurred excitement in this field, and many research groups are putting significant research effort into this area. At optical frequencies, one of the present challenges lies in the fabrication of photonic crystals. However, the advance- ment of photolithographic techniques and other micromatch- ing technologies allows photonic crystals with applications up Ž . to sub millimeter wavelengths to be easily fabricated. In the field of antennas, photonic bandgap structures can be used as substrates for different kinds of antenna configurations. Promising results with two-dimensional PBG substrates for patch antenna configurations and three-dimensional PBG substrates for dipole or slot structures have already been   obtained 4  6. For some aircraft, spacecraft, satellite, mobile-radio, and wireless-communications applications, the size, weight, cost, and ease of installation of the antenna system are constrain- ing factors, and generally, low-profile antennas are required. Microstrip antennas are often preferred because they are compatible with planar and nonplanar surfaces, simple, inex- pensive to manufacture, and robust. These advantages must be weighed against the disadvan- tages of their relative low efficiency and operational band- width. Insufficient operational bandwidth can be overcome by thickening the substrate, although this leads to an increase of the fraction of total power going into surface waves. These extract power from the direct radiation, thus degrading the pattern and efficiency of the antenna. In order to solve this  problem, a photonic bandgap substrate can be used 6 . A two-dimensional structure formed by a patch antenna on a PBG substrate has been studied. Considerable reduction in the surface-wave levels has been observed in the simula- tions with the commercial HP-HFSS finite-element software, which improves the gain, efficiency, and sidelobe levels. II. DESIGN AND RESULTS There are many ways to feed a microstrip antenna, but the one used in this letter consists of a microstrip line going up Ž . to the 50  feeding point see Fig. 1 . The patch antenna dimensions are 31.98  15.527 mm, and the thickness of the Ž . substrate   10 is 12 mm. With these parameters, a r Ž . working frequency of 1.78 GHz has been obtained see Fig. 2 . The ratio h is 0.07, which leads to an appreciable excita- 0  tion of surface-wave modes 7 . To avoid this problem, an antenna on a PBG substrate has been designed. The selected PBG structure is a square lattice    of air holes 6 . Following the procedure described in 6, 8 , the distance obtained between holes and their radius is 44.6 and 21.4 mm, respectively. The input return loss for this antenna is shown in Figure 2. The obtained working fre- quency has slightly shifted to 1.81 GHz. This small frequency shift with respect to the conventional patch antenna can be attributed to the variation of the relative dielectric constant around the antenna. Figure 1 Schematic of the PBG antenna Ž . Figure 2 Input return loss S for the conventional patch antenna 11 Ž . Ž . dashed line and the PBG antenna solid line MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 24, No. 4, February 20 2000 213