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In this communication, we present a theoretical analysis, design, and fabrication of a limited-diffractive planar Bessel beam launcher, that exhibits a zeroth-order Bessel profile in the transverse electric field component with respect to the z-propagation axis. The launcher is designed by synthesizing a finite zeroth-order, first kind Hankel aperture distribution, polarized along a fixed polarization unit vector. The field radiated by such an aperture distribution is derived by following an approximate model based on the geometric theory of diffraction, thus allowing to highlight the relevant wave constituents involved in transverse Bessel beam generation, also including the effect of aperture truncation on the radiated beam. Moreover, the theoretical analysis has been profitably applied to the design of a circular-polarized planar transverse Bessel beam launcher by means of a slotted radial waveguide. A prototype of right-handed circular polarized (RHCP) transverse Bessel beam launcher has then been fabricated at ƒ = 30 GHz. The measured transverse electric field component shows a satisfactory agreement with full-wave numerical simulations. [ABSTRACT FROM AUTHOR]

The authors present the design, fabrication and testing of a W‐band monopulse antenna. This antenna consists of three main building blocks: an array with 48 × 48 slots, a tapered corporate‐feed network and a comparator. The comparator has four input ports to generate sum and difference patterns in orthogonal cardinal planes for monopulse operation. A novel octuple excitation is adopted for the corporate network to achieve a side lobe level lower than −20 dB for the sum patterns and simultaneously reduce drastically the manufacturing complexity. Diffusion bonding technology is used for fabrication. Twenty‐four etched copper sheets with a thickness of 0.2 mm are stacked to realise the prototype. The total size of the antenna is 40.76×40.76×1.51λ03 $40.76\times 40.76\times 1.51{\lambda }_{0}^{3}$, with λ0 being the wavelength at the centre frequency (94 GHz). The antenna presents an isolation better than 12 dB among the input ports in the 84–100 GHz band. The realised measured gain is 40.7 dBi at 94 GHz. The measured 3‐dB gain bandwidth is 17.4%. The difference patterns at 94.0 GHz present null depths of −24.0, −18.0, −17.5, and −17.4 dB, in the E‐, H‐, 45°‐, and 135°‐planes, respectively. [ABSTRACT FROM AUTHOR]

Multi-beam antennas based on quasi-optical systems feeding a single radiating aperture generate orthogonal beams with a low crossover level between adjacent beams (around −13 dB at most). To circumvent this limitation, we present a circuit architecture allowing the excitation of a quasi-optical system (pillbox system) with two feeds per beam to reach much higher beam crossover levels. To this end, a specific eight-beam passive circuit is designed to cover the 76–86 GHz frequency band. It is based on a single-layer substrate integrated waveguide (SIW) coupler followed by equiphase SIW lines. The maximum phase and amplitude imbalance between the sources are only 35° ($\lambda _{g} / {1g}$ , with $\lambda _{g}$ the wavelength in the SIW lines at the design frequency) and 1.3 dB, respectively, and the isolation is better than −19 dB. The measured beam crossover level of the corresponding antenna is better than −3.2 dB, corresponding to an improvement of 16 dB with respect to single-feed-per-beam pillbox systems. The proposed passive architecture offers beam crossover levels suitable for low-cost electronically controlled multi-beam applications, as for next-generation 5G backhauling systems. [ABSTRACT FROM AUTHOR]

We present here a novel architecture of high-gain continuous transverse stub (CTS) antenna array operating at E-band (71–86 GHz) with reduced sidelobe level (SLL). The antenna consists of an array of 16 long slots excited by a nonuniform parallel-plate waveguide (PPW) corporate feed network. The nonuniform excitation is achieved by the design of asymmetrical power dividers in PPW technology, for which simple analytical models are provided. Full-wave simulations are validated by measurements of a prototype fabricated in aluminum. The SLL is lower than −28 dB in the E-plane. Such an SLL is 15 dB lower than classical parallel-fed CTS arrays. The overall antenna SLL remains below −18 dB over the 75–86 GHz band. Its mean radiation efficiency is 60%, and the maximum efficiency of 85% is reached at 86 GHz. [ABSTRACT FROM AUTHOR]

We present a 2-D Van Atta array for far-field wireless power transfer systems. The antenna array is made up of wideband, wideangle long slots. The monostatic radar cross section (RCS) of the finite array is derived by windowing the aperture field distribution of the corresponding infinite case. It is shown that for oversampled apertures, the array behaves as an ideal Van Atta reflector. Therefore, the proposed solution can collect and reradiate the total power impinging on it from a remote source over a large field of view and bandwidth. The array design is presented and validated through measurements over a relative bandwidth of 24% centered at a frequency of 5.8 GHz. The array consists of four slots, each fed by eight coaxial ports. Coaxial cables are used to connect pairs of feeds in a Van Atta configuration. A bidirectional amplifier is also introduced into the Van Atta scheme to amplify the collected energy before retransmission to a possible moving user for wireless power transfer. Off-the-shelf components are used to implement the bidirection amplifier. The amplifier is matched across the bandwidth of the array and provides 5 dB gain. Monostatic RCS measurements show that the array exhibits −10 dB beamwidths of about 118° and 80° in the E- and H-planes, over the entire bandwidth. In addition, it is shown that when the array ports are terminated with matched loads, the antenna RCS is reduced by approximately −15 dB with respect to the Van Atta case. This experimentally confirms that the array collects the total energy impinging from a remote source. [ABSTRACT FROM AUTHOR]

A dual-layer Rotman lens with reduced footprint and wide field of view is introduced here in substrate integrated waveguide (SIW) technology in the 24-GHz band. Both layers are connected by a high-efficiency transition made by an SIW integrated reflector and several coupling vias. The delay lines of the Rotman lens are implemented in ridged waveguides for compactness and broadband operation over a wide angular range, and the radiating part is an SIW slotted waveguide array made of 15 resonant waveguides. Design guidelines and performance are provided for the transition and the ridged delay lines. Experimental results show very good scanning performance over an angular sector of ±48°. [ABSTRACT FROM AUTHOR]

An amplitude-comparison monopulse slotted waveguide antenna array with a pillbox parabolic quasi-optical beamforming network is proposed in substrate integrated waveguide (SIW) technology in the 24 GHz band. The beamformer is illuminated by a cluster of horn pairs located in the focal plane of the pillbox reflector. Each horn pair is composed of two H-plane sectorial horns connected to a coupler. In-phase and out-of-phase operations of each horn pair allow radiation of the $\Sigma $ and $\Delta $ beams along the antenna scanning plane (E-plane). At the design frequency of 24.15 GHz, the experimental results demonstrate an antenna field of view of ± 26°, with null depth better than −20 dB for all the measured $\Delta $ beams. The antenna gain varies between 21.6 and 20.5 dBi for the $\Sigma $ beams and the measured input reflection bandwidth is about 2.4% for VSWR < 2. [ABSTRACT FROM PUBLISHER]

An integrated solution for the development of multilayer antenna modules for fifth-generation (5G) communications, based on low temperature cofired ceramic (LTCC), is presented. The design exploits the 3-D integration capabilities of the LTCC process, enabling the realization of a full-corporate feed network (CFN) in vertical configuration. A novel implementation of the CFN employing dielectric-embedded parallel plate waveguides (PPWs) is proposed. The PPW lines are delimited by via-rows. As opposed to standard substrate-integrated waveguide feed networks, guided fields are orthogonal to the via-rows and propagate along the vertical axis of the structure. The CFN feeds four long slots, without any coupling structure, and provides broadband operation. The final prototype comprises 18 LTCC tapes, with a total thickness of 3.4 mm. The measured −10-dB impedance bandwidth spans from 51.2 to 66 GHz (>25.2%). The module generates a fixed broadside beam, but multibeam operation in $H$ -plane can be easily achieved. In the 50–66-GHz band, the peak gain is 14.25 dBi and the average first side-lobe level in $H$ -plane is −20.6 dB. The proposed technology and the design concept are suited for highly integrated millimeter-wave systems, such as access points in the future $V$ -band high data-rate wireless networks. [ABSTRACT FROM PUBLISHER]

The paper presents the analysis and design of Bessel beam launchers using a finite inward cylindrical traveling wave aperture field distribution. The launcher radiates an electric field whose normal or longitudinal component takes the form of a zeroth-order Bessel function. The nondiffractive behavior of the structure in a well-defined area close to the radiating aperture is analyzed by decomposing the radiated field in its geometrical optics (GO) and diffractive (D) contributions. A closed-form expression is provided for the GO contribution whereas an asymptotic approximation is provided for the diffractive part. Such theoretical analysis allows a precise definition of the nondiffractive region for the generated Bessel beam. At the same time, it also highlights and predicts the oscillating behavior of the longitudinal component of the electric field along the z-axis due to the diffraction from the edges of the aperture. The proposed analysis is validated by a prototype at 30 GHz made by a radial waveguide loaded with metallic gratings and centrally fed by a coaxial probe. Measurement results for the longitudinal component of the electric field are in excellent agreement with full-wave results. In addition, the nondiffractive behavior for the radiated beam is reported over a bandwidth larger than 6.5% around 30 GHz. This behavior is peculiar of the nonresonant first kind Hankel aperture field distribution used for the generation of the Bessel beam. [ABSTRACT FROM PUBLISHER]

We proposed a synthesis method to design thinned leaky-wave-based arrays. The array elements are small waveguide apertures opened on a ground plane. A superstrate (impedance sheet, dielectric slab, etc.) is placed above the array. The array elements excite leaky-wave modes between the ground plane and the superstrate, enhancing the aperture efficiency of the final antenna. An iterative optimization procedure based on a convex $\ell_1$ minimization is used to reduce the number of array elements, maintaining the radiated field within a predefined mask. All the embedded element patterns are evaluated for each source through a fast Green’s function spectral approach, taking into account the effects of the mutual coupling during the optimization procedure. The use of a superstrate provides array structures with a reduced number of elements and enhanced aperture efficiency with respect to configurations without superstrate. The level of the grating lobes is also controlled during the optimization. A parametric study is performed to understand the capabilities of the leaky-wave-thinned array varying the reflectivity of the superstrate. As an example, an array of 18 elements working over a 7% fractional bandwidth is synthesized, providing a reduction by a factor of 4 in the number of elements when compared to a configuration with the same requirements, but without superstrate. [ABSTRACT FROM PUBLISHER]

A multibeam antenna system fed by a compact Rotman lens is presented in multilayer substrate integrated waveguide (SIW) technology. The lens is implemented in two layers using a new transition based on several star-shaped coupling slots and an SIW integrated reflector. Compared to standard rectangular coupling slots, this transition is broadband and maximizes power transfer between the two layers of the lens regardless the position of the beam port along the focal arc of the Rotman lens. The radiating part is an array of 15 slotted waveguides. These waveguides are centrally fed to enhance their bandwidth and reduce the “long line effect.” To validate the proposed concepts, an antenna system has been prototyped at 24.15 GHz. The designed transition leads to a physical size reduction of about 50% for the Rotman lens, and more than 33% for the antenna footprint compared to standard single-layer implementations. The experimental results demonstrate a scanning range of \pm 33^\circ and a 4% bandwidth for \textVSWR < 2; the isolation between input ports is better than - 10\;\textdB. [ABSTRACT FROM PUBLISHER]

We describe here an optimization procedure to shape the radiation pattern of planar two-dimensional (2-D) leaky-wave antennas. The structures under investigation are superstrate configurations made by partially reflecting screens (impedances sheets) over a ground plane and excited by open-ended waveguides. The antenna configuration providing the required radiation pattern is obtained by resorting to an ad hoc optimization procedure that minimizes the mean squared error between the target pattern and the actual radiated far field. The radiated field is analytically evaluated using a Green’s function spectral approach to speed up the optimization process. Several kinds of radiation patterns can be obtained using the proposed algorithm. In this work, we focus our attention on flat-topped radiation patterns, suitable as element pattern for phased array antennas covering a limited angular field of view such as those considered for space applications. The proposed procedure is validated by full-wave results and measurements. [ABSTRACT FROM PUBLISHER]

This paper presents a flat, high gain, wide scanning, broadband continuous transverse stub (CTS) array. The design procedure, the fabrication, and an exhaustive antenna characterization are described in details. The array comprises 16 radiating slots and is fed by a corporate-feed network in hollow parallel plate waveguide (PPW) technology. A pillbox-based linear source illuminates the corporate network and allows for beam steering. The antenna is designed by using an ad hoc mode matching code recently developed for CTS arrays, providing design guidelines. The assembly technique ensures the electrical contact among the various stages of the network without using any electromagnetic choke and any bonding process. The main beam of the antenna is mechanically steered over $\pm 40^\circ$ in elevation, by moving a compact horn within the focal plane of the pillbox feeding system. Excellent performances are achieved. The features of the beam are stable within the design 27.5–31 GHz band and beyond, in the entire Ka-band (26.5–40 GHz). An antenna gain of about 29 dBi is measured at broadside at 29.25 GHz and scan losses lower than 2 dB are reported at $\pm 40^\circ$. The antenna efficiency exceeds 80% in the whole scan range. The very good agreement between measurements and simulations validates the design procedure. The proposed design is suitable for Satcom Ka-band terminals in moving platforms, e.g., trains and planes, and also for mobile ground stations, as a multibeam sectorial antenna. [ABSTRACT FROM PUBLISHER]

A novel dual-layer multibeam pillbox antenna with a slotted waveguide radiating part in substrate-integrated waveguide (SIW) technology is proposed. In contrast to previous works, the design goal is to have a multibeam antenna with arbitrary low sidelobes and at the same time a high crossing level between adjacent radiated beams. These two constraints cannot be satisfied simultaneously for any passive and lossless multibeam antenna systems with a single radiating aperture due to beam orthogonality. Here, this limitation is overcome using the “split aperture decoupling” method which consists in using two radiating apertures. Each aperture is associated with a pillbox quasi-optical system with several integrated feed horns in its focal plane so as to steer the main beam in the azimuthal plane. The antenna operates at 24.15 GHz and presents very good scanning performance over an angular sector of \pm 40^\circ , with a good agreement between full-wave simulations and measurements. The crossover level between adjacent beams is about - 3\;{\text {dB}} with a sidelobe level lower than - 24\;{\text{dB}}$ for the central beam and better than - 11\;{\text{dB}} for the extreme beam positions. The isolation between feed horns in the same pillbox system is better than 20 dB. [ABSTRACT FROM PUBLISHER]

A multibeam pillbox antenna system incorporating monopulse phase comparison technique is proposed in the 24-GHz band. This low-profile antenna architecture combines the scanning capabilities of pillbox configurations and enhanced resolution of two-quadrant monopulse technique; this approach avoids mechanical orientation of the antenna system for tracking applications. The radiating panel consists of two subarrays of 23 slotted waveguides, with 8 slots per waveguide. The beam is scanned in E-plane over a field of view of \pm \bf 40^\circ , and sum/difference patterns are generated in H-plane. Around the design frequency, \bf f = \bf 24.\bf 15\,\bf GHz, the antenna gain varies between 25 and 21.5 dBi for the central and extreme beams, respectively. The measured null depth is better than - \bf 16\;\textdB for all difference beam patterns. The antenna bandwidth for \bf VSWR < \bf 2 (\textbfS\textbf{ii} < - {\bf 10}\;\textbf{dB}) and isolation better than \bf 10\;\bf dB (\textbfS\textbf{ij} < - {\bf 10}\,\textbf{dB}) is equal to 4.5%. [ABSTRACT FROM PUBLISHER]

This paper introduces a rigorous and effective modeling framework for the continuous transverse stub (CTS) array, based on a spectral Green’s function approach. A multimodal expansion of the equivalent currents over the slots is used to derive an accurate expression for the active impedance of an infinite array. This formula is validated through comparisons with full-wave simulations when the array scans are in principal planes. The mathematical formulation highlights the broadband properties of CTS arrays and accurately predicts the impact of the geometrical parameters on the active impedance, as a function of frequency and scan angle. The discussion of the results provides physical insights to outline effective design strategies. [ABSTRACT FROM PUBLISHER]

In this paper, we prove experimentally that nondiffractive Bessel beams can be generated by using inward cylindrical traveling wave aperture distributions. An azimuthally invariant inward traveling wave distribution is defined over the aperture of a radial line slot array (RLSA) to launch a Bessel beam whose normal electric-field component assumes a truncated, zeroth-order Bessel function. An optimization procedure based on a holographic approach is used for tuning the position and size of the slots of the RLSA. The antenna is centrally fed by a coaxial probe transition. The final structure operates at 12.5 GHz. Full-wave simulations and measurements of the vertical component of the electric field show that a non-diffractive radiation is obtained within a range larger than 12 wavelengths in front of the antenna. The generated Bessel beam presents a stable half-power beamwidth of about 20 mm within this range. The proposed system may open new opportunities for planar, low-profile Bessel beam generators at millimeter waves, Terahertz, and optics. [ABSTRACT FROM PUBLISHER]

We describe the design of a radial line slot array antenna with a shaped and focused near field. The antenna is designed in such a way to control the side lobe level and beamwidth of the normal component of the electric field with respect to the radiating aperture. The design procedure consists of two steps. In the first step, the requirements on the near-field pattern are provided over a focusing plane at a given distance from the radiating aperture. A set theoretic approach is then used to derive the aperture field distribution fitting the requirements over the near field. In the second step, the aperture field distribution is synthesized by accurately placing and sizing the slots of the antenna. The spillover efficiency is maximized during the design process. The antenna is centrally fed by a simple coaxial probe. The antenna design is validated by a prototype and measurements at 12.5 GHz. [ABSTRACT FROM PUBLISHER]

A lumped-element unit cell for designing compact beam-forming networks (BFNs) is proposed. It is a reciprocal four-port network, which is lossless, isolated, and matched at all ports. The unit cell is designed to behave as planar and compact directional couplers with both arbitrary output amplitude and phase distributions, as well as crossovers with arbitrary output phase distributions. For both cases, design equations are derived. The design procedure is validated numerically and experimentally through several examples. The versatility of the unit cell allows the design of BFNs using only the proposed network topology. As an example, a 4\,\times\,4 Butler matrix is implemented. A strong size reduction is also achieved with respect to classical designs thanks to the lumped-element implementation. The prototype footprint is approximately 25 times smaller than typical transmission-line-based Butler matrices. The structure shows also a very small area compared to state-of-the-art miniaturized 4\,\times\,4 Butler matrix designs. Measurement results confirm the behavior of the Butler matrix as a BFN. The achieved bandwidth is approximately 8%. [ABSTRACT FROM AUTHOR]

The generation of Bessel beams using a leaky radial waveguide is presented. The radial waveguide consists of a capacitive sheet over a ground plane. It supports an azimuthally invariant leaky-wave mode whose normal electric-field component is a truncated, zeroth-order Bessel function. The annular spectrum and nondiffractive extent of the Bessel beam is clearly linked to the complex wavenumber of the leaky-wave mode. The fields inside the radial waveguide are derived using classical vector potential techniques. A vector approach is employed to avoid paraxial approximations of earlier works and the associated limitations on shaping the Bessel beam. Design rules are provided to synthesize a desired propagating Bessel beam. A simple coaxial feed is proposed for the radial waveguide and its input impedance is derived analytically. The analytical results are also validated numerically. The proposed structure and design procedure can be used for generating arbitrary zeroth-order propagating Bessel beams at microwave and millimeter-wave frequencies. [ABSTRACT FROM PUBLISHER]

We present the experimental generation of Bessel beams using a leaky radial waveguide. The radial waveguide consists of a capacitive sheet over a ground plane. The capacitive sheet is composed of patch elements printed on both sides of a dielectric substrate. The radial waveguide is coaxially fed and supports an azimuthally invariant leaky-wave mode whose normal electric-field component is a truncated, zeroth-order Bessel function. Two prototypes are presented with the same propagation constant and lateral extent, but different attenuation constants. 2D electric field measurements and their respective Fourier transforms validate the operation of the prototypes as Bessel-beam launchers at two frequency bands. Cleaner patterns are achieved by the prototype with lower attenuation constant. The dual-band capability and associated frequency dependent resolution can be useful in near-field planar focusing systems. The proposed structure can be used for generating arbitrary zeroth-order propagating Bessel beams at microwave and millimeter-wave frequencies. [ABSTRACT FROM PUBLISHER]

The use of dielectric super-layers for shaping the radiation pattern of focal plane feeds of a multibeam reflector system is discussed. Using the super-layers, it is possible to reduce the spillover from the reflectors without increasing the dimension of each aperture. The effect has been demonstrated using a realistic array configuration. The experimental demonstration is obtained with configurations which are typical for satellite based multi beam telecommunication links. Thanks to the adoption of a \times \,4 reuse scheme, based on frequency and polarization orthogonal channels, the measured edge of coverage directivity has improved, with respect to a standard technology case, by 2 dB over an operational bandwidth of about 2%. [ABSTRACT FROM PUBLISHER]

This work proposes a novel multi-beam leaky-wave pillbox antenna. The antenna system is based on three main parts: feeding part (integrated horns), quasi-optical system and radiating part. The radiating and input parts are placed in two different stacked substrates connected by an optimized quasi-optical system. In contrast to conventional pillbox antennas, the quasi-optical system is made by a pin-made integrated parabola and several coupling slots whose sizes and positions are used to efficiently transfer the energy coming from the input part to the radiating part. The latter consists of a printed leaky-wave antenna, namely an array of slots etched on the uppermost metal layer. Seven pin-made integrated horns are placed in the focal plane of the integrated parabola to radiate seven beams in the far field. Each part of the antenna structure can be optimized independently, thus facilitating and speeding up the complete antenna design. The antenna concept has been validated by measurements (around 24 GHz) showing a scanning capability over \pm 30^\circ in azimuth and more than 20^\circ in elevation thanks to the frequency scanning behavior of the leaky-wave radiating part. The proposed antenna is well suited to low-cost printed circuit board fabrication process, and its low profile and compactness make it a very promising solution for applications in the millimeter-wave range. [ABSTRACT FROM AUTHOR]

An accurate optimization tool has been developed to design arbitrarily-shaped axis-symmetrical devices. It is based on the combination between a genetic algorithm (GA) and a finite-difference time-domain (FDTD) solver formulated in a cylindrical coordinate system. For validation purposes, this tool is applied to design several smooth-walled compact conical horns with a large flare angle (90^\circ) in the 60-GHz band. The unavoidable phase distortions appearing in the horn aperture are compensated by loading the horn mouth with thin shaped dielectric lenses made in Rexolite. Based on these test cases, several synthesis strategies are discussed and compared in order to select the best antenna prototype among those synthesized. A scaled version of this prototype has been fabricated to operate around 29.5 GHz. The numerical and experimental results are in excellent agreement. Horn size reduction rates up to 80% have been reached. [ABSTRACT FROM PUBLISHER]

A leaky-wave slot array antenna fed by a dual offset Gregorian reflector system is realized by pins in a parallel plate waveguide. The radiating part of the antenna is composed by parallel slots etched on one side of the same parallel plate waveguide. The dual offset Gregorian reflector system is fed by an arrangement constituted by two vias and a grid, also constituted by pins. Also this feed arrangement realizes a leaky type of radiation, this time inside the parallel plate waveguide. A prototype of the antenna has been designed, manufactured and successfully tested. The low profile, low cost and high efficiency of the antenna render it suited for a variety of radar or telecom applications. [ABSTRACT FROM AUTHOR]

In this article, we propose the implementation of a triangular mesh along with Rao-Wilton-Glisson (RWG) basis functions to improve the performance and flexibility of a mixed mode-matching (MM)/method-of-moments (MoM) algorithm dedicated to substrate integrated waveguide (SIW) antennas analysis and optimization. These basis functions represent equivalent magnetic currents on either coupling or radiating apertures. In the initial implementation, the entire domain sine basis function was presented, providing fast and accurate simulations for electrically thin rectangular slots. In order to treat wider slots and other geometries, we add here RWG functions, present the theoretical framework of this approach, and highlight its potential through concrete examples. [ABSTRACT FROM AUTHOR]

A methodology for the design of printed magnetically coupled resonant wireless power-transfer (WPT) systems is proposed. The design methodology aims at a well-matched system with a maximized power-transfer efficiency for mid-range applications. The system consists of two identical resonant coils driven by high-quality factor loops. The proposed design criteria allows obtaining maximum achievable transfer efficiency and impedance matching at any defined distance without any external matching circuits connected to the driven loops. For validation purposes, a printed WPT system at 10 MHz is fabricated targeting 1-m distance between the transmitting and receiving sides. Its performances in terms of reflection and transmission coefficients, as well as in terms of generated electric and magnetic fields, have been characterized numerically and experimentally. The impact of human body presence on the system has also been investigated observing the splitting in two frequencies of the common resonant frequency of the coils. A dosimetric study has been conducted using a detailed high-resolution anatomical human body model and considering E99 , J1~\text {cm^{2}} , and specific absorption rate (SAR) (local and whole body averaged SAR) as exposure metrics. It has been observed that peak exposure levels appear in different tissues depending on the body location. Compliance with the international commission on nonionizing radiation protection (ICNIRP) reference level as well as basic restrictions has been studied followed by computing the maximum allowable input power. It has been found that, with the body located 1 m away from the transmitting coil, the maximum allowable input power satisfying E99 is in the order of MW, whereas it reduces to tens of kilowatt when considering SAR and J1~\text {cm^{2}} . The latter has been noticed to be the most restrictive dosimetric quantity. [ABSTRACT FROM PUBLISHER]

This paper deals with the design of scalar metasurface antennas capable of radiating a well-polarized beam in the far-field or near-field zones. The equivalent electric current over the metasurface is used to derive design equations to generate the desired field pattern based on the scalar impedance condition. In particular, it is shown that scalar metasurfaces can be used to generate linear and circular polarizations for a fixed pointing direction in the far field by properly changing locally the scalar impedance boundary condition. In addition, they can also be used to generate normal polarized Bessel beams in the near-field region. Several solutions are presented at 20 GHz, with different polarizations and feeders developed in the framework of a two-year research project financed by the French space agency (Centre National d’Etudes Spatiales). Measurements and full-wave results validate the proposed approach. [ABSTRACT FROM PUBLISHER]

Modern K-/Ka-band satellite communications have become increasingly demanding in recent years with regard to the performance of the terminal antenna used in the link. These terminals should be high gain and broadband, as well as guarantee polarization diversity over a large field of view. A low profile is also highly desired to integrate the antenna system into a variety of moving platforms, such as aircraft and trains. An ultra-low-profile continuous transverse stub array is proposed here to tackle these challenges. The antenna array is realized in printed circuit board technology by adopting a contactless transition between the stacked-up layers without resorting to blind or buried vias. The proposed antenna is wideband (impedance matched within the 19–31 GHz band, i.e., ~50%) and provides multibeam radiation for polar angles as far as ±22.5°. The peak value of the gain is 19.7 dBi, and the maximum radiation efficiency equals 50%. Furthermore, the antenna array is combined with a dual-band polarization converter. The latter provides orthogonal circular polarization (CP) in two non-adjacent bands. The overall antenna system performs right- and left-hand CPs within the SatCom downlink and uplink in the K-/Ka-band, respectively. For each polarization, the axial ratio is below 3 dB over 9.47% and 4.42% fractional bandwidths, respectively. [ABSTRACT FROM AUTHOR]

This article presents a parallel-plate lens beamformer for continuous wide-angle scanning. The design is based on a compact dual-lens system with extended scanning range and is optimized using a previously developed geometrical optics technique. Beam steering is accomplished with a mechanical feed system based on the noncontact characteristic of groove gap waveguides, offering large bandwidth, low profile, and mechanical ruggedness. The proposed concept is validated by an all-metal prototype of a $20.5\lambda $ lens operating in the uplink Ka-band allocated to satellite communications (27.5–31 GHz). Good agreement is obtained between the simulated and measured performances. The measured return loss is greater than 12 dB over the entire frequency band and beyond. High scanning performances are achieved over an angular range of ±50° (±14 beamwidths), with maximum scan losses in the order of 3 dB and good pattern stability over the entire band. The proposed solution is particularly suited for next-generation satellite terminals requiring compact broadband antennas with continuous beam-steering capability over a large angular range. [ABSTRACT FROM AUTHOR]

Mm-wave dielectric waveguides are a promising and low-cost technology for the transmission of ultra-high data rates. Besides the attenuation (losses) and group delay, the bending loss of the dielectric waveguides is also one of the key parameters to establish the capacity and energy efficiency of such wired links, when deployed in realistic scenarios. In this context, we report the experimental characterizations of bending effects for various solid and hollow commercially available dielectric waveguides at V-band (50–75 GHz). A wide-band transition has been designed to carry out the measurements using a Vector Network Analyzer (VNA) and extension modules. The measured results are in very good agreement with full-wave simulations. Our experimental results show an average bending loss of 1.46 dB over the entire V-band for the fundamental HE 11 y mode of a PTFE solid dielectric waveguide (core diameter of 3.06 mm) with a 90° bending angle and 25 mm radius of curvature. This value rises up to 2.88 dB (or 3.25 dB) when bending radius is changed to 15 mm (or bending angle grows up to 140°). The measurements also show that the measured bending losses increase significantly for hollow dielectric waveguides, in particular when the inner to outer diameter ratio gets larger. [ABSTRACT FROM AUTHOR]

In this work, we analyze the spatial and temporal features of electromagnetic X-waves propagating in free space and generated by planar radiating apertures. The performance of ideal X-waves is discussed and compared to practical cases where the important effects related to the finiteness of the radiating aperture and the wavenumber dispersion are taken into account. In particular, a practical device consisting of a radial waveguide loaded with radiating slots aligned along a spiral path is considered for the practical case in the millimeter-wave range. A common mathematical framework is defined for a precise comparison of the spatiotemporal properties and focusing capabilities of the generated X-wave. It is clearly shown that the fractional bandwidth of the radiating aperture has a key role in the longitudinal confinement of an X-wave in both ideal and practical cases. In addition, the finiteness of the radiating aperture as well as the wavenumber dispersion clearly affect both the transverse and the longitudinal profiles of the generated radiation as it travels beyond the depth-of-field of the generated X-wave. Nevertheless, the spatiotemporal properties of the X-wave are preserved even in this "dispersive-finite" case within a defined region and duration related to the nondiffractive range and fractional bandwidth of the spectral components of the generated X-wave. The proposed analysis may open new perspectives for the efficient generation of X-waves over finite radiating apertures at millimeter waves where the dispersive behavior of realistic devices is no longer negligible. [ABSTRACT FROM AUTHOR]

We demonstrate a silicon (Si)–gallium arsenide (GaAs) semiconductor-based holographic metasurface antenna operating at 94 GHz. The metasurface antenna molds the radiated beam by resorting to a holographic approach involving the modulation of a guided-mode generated by a pillbox beamformer. The pillbox beamformer is fed by three integrated horns in substrate-integrated waveguide (SIW) technology located in the focal plane of the parabolic reflector of the pillbox beamformer. The three horns correspond to three independent beams and allow beam steering by switching from one feed to another. The proposed metasurface antenna is planar and extremely low-profile, thus suitable for small platforms such as CubeSats/SmallSats. A prototype validates the concept demonstrating a directivity as high as 31.9 dBi and a reflection coefficient lower than −15 dB at 94 GHz. [ABSTRACT FROM AUTHOR]

We present the experimental validation of the dual-mode, polarization-agile parallel-fed continuous transverse stub (CTS) antenna architecture introduced in Part I of this two-part paper. The Ka-band dual-mode CTS array described in Part I is characterized when it radiates in horizontal and vertical polarization. To this end, it is combined with two different quasi-optical beamformers operating in a quasi-transverse electromagnetic (quasi-TEM) and in a quasi-transverse electric (quasi-TE1) mode, respectively. The scanning capabilities of both multibeam antenna systems are demonstrated. The CTS array and its feed network comprising corrugated parallel-plate waveguides (CPPWs) are fabricated by additive manufacturing. Measurements show that the dual-polarized CTS antenna works between 29 and 32 GHz in a field of view of about 45°, achieving a peak gain of 31.3 dBi and very low cross polarization. These promising results pave the way for the realization of dual-circularly polarized beam-scanning antennas with application to broadband and compact Ka-band ground terminals. [ABSTRACT FROM AUTHOR]

Continuous transverse stub (CTS) arrays represent a high-performance and affordable solution for fixed-beam and scanning flat antenna systems, though intrinsically linearly polarized. In these systems, the polarization is controlled using add-on polarization converters or combining cross-connected CTS arrays. This two-part article presents a novel parallel-fed CTS antenna which achieves dual-linear polarization using, instead, a single array and two orthogonally polarized excitation fields. In this article, it is shown that, due to modal coupling, standard parallel-plate waveguides (PPWs) are not suitable as building blocks of the proposed antenna. Therefore, an alternative waveguide, the corrugated PPW (CPPW) is introduced. The dispersion characteristics of such CPPW are investigated. Guidelines to independently design the dispersion properties of the two propagating modes and avoid the excitation of higher order modes are provided. Based on this study, the design and simulated performance of a $Ka$ -band array of eight long slots parallel-fed by a dual-mode CPPW corporate-feed network are described. An extensive experimental validation of the concept is presented in Part II. [ABSTRACT FROM AUTHOR]

Objective: The potentialities of improving the penetration of millimeter waves for breast cancer imaging are here explored. Methods: A field focusing technique based on a convex optimization method is proposed, capable of increasing the field level inside a breast-emulating stratification. Results: The theoretical results are numerically validated via the design and simulation of two circularly polarized antennas. The experimental validation of the designed antennas, using tissue-mimicking phantoms, is provided, being in good agreement with the theoretical predictions. Conclusion: The possibility of focusing, within a lossy medium, the electromagnetic power at millimeter-wave frequencies is demonstrated. Significance: Field focusing can be a key for using millimeter waves for breast cancer detection. [ABSTRACT FROM AUTHOR]

We explore the use of a new modulated metasurface (MTS) antenna topology as solution for wireless backhaul at K band. The proposed structure is composed of a quasi-optical beamformer, which feeds the modulated MTS radiating aperture. These two elements are vertically stacked in a two-layer pillbox architecture to produce a very compact antenna. Furthermore, our design is able to provide several beams at different pointing angles and, hence, it offers the possibility of discrete beam steering by beam switching. The employment of a modulated MTS and the compactness given by the pillbox approach lead to a high-gain and low-profile antenna that could be an appealing solution for mobile backhaul networks. [ABSTRACT FROM AUTHOR]

This article presents an efficient analysis method for a novel continuous transverse stub (CTS) antenna. As opposed to state-of-the-art CTS antenna designs, the proposed architecture achieves circular polarization using a single CTS array and without any polarization converter. The structure consists of long radiating slots/stubs fed by over-moded parallel-plate waveguides. More precisely, the transverse electromagnetic (TEM) mode and the first transverse electric (TE1) mode of the feeding waveguides are used to generate a circularly polarized field. The array is analyzed using a spectral mode-matching method. First, the active reflection coefficient of the infinite array is derived in closed form. A windowing approach is then adopted to compute the radiation patterns of finite-size arrays. Numerical results obtained with this method are in excellent agreement with full-wave simulations, carried out with a commercial software. The performance of this class of CTS antennas has been investigated using the developed model. It is theoretically demonstrated that the proposed array can be designed to attain an axial ratio (AR) lower than 3 dB over a 52.9% relative bandwidth at broadside. Furthermore, the active input reflection coefficient is lower than −10 dB over a 40.8% relative bandwidth, when the array steers its main beam at $\theta _{0}=45^\circ $. This solution is an attractive candidate for next generation satellite communication terminals. [ABSTRACT FROM AUTHOR]

A very low-profile sub-THz high-gain frequency beam steering antenna, enabled by silicon micromachining, is reported for the first time in this paper. The operation bandwidth of the antenna spans from 220 to 300 GHz providing a simulated field of view of 56°. The design is based on a dielectric filled PPW leaky-wave antenna fed by a pillbox. The pillbox, a two-level PPW structure, has an integrated parabolic reflector to generate a planar wave front. The device is enabled by two extreme aspect ratio, 16 $\times$ 16 mm large perforated membranes, which are only 30 $\mu \text{m}$ thick, that provide the coupling between the two PPWs and form the LWA. The micromachined low-loss PPW structure results in a measured average radiation efficiency of −1 dB and a maximum gain of 28.5 dBi with an input reflection coefficient below −10 dB. The overall frequency beam steering frontend is extremely compact (24 $\times$ 24 $\times$ 0.9 mm) and can be directly mounted on a standard WM-864 waveguide flange. The design and fabrication challenges of such high performance antenna in the sub-THz frequency range are described and the measurement results of two fabricated prototypes are reported and discussed. [ABSTRACT FROM AUTHOR]

This paper presents the spectral Green’s function (GF) to analyze Fabry–Perot antennas to generate circular polarization (CP). The structure consists of a single cavity created between a partially reflective surface and a high impedance surface, fed by a low directive, linearly polarized source. Elements in the antenna are characterized by tensor impedances. It allows conceiving a circularly polarized antenna supporting a single hybrid leaky-wave mode. Residue analysis is used to demonstrate that a single hybrid leaky-wave mode is sufficient to generate CP. The tool is validated by an antenna design matched over a relative bandwidth of 28.5% with an axial ratio lower than 3 dB. [ABSTRACT FROM AUTHOR]

The generation of several beams with strict radiation requirements in the field of a radiating aperture is demonstrated here. An optimization scheme is adopted to derive the aperture-field distribution generating the required near-field radiation. The optimization tool is based on a set-theoretic approach and takes into account field constraints on the radiation profile and polarization. The challenging case of generating four independent beams in the near field of a radiating aperture and with different polarizations is then considered. A tensorial metasurface is adopted to synthesize the derived aperture field profile to fully control the amplitude and phase distribution of the radiating aperture. Measurement results at 20 GHz validate the proposed approach. [ABSTRACT FROM AUTHOR]

Wide-angle coverage, fine angular resolution, and low-power consumption are the key characteristics of millimeter-wave short-range wireless systems, such as access points in fifth-generation cellular networks and automotive radars. In this paper, we present a 60 GHz switched-beam antenna system addressing all these requirements. It consists of two identical parallel-fed arrays of eight slots, each fed by a passive pillbox beamformer. A switch network comprising four single-pole-multiple-throw switches excites one of the two arrays at a time and selects the radiated beam. The whole system is fully integrated into a multilayer low-temperature cofired ceramic module. The antenna covers a scan range of about ±39° in one plane using 11 beams between 57 and 66 GHz. The proposed architecture enables the simultaneous achievement of continuous coverage (beam crossover levels of about −3 dB) and of low sidelobe levels. Dedicated numerical tools are used for a preliminary design of the antenna. The design procedure, the technological development, and the experimental results are discussed in detail. [ABSTRACT FROM AUTHOR]

In metasurface (MTS) antennas, a surface wave (SW) is gradually transformed into a leaky wave, owing to the SW interaction with a reactance tensor periodically modulated in the aperture plane. In such antennas, the modulated reactance is typically obtained by subwavelength patches printed on a dielectric substrate. As an alternative technology, we propose the use of a class of metal-only MTSs in the synthesis of the reactance tensor. The absence of dielectric is useful to withstand harsh environments in space applications. The proposed element consists of a metallic cylinder with elliptical cross section, placed on a ground plane and arranged in a square lattice with subwavelength unit cells. This structure is well suited for fabrication by metal additive manufacturing. We have applied the metal cylinders in the design of a right-handed circularly polarized antenna, with broadside pencil beam at Ka-band. The obtained performance has been verified by full-wave simulations. A prototype has been successfully manufactured and measured, showing a good performance. [ABSTRACT FROM AUTHOR]

In this communication, it is shown that a nondiffracting vortex beam (i.e., a higher order Bessel beam with azimuthal phase variation) can be generated in the near field by synthesizing an inward cylindrical traveling-wave distribution over a finite aperture antenna. A radial line slot array (RLSA) is then designed to prove the concept. The collimated vortex beam is excited in the proximity of the RLSA, within a region properly defined by the nondiffracting range of the generated beam. The radial dependence of the longitudinal electric field of the vortex-beam magnitude follows a first-order Bessel function, and its phase presents a linear azimuthal variation. Full-wave results validate the generation of the nondiffractive higher order Bessel beam within the radiative near field of the launcher. [ABSTRACT FROM PUBLISHER]

The manipulation of the near-field (NF) is a challenging topic in electromagnetics, carrying essential importance for several applications. In this paper, we develop an algorithm capable of shaping the NF in 3-D domains according to given specifications. The algorithm is based on a set theoretic approach and a front-and-back iterative propagation scheme. The latter is realized using fast Fourier transforms (FFT), reducing the required computational time. The algorithm is applied on a focal shaping scenario. In this case, a full control of the depth of focus, spot diameter, and sidelobe level of the generated radiation is achieved. In particular, the proposed procedure is successfully adopted to control the normal component of a radiating aperture over a range of 12 \lambda at 30 GHz. The derived aperture field distribution is then synthesized by a radial line slot antenna (RLSA) by means of an in-house method of moments (MoM) code. NF measurements have validated the proposed approach and prototype. [ABSTRACT FROM PUBLISHER]

We present a Bessel-beam launcher based on a leaky radial waveguide consisting of a capacitive sheet over a ground plane that supports higher-order leaky modes. A propagating Bessel beam is generated above the radiating waveguide. The Bessel beam is transverse-magnetic (TM) polarized with a vertical component of electric field that is a zeroth-order Bessel function of the first kind. A higher-order leaky-wave mode is used to reduce losses at millimeter waves and, at the same time, avoid the thin dielectric layers used in previously proposed lower order leaky-wave Bessel launchers. Closed-form design equations are provided for the proposed structure. In addition, the operating bandwidth of the launcher is defined using dispersion analysis. Near-field measurements of a prototype operating in the frequency range 38–39.5 GHz validate the concept. The measured launcher generates a Bessel beam with a stable spot size of about 4.3 mm ($0.57\,\lambda$) over a nondiffractive range of about 16.4 mm ($2.2\,\lambda$), within about a 4% fractional bandwidth. [ABSTRACT FROM PUBLISHER]

A novel multibeam antenna based on a \mathbf4 \times \mathbf4 Butler matrix with sidelobe level (SLL) control is proposed in the 60-GHz band. The SLL is controlled using a dedicated circuit composed by four 90-degree couplers with different coupling factors and several cross couplers. This dual-layer antenna is made by hollow waveguides and is assembled using diffusion bonding technique. The radiating part is a compact array of ridged waveguides designed to avoid appearance of grating lobes in the visible range for large scanning angles. The antenna field of view is \pm \mathbf43^\circ with sidelobes lower than - \mathbf17.5\;\mathbfdB over the entire scanning range. The isolation between the input ports of the Butler matrix is better than 25 dB at the design frequency of 61.5 GHz. The reflection bandwidth for VSWR < \mathbf2 is larger than 6.5%. At the design frequency, the measured conductor losses of the antenna are estimated lower than 0.64 dB for all ports. [ABSTRACT FROM PUBLISHER]

We propose a fast and accurate full-wave code capable of analyzing electrically large substrate integrated waveguides consisting of stacked parallel-plate waveguides hosting dielectric or metallic posts and coupling and/or radiating slots. Boundary conditions enforced on posts yield scattering amplitudes, while slots are modeled by equivalent magnetic currents, solved by a method of moments. Substantial accelerations are proposed to exploit various symmetries of the structures and to select the optimal number of modes according to the relevant geometrical and physical parameters. The formulation is validated by full-wave simulations with a commercial software and measurement results. [ABSTRACT FROM PUBLISHER]

This paper deals with the use of frequency selective surfaces (FSS) to increase the efficiency x bandwidth product in wideband antenna arrays, whose efficiency is limited by the front-to-back ratio. If the backing reflector for the antenna is realized through a single metal plane solution, its location will be suitable only on a relatively limited frequency range especially if wide angle scanning is required. In order to extend the frequency range of usability, an FSS can be sandwiched between the antenna and the ground plane, providing an additional reflecting plane for an higher frequency band. The possibility to integrate in the antenna different functionalities, otherwise performed by several antennas, is also discussed in the paper. The proposed backing structure composed by the FSS and the ground plane has been designed to be used in conjunction with a wideband antenna consisting of an array of connected dipoles. A hardware demonstrator of the backing structure has also been manufactured and tested. [ABSTRACT FROM AUTHOR]