Circularly Polarized Post-wall Waveguide Slotted Arrays

Transcription

1 Circularly Polarized Post-wall Waveguide Slotted Arrays Hisahiro Kai, 1a) Jiro Hirokawa, 1 and Makoto Ando 1 1 Department of Electrical and Electric Engineering, Tokyo Institute of Technology Ookayama Meguro-ku, Tokyo, , JAPAN a) hisahiro@antenna.ee.titech.ac.jp Abstract: The circularly polarized post-wall waveguide array is designed and fabricated for the first time. The aperture illumination and the characteristics of the antenna are investigated by using analysis/design method developed for oversized rectangular waveguide slot arrays. The uniformity as well as the polarization is perturbed along the periphery of the aperture; the narrow walls are identified to be the dominant factors for degrading the amplitude uniformity and the polarization purity in the aperture illumination. The peak gain of 29.3 dbi with 39.7% efficiency is realized at 77 GHz band for the antenna with about 5 cm square aperture. Keywords: post-wall waveguide array, oversized rectangular waveguide slotted array, circular polarization, millimeter-wave, reflection canceling slot pair. Classification: microwave and millimeter wave devices, circuits, and systems References [1] Jiro Hirokawa, Makoto Ando, and Naohisa Goto, Waveguide-fed Parallel Plate Slot Array Antenna, IEEE Trans. Antennas. Propagat., vol. 40, pp , Feb [2] Jiro Hirokawa and Makoto Ando, Single-layer feed waveguide consisting of posts foe plane TEM wave excitation in parallel plates, IEEE Trans. Antennas. Propagat., vol. 46, pp , May [3] Jiro Hirokawa and Makoto Ando, Sidelobe Suppression in 76-GHz Post Wall Waveguide-fed Parallel Plate Slot Arrays, IEEE Trans. Antennas. Propagat., vol. 48, pp , Nov [4] Jiro Hirokawa and Makoto Ando, Efficiency of 76-GHz Post Wall Waveguide-fed Parallel Plate Slot Arrays, IEEE Trans. Antennas. Propagat., vol. 48, pp , Nov [5] Jiro Hirokawa and Makoto Ando, 45 degree Linearly Polarized Post Wall Waveguide-fed Parallel Plate Slot Arrays, IEE Proc. -Microw. Antennas. Propagat., vol. 147, pp , Dec [6] Hisahiro Kai, Jiro Hirokawa, and Makoto Ando, Analysis of Inner Fields and Aperture Illumination of an Oversize RectangularSlotted Waveguide, IEE Proc. Microwave, Antennas and Propagation, vol. 150, no. 6, pp , Dec

2 [7] Hisahiro Kai, Jiro Hirokawa, and Makoto Ando, Perturbation Analysis of Aperture Illumination in Oversized Rectangular Slotted Waveguide Arrays, IEEE AP-S/URSI, vol. 3, pp , San Antonio, June 16 21, [8] Kunio Sakakibara, Jiro Hirokawa, Makoto Ando, and Naohisa Goto, A Linearly-Polarized Slotted Waveguide Array Using Reflection Canceling Slot Pairs, IEICE TRANS. COMMUN., vol. e77-b, no. 4, April [9] J. Hirokawa, M. Ando, and N. Goto, Analysis of slot coupling in a radial line slot antenna for DBS reception, IEE Proc., vol. 137, pt. H, no. 5, pp , Oct [10] S. Kobayashi, K. Sudo, J. Hirokawa, and M. Ando, Efficiency of Millimeter-wave Band Radial Line Slot Antennas using double layer dielectric, IEICE Tech. Rep., Jan [11] M. Ando, J. Hirokawa, T. Yamamoto, A. Akiyama, Y. Kimura, and N. Goto, Novel Sigle-Layer Waveguides for High Efficiency Millimeter Wave Arrays, IEE Trans. MTT, Jun [12] K. Sudo, T. Mizutani, J. Hirokawa, and M. Ando, Radial LineSlot Antennas for Space Soler Power Satellite and Grating lobe Suppression, JUSPS 03, pp , July Introduction Rectangular parallel plate slotted arrays using dielectric substrate, named post-wall waveguide arrays shown in Fig. 1, are attractive candidates for high efficiency and mass producible planar array antennas in millimeter wave applications [1, 2, 3, 4, 5] such as car warning radar system (76-77 GHz) and wireless LAN system (60-61 GHz). A large rectangular parallel plate is shorted by densely arrayed metal-plated via-holes and works as the antenna aperture with slots on it. A TEM-like wave is excited at one end of the aperture by a feed waveguide through the coupling windows, which consists of posts as well. Until now, 0 degree [2] and 45 degree [5] linear polarization antennas are proposed and developed. The gain of 29.7 dbi with 44.6% efficiency and the gain of 29.8 dbi with 46.4% efficiency were reported for the 0 degree and 45 degree linear polarization antenna with about 5 cm square aperture, respectively. The oversized post-wall waveguides are low loss TEM-like waveguides which are much larger than the wavelength, but the aperture size is still finite (10-40 wavelengths). So several peculiar behaviors have been observed, which are unique and inherent to the oversized waveguide arrays. For example, aperture illumination of 45-deg linearly polarized parallel plate slotted array suffers from the defects of a triangular area with extremely low illumination [5]. For enhancing the quality and the accuracy of the array design, an accurate analysis model of the inner field and aperture illumination was developed by the authors for the oversized waveguide arrays. In this paper, the authors design the circularly polarized array consisting of slot pairs [1] and fabricate it for a validation. We found that the perturbation due to the narrow walls at the aperture periphery is the dominant degradation of the polarization in this antenna: the peak gain 8

3 Fig. 1. Structure is 29.3 dbi with 39.7% efficiency at 77 GHz band for the antenna with about 5cmsquareaperturesize. 2 Antenna structure and model antenna 2.1 Antenna structure Figure 1 shows the structure of this antenna. A large parallel plate is shorted by densely arrayed metal-surface via-holes and works as the antenna aperture with slots on it. A TEM-like wave is excited at one end of the parallel-plate by a feed waveguide through the coupling windows, which consist of posts as well. An input aperture is cut on the bottom of the parallel plates at the centre of the feed waveguide. The dimension of the plate in transverse direction is very large compared with the wavelength. So the structure is fully oversized. The two-dimensional array consists of radiation units as element, each one of which satisfies the reflection canceling design rule [8]. Since the reflection from units is negligible, traveling wave operation is expected in the oversized waveguide. 2.2 Configuration of radiation unit The radiation unit consists of slots and its shape depends upon the polarization requirement. In this paper, the slot configuration for circular polarization similar to that of radial line slot antennas [9] is adopted. Figure 1 includes the configuration of the radiation units as well. Two slots are put perpendicular to each other, each one of which is making the angle 45 degree with the z direction. The spacing q between slots in the z direction is approximately λ g /4 to suppress the reflection from the unit. The unit thus designed radiates the right hand circular polarization field. In addition, slot offset p in the x direction can be chosen arbitrarily provided that the units should not perturb the illumination uniformity excessively. In the design of slot, we assume the infinite arrays and utilize the linear array model with periodic boundary walls [1, 2, 3, 4, 5]. Here, the uniform aperture illumination along the z direction is designed and the slot pairs are arrayed with one wavelength spacing to radiate a boresight beam. Analysis of the two-dimensional finite array, on the other hand, is conducted by using the model specially developed for oversized waveguide arrays [6, 7]. 9

4 2.3 Model antenna A circular polarization parallel plate slot array is designed and fabricated at 76.5 GHz. The aperture size is about 5 cm square. The dielectric constant is 2.17 and the loss tangent tanδ= is specified at 10 GHz. The height of waveguide is mm, about 0.29 wavelength at 76.5 GHz. Total of radiation units are placed on the waveguide. The parameters p = mm is used. Figure 2 (a) shows the predicted aperture illumination of test antenna. The horizontal axis is perpendicular to the feed waveguide while vertical one lies parallel to it. The incident field propagates from the right to the left. The predicted aperture illumination has the ripple in the center and weak amplitude areas appear near the narrow wall; the perturbation due to narrow walls is dominant in this antenna and the serious illumination defects due to scattering from radiation units are not observed for this polarization. 3 Experimental results 3.1 Measured aperture illumination Figure 2 (b) presents the measured amplitude of aperture illumination for principal polarization of test antenna at 76.5 GHz. The amplitude of aperture illumination has the ripple around the center region and weak amplitude near the narrow wall. The measured results are well explained by the predicted ones. Figure 2 (c) shows the cross polarization components over the aperture. It is about 18 db lower than the principal one but the purity is low around the periphery. 3.2 Reflection The reflection of about-10 db is observed around design frequency. This reflection mainly comes from the antenna input port and the match junction at the ends of the feed post-wall waveguide. Reduction of the reflection is the future study in the design of this antenna [1, 2, 3, 4, 5]. 3.3 Gain and axial ratio The gain, axial ratio and radiation patterns are measured. Figure 3 (a) shows the measured gain and the axial ratio as functions of frequency. The peak gain is 29.3 db with 39.7% efficiency for 5 cm square aperture size of this antenna. This efficiency is almost comparable to that for the linearly polarized one [2, 5]. The elimination of loss, especially the dielectric loss which is dominant in oversized waveguide arrays [4], is an important study for the future [10, 11, 12]. The axial ratio is below 1.0 db at 76.5 GHz and 3 db band width of axial ratio is larger than 2 GHz. 3.4 Radiation pattern Figure 3 (b) shows the far field radiation pattern in the plane parallel to the feed waveguide. The red dotted line shows the measured patterns of principal polarization. The 3 db beamwidth is 3.6 degree in the plane. The first sidelobe level is db, but the sidelobe envelopes are slightly deformed 10

5 Fig. 2. Aperture Illumination from that for uniform illumination, due to the perturbation in aperture illumination. The green solid line shows the calculated result which explains the measurements beautifully. The blue dotted line shows the measured cross polarization pattern, which is lower than -21 db. 4 Conclusion The authors have designed and fabricated the circular polarization parallel plate slot array. A slot pair for circular polarization similar to that of RLSA is adopted. The aperture illumination is assessed and the stable uniformity is predicted though the perturbation due to the narrow walls is observed. 11

6 Fig. 3. Gain, Axial ratio and Radiation pattern Experiments are conducted and the peak gain of this antenna is 29.3 dbi with 39.7% efficiency for at 77 GHz band 5 cm square aperture size of this antenna. Acknowledgements We would like to thank Mr. Yukikazu Arai of Taisei Co. Ltd. for the fabrication of the antennas. 12