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Designing a sparse array with reduced transmit/receive modules (TRMs) is vital for some applications where the antenna system's size, weight, allowed operating space, and cost are limited. Sparse arrays exhibit distinct architectures, roughly classified into three categories: Thinned arrays, nonuniformly spaced arrays, and clustered arrays. While numerous advanced synthesis methods have been presented for the three types of sparse arrays in recent years, a comprehensive review of the latest development in sparse array synthesis is lacking. This work aims to fill this gap by thoroughly summarizing these techniques. The study includes synthesis examples to facilitate a comparative analysis of different techniques in terms of both accuracy and efficiency. Thus, this review is intended to assist researchers and engineers in related fields, offering a clear understanding of the development and distinctions among sparse array synthesis techniques.

In this paper, the design methodology and implementation of single-band and multiple-band elliptic function bandpass filters (BPFs) are presented, based on the concept of bandstop resonator (BSR) sections. One or more single-mode and multiple-mode BSRs can be dangled from a non-resonant node. Each BSR can generate one reflection zeroes (RZ) and one transmission zeroes (TZ). Multiple BSR sections are used to flexibly and independently control the location and bandwidth of the stop bands and therefore the same of the passbands. The method to design single- and multiple-band elliptic function BPFs has been detailed using a number of examples based on waveguide technology. For proof of concept, a 6th-order single-band BPF with six BSR = 2 sections and a 3rd-order dual-band BPF using three BSR = 3 sections are designed and fabricated monolithically using a selective-laser-melting (SLM) 3-D printing technique. Excellent agreement between simulated and measured results verifies the proposed design methodology and its versatility as well as the additive-manufacture approach.

The conventional monopulse comparator, which commonly consists of a power divider, a standard 3dB-coupler and a 90-degree phase shifter, occupies a significantly large area of the antenna aperture. This results in a low efficiency. In order to improve the antenna's aperture efficiency and keep a planar single-layer structure simultaneously, a substrate integrated waveguide (SIW) slots array monopulse antenna excited only by a square dual-mode (TE120 and TE210) SIW resonator is proposed, fabricated and measured in this communication. The sum and difference beams are obtained when the feeding cavity operates at two high-order modes, respectively. The needless of complex feeding network reduces the antenna's structure complexity and improves its aperture efficiency. The aperture efficiency of the antenna can reach 42%. Besides, the sum and difference ports have good isolation due to the orthogonality between the two modes. The proposed antenna obtains a 100 MHz impedance bandwidth, determined by the reflection coefficient less than -10 dB. The measured gain of the sum pattern is 11.9 dBi at 10 GHz, while the null depth of the difference pattern is lower than -27 dB.

The interaction of light with micro-scale dielectric structures has garnered significant attention in recent years due to its potential applications in various fields ranging from photonics to biomedical imaging. In this context, this study investigates the generation of twin photonic jets from a dielectric micro-cylinder illuminated by two point-sources. The effect of different parameters on the jet's shape profile and length was analyzed. Those parameters include the cylinder's refractive index and radius, the source distance from the cylinder, and the vertical displacement between the sources. Innovatively, this study introduces the concept of ‘Horn Jets' to characterize the twin photonic jets resulting from the illuminated dielectric micro-cylinder, marking a significant conceptual advancement in the field. Moreover, the generation of whispering gallery modes (WGMs) is investigated. The outcomes obtained through the spectral element method highlight the possibility of generating WGMs by meticulously selecting the cylinder's radius and refractive index, under the scenario of two-point-source illuminations. The results show that the magnitude of the WGMs resonance significantly lies on the vertical displacement due to the resulting interference pattern's dark and bright fringes with the cylinder edges.

Due to the divergence characteristics of vortex microwaves, it is difficult to determine orbital angular momentum (OAM) orders through far fields, while the spiral spectrum analysis method based on the area integral of antenna near fields is complicated. In this paper, combined with near-field approximation processing, a spiral spectrum analysis method based on loop integration is proposed. According to the Laguerre–Gaussian vortex beam model, the key parameter estimation formulas suitable for low-order OAM microwave beams are derived, including the position of the reliable observation plane and the size of the annulus area. On this basis, the radius of the integral loop used for the spiral spectrum calculation is provided. Since only field data on the loop are collected in this method, the calculation amount is reduced and the analysis efficiency of the spiral spectrum is greatly improved. Due to the combination of near-field and far-field considerations, this method can fully reflect the spiral spectrum characteristics of antenna vortex waves.