## 2 Beam Steering Antenna Technologies  
Based on the steering principles involved, beam steering antennas can be broadly classified as mechanical or electrical. [13]. Electrical antenna systems can be further classified as phased arrays or beamforming antennas, including digital, hybrid and analog beamforming [13; 14]. In mechanical beam steering, the antenna system is physically rotated with the help of electric motors to direct the beam in any direction. High-gain antennas such as reflector dishes or arrays of horns are typical examples of mechanically steered antennas [5; 15]. Various mechanical steering antennas for SATCOM On-The-Move (SOTM) are commercially available [16]. Although mechanical parabolic dish-based steering methods maintain antenna gain and offer flexible steering ranges, such systems are undesirable in certain applications due to antenna weight and size, weather effects, and steering speeds. Some of the mechanical steering systems are bulky and expensive. Power requirements are also increased when the complete system has to be rotated. Such designs may not be feasible when the whole system has to be incorporated into a smaller space to reduce the payload. Likewise, for mobile platforms such as cars, trains, or aircraft, integrating a mechanically rotating system proves impractical. In these scenarios, where aerodynamic considerations are paramount, compact and planar antenna systems reign supreme over their bulkier, mechanically rotated counterparts. The other technique uses phased arrays, an electrical beam steering technique that can provide adaptive beam scanning and beam shaping capabilities [17]. The currently available active RF-based electrical steering antennas, including [18; 19], utilize electrical beam steering by incorporating active metasurfaces for full-duplex shared aperture antenna operation. Several other antenna systems are available for SOTM and COTM [20; 21]. Figure 1: The modern wireless communication landscape. Although electrical beam steering systems provide faster scanning speed and compact antenna designs, they require complex feed networks due to the distributed amplifiers and phase shifters in the feed network compared to passive systems with mechanical means to steer the beam. Lately, there has been a notable increase in research interest in metasurface-based beam steering techniques [22; 23; 24; 25]. These techniques are particularly attractive because they offer cost-effective solutions for planar antennas well-suited for large-scale deployment. Metasurface-based beam steering techniques function similarly to phased arrays, but instead of using RF phase shifters, they employ phase delay unit cells [26; 27] or phase rotation unit cells [28]. This innovation enables flexible beam manipulation while reducing the complexity associated with conventional phased-array implementations. Since the passive metasurface does not include any active circuits to introduce phase shifts or control the direction of the beam, it does not require any RF power for its operation. Additionally, because the entire surface is lightweight, low-power DC motors can be used to rotate it. Compared to some reported phased arrays [29; 30], the antenna's gain is higher in [31; 32; 33], which means less RF input power is needed to achieve the same effective isotropically radiated power (EIRP) as reported in [29; 30]. The transmitarrays (TA), refelctarrays (RAs), and near-field meta-steering (NFMS) antennas reported in the literature, have aperture efficiency values ranging from 15% to 63% [31; 32; 33]. Therefore we calculated the gain-to-noise temperature, \(G/T\), assuming an aperture efficiency of 34%, which is the median value. For an antenna with aperture efficiency 34% and the same physical area as the one reported in [29], the calculated gain (G) is 36.14 dB. Assuming a similar receiving system is connected at the back end of the antenna, the calculated noise temperature (T) for a complete receiver is 23 dBK, resulting in a \(G/T\) of 13.14 dB/K--higher than that reported in [29; 30].