Two-dimensional array design techniques are developed for microstrip comb-line antenna array. Low sidelobe design is proposed in the plane parallel to the feeding line. Element radiation is controlled by both widths of the radiating element and the feeding line simultaneously. Beam-tilting design is proposed in the perpendicular plane to the feeding line even though only two feeding lines are arranged. Microstrip slit structure is located around each radiating element in order to cancel reflections from element and slit and to achieve high design accuracy. By using these items, three antennas with different beams are developed for three-beam switching system. Performance of these antennas is evaluated by experiments in the millimeter-wave band.
 Broadband and high data rate capability in millimeter-wave communication systems is attractive for applications of wireless LAN and home servers. High sensitivity and high angular resolution are expected for millimeter-wave sensor systems since high gain and narrow beam width are possible for antennas in the millimeter-wave band [Tokoro, 1996; Fujimura, 1995; Asano, 2000]. Forward-looking and backward-looking automotive radars have already been in practical use. More than one sensor will be installed in order to cover all the directions around a car such as side and obliquely backward directions and short-range forward-looking with wide field-of-view. Not only high efficiency and low cost but also high design flexibility of radiation pattern are very important for the antennas to perform required radiation patterns even in severe installation conditions for these sensor systems into cars. Furthermore, beam-scanning function is one of the solutions to cover wide area with high gain. Beam-switching systems can be replacement with lower cost from such beam-scanning systems. We developed two-dimensional array design techniques of microstrip comb-line antennas [James and Hall, 1989] with travelling-wave excitation to extend design flexibility. Microstrip comb-line antennas are expected to perform high efficiency because of relatively low feeding loss [Iizuka et al., 2003]. Low sidelobe design in the parallel plane to the feeding line and beam-tilting design in the perpendicular plane to the feeding line are two-dimensional array design techniques that are mentioned in sections 2 and 3, respectively [Owa et al., 2005, 2006; Kashino et al., 2006, 2007]. Microstrip slit structure is located around each radiating element in order to cancel reflections from element and slit and to achieve high design accuracy. By using these items, three antennas with different beams are developed for three-beam switching system by using these techniques in this paper. Performance of these antennas is evaluated by experiments in the millimeter-wave band.
2. Low Sidelobe Design of Microstrip Comb-Line Antenna
 A microstrip comb-line antenna is composed of several rectangular radiating elements directly attached to a straight feeding microstrip line printed on a dielectric substrate (Teflon, thickness t = 0.127 mm, relative dielectric constant ɛr = 2.2 and loss tangent tan δ = 0.001) with a backed ground plane as is shown in Figure 1. The radiating elements are inclined by 45 degrees from the feeding microstrip line for the polarization requirement of the automotive radar systems. The radiating elements are arranged on the both sides of the feeding line, which forms an interleave arrangement. Element spacing is approximately a half guided wavelength so that all the elements in the both sides are excited in phase. An ordinary patch antenna is connected at the termination of the feeding line for a matching element in order to radiate all the residual power in the microstrip line. In order to design the array antenna with Taylor distribution for low-sidelobe radiation pattern such as −30 dB, radiating elements with extremely low radiation are required around the input port where high input power flows in the feeding line. Radiation from the array elements has been controlled only by modification of the element width in the conventional comb-line array antennas where the element length is the resonant length. However, it is difficult to realize small radiation in the millimeter-wave band since the width of the radiating element is limited wider than 0.1 mm in etching process. Then, we propose to extend a width of the feeding line for small radiation from the elements [Owa et al., 2005, 2006]. The feeding line width near the input port is gradually changed by 0.1mm at the connection of each radiating element up to the eighth element without impedance transformer to prevent discontinuities on the feeding line as shown in Figure 1. Impedance transformer from the feeding line width of 1.0mm to 0.3 mm is provided at the input of the feeding line. Radiation and phase perturbation of each element are analyzed by electromagnetic simulator of finite element method for array design. Small impedance mismatch at each element is acceptable since a few degrees beam tilting is applied in order to improve reflection characteristics of the travelling-wave array.
 A linear array antenna with 27 radiating elements is developed as shown in Figure 1. Figure 2 shows radiation pattern in the parallel plane to the feeding line at 76.0 GHz. Beam-tilting of a few degrees is adopted to improve the reflection characteristics. Low sidelobe of −27.1 dB is observed in the measured results. Applicability of the proposed structure is demonstrated in this work.
3. Beam-Tilting Technique in Perpendicular Plane to Feeding Line
 Here is regarded with pattern synthesis in perpendicular plane to the feeding line of comb-line antenna composed of two feeding lines as shown in Figure 3. According to the conventional design for required beam-tilting in the perpendicular plane to the feeding line, a different excitation phase is supplied to the two feeding lines. However, the design flexibility is low when the number of the feeding lines is small. Grating lobe grows up when beam-tilting is applied since a distance between the feeding lines is long compared with a wavelength. Here, we focus on the interleave arrangement of the radiating elements on the both sides of the feeding line. In order to improve the design flexibility, individual phase values are assigned to the radiating elements on the different sides of the same feeding line [Kashino et al., 2006, 2007]. The excitation phase of array elements in travelling-wave excitation is controlled by connected location of the element on the feeding line. The flexibility of the proposed design is twice as high as that of the conventional one, in which the same phase is assigned to the elements connected to the same feeding line. Furthermore, re-radiation of reflection wave from radiating elements degrades the design accuracy of radiation patterns. Reflection-canceling slit structure is located on the feeding line around each radiating element to reduce reflection from the element. Dimensions and locations of the rectangular slit are optimized for each element to minimize reflection at the design frequency by using electromagnetic simulator of finite element method.
 We fabricated microstrip comb-line antenna array consisting of two feeding lines with 26 radiating elements for each to demonstrate feasibility of the proposed design in the millimeter-wave band. Figure 3 shows a photograph of the developed antenna. Matched load is connected at the termination. Reflection-canceling slit is closely located at each radiating element. Figure 4 shows radiation patterns in the plane perpendicular to the feeding line. Beam-tilting of 30 degrees is obtained. Grating lobes are not observed due to the proposed phase assignment. Unwanted radiation is observed in −30 degrees direction when slits are not used. It is re-radiation due to the reflection from all the radiating elements without slits. On the other hand of the radiating element with slit, the measured curve agrees well with the design over all the angles. It is confirmed that the design accuracy is improved by the effect of the reflection-canceling slits.
4. Antennas for Three-Beam Switching
 Three microstrip comb-line antennas with different beams are developed for three-beam switching. In the previous section, we have mentioned beam-tilting antenna in perpendicular plane of feeding line. Here, we applied this technique to the design of three microstrip antennas with different beams of −30, 0, +30 degrees directions. Excitation phase is controlled by the location of the element connection along feeding line for beam tilting. Microstrip antenna with +30 degrees beam tilting has been developed and the design technique is proposed in the previous section. This technique is applied to the antenna with −30-degrees beam-tilting. Three antennas with different beams of −30, 0, +30 degrees are developed as shown in Figure 5. Since excitation phase of each radiating element is controlled by connected location along the feeding line, arrangements of radiating elements in three antennas are different from each other. Figure 6 shows the measured radiation patterns of three antennas. Both 30 degrees beam-tilting are obtained by the beam-tilting design. 2 dB gain reduction is observed in the beam-tilting antenna compared with broadside beam. Cross over level between the adjacent beams is approximately 5 dB lower than the peak gain of broadside beam.
 Two-dimensional array design techniques are developed for microstrip comb-line antenna array. Low sidelobe −27.1 dB is obtained in the plane parallel to the feeding line. Beam-tilting of 30 degrees is designed and high design accuracy is confirmed by the effect of reflection-canceling slits. By using these techniques, three antennas with different beams are developed for three-beam switching. Feasibility of the proposed designs is confirmed by the experiments in the millimeter-wave band.