# Low-Profile High-Efficiency Transmitarray Antenna for Beamforming Applications
###### Abstract  
A low-profile high-efficiency transmitarray antenna (TA) for beamforming applications is proposed and investigated in this paper. The partial H-plane waveguide slot array antenna is employed as the compact low-profile feeding structure of the beamforming TA. The designed TA can achieve a high taper efficiency due to the multi-array sources and the compactness of the partial H-plane waveguide. Moreover, the proposed TA can inherently have a high spillover efficiency because the frequency selective surface (FSS) for beamforming is located just above the radiating slot. The FSS with a transmission phase variation of 2\(\pi\) is designed by a square patch array and used to manipulate the wave-front of the transmitted electromagnetic wave instead of a complicated feed network and phase shifters. To verify its beamforming characteristic, three types of FSSs to operate a forming angle of \(-40^{\circ}\), \(-20^{\circ}\), \(0^{\circ}\), \(+20^{\circ}\), and \(+40^{\circ}\) are designed at 12 GHz. The distance between the FSS and the slot array antenna is \(0.10_{0}\), and the aperture efficiency is measured to be about 69%. The measured results, such as the reflection coefficient and the far-field radiation pattern, are in good agreement with the simulated results. From the measured results, the proposed TA is confirmed to have good beamforming characteristics and high aperture efficiency. beamforming; low profile; high efficiency; transmitarray antenna (TA) +
Footnote †: journal: Electronics Letters  
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Footnote 1: email: {[PERSON], j-G. [PERSON], j-H.  
To design a highly efficiency TA, there are two structurally important parameters, such as the diameter (\(D\)) of the MS and the distance (\(F\)) from the feed antenna to the MS. The spillover and taper efficiencies, which are determined by the amount of power reaching the MS from the feed source and their uniformity, respectively, depend on the \(F/D\) ratio. As the diameter of an MS increases, the spillover efficiency increases. Conversely, as the distance of \(F\) increases, the taper (spillover) efficiency increases (decreases), respectively. Therefore, \(F/D\) should be optimized to achieve high aperture efficiency. Most recently studied TAs have an \(F/D\) ratio of 0.2 to 1, and \(F\) is greater than \(1\lambda_{0}\). Therefore, many TAs are inherently bulky and cannot be designed with a low profile. Another type of TA has a multi-array source antenna and MS. Even if the MS is placed close to the source antenna, this type of TA can inherently have high spillover efficiency and high taper efficiency because of the uniform power distribution to reach the MS. As a result, a low-profile beam-steering TA can be designed using a transmissive phase gradient MS. In [17], a conventional waveguide slot array antenna is utilized to achieve the low-profile beam-steering TA for satellite communication. To obtain a high directivity, large areas to implement many radiating slots are required. In this paper, a low-profile high-efficiency TA for beamforming applications is proposed and investigated. The partial H-plane waveguide slot array antenna is utilized as the compact low-profile feeding structure of the beamforming TA. The partial H-plane waveguide is a transversely folded rectangular waveguide that has a partially inserted metal vane in the H-plane [18]. As the partial H-plane waveguide is more compact than the conventional waveguide, the number of radiating slots of the partial H-plane waveguide slot array antenna is larger than that of the conventional waveguide slot array antenna in the same design area. Therefore, the partial H-plane waveguide slot array antenna can have a higher taper efficiency compared to the conventional waveguide slot array antenna. To design an 8 \(\times\) 10 array antenna, a series type of 8-way power divider for the partial H-plane waveguide slot array antenna is proposed.