Millimeter-wave antennas have been developed for various applications such as broadband highspeed wireless communication systems and automotive radar systems. Microstrip antennas are more advantageous than other millimeter-wave antennas at the viewpoints of low profile and low cost. On the other hand, feeding loss due to transmission loss of microstrip line is significant problem in array feeding. So, microstrip array antennas are suitable for relatively low gain applications such as a subarray of digital beam forming (DBF) systems. A comb-line feeding system is effective at the point of relatively low loss compared with other microstrip patch array antennas (MSA) fed by parallel or ordinary series feeding [1]. A travelling wave array antenna has a significant problem that gain is degraded due to beam shift in frequency changes when the array antenna is fed from one end of the feeding line. A center feeding microstrip comb-line antenna (MSCLA) is one of the solutions to reduce the gain degradation due to frequency change [2]. However, a blank area exists at the center above the microstrip-to-waveguide transition in the aperture radiation distribution, which causes elevation of sidelobe level (SLL). To fill the radiation source in the blank area, we designed four-microstrip-port waveguide transition with slot radiators. We proposed a center feeding 2 × 2 MSA and a 2-line 6-element MSCLA fed by the transition in this paper. We compared the simulated radiation patterns of the antennas fed by the transitions with slot radiators and without slot radiators in the computer analysis.
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