High-directivity antenna systems that provide 2D beam steering by rotating a pair of phase-gradient metasurfaces in the near field of a fixed-beam antenna, hereafter referred to as Near-Field Meta-Steering systems, are efficient, planar, simple, short, require less power to operate and do not require antenna tilting. However, when steering the beam, such systems generate undesirable dominant grating lobes, which substantially limit their applications. Optimizing a pair of these metasurfaces to minimize the grating lobes using standard methods is nearly impossible due to their large electrical size and thousands of small features leading to high computational costs. This paper addresses this challenge as follows. Firstly, it presents a method to efficiently reduce the strength of "offending" grating lobes by optimizing a supercell using Floquet analysis and multi-objective particle swarm optimization. Secondly, it investigates the effects of the transmission phase gradient of PGMs on radiation-pattern quality. It is shown that the number of dominant unwanted lobes in a 2D beam-steering antenna system and their levels can be reduced substantially by increasing the transmission phase gradient of the two PGMs. This knowledge is then extended to 2D beam-steering systems, where we demonstrate how to substantially reduce all grating lobes to a level below −20 dB for all beam directions, without applying any amplitude tapering to the aperture field. When steering the beam of two Meta-Steering systems with peak directivities of 30.5 dBi and 31.4 dBi, within a conical volume with an apex angle of 96 • , the variation in directivity is 2.4 dB and 3.2 dB, respectively. We also demonstrate that beam-steering systems with steeper gradient PGMs can steer the beam in a wider range of directions, require less mechanical rotation of metasurfaces to obtain a given scan range and their beam steering is faster. The gap between the two metasurfaces in a Near-field Meta-Steering system can be reduced to one-eighth of a wavelength with no significant effect on pattern quality.

Context. The sensitivity of a radio interferometer is a key figure of merit (FoM) for a radio telescope. The sensitivity of a single polarized interferometer is typically given as an antenna effective area over a system temperature, Ae/Tsys, assuming an unpolarized source. For a dual-polarized polarimetric interferometer intended to observe sources of unknown polarization, the state of polarization must not be assumed a priori. Furthermore, in contrast to the narrow field of view (FoV) of dish-based interferometers, the sensitivity of a polarimetric low-frequency radio interferometer warrants a careful review because of the very wide FoV of the dual-polarized antennas. A revision of this key FoM is particularly needed in the context of the Low-Frequency Square Kilometre Array (SKA-Low) where the sensitivity requirements are currently stated using Ae/Tsys for a single-polarized antenna system, which produces ambiguity for off-zenith angles. Aims. This paper aims to derive an expression for the sensitivity of a polarimetric radio interferometer that is valid for all-sky observations of arbitrarily polarized sources, with neither a restriction on FoV nor with any a priori assumption regarding the polarization state of the source. We verify the resulting formula with an all-sky observation using the Murchison Widefield Array telescope. Methods. The sensitivity expression was developed from first principles by applying the concept of system equivalent flux density (SEFD) to a polarimetric radio interferometer (not by computing Ae/Tsys). The SEFD was calculated from the standard deviation of the noisy flux density estimate for a target source due to system noise. Results. The SEFD for a polarimetric radio interferometer is generally not 1/√2 of a single-polarized interferometer as is often assumed for narrow FoV. This assumption can lead to significant errors for a dual-polarized dipole based system, which is common in low-frequency radio astronomy: up to ∼15% for a zenith angle with a coverage of 45° and up to ∼45% for 60° coverage. The worst case errors occur in the diagonal planes of the dipole for very wide FoV. This is demonstrated through theory, simulation, and observations. Furthermore, using the resulting formulation, the calculation of the off-zenith sensitivity is straightforward and unambiguous. Conclusions. For wide FoV observations pertinent to a low-frequency radio interferometer such as SKA-Low, the narrow FoV and the single-polarized sensitivity expressions are not correct and should be replaced by the formula derived in this paper.

No abstract