As lower-frequency bandwidths are consumed by communications and other applications, researchers attempt to find cost-effective methods to make use of millimeter-wave frequencies. Using the 60- and 120-GHz bands for short-range, high-data-rate communications is one such example. Dongjun Kim, Jiro Hirokawa, and Makato Ando from the Department of Electrical and Electronics Engineering of the Tokyo Institute of Technology (Tokyo, Japan), along with Jun Takeuchi and Ahihiko Hirata from the NTT Microsystem Integration Laboratory (Kanagawa, Japan), have proposed the design of a 4 × 4 element corporate-feed waveguide slot array antenna for the 120-GHz band.
This antenna achieves impressive performance levels, with gain of 21.1 dBi and efficiency of 80%. The antenna features a 1-dB-down bandwidth of 22 GHz from 118 to 140 GHz. The array antenna features two types of cavities to adjust mutual coupling effects. The outer cavities of the design provide sufficient mutual coupling to achieve effects similar to that of an infinite array. The outer cavities also work as a choke and suppress contributions from neighboring radiating elements, enabling the characteristics of each radiating element with an outer cavity to be maintained in the array.
Elements are arranged at 0.86-wavelength intervals, and the aperture size of the array is defined as 8.4 mm square (2.1 mm × four elements) for the 120-GHz band. The structure has four sets of 2 × 2 element subarrays with outer cavities and an H-junction feed network. The peak gain of the 4 × 4 element array with the outer cavities is 21.2 dBi with 80% antenna efficiency; the peak gain is only 20.2 dBi with 71% efficiency without the outer cavities. Simulations were performed using the High-Frequency Structure Simulator (HFSS) electromagnetic (EM) simulation software from Keysight Technologies. Based on the standard value of copper conductivity, the conductivity loss of the antenna array is only 0.1 dB at 125 GHz.
With and without the outer cavities, the array antenna achieved uniform aperture distribution. Measured gain appeared to be stable in spite of some instability in the measurement system—e.g., less-than-adequate electromagnetic-interference (EMI) shielding. Radiation patterns were measured only for the center frequency at 125 GHz, although the simulations indicate a wide operating bandwidth with high gain. See “4 × 4 Element Corporate-Feed Waveguide Slot Array Antenna with Cavities for the 120 GHz Band,” IEEE Transactions on Antennas and Propagation, December 2013, p. 5,968.