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The intensities and phase profiles of the elliptic optics vortex beam both are determined by the topological charge (TC) and ellipticity. This paper presents an efficient and simple method for measuring the TC and ellipticity of elliptic vortex. By observing the diffraction patterns, the TC modulus and ellipticity can be probed with the suitable rectangular aperture, and the TC sign can be detected with the suitable right triangular aperture. The scheme works well even for high-order elliptic vortex with topological charge value as high as ± 18, and ellipticity range from 0.5 to 2 at least. Obviously, the measurement of TC and ellipticity have important research value for promoting the application of elliptical vortex.

Wireless communications with extremely large-scale array (XL-array) correspond to systems whose antenna sizes are so large that conventional modelling assumptions, such as uniform plane wave (UPW) impingement, are longer valid. This paper studies the mathematical modelling and performance analysis of XL-array communications. By deviating from the conventional modelling approach that treats the array elements as sizeless points, we explicitly model their physical area/aperture, which enables a unified modelling for the classical discrete antenna arrays and the emerging continuous surfaces. As such, a generic array/surface model that accurately takes into account the variations of signal phase, amplitude and projected aperture across array elements is proposed. Based on the proposed model, a closed-form expression of the resulting SNR with the optimal single-user MRC/MRT beamforming is derived. The expression reveals that instead of scaling linearly with the antenna number M as in conventional UPW modelling, the SNR with the more generic model increases with M with diminishing return, which is governed by the collective properties of the array, such as the array occupation ratio and the physical sizes of the array along each dimension, while irrespective of the properties of the individual array element. Additionally, we have derived an alternative insightful expression for the optimal SNR in terms of the vertical and horizontal angular spans. Furthermore, we also show that our derived results include the far-field UPW modelling as a special case. One important finding during the study of far-field approximation is the necessity to introduce a new distance criterion to complement the classical Rayleigh distance, termed uniform-power distance (UPD), which concerns the signal amplitude/power variations across array elements, instead of phase variations as for Rayleigh distance.

This communication presents a design of a dual-band circularly polarized antenna with structure reuse for 1U CubeSat applications. It is a challenging to design an antenna with the limited space of a CubeSat platform. To improve the space utilization, an antenna is designed inspired by the concept of structure reuse. A meshed-like-structure is designed, which works as the radiator of a microstrip antenna at UHF band. However, it works as a partial reflective sheet for a Fabry-Perot resonator cavity antenna at C band. Therefore, a microstrip patch antenna and a Fabry-Perot resonator cavity antenna share only single aperture. A shaped ground is used to decrease the resonance at the lower band. Then, the aperture of the antenna is only 0.3λ0 × 0.3λ0, which makes it easy to integrate with a 1U CubeSat. The measurement reveals that the antenna has a common bandwidth of (|S11| < -10 dB, AR < 3 dB) 1% and 1.4%, an isolation higher than 35 dB and 20 dB, a peak gain of 5.82 dBic and 13.75 dBic at UHF and C band, respectively. The frequency ratio of these two bands reaches 6.4. The above properties make the proposed antenna suitable for a CubeSat system.

Compton imaging is the main method for locating radioactive hot spots emitting high-energy gamma-ray photons. In particular, this imaging method is crucial when the photon energy is too high for coded-mask aperture imaging methods to be effective or when a large field of view is required. Reconstruction of the photon source requires advanced Compton event processing algorithms to determine the exact position of the source. In this study, we introduce a novel method based on a Deep Learning algorithm with a Convolutional Neural Network (CNN) to perform Compton imaging. This algorithm is trained on simulated data and tested on real data acquired with Caliste, a single planar CdTe pixelated detector. We show that performance in terms of source location accuracy is equivalent to state-of-the-art algorithms, while computation time is significantly reduced and sensitivity is improved by a factor of ∼5 in the Caliste configuration.

We describe a new type of ultra-wideband feed antenna based on nested coaxial waveguides. The feed is for radio astronomy applications and covers two closely spaced bands: 400–800 MHz and 900–1800 MHz. New design techniques are described for a three-cavity choke, a wideband iris matching network, and printed circuit board excitation structures. The design methods were developed to improve broadband impedance matching, to control radiation patterns, to separate bands, and to suppress unwanted waveguide modes. The feed has nearly constant beamwidth over the entire 4.5:1 frequency range and a stable phase center. On a f/D = 0.43 reflector antenna, the aperture efficiency exceeds 70% and the phase efficiency exceeds 95%. Other features include low ohmic loss and a compact, lightweight, and low-cost structure. Experimental results are shown for a scaled prototype to verify the design methodology. The methods described are general and can be applied to continuous frequency coverage on reflectors of different geometries.

We experimentally demonstrate an elliptical-aperture multimode diversity receiver to mitigate the impairment from anisotropic turbulence, which is emulated by employing an electrically-controlled heater. The anisotropy gradually decreases as the flow-level of the introduced airflow from fans increases. In contrast to circular-aperture reception, the elliptical-aperture multimode diversity receiver can effectively resist beam wander under anisotropic turbulence, reducing outage probability from 14.63% to 0.38% for detecting a dual-polarization 30-Gbaud QPSK signal. Without the aid of an adaptive optics system, this work demonstrates the feasibility of realizing an anisotropic-turbulence-resistant FSO link using multimode and multi-aperture diversity reception employing digital coherent combining techniques.

A synthesized strategy for achieving polarization-insensitive metalens is proposed by imposing identical parabolic phase patterns individually on two decoupled left-handed (spin up, σ+) and right-handed (spin down, σ-) circular polarization (CP) channels of an anisotropic scheme. This is enabled by synergizing dynamic phases and local geometric phases within 0° and 90° rotations. As a consequence, similar focusing behavior is expected when it is illuminated by a plane wave of arbitrary polarization since any incoming wave is a superposition of σ+ and σ- wave. Such an intriguing feature can be utilized to engineer a polarization-insensitive reflectarray/transmitarray in a shared aperture by placing a feeding source of different polarizations at its focal spot without altering the array aperture. Therein, the polarization of the meta-array is determined by its feeding which is readily to be changed in practice. For demonstration and possible applications, two reflectarrays composed of 50 × 50 elements at Ku band with one in normal and the other in offset configuration were designed and experimentally characterized. Numerical and experimental results illustrate that both reflectarrays are able to work in dual-linear (LP) and dual CP states by utilizing LP and CP horns of different orientations. Moreover, the versatile polarization-agile property does not pose significant penalty on its radiation performance and both reflectarrays exhibit a measured peak aperture efficiency of 63.8 and 39.6%, respectively near 14 GHz, and a 1dB gain bandwidth more than 11.8 %.

This work investigates the spectral quantum efficiency and inter-pixel crosstalk of a MWIR-LWIR dual band, HgCdTe-based focal plane array (FPA) photodetector (MWIR and LWIR stand for mid- and long-wavelength infrared bands). Pixels are $\text{10}\,\mu$ m-wide with truncated pyramid geometry and separated by deep trenches. Three-dimensional combined full-wave electromagnetic and electrical simulations in the drift-diffusion approximation allowed to describe the complex, standing-wave-like spectral features resulting from the light interference and diffraction due to the pixels and illuminating beam aperture. The inter-pixel crosstalk for the MWIR operation demonstrated to be very sensitive to the trench depth, in contrast to the LWIR electrooptical response, left almost unchanged. The present work also investigates the causes of performance worsening in the two IR bands when pixel pitch is reduced to $\text{5}\,\mu$ m, hence well below typical LWIR wavelengths and close to the diffraction limited operation.

This paper presents a shared-aperture dual-band dual-polarized antenna array for fifth-generation (5G) mobile communication. Within one radiating aperture, four higher-band (HB) patch antenna elements are placed over the lower-band (LB) full-wavelength slot antenna. The proposed array is miniaturized by bending the ground plane of the slot antenna. Suppression of cross-band mutual coupling and scattering is achieved by using the filtering performance of the slot antenna and a dual-function metasurface. The filtering performance of the slot antenna is realized using coupled microstrip lines inside the radiating slot and open-ended stubs on the feeding lines. The dual-function metasurface operates as an artificial magnetic conductor reflector in the LB and bandpass frequency selective surface in the HB. By placing the dual-function metasurface behind the slot antenna, the radiation of the slot antenna is enhanced in the LB but not HB, and the negative effect of its HB scattering on the HB antennas is reduced. The working mechanism is explained in detail. The measured and simulated results demonstrate that the proposed array has stable radiation patterns in the bands of 0.69-0.96 and 3.4-3.7 GHz with a high cross-band isolation of over 32 dB, and is more compact than the previous works.

The problem of transient free convection in an open cavity containing two elastic thin fins is studied in this paper. The left surface is heated, and the right surface is in opening, which is in contact with the environment air. The dimensionless forms of the governing equations for continuity, momentum and heat transfer in the fluid and structures domains are written in Arbitrary Lagrangian–Eulerian (ALE) formulation. The governing equations are solved by the finite element method (FEM). The governing parameters considered are the oscillating fin direction, fin amplitude and length with several flexibility values and aperture sizes. The results show that a maximum heat transfer enhancement occurs at the suitable parameters between the aperture size and the fins oscillating direction.

This paper presents a dual-polarized Frequency-scaled Ultra-wide Spectrum Element (FUSE) array on a triangular lattice, precision-machined from an Aluminum block as a self-supporting structure, similar to all-metal Vivaldi arrays reported by the authors in past work. Whereas dual-polarized FUSE arrays normally consist of offset radiator pairs conforming to the ubiquitous square-lattice egg-crate configuration and capacitively coupled at the ends to achieve ultra-wideband operational frequency ranges, this work is novel in that it uses conventional manufacturing to directly machine the dual-polarized FUSE array on a triangular lattice instead. Triangular lattices for dual-polarized wideband arrays are uncommon in the literature and have the benefit of 15.5% reduction in element counts over a square lattice aperture of the same footprint and upper frequency of operation, making this a useful development for applications requiring minimum element counts, i.e., reduced cost/weight. In this paper, embedded element performance predictions for the dual-polarized triangular-lattice FUSE array are presented and validated with measurements of a hardware prototype. Results show similar overall grating-lobe-free performance to a square lattice FUSE array of the same footprint and operational frequencies reported in earlier work. The asymmetries of the triangular lattice result in relative performance degradation vs. square lattice FUSE arrays as a trade-off to the benefit of reduced element counts for apertures of similar size.

Real-time, three-dimensional (3D), passive acoustic mapping (PAM) of microbubble dynamics during transcranial focused ultrasound (FUS) is essential for optimal treatment outcomes. The angular spectrum approach (ASA) potentially offers a very efficient method to perform PAM, as it can reconstruct specific frequency bands pertinent to microbubble dynamics and may be extended to correct aberrations caused by the skull. Here we assesses experimentally the abilities of heterogeneous ASA (HASA) to perform trans-skull PAM. Our experimental investigations demonstrate that the 3D PAMs of a known 1 MHz source, constructed with HASA through an ex vivo human skull segment, reduced both the localization error (from 4.7±2.3 mm to 2.3±1.6 mm) and the number, size, and energy of spurious lobes caused by aberration, with modest additional computational expense. While further improvements in the localization errors are expected with arrays with denser elements and larger aperture, our analysis revealed that experimental constraints associated with the array pitch and aperture (here 1.8 mm and 2.5 cm, respectively) can be ameliorated by interpolation and peak finding techniques. Beyond the array characteristics, our analysis also indicated that errors in the registration (translation and rotation of ±5 mm and ±5°, respectively) of the skull segment to the array can led to peak localization errors of the order of a few wavelengths. Interestingly, errors in the spatially dependent speed of sound in the skull (±20 %) caused only sub-wavelength errors in the reconstructions, suggesting that registration is the most important determinant of point source localization accuracy. Collectively, our findings show that HASA can address source localization problems through the skull efficiently and accurately under realistic conditions, thereby creating unique opportunities for imaging and controlling the microbubble dynamics in the brain.

In this paper, we propose a traveling-wave endfire antenna based on the first higher-order transmission line (TL) mode, and it features wide bandwidth, high gain, and large aperture efficiency. We propose two essential conditions for the realization of the proposed wideband endfire antenna. Firstly, the dispersion curve of the first higher-order mode should be made close to that of the air line across a wide frequency band. Secondly, the fundamental mode should be suppressed to allow single higher-order mode operation across a wide frequency band. Following these principles, we provide a design for an endfire antenna based on a periodically slotted parallel stripline, where the periodic meandered slots suppress the fundamental mode and hence allow single higher-order mode radiation. To verify the proposed approach, a prototype has been designed and fabricated. The antenna can achieve endfire radiation within a 1:2 frequency band (8.5-17.5 GHz), with an average gain of 10.7 dBi, and large aperture efficiency up to 6.38 dBi/λ20.

A broadband low-profile folded transmitarray is proposed and investigated, in which a novel wideband transmission polarizer (TP) is used to realize low-profile design. The proposed TP, mainly consisting of a differential-fed grid-slotted patch antenna, can realize high-efficiency transmission and polarization selection simultaneously within a wideband frequency range. Moreover, the proposed TP can realize linear transmission phase because of the true-time delay (TTD) technique. Owing to the function of polarization selection and linear transmission phase, the TP can be used as antenna element for a folded transmitarray. A 140-element folded transmitarray (FTA) is designed and fabricated by combining the TP with a broadband polarization rotation reflecting surface (PRRS). The measured radiation patterns agree well with the simulated results, and a peak gain of 24 dBi and maximum aperture efficiency of 44.7% are achieved. Furthermore, -1 dB and -3 dB gain bandwidth with 25.9% and 33.3% are also experimentally demonstrated in the proposed FTA, respectively.

High-gain single-feed antennas are desired for current and future wireless systems. Slot-based highly overmoded cavity antennas are one solution, but the fact that they can only radiate broadside beams limits their applications. An innovative phase-control technique is developed for slot-based resonant cavity antennas that utilizes a planar array of closely-spaced, appropriately-sized metallic patches integrated into the very near field of their slot arrays. It facilitates the design and realization of the phase distribution over their radiating aperture that is required for them to generate single and/or multiple tilted beams pointed at specified directions. A TE(10)(11)(0)-mode slot-based cavity antenna is selected as the pathfinder system. A TE110-mode single-slot cavity antenna that is equivalent to the half-wavelength subsection of that cavity is introduced initially to monitor the phase radiated from each slot. The phase control surface is then carefully engineered to facilitate tilted output beams from the base design. The approach is validated with two subsequent tilted-beam systems. The first system achieves a single beam tilted to 30° with respect to the broadside direction. The second system is extended to produce two independently-directed titled beams pointing at –10° and 20°, respectively. All of the measured results agree well with their simulated values.

We present a study on beam coupling in radiating structures that support multiple simultaneous beams. The formation of multiple beams is relevant in modern wireless communication applications when diverse data streams can be sent from a single transmitter to users located in different directions. General expressions are provided that relate the coupling between radiated beams to the associated current distributions on the radiating aperture. When expanding the currents in terms of basis functions, the beam coupling can be also written in terms of coupling contributions due to each pair of basis functions. The analysis is applied to antenna arrays with different levels of mutual coupling between the array elements. More specifically, arrays of resonant elements, characterized by very low mutual coupling, are compared with arrays of connected elements, which are strongly coupled. We show that, even for a very high level of mutual coupling between individual array elements, it is possible to obtain orthogonal beams if the total current distributions associated with the beams are uncoupled.

This paper proposes a shared-aperture self-diplexing antenna using the composite parallel-plate waveguide (PPW). The long-straight slots (LSS) etched on the top surface of the PPW are utilized as the common radiation aperture for two operating bands. At the high frequency band, the proposed antenna, excited by the alternative-phase TE10-mode wave, operates as a leaky-wave antenna (LWA). Meanwhile, it serves as a continuous transverse stub (CTS) array antenna when excited by the TEM wave at the low frequency band. Two kinds of feeding networks are adopted to generate the alternative-phase TE10-mode wave and TEM wave, respectively. To verify the proposed method, a millimeter-wave prototype is designed, fabricated and measured. The measured results show that -10 dB fractional bandwidths are 3.42% (76.25-78.9 GHz) and 14.5% (83.2-96.2 GHz), respectively, and the port isolation is higher than 40 dB. Additionally, the peak gains are 27.1 dBi at 78.5 GHz and 25.3 dBi at 90 GHz, respectively. The proposed antenna can be used to develop compact frequency-division system without using duplexers and multi-band wireless systems.

Lumped-element grids provide an attractive option for wave propagation control in electromagnetic compatibility (EMC) engineering. This article investigates the peak shielding effectiveness (SE) of a diode grid used for protection of enclosures with apertures against high-intensity radiation fields (HIRFs). The nonlinearly loaded aperture is investigated with an efficient hybrid field-circuit simulation approach. Numerical experiments show that design aspects, e.g., aperture and enclosure size, grid density, impedance characteristic of lumped loads, play an important role in the field transmission through a diode grid which is nonlinear and time-variant. With a physics-based analysis of the interaction between the grid and the enclosure-backed aperture, nonlinear shielding techniques are identified that allow a control of the peak SE between 0 and 26 dB in novel ways. For the first time, the peak SE curve of a diode grid is demonstrated with four different field intensity dependencies, i.e., intensity low-pass, intensity high-pass, intensity bandpass, and intensity band-stop selectivities from tens V/m to hundreds of V/m. By considering design aspects into a two-step optimization procedure, practical guidelines are provided for the nonlinear shielding implementation.

A novel dual-band circularly polarized (CP) shared-aperture antenna is proposed in this letter. The overall structure consists of an SIW slot antenna working in X-band which also serves as the top patch of a C-band microstrip-like antenna. The SIW slot antenna achieves CP radiation by etching a circular slot on the top surface of a dual-mode SIW cavity. The patch antenna realizes CP radiation by loading two pairs of asymmetric shorting vias. With this design method, the proposed antenna can achieve dual-band CP operation and high aperture reuse efficiency. Besides, feeding-pin of the SIW slot antenna inserts through one of asymmetric shorting vias, that is, the shorting vias serves as both perturbations for C-band operation and a feeding path for the X-band operation. This new feeding scheme can prevent signal crosstalk between the two bands.

A new architecture is provided for the efficient generation of shaped beams and multiple shaped beams (in amplitude, phase, and polarization), with leaky-wave (LW) metasurfaces (MTSs). Those antennas are usually fed by a surface-wave (SW) launcher, which evenly illuminates the MTS versus azimuth, through a cylindrical SW. When the interest lies in generating pencil beams or particular types of shaped beams, the required radiating aperture field (linked to the Fourier transform of the desired radiation pattern) on the MTS is almost azimuthally symmetric in amplitude. However, for a wide class of shaped beams, the desired aperture field is far from being azimuthally symmetric. This leads to an MTS design in which some sectors exhibit a very small surface impedance modulation. Those sectors tend to leave the SW intact until it reaches the rim of the surface, where the power is either reflected or diffracted at the slab rim. We solve this efficiency problem by reducing the amount of power fed into some sectors, through the use of an array of feeding monopoles. The azimuthal illumination of the feeder should be designed to match the azimuthal asymmetry of the desired aperture field. The corresponding MTS surface impedance is then determined using a method of moments (MoM)-based synthesis technique. Several examples are provided to illustrate the concept. The improvement in terms of antenna efficiency is numerically demonstrated.

This letter presents an end-fire fan-beam radiation antenna by utilizing a gradient refractive index metamaterial lens. The lens includes one core and two impedance matching layers which are both based on the proposed metamaterial unit cells with a large refractive index variation range. An incident wave is excited from the antenna source and propagates to the lateral side of the lens. Two parallel plates set for the TE1 mode condition are utilized such that E-plane radiating beam can be controlled. To validate our proposed design, a prototype working at 10 GHz is fabricated and measured. Measured results are in good agreement with the simulation. The height and thickness of the metamaterial lens are about 0.44 0 and 0.36 0, where 0 is the wavelength of the center frequency in free space. The measured E- and H- planes 3-dB beamwidths are 25o and 82o at the center frequency, respectively. The measured peak gain of the antenna is about 11.1 dBi, which implies a 5.4 dB gain enhancement compared with initial radiation source. Furthermore, this lens offers a high aperture efficiency of 72.4%.

The explicit expression of orbital angular momentum (OAM) spiral spectrum of Laguerre-Gaussian (LG) beam propagating in anisotropic plasma turbulence is derived based on Rytov theory. The relationship between the detection probability of OAM and the crosstalk with the anisotropy parameters, topological charge, radial index, wavelength, beam width, receiving aperture, inner and outer scales are discussed in detail. In order to quantify the effect of plasma turbulence, we compared it with atmospheric turbulence based on restriction function. To optimize OAM detection probability, we also compared the effects of circular aperture method (CAM) with focusing mirror method (FMM) on the propagation characteristics of the spiral spectrum. The results show that even in the strong turbulence region of atmospheric turbulence, the radial dimension of LG beam in plasma turbulence is nearly twice that of atmospheric turbulence and the modes crosstalk has almost reached its upper bound. Besides, by comparing the two optimization methods, we found the ability to reduce crosstalk is related to the choice of beam width and receiving aperture. Therefore, we give a method to optimally select between the CAM and the FMM. We found the corresponding relationship and gave an analytical formula. The FMM has a greater ability to reduce crosstalk than the CAM when the beam width ≥4 cm and the aperture ≥3 cm. We quantified the effect of plasma turbulence and provide a guideline for the selection of beam width and receiving aperture under these two optimization methods. The research results may be helpful in the field of optical communication.

In order to effectively exploit the orbital angular momentum (OAM) characteristics, it is essential to have devices which are capable of generating beams with multiple OAM modes simultaneously. Most of the proposed techniques commonly require time division multiplexing at the transmitter side or beam splitters at the reception side for separating OAM modes and directing beams of different OAM modes to different directions. In order to overcome this problem, here, the concept of aperture sharing for generating concentrically embedded OAM carrying beams is presented. Such a methodology provides embedded beams of various OAM modes which are launched toward a single direction through a shared aperture of transmitarray (TA), while having a spatial division between them. In this manner, the transmitted beams are initially separated by the hardware at the transmitter level. A novel circularly-polarized (CP) substratecoupled unit-cell is designed and used as the elemental components on the aperture of the TA. Three prototype TAs are realized for investigating different cases of the OAM modes embedding and verifying the effectiveness of the proposed method. The obtained simulated and experimental results are consistent which validates the novel technique.

A high-gain low-profile leaky-wave antenna (LWA) for vertically-polarized (VP) fixed-beam endfire radiation is proposed in this article. The LWA is composed of two back-to-back half-mode waveguides with grooves etched at the margins of both radiation edges. By etching grooves, the phase velocity of the traveling wave can be tailored to slower than the wave velocity in free space for fitting the Hansen-Woodyard condition. Compared with periodic LWAs, the proposed antenna continuously radiates at two symmetric apertures, contributing to a high endfire gain. A prototype is fabricated and tested to validate the concept. The measured and simulated results are in good agreement. A simulated gain of 14.29 dBi and a measured gain of 13.37 dBi is achieved within an antenna length of 3.05 λ0. Moreover, a measured overlapping bandwidth of 2.74 -3.17 GHz (14.3%) is realized with S11 < -10 dB and gain variation less than 3 dB. To the best of the authors’ knowledge, the proposed VP endfire LWA realizes the highest aperture efficiency and shows great advantages in high gain, low profile, and wide bandwidth.

Based on simultaneous control of the phase and polarization, metallic waveguide transmiarray antennas (MWTAs) with a single feed are proposed to generate multibeams with both high gain and optional polarized states (circular, elliptical, or linear polarization) in the F-band. The phase delay of two orthogonal components can be adjusted in the full range of 0-360° by varying the dimension of each hole in the array. The Gerchberg-Saxton algorithm based on Fresnel diffraction theory is used to calculate the required phase distributions for generating multibeams, which are further achieved by the phase control of unit cells in the MWTA. The phase difference of the two orthogonal components is set to be π/2, which allows us to achieve the required radiating polarized states by selecting the polarization direction of the incident wave. As a proof of concept, the circularly polarized dual- and triple-beam MWTAs with a single feed are designed and simulated. The peak gains are up to ∼29 and ∼28 dBi, the axis ratios are lower than 1.9 dB and 1.6 dB, and the aperture efficiencies are 50.6% and 63.4%, respectively. According to the design, the MWTA prototypes are fabricated for generating the dual- and triple-beams. The measured results in the far field agree well with the simulation. The proposed MWTAs are very promising for applications in wireless and satellite communications in the terahertz and millimeter wave bands.

The dynamics and control of a tetrahedral spacecraft formation flying in the Sun-Earth L2 region is initiatively studied, based on the circular restricted three-body problem (CR3BP). Driven by the science goal of identifying extra-solar terrestrial planets and the requirement of imaging optics, a conceptional four-spacecraft triangular pyramid configuration has been proposed for the Multiple-spacecraft Exoplanet Aperture sYnthetic INterferometer (MEAYIN) project, China’s first mid-infrared interferometric imaging mission. Although it looked promising from an optical perspective, the configuration has not been verified dynamically. The formation is required to be virtually “rigid”, because its mutual distances and inertial pointing direction must be maintained with very high accuracy during each observation. In this study, the spatial geometrical relationship between the four spacecraft was established by introducing the parameters of lengths, angles, and a reference vector. The first contribution is that a compact set of normalized factors and critical time indices are defined, which can provide a complete description of the drift of the shape and pointing direction of the configuration, caused by the unstable dynamical environment. Five design variables are isolated and analyzed, and their individual impacts on the uncontrolled evolution of the formation are studied. The main results obtained reveal that the dimensions of the rigid configuration allow a free drift for a time period on the order of tens of hours, while the inertial pointing direction will be lost within merely tens of seconds. Therefore, to form a rigid configuration, the control challenge lies in the fact that control efforts are frequently required for each spacecraft in the fleet, owing to the diverging dynamics. As a second contribution, a simple and feasible control algorithm is proposed to maintain the rigidity of the formation configuration. The results indicate that the associated energy cost is merely 0.05 m/s per observation on average.

The design of a 1 × 0.33 m dual-polarized reflectarray antenna (RA) operating in Ku-Band (17.2 GHz) for a synthetic aperture radar (SAR) mission using a 12 U CubeSat is presented. The SAR RA has larger electrical size, larger aspect ratio, and smaller focal to aperture length ratio (F/L) compared to existing CubeSat RAs designed for communication payloads. Electrically small unit cells accounting for large incidence angle variations, improving phase resolution, and preventing grating lobes are designed. To illuminate the RA with the desired elliptical beam, a compact aperture-coupled microstrip patch array antenna was designed. Tests done on a CubeSat mockup integrating the dual-polarization feed source and a five-panel reflectarray showed good agreement between the simulated and the measured beamwidths, with low sidelobe and cross polarizations levels.

Synthetic impulse and aperture radar (SIAR) is an orthogonal multiple input multiple output radar. It has several advantages, including the ability to detect multiple targets, Antiradiated Missiles (ARM), and stealth targets. Despite its four-dimensional (4-D) nature, it exhibits a strong coupling between range and angle, which dictates how well SIAR performs in the range-angle domain. Not only does the SIAR use frequency codes to separate the antennas, but also to ensure orthogonality. They can, however, mitigate the strong coupling between range and angle. Pulse-to-pulse frequency code agility is a technique for suppressing the SIAR radar's sidelobes. In this letter, we propose a method for reducing the coupling between range and angle by combining sequential positive, negative and random code with the dot product. The idea of coupling reduction presented in this letter is based on the ratio of each antenna position and frequency codes, which is referred to $\alpha $ k , and multiple $\alpha $ k in an array provides new advantages for target detection. This technique has the potential to significantly reduce the sidelobe levels.

This article proposes a periodic leaky-wave antenna that is based on a microstrip-fed periodically modulated transverse-slot array in the ground plane. The profile of the transverse-slot array is periodically modulated to introduce a fast space harmonic ( $n = -1$ ) that generates a narrow beam scanning in the backward and the forward quadrants. Different profile modulations are studied for suppressing the open stopband and the $n = -2$ space harmonic. If the slot profile is designed to have the impedance matched at the broadside frequency, the leaky-wave antenna has the open stopband suppressed and allows continuous scanning of the main beam through the broadside from the backward endfire to the forward direction. However, it cannot avoid the $n = -2$ space harmonic, which causes another stopband that limits the scanning range in the forward quadrant. If the slot profile is modulated with a profile such that the aperture illumination excludes the $n = -2$ space harmonic, the leaky-wave antenna avoids the stopband and grating lobe caused by the harmonic and thereby has the scanning range in the forward quadrant enlarged, but its open stopband cannot be suppressed simultaneously. Prototypes with the two profile modulations are fabricated and measured for validation.

We present the optical layout and actual design of a compact high-aperture referenceless point diffraction interferometer with a diffraction reference wave. The basic concept of this interferometer is based on a single-mode optical fibre with a subwave output aperture, which is used as a source of spherical reference waves. The interferometer is designed for high-precision measurements of the surface shape and aberrations of optical elements. It makes use of a diffraction reference wave and is developed for mass industrial applications. The proposed design does not require the involvement of special external conditions, and reduces the need for calibrated reference surfaces. We evaluate the performance of our interferometer for highprecision measurements and demonstrate its use by presenting results obtained from measuring the shapes of spherical references from various manufacturers and the aberration of a five lens objective.

We report on the realization of a multichannel imaging radar that achieves uniform 2-D cross-range resolution by means of a linear array of a special form of leaky-wave antennas. The presented aperture concept enables a tradeoff between the available range resolution and a reduction in the number of channels required for a given angular resolution. The antenna front end is integrated within a multichannel radar based on stepped-carrier orthogonal frequency-division modulation, and the advantages and challenges specific to this combination are analyzed with respect to signal processing and a newly developed calibration routine. The system concept is fully implemented and verified in the form of a mobile demonstrator capable of soft real-time 3-D processing. By combining radio frequency (RF) components operating in the $W$ -band (85–105 GHz) with the presented aperture, a 3-D resolution of less than ${1.5}^\circ \times {1.5}^\circ \times $ 15 cm is demonstrated using only eight transmitters and eight receivers.

A W-band low-profile dual-polarized Cassegrain reflectarray with an integrated feed is designed in this work. High isolation, which is necessary for the in-band full-duplex (IBFD) applications, is realized between two feeding ports corresponding to the orthogonal polarizations. The antenna is composed of two parts, i.e., the dual-polarized planar-array feed and the Cassegrain reflectarray with a small focal-length to diameter ratio ( $F/D)$ . The dual-polarized planar array feed is appressed behind the main-reflectarray, and the $F/D$ is reduced to 0.2 to realize the integration and the low-profile property. For the array feed, high isolation is achieved through the orthogonal-mode isolation and the differential signal cancellation. Its radiation pattern is also manipulated to fit the reflectarray considering the taper and spillover efficiency. For the reflectarray, the rotational symmetry configuration and the dual-layer-patch cell are applied to suit the dual-polarization operation and the large incident angle, respectively. The measured operation band, where the gain variation is within 1 dB, is 93.4–95.6 GHz. At the center frequency, the gains of the Cassegrain reflectarray for the two ports are 31.86 and 30.88 dBi, and the corresponding aperture efficiencies are 31.4% and 25.1%, respectively. The isolation between the two ports is higher than 55 dB within the operation band. This work is a low-profile candidate for W-band point-to-point IBFD communication.

This article presents a novel miniaturized circularly polarized (CP) antenna based on periodically structured surfaces inspired by the vertical split-ring resonator (VSRR). The VSRR is considered as a magnetic dipole placed above the ground, which could excite strong boresight radiation with a reduced size. To enhance the aperture gain, multicapacitors and inductors (Multi-C&L) are loaded on the loop, which replace the single capacitive gap of the conventional split-ring resonator (SRR). Here an SRR-inspired periodic structure with a relatively uniform field distribution is achieved. The stepped impedance resonator (SIR) structure, as the unit cell of the SRR-inspired surface, is analyzed and investigated using the circuit models derived from the SRR. By cascading multiple high-value capacitors, a larger aperture with a uniform field distribution is obtained. Linearly polarized (LP) and CP antennas are studied. The proposed CP antenna is made of thin substrates and metallic screws, resulting in a negligible dielectric loss. It offers attractive features including a compact size, a low profile ( $0.191\lambda _{0} \times 0.191\lambda _{0} \times 0.032\lambda _{0}$ ), good radiation efficiency, novel working principle, flexible performance control, and a bandwidth well covering the 902–928 MHz UHF band, which is well situated for radio frequency identification (RFID) application.

A dual-broadband dual-polarized magnetoelectric dipole (ME-dipole) antenna with a shared aperture for 5G applications is proposed in this communication. The antenna is operated at the 2.35–3.93 GHz (N41 and N78) and 24–34 GHz (N257 and N258) dual fifth-generation (5G) bands. Each band exhibits a dual-polarized radiation, and more importantly, the two bands shared the same radiation aperture. In the millimeter-wave (MM-wave) band, the proposed antenna can generate two-dimensional (2-D) multiple beams with dual-polarization, and the switching range at each plane is about ±20°. The antenna is composed of two kinds of ME-dipole. The MM-wave band uses an ME-dipole feeding through the substrate-integrated waveguide (SIW) as the source to excite four horn antennas with inclined inner walls to achieve high-gain 2-D switching with dual-polarization. The lower frequency band combines the four horn antennas to achieve a large dual-polarized ME-dipole antenna. The antenna operating mechanism and the design procedure are illustrated in detail. The measured results show that a −10 dB impedance bandwidth of 33.91% (24–33.91 GHz) and 50.31% (2.35–3.93 GHz) is achieved. The peak gain of the proposed antenna in the two bands is 10.67 dBi (3.8 GHz) and 14.85 dBi (32.2 GHz), respectively. Due to the robust characteristics of the ME-dipole and horn antenna, the antenna shows a stable pattern and a wide impedance bandwidth, which covers most of the current 5G microwave and MM-wave bands. The antenna features a shared aperture, large frequency ratio, dual-broadband, dual-polarization, and 2-D multiple beams, which are well suited for the 5G base station applications.

This paper investigates tilt decorrelations due to atmospheric anisoplanatism occuring when observing wavefronts emerging from distinct line of sight. The targeted application is ultimately the (pre-)compensation of the atmospheric turbulence experienced by a laser beam during ground-to-satellite optical links. The purpose of the study is to evaluate the effectiveness of the uplink pre-compensation, if the downlink signal (received from the satellite) is used as a reference. Because of the point-ahead angle of the satellite, one expects some decorrelation between the downlink and the uplink signals, which, in turn, impacts the efficacy of the pre-compensation. The larger the beam, the smaller its divergence and the more sensitive it is to pointing errors. In this framwork, a test campaign was carried out in May 2018 at the Optical Ground Station (OGS) of the European Space Agency (ESA), to perform measurements of double stars featuring angular separations representative of the point-ahead angle of GEO/LEO satellites. The differential Tip/Tilt distortion between the double stars is used as an estimator of the typical decorrelation between the downlink and the uplink signal, hence the present study. The algorithm used to extract the tip-tilt error due to anisoplanatism is described, and the experimental results are compared to the numerical predictions. It is then shown how to estimate the jitter of the telescope, based on the common motion of two independent stars as seen in the focal plane of the telescope. Finally, the paper provides a methodology to determine the maximum transmitter aperture of a ground-based terminal, in case a tilt pre-compensation is applied based on the satellite signal.

The Shack-Hartmann wavefront sensor restricts the dynamic range and detection accuracy of wavefront detection due to the limitation of the lens array aperture. This study proposes a system using signal light nutation to detect wavefront distortion, providing high spatial resolution and detection dynamic range. Simulations show that the root mean square (RMS) and mixing efficiency of the compensated wavefront are optimized. In the case of strong turbulence, the mixing efficiency is increased by nearly 105 times, and the RMS is reduced to the original value of 0.048.

In order to evaluate the transmission performance of orbital angular momentum (OAM) beams with a parabolic antenna, this letter investigates the attenuation of OAM beams reflected by the parabolic reflector. Specifically, the system structure and mathematical models of the OAM transmission with a parabolic antenna are established. According to the geometrical optics approximation and aperture field method, the electric field strength after the parabolic antenna is solved. In the case of the paraxial approximation, for the zero-order OAM mode, the receiving power is inversely proportional to the square of the transmission distance. However, for all the nonzero-order OAM modes, the receiving power is inversely proportional to the fourth power of the transmission distance. Compared with OAM beams generated by the traditional antenna array without the parabolic reflector, the receiving power can be improved greatly by the proposed structure with the parabolic reflector used for the OAM beam convergence. Especially for OAM beams with the high orders, the attenuation is no longer related to the OAM mode.

A broadband folded transmitarray antenna (FTA) with ultralow-profile, high-gain, and high polarization purity is proposed in this communication. It consists of a focusing metasurface (MS) with the function of reflecting and transmitting two orthogonal linear-polarized waves, a reflection-type linear polarization conversion MS, and an open waveguide as a feed. With elaborate design, a −3 dB gain bandwidth of 43.5% from 9 to 14 GHz, a peak gain of 23.5 dBi, and a maximum aperture efficiency of 32.8% are achieved. Meanwhile, the −20 dB cross-polarization level at the broadside direction covers the entire operating band. Furthermore, it is worth mentioning that the profile of the proposed FTA is reduced to about 1/3 of the focal length and the profile-to-diameter ratio is only 0.22. The significant promotions in performance indicate the remarkable application value in portable communication systems.

In this work, a method of focal region flux characterization of Scheffler fixed focus concentrator is presented. Being a fixed focus concentrator, Scheffler dish serves as an ideal focusing unit for on-Sun testing of scaled down high pressure receiver designs under actual boundary conditions. Flux characterization is performed numerically by tracing the ray trajectories from their inception till the focal region and experimentally using point-based heat flux sensors. A circular plate is mounted at the focal region with five water cooled heat flux sensors attached. Four of these sensors are positioned around a diameter of size corresponding to an existing cavity receiver aperture diameter and one sensor is located at the plate centre. These sensors help to obtain the point based flux measurements in the region. Since the dish is not parabolic, a non-Gaussian flux profile is obtained, and spatial averaging of the flux data is performed analytically to obtain an averaged heat flux value. This averaged flux density is then matched with the spatially resolved flux map average value obtained from the numerical analysis. Average thermal power intercepted on the target surface is also measured, which serves as a useful procedure for estimating the radiative power incident on the receiver aperture. Further, as a demonstration exercise of the present fixed focus dish characterization method, a receiver testing methodology with an existing hybrid cavity and tubular design using compressed air is presented.

A novel antenna with the combination of the shorted-edge patch and the coupling feed is proposed in this letter. This new combination still keeps the good features of patch antenna, such as low cost and easy integration. Moreover, it simultaneously possesses several useful features: low cross polarization, wideband, high gain, and adjustable beamwidth in design. To show the effectiveness, a prototype operating at the central frequency of 12.5 GHz is designed and measured. Good agreement between the simulated and measured results has been obtained. The gain and the cross-polarized level are 11.2 dBi and −40 dB, respectively, and the bandwidth is 35%.

We propose an approximate analytical model for magnetic field penetration through a circular aperture on a perfectly conducting plate. The magnetic field source is a circular loop current placed parallel to the plate and coaxial with the aperture. It is shown that the aperture is equivalent to a magnetic quadrupole normal to the plate when the field point is far from the aperture. The model is validated using numerical simulations and a minor disagreement with experimental data is explained.

A transmitarray antenna with an elliptical cylindrical shape is presented for a wide-angle multibeam radiation in this communication. The transmitarray has a cylindrical radiating aperture with an elliptical cross-section, namely, elliptical cylindrical shape. Multiple feeds can be placed on the middle horizontal plane to realize multiple beams. Inspired by a 2-D Ruze lens, the antenna shape and the phase compensation are jointly designed according to the desired maximal beam direction. Innovative methods including a feed refocusing analysis and a virtual focal length are utilized to achieve the phase compensation across the 3-D aperture for multiple beam radiations with a small scanning loss. In order to validate the proposed antenna, a prototype operating in the millimeter-wave (mm-wave) $E$ -band has been designed, fabricated, and measured. By changing the position of the feeding gain horn along the refocusing arc, the main beam of antenna can be scanned in 11 directions. The measured peak boresight realized gain is 27 dBi at 70.5 GHz and a beam coverage of ±43° with a less than 2.7 dB scanning loss is obtained.

Light detection and ranging (LiDAR) has been widely used for airborne surveillance and autonomous driving. The ability of robotics or even micro-robotics can be drastically enhanced if equipped with LiDAR, but very light and small LiDAR must be used. Micro-robots have sizes close to birds or insects, and almost all existing LiDAR are too heavy and too large for them. In this work, a novel MEMS LiDAR, with its optical scanner separated from its base, is proposed and demonstrated. The scanner head will be carried by a moving micro-robot while the LiDAR base is fixed on the ground. There is a thin, flexible optical/electrical cable connecting the scanner head to the base. The scanner head consists of a MEMS mirror and a rod lens, which weighs only 10 g and measures under 4 cm long. The MEMS mirror has an aperture of 1.2 mm $\times \,\,1.4$ mm and can scan a field of view (FoV) of $9^{\circ } \times 8^{\circ }$ . As the micro-robot and the optical scanner head moves with respect to the optical receiver, an IMU (inertial measurement unit) has been embedded in the scanner head to track the motion and an algorithm has been developed to reconstruct the true point clouds. The movable head LiDAR can acquire 400 points per second and detect targets up to 35 cm away. A micro-robot can carry the scanner head while it is moving, and the point clouds can be generated at the LiDAR base. This new LiDAR configuration enables ranging, mapping, tracking, and zoom-in scanning for micro-robots.

Two small multifunctional antennas based on the negative order resonance (NOR) are proposed for the 5G new radio (N78 band) application in this article. A novel ring-type resonator (RTR) inspired by the mushroom-type metamaterial structure is first proposed and studied by the equivalent circuit models and dispersion diagrams. The RTR comprises small patches to enhance the aperture efficiency and gain. By loading the left-handed (LH) components (gap capacitors and metallic vias), the first NOR (−1st NOR) mode is successfully excited, which leads to significant antenna size reduction. The designs overcome the bandwidth limitation of the conventional miniaturized antennas. A dual-polarized antenna with a compact size of $0.30\lambda _{0} \times 0.30\lambda _{0} \times 0.04\lambda _{0}$ and a multifunctional antenna with a low profile of $0.06\lambda _{0}$ are developed with a shared aperture configuration. Moreover, they achieve a good bandwidth that well covers the 3.3–3.8 GHz N78 band, a high radiation efficiency that is larger than 80%, and flexible multipolarized radiation. These features make them preferable candidates for microcells to meet the 5G communication demands.