This paper presents a design technique to achieve a broadband planar microstrip-to-waveguide transition in a millimeter-wave (mmWave) band. In the conventional planar microstrip-to-waveguide transition, via holes are located around the rectangular waveguide and microstrip line to prevent power leakage due to the generation of a multi-transmission mode. Therefore, a single-transmission mode is dominant at the input port of the transition, with a narrow bandwidth of the single resonance. In the broadband planar microstrip-to-waveguide transition, via-hole positioning is utilized to add inductance to constrain the predominance of the single-transmission mode at the input port of the transition. The double-resonant frequency yielded by excitation of the grounded coplanar waveguide transmission mode and parallel plate transmission mode is obtained by controlling the positions of holes adjacent to the microstrip line. Moreover, to simplify the structure and meet the requirement of high assembly accuracy in fabrication, two holes adjacent to the microstrip line are maintained, but the remaining holes are replaced by a choke structure that performs the equivalent function to the via-hole arrangement. The influences of the multi-transmission mode and choke structure on the characteristics are investigated by electromagnetic analysis, and the feasibility is confirmed by experiments in this work. A double-resonant frequency and a broad bandwidth of 10.6 GHz (13.8%) are obtained. The measured results of the broadband planar microstrip-to-waveguide transition using via-hole positioning show an insertion loss of 0.41 dB at the center frequency of 76.5 GHz.

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Bandwidth Extension of Planar Microstrip-to-Waveguide Transition by Controlling Transmission Modes Through Via-Hole Positioning in Millimeter-Wave Band

The conventional planar microstrip-to-waveguide transition has been developed. Via-holes are arranged to reduce the insertion loss due to the leakage power between parallel plates in dielectric substrate. However, the transition obtained quite narrow frequency bandwidth in the millimeter-wave band. Therefore, a design technique of via-hole locations at both sides of the microstrip line is proposed for bandwidth extension of planar microstrip-to-waveguide transition used at a frequency of 76.5 GHz. The double-resonant frequency is formed by controlling the via-hole locations at both sides of the microstrip line. The broadband of 10.6 GHz (13.8%) is obtained where the reflection level is lower than −15 dB. Characteristics of broadband planar microstrip-to-waveguide transition depending on via-hole positions are investigated in this study.

Bandwidth Extension of Planar Microstrip-to-Waveguide Transition by Via-Hole Locations at Both Sides of Microstrip Line

We present the design and characterization of planar mm-wave patch antenna arrays with waveguide-to-microstrip transition using both near- and far-field methods. The arrays were designed for metrological assessment of error sources in antenna measurement. One antenna was designed for the automotive radar frequency range at 77 GHz, while another was designed for the frequency of 94 GHz, which is used, e.g., for imaging radar applications. In addition to the antennas, a simple transition from rectangular waveguide WR-10 to planar microstrip line on Rogers 3003™ substrate has been designed based on probe coupling. For determination of the far-field radiation pattern of the antennas, we compare results from two different measurement methods to simulations. Both a far-field antenna measurement system and a planar near-field scanner with near-to-far-field transformation were used to determine the antenna diagrams. The fabricated antennas achieve a good matching and a good agreement between measured and simulated antenna diagrams. The results also show that the far-field scanner achieves more accurate measurement results with regard to simulations than the near-field scanner. The far-field antenna scanning system is built for metrological assessment and antenna calibration. The antennas are the first which were designed to be tested with the measurement system.

Near- and Far-Field Characterization of Planar mm-Wave Antenna Arrays with Waveguide-to-Microstrip Transition

In this paper, a novel waveguide-to-microstrip line transition in the multi-layered substrate which is operated at 77GHz forward-looking automotive collision warning radar band, is presented. The proposed waveguide-to-microstrip line transition is fed by a standard WR10 rectangular waveguide, the power is coupled from the waveguide into the patch which is located at the center of waveguide and transmit to antenna. Compare to the conventional W-band waveguide-to-microstrip line transition, the feeding direction of the proposed transition is inverse to those conventional design. The design of freedom of conventional type transition is limited cause by this fed direction. Moreover, the stacked-layer structure's via size and spacing is re- strict by the fabrication rule, via size and spacing is difierent in each dielectric layer, thus, the complexity of the design increase. Finally, the proposed waveguide-to-microstrip line transition is well design, this design possess several advances such as small size, low cost, and could easy introduced in the automobile radar application.

A Waveguide-to-microstrip Line Transition in Multi-layered Structure for 77GHz Band Automotive Radar Applications

A microstrip-to-waveguide transition can be used for connecting antennas and backed RF circuits. Loss of the transition due to the leakage from the gap between the substrate and the waveguide has been a serious problem in the millimeter-wave band. A choke structure is provided in the gap at the quarter wavelength away from the waveguide edge to suppress the leakage. The effect to use the choke structure is demonstrated by simulation in this paper.

Loss reduction of microstrip-to-waveguide transition suppressing leakage from gap between substrate and waveguide by choke structure

Millimeter-wave technologies have been developed for applications to high-speed wireless communication systems and high angular resolution microwave sensors. Figure 1 shows one of the possible configurations of the millimeter-wave RF module when a microstrip antenna is used. The microstrip-to-waveguide transitions are necessary at the connection of the waveguide with the antenna feed and with the planar RF circuit. We have developed planer microstrip-to-waveguide transitions on a single-layer dielectric substrate in the millimeter-wave band [1], [2]. Via holes surround a rectangular waveguide to prevent leakage of parallel plate mode transmitting in the substrate. However, an additional process for via holes increases production cost and manufacturing errors of via-hole positions increases insertion loss of the transitions. Then, we have proposed a choke structure for replacement from via-hole arrangement [3]. The size of the waveguide short pattern composed of the metal plate on the substrate is designed to form choke structures in the substrate around the waveguide. The impedance of the open circuit with a quarter wavelengths transmission line is equivalent to the short circuit. Therefore, the impedance at the point along the waveguide profile is equivalent to the short circuit, when the open circuit at the edge of the metal plate is spaced by a quarter wavelengths from the waveguide profile in the substrate. First, the planar microstrip-to-waveguide transition is designed. Then, we optimized the choke structure for the transition in this work. The perturbed characteristics of the transition are discussed and the simulated performance is presented in this paper.

http://ap-s.ei.tuat.ac.jp/isapx/2012/pdf/2B1-5.pdf

Keisuke Murase

Papers

Design of via-less planer microstrip-to-waveguide transition with choke structure