High-Gain Millimeter-Wave Planar Array Antennas with Traveling-Wave Excitation (2010)

With regard to the microstrip antennas, comb-line antennas are developed in the millimeterwave band. In the comb-line antenna, since radiating array-element is directly attached to the feeding line, feeding loss could be quite small in comparison with other ordinary patch array antennas connected via microstrip branch from feeding lines. The branch of the combline antenna is no longer feeding circuit but radiating element itself. Radiation from the discontinuity at the connection of the radiating element joins to the main radiation from the element. Consequently, equivalent circuit model can not be used in the design any more. Electromagnetic simulator must be used to estimate the amplitude and phase of radiation from the elements accurately. Traveling-wave excitation is assumed in the design of comb-line antennas. Reflection waves from the elements in the feeding line degrade the performance of the antenna. When all the radiating elements are excited in phase for broadside beam, reflection waves are also in-phase and return loss grows at the feeding point. Furthermore, reflection waves from elements re-radiate from other elements. Radiation pattern also degrades since it is not taken into account in the traveling-wave design. Therefore, reflectioncanceling slit structure is proposed to reduce reflection from the radiating element. Feasibility of the proposed structure is confirmed in the experiment On the other hand in the design of conventional shunt and series slotted waveguide array antennas for vertical and horizontal polarization, radiation slots are spaced by approximately a half guided wavelength for in-phase excitation. Interleave offset and orientation from waveguide center axis are necessary to direct the main beam toward the broadside direction Since the spacing is less than a wavelength in free space, grating lobes do not appear in any directions. For bidirectional communication systems in general, two orthogonal polarizations are used to avoid interference between the two signals. In the case of automotive radar systems, 45-degrees diagonal polarization is used so that the antenna does not receive the signals transmitted from the cars running toward the opposite direction (Fujimura 1995). However, in the design of the slotted waveguide array antenna with arbitrarily linear polarization such as 45-degrees diagonal polarization for the automotive radar systems, slot spacing is one guided wavelength which is larger than a wavelength in free space. All the slots are located at the waveguide center with an identical orientation in parallel unlike conventional shunt and series slotted waveguide array. Consequently, grating lobes appear in the radiation pattern. Antenna gain is degraded significantly and ghost image could be detected in the radar system toward the grating-lobe direction. In order to suppress the grating lobes, dielectric material is usually filled in the waveguide However, it would cause higher cost and gain degradation due to dielectric loss in the millimeter-wave band. We have proposed a narrow-wall slotted hollow waveguide planar antenna for arbitrarily linear polarization Here, we developed two different slotted waveguide array antennas with 45-degrees diagonal linear polarization. One is quite high gain (over 30 dBi) two-dimensional planar array antenna Microstrip comb-line antenna is also developed for lower-gain applications of the sub-array. Both waveguide antennas consist of the same waveguides with radiating slots designed by traveling-wave excitation. The number of the radiating waveguide and the structures of the feeding circuits are different in the two antennas. Traveling-wave excitation is common technique in the designs of the slotted waveguide array antennas and the microstrip comb-line antenna. Array fed by www.intechopen.com High-gain Millimeter-wave Planar ArrayAntennas with Traveling-wave Excitation