Design of Slit-Loaded Series Feed Network for mm-Wave Array Antenna with a Low Sidelobe Level Over a Wide Bandwidth

Abstract

Microstrip array antenna is widely used in the millimeter-wave (mm-wave) band because it is suitable for mass production, thus, cost effective, and easy to integrate with other circuits. Series-fed microstrip array comprising periodic antenna elements along feed line is compact and has low Ohmic and spurious radiation losses relative to corporate-fed array. However, the incident power gradually reduces along the feed line so that the coupled power on antenna element is insufficient at the end of an array. Moreover, the multiple reflections caused by mismatch of antenna elements and feed line are accumulated with in-phase at the input of an array so that it results in high return loss. Thus, to obtain the desired array performance, such as sidelobe level (SLL) and impedance bandwidth, it is both necessary that the coupled power on each antenna element can be accurately controlled and that matching between antenna elements and feed line is achieved. These characteristics can be resolved by changing dimensions of antenna element and feed network in series-fed array (SFA) antenna. The proposed feed network controls the power distribution of an array and achieves matching by changing dimensions of slit structures, which are loaded on the feed lines. Four types of slit-loaded feed units are developed to control a wide range of power distribution, which is necessary to obtain low SLL, regardless of limitation in standard printed circuit board technology. Moreover, the amount of coupled power and phases in each port of proposed feed network are accurately controlled by designing it as follows a systematic design procedure. To validate the control performance of the feed network, two types of SFA, i.e., end-fed 1-D array and center-fed 2-D array, are designed at 79 GHz. The feed network and antenna elements are separately designed on two layers of dielectric substrates such that it provides a great isolation for the accurate validation. The 1-D SFA antenna operates with stable radiation patterns, i.e., flat peak gain and lows SLL under -20 dB, over a wide band in spite of main beam tilting along the frequency variation. The 2-D SFA antenna produces high peak gain and narrow half-power beamwidth relative to 1-D array with fixed main beam direction along the frequency variation. The 2-D SFA also operates with stable radiation patterns from 77 to 81 GHz such that it can be used as high resolution automotive radar, which requires 4 GHz of bandwidth with 79 GHz of center frequency. The designed SFA antennas are compared with other mm-wave SFAs and array antennas for automotive radar through literature survey, and the excellence of proposed feed network is validated. The proposed slit-loaded series feed network can be widely used for designing feed networks or array antennas with adopting proper antenna element depending on the desired applications.