In this paper, a new super-resolution and highly stable DOA estimation technique of coherent sources is introduced. Furthermore, the proposed technique is applied to the data collected from the AWR1243 mm-wave 76-81 GHz frequency modulated continuous wave (FMCW) radar to estimate the DOAs of real targets. A virtual antenna array is proposed to increase the array aperture size and the dimension of the data covariance matrix which effectively helps in de-correlating the received signals and in increasing the number of detectable sources and hence improving the detection resolution. Moreover, a significant improvement in the DOA estimation capability is achieved by handling the frequency domain of the received signals instead of their time-domain representations. That is because the signal to noise ratio (SNR) is increased by a multiplication factor when it is transformed using FFT which acts as a filter for the noise. The simulation results proved the superiority of the proposed technique compared to the state of the arts in this field, especially at low SNR that approaches -35 dB.

A new class of the wideband series-fed microstrip array antenna is presented for X-band applications. A novel configuration of the reflector-slot-strip-foam-inverted patch (RSSIP) is proposed to provide high efficiency and wide operating frequency band. To improve the front to back ratio (FBR) and enhance the gain, a reflector is used. The series-fed configuration is selected for the array to simultaneously provide a very high efficiency and reduce the side lobe level. To experimentally verify the performance, a prototype of the array antenna is fabricated, and measurement is performed. This array consists of 12 sub-linear arrays with series-fed microstrip excitation. Also, each of these subarrays consists of 16 RSSIP antennas. An excellent agreement exists between measurement and simulation. The measured gain and efficiency of the fabricated antenna are 28.5 dB and 67% at 10 GHz, respectively. The measured impedance matching bandwidth (S₁₁<-10 dB) is 24% which confirms the wideband characteristic of the antenna. The series-fed configuration results in very low measured SLL of -24.5dB at H-plane. The proposed 16 x 12 array antenna is a proper candidate for applications in MIMO systems and synthetic aperture radars (SAR).

Double Elliptical Micro-strip Patch Antenna (DEMPA) is a newer family of patch antennas which possesses higher design flexibility and has greater potential for getting miniaturized than Elliptical Micro-strip Patch Antenna (EMPA). The DEMPA is made out of a Double Elliptical Patch (DEP) which is designed as a combination of two half-elliptical patches either with a common minor axis and two different semi-major axes or with a common major axis and two different semi-minor axes. There are only two design parameters for an EMPA, its semi-major axis and semi-minor axis, whereas a DEMPA has three because of either two different semi-major axes or two different semi-minor axes. A parametric study is required to understand the relationship among these three design parameters and antenna characteristics such as return loss, impedance, resonant frequency and gain. The present work is a statistical study, using the concept of Design of Experiments (DOE), of the impact of these design parameters on the return loss at resonant frequency within the frequency band of 8.50 GHz-10.55 GHz which has been earmarked for radiolocation applications by regulating agency. The Central Composite Design (CCD) technique in the Response Surface Methodology (RSM) of DOE has been employed here to develop empirical relationship between the design parameters and response variable. Numerical models were developed using Ansoft's HFSS as per the design matrix provided by Minitab. The concept of DOE helped to establish statistically significant parametric relationship between the design parameters and antenna return loss with the minimum amount of design effort. The predictive ability of regression model was confirmed by using numerical models of two DEMPAs that were not utilized to build the empirical relationship, one among which had been fabricated, tested and reported in literature.

In order to reduce the cost and blindness of antenna design, the application of electromagnetic field similarity principle in the working environment of underwater receiving antenna was studied and verified. The field distribution and electrical parameters of the underwater receiving antenna and its reduced-scale model were calculated and proved to be in accordance with the similarity principle. The simulation analysis of the receiving antenna and its reduced-scale model for receiving the airborne electromagnetic wave signal in the seawater shows that the underwater receiving antennas before and after the scale-down are similar. The simulation is verified by measuring the receiving signal amplitude of the underwater receiving antenna and its reduced-scale model. The results of theoretical derivation and simulation analysis show that the electromagnetic field similarity principle can be applied to the underwater receiving antenna system.

In this paper, two different architectures based on fully and partially clustered arrays are proposed to optimize the array patterns. In the fully clustered arrays, all the elements of the original array were divided into several equal subarrays, while in the partially clustered arrays, only the side elements were grouped into subarrays, and the central elements were left individually. The second architecture enjoys many advantages compared to the first one. The proposed clustered arrays use quantized amplitude distributions, thus, their corresponding patterns were associated with high side lobes. To overcome this problem, a constraint mask was included in the pattern optimization process. Simulation results show that the peak sidelobe level and the complexity of the feeding network in the partially clustered arrays can be reduced to more than -28 dB and 70.833% respectively, for a total of 48 array elements, number of individual central elements = 24, number of clusters on both sides of the array Q = 4, and number of elements in each side cluster M=6. Finally, the principles of the proposed clustered arrays were extended and applied to the two dimensional planar arrays.

The aim of this work is to discuss the possibility of resistive thin-film coatings used for vacuum electron devices. Following a short review, results on radiophysical parameters studies in millimeter-band of such coatings are presented. Resistive Sn-O coatings with varied oxygen content were fabricated on quartz substrates by reactive magnetron sputtering. Morphology and elemental composition of prepared coatings were studied by means of scanning electron microscopy and secondary ion mass spectrometry, while four-probe method was utilized to study the resistivity of those. Dielectric properties were measured in the V-band (50-70 GHz) in free space using vector network analyzer. It was demonstrated that resistivity and the dielectric properties of coating in millimeter-band can be widely varied by controlling coating composition.

This letter presents a uniplanar two-port UWB-MIMO antenna with high isolation for wireless communication applications. The designed antenna is composed of a single metal layer and a thin substrate. The single metal layer acts as radiators and a ground plane. The radiator of each element consists of a modified dual-L-shaped feeding structure and a defected rectangular patch, which is shared by the ground plane. The modified dual-L-shaped feeding structure is introduced to broaden the bandwidth. Furthermore, two fork-shaped slots and bent slots are embedded in the rectangular shared structure for further improving the bandwidth and decreasing the mutual couplings without any other additional decoupling structures. The experimental results show that the proposed antenna achieves the ultra-wide impedance bandwidth (3.0-12.4 GHz), high isolation ( > 20 dB at entire impedance bandwidth), very small ECC (< 0.01), high multiplexing efficiency (> -1.9 dB), stable realized gain and radiation patterns. Therefore, the designed antenna is suitable for most wireless UWB communication applications.

In this paper, a planar, compact, and low cost printed microstrip line fed pentagon-shaped ultra-wideband antenna offering dual band notched characteristics response is proposed and investigated. By introducing modified V-shaped slots in the pentagonal patch and hexagonal electromagnetic band gap structures near the feedline, dual band notched response can be realized. The proposed antenna is successfully simulated, designed, and fabricated on an FR-4 substrate. The measured results show that the proposed antenna having dimensions of 35 × 33 × 1.6 mm³ has a bandwidth over the frequency band 2.7-10.6 GHz with magnitude of S₁₁ ≤ -10 dB (VSWR ≤ 2), except 3.7-4.6 GHz (C-band Satellite Communication) and 5.16-6.08 GHz (WLAN) frequency bands. The presented antennas show small group delay variation, nearly omnidirectional radiation pattern and stable gain at working frequencies. Satisfactory results have been obtained in frequency and time-domain analysis of the proposed antenna. The formulation of the center frequency of dual notched frequency band is also proposed.

The Ultra-Wideband Software defined microwave radiometer (UWBRAD) was developed to probe internal ice sheet temperatures using 0.5-2 GHz microwave radiometry. The airborne brightness temperature data of UWBRAD show a significant reduction due to reflections of surface layering of density fluctuations making difficult the retrieval of subsurface temperature in the kilometer range of depth. Such reflections can be measured by the ultra-wideband radar in the same frequency range suggesting a combined active and passive remote sensing of polar ice sheets. In this paper, we develop a coherent reflectivity model for both ice sheet thermal emission and backscattering. Maxwell equations are used to calculate the coherent reflections from the cap layers, and the WKB approximation is used to calculate the transmission for the slowly varying profile below the cap layers. Results are then shown to demonstrate the use of radar measurements to compensate reflection effects on brightness temperatures. It is shown that the reflections corrected brightness temperature is directly related to the physical temperature and absorption profile making possible the retrieval of subsurface temperature profile with multi-frequency measurements

Recently, magnetic-resonance-based electrical properties tomography, by which the electrical properties (EPs), namely conductivity and permittivity, of biological tissues are reconstructed, has been an active area of study. We previously proposed an explicit reconstruction method based on the Dbar equation and its explicit solution given by the generalized Cauchy formula. In this method, as in some other conventional methods, the values of EPs on the boundary of the region of interest must be specified by the Dirichlet boundary condition of the partial differential equation. However, it is difficult to know the precise values in practical situations. In this paper, we propose a novel method that reconstructs EPs without the prior information of boundary EP values by deriving a new representation formula of the solution of the Dbar equation with the complex-derivative boundary condition. Numerical simulations and phantom experiments show that the proposed method can reconstruct EPs without knowledge of the boundary EP values. Therefore, the proposed method greatly enhances the applicability of the current EPT methods to practical situations.

A single-layer single-feed wideband omnidirectional microstrip antenna with rotating square patches is investigated in this paper. To obtain wide impedance band, the proximity-fed structure are introduced to the antenna to generate dual resonating modes. Two square patches with rotating angle of 90 degrees are used as the main radiator to improve the omnidirectional feature of the radiation pattern. For verifying the design, an antenna prototype with wide operation bandwidth is designed, simulated, fabricated, and measured. The results indicate that the impedance bandwidth (|S₁₁| < -10 dB) as high as 48.6% (3.02-4.96 GHz) is obtained. Meanwhile, the height of the antenna is only 0.11λ₀ (λ₀ is the free space wavelength corresponding the lowest frequency). What's more, omnidirectional radiation pattern is obtained over the operation band. The advantages ofwideband, low-profile and simple feed structure make the antenna good candidate in modern communication systems.

Enhancing the scattering of light from subwavelength structures is of both fundamental and practical significance. While the scattering cross section from each channel cannot exceed the single-channel limit, it is recently reported that the total cross section can far exceed this limit if one overlaps the contribution from many channels. Such a phenomenon about enhancing the scattering from subwavelength structures in free space is denoted as the superscattering in some literature. However, the scatterer in practical scenarios is not always in free space but may be embedded in environments with non-unity refractive index n. The influence of environments on the superscattering remains elusive. Here the superscattering from subwavelength structures in the isotropic environment with near-zero index are theoretically investigated. Importantly, a smaller n can lead to a larger total cross section for superscattering. The underlying mechanism is that a smaller n can give rise to a larger single-channel limit. Our work thus indicates that the scattering from subwavelength structures can be further enhanced if one simultaneously maximizes the single-channel limit and the contribution from many channels.

This article presents a metamaterial-based microwave sensitive sensor with a complementary split-ring resonator (CSRR) structure for nondestructive surface crack detection in pipelines. The CSRR resonator is etched in the ground plane of a microstrip line and is produced using printed circuit board technology. The novelty of the proposed sensor is its structure that allows it to be directly used for nondestructive crack detection in pipelines, based on frequency and Q factor variations, even for cracks under a coating. A measurement setup was used to test the proposed sensor in pipelines of different materials: steel, PVC, and aluminum. The sensor could detect cracks of 1 mm. For a crack of 1 mm, the frequency shift was 6.10 MHz in steel, 2.62 MHz in polyvinyl chloride (PVC), and 1.70 MHz in aluminum. In some conditions, the Q-factor shift measurements were 6.72, 5.18, and 7.15 for steel, PVC, and aluminum, respectively. The proposed sensor features high sensitivity, small dimension, simple design, and easy fabrication.

A novel high-gain polarization reconfigurable antenna composed of a polarization conversion metasurface (PCM) and a linearly polarized source patch antenna is presented in this article. The PCM is placed above the source patch antenna with an air gap. The proposed PCM can convert the linear polarization (LP) wave radiated by the source patch antenna to LP wave, right-hand circular polarization (RHCP) wave and left-hand circular polarization (LHCP) wave by rotating the PCM around the center of the antenna. Meanwhile, the proposed PCM can serve as the partially reflective surface (PRS) of a Fabry-Perot (FP) resonant cavity which can achieve gain enhancement. In order to validate the performance of the proposed design, a prototype antenna is fabricated and measured. Simulated and measured results agree well. From 10.43 GHz to 11.2 GHz, the polarization reconfiguration can be achieved by rotating the PCM to different angles while maintaining the high gain performance simultaneously.

The efficacy of including a defected ground structure (DGS) for mobile communication on a mobile phone PCB in improving the multiple input multiple output (MIMO) system performance is evaluated and demonstrated in the context of single and multiple-element two-port planar inverted-F antennas (PIFAs). The proposed scheme designed and developed for operation in the 3.5 GHz long term evolution (LTE) and future 5G frequency bands demonstrates efficiency improvement by 15% and capacity improvement by around 7% because of the significant reduction in mutual coupling between the antenna ports. Results in free space as well as next to a human head and hand phantom are presented.

The zeroing of second order correlation functions between output fields after interferences in a 50/50 beam splitter has been accepted decades-long in the quantum optics community as an indicator of the quantum nature of lights. But, a recent work [1] presented some notable discussions and experiments that classical electromagnetic fields can still exhibit the zero correlation under specific conditions. Here, we examine analytically classical and quantum electromagnetic field interferences in a 50/50 beam splitter in the context of the second order correlation function for various input conditions. Adopting the Heisenberg picture in quantum electromagnetics, we examine components of four-term interference terms in the numerator of second order correlation functions and elucidate their physical significance. As such, we reveal the fundamental difference between the classical and quantum interference as illustrated by the Hong-Ou-Mandel (HOM) effect. The quantum HOM effect is strongly associated with: (1) the commutator relation that does not have a classical analogue; (2) the property of Fock states needed to stipulate the one-photon quantum state of the system; and (3) a destructive wave interference effect. Here, (1) and (2) imply the indivisibility of a photon. On the contrary, the classical HOM effect requires the presence of two destructive wave interferences without the need to stipulate a quantum state.

The transmission spectrum of electromagnetic waves through a one-dimensional photonic star waveguides (SWGs) structure is obtained using the Green function method (GFM). The proposed structure is composed of a finite number of periodic cells; each cell contains a segment of length d1 grafted in its extremity by resonators of length d2. This periodic system is altercated by insertion of defects resonators located in three different sites, which have lengths d02, d04, and d06 different from that of the perfect resonators. The properties of such a multi-defects structure are suitable for tunable very narrow band multichannel filter applications in microwave domain. The perfect photonic structure demonstrates large microwave photonic band gaps (PBGs) in which the propagation of electromagnetic waves is prohibited. For the sake of comparison, we first introduce single defect resonators located in one site; this defect creates hence two transmission peaks (defect modes) in the gaps. These induced modes of noticeable transmissions and good quality factors permit our proposed SWGs to behave like a single narrow band channel filter. Several localized modes appear in the band gaps whenweintroduce three resonators defects of different lengths and located in three different sites. With an appropriate choice of the geometrical parameters d₁ = 1 m,d₂ = 0.5 m, d₀₂ = d₀₄=d₀₆ = 2 m, our thrice defectives system can create, in the microwave frequencies range, till eight transmission peaks in the gaps when the defects are located in three consecutive sites.These peaks have very high values of transmission rates and important quality factors reaching Q=610000. Therefore, our proposed system acts as very narrow band multichannel filters for which defects modes shift towards the low frequencies when the defects resonators lengths increase. The results obtained demonstrate the dependence of the defect modes frequencies, transmission coefficient, and the quality factor on the resonators defects lengths, their numbers, and their positions inside the defective structure.

A very compact Multiple-Input-Multiple-Output (MIMO) radiator providing dual notches is presented in this paper. It comprises circular ring monopoles fed by a microstrip line embedded with a slot of inverted U-shape. By etching a crescent slot, a split ring resonator slot, a circular slot in the circular monopole, a reversed U-shaped slot inserted along the feeding line, two notches are attained. It operates from 3.5 GHz to 12 GHz that overcomes interference from the Wireless Local Area Network(WLAN) (5.15-5.85 GHz) and X-band (7-8.1 GHz). MIMO antenna has a very compacted dimension of x 44 mm x 1.6 mm at the lowest operating frequency and hence is suitable for portable devices. It has dimensions of 0.51λ x 0.51λ x 0.018λ where λ is the wavelength of the lowest operating frequency 3.5 GHz.

A wideband 10-port multiple input multiple output (MIMO) antenna array operated below 6 GHz for the fifth generation (5G) metal-frame smartphones is presented and discussed in this paper. The proposed MIMO antenna array element is composed of a microstrip line with a tuning stub and rectangular slot. The size of the rectangular slot is only 15 mm x 2 mm (0.211λ₀ x 0.028λ₀, and λ₀ is the wavelength with the resonance frequency of 4.2 GHz). U-shaped slots on the substrate are used to reduce the mutual coupling between the antenna elements. At the same time, in order to improve the radiation characteristics of the antenna arrays, narrow slits are etched in the metal-frame. The proposed antenna covers 3.3-5.5 GHz (S₁₁ < -6 dB), which is ultra-wide bandwidth for the 5G communications. The proposed MIMO antenna array is fabricated and measured. Results show that in the desired wide frequency band, the proposed antenna array can achieve desirable performances, including antenna isolation better than -13 dB with decoupling structures, efficiency, and envelope correlation coefficient (ECC)<0.06. Moreover, radiation pattern, calculated ergodic channel capacity, the effects of the user's hand effects and head specific absorption rate (SAR) are also given in the paper. Good agreement between measured and simulated results is obtained, which means that the proposed MIMO array is a good candidate for 5G metal-frame smartphone applications.

In this paper, a novel single-layer anisotropic unit with both reflection and transmission functions is proposed. The unit is a ring-encircled two mirror-symmetry fan-shaped patches, and is fabricated in one side of an F4B substrate. The unit structure is asymmetry with respect to x- and y-axes, and both transmitted and reflected cross-polarized fields are generated simultaneously when the co-polarized field is incident on the symmetry broken surface. Full 360° phase shift range is achieved by utilizing the cross-polarized field, and the transmitted and reflected coefficient magnitudes are above 0.49 close to the theoretical limit. Using this anisotropic unit, three single-layer transmit-reflect-arrays are designed: (1) Two high-gain beams in (θ₁ = 0°, φ₁ = 0°) and (θ₁ = 180°, φ₁ = 0°) directions. The gain is 20.9 dBi, and the 3 dB beam width is 8.9°. (2) Two OAM beams with l = 1 at (θ1 = 45°, φ₁ = 0°) and (θ₂ = 135°, φ₂ = 0°). (3) Four OAM beams with l = 1 at (θ₁ = 30°, φ₁ = 0°), (θ₂ = -30°, φ₂ = 180°), (θ₃ = 150°, φ₃ = 0°) and (θ₄ = -150°, φ₄ = 180°). The simulated and measured results agree well and validate the design principle. The proposed metasurface has the following advantages: single-layer, transmission and reflection dual-functions, multi-beam, and high gain.