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There has been a surge of emphasis on polarimetric imaging modalities recently, in which Mueller matrix polarimetry (MMP) is at the heart of our understanding of polarization properties of an imaged sample. However, the historiography of MMP, predominantly undertaken in optics, largely lost sight of the potential of terahertz radiation on interactions with the imaged samples, especially biological ones. In addition, studies of MMP are generally concerned with intensity-only measurements requiring at least 16 polarization combinations, which dramatically add to the time cost in the process of measurements. The proposed study sets out to examine the complete 4 × 4 MMP from 0.2 to 1 THz using only orthogonal polarizations for two-dimensional (2-D) and 3-D imaging on leaf. A modified THz time domain spectrometer with self-designed linear polarizers is utilized for measurements and data acquisition. The Mueller matrix polar decomposition and Mueller matrix transform are employed to facilitate extraction of polarization characteristics of the imaged leaf. The time-of-flight technique is adopted to assist in the 3-D reconstruction of the leaf. The results demonstrate the effectiveness and feasibility of the novel measurement synthesis approach and the modified system for accurate characterization of the sample. Meanwhile, the findings indicate that MMP is superior in feature extraction as well as contrast enhancement of the sample. The insights gained from this article may be of assistance to the characterization of biological samples with MMP at higher frequencies for future research.

In this letter, an appropriate approach is proposed for modeling the electromagnetic (EM) scattering from composite rough surfaces made up of wake due to a submerged body and wind-driven sea waves. The computational fluid dynamics (CFD) method is used to extract the air–seawater surface wake generated by an underwater moving body at different speeds and depths. Then, the wake is superimposed on the randomly rough wind-driven sea surfaces that obey the Pierson–Moskowitz power spectrum. The small slope approximation (SSA) method is adopted to calculate the EM scattering by the composite surfaces. The simulation results are obtained and justified.

Conformal antennas are practicable candidates to the aerodynamic platforms. A wide-angle scanning conformal phased array antenna for an unmanned aerial vehicle (UAV) platform is designed and presented in this article. An eight-element linear phased array loaded with frequency selective surface (FSS) operating in $S$ -band is investigated. The horizontally polarized antenna element is designed based on a tapered slot feeding dipole. The antenna aperture can be easily conformed to the front wing airframe of the UAV due to the flexibility of the employed polyimide film. To fulfill the requirement of the airborne communication system, two kinds of FSSs are loaded on the dipole antenna to enhance the gain and reduce the 3 dB beamwidth of the main lobe in the elevation plane. To improve the active impedance matching when the beam scans to the large angles, a novel parasitic structure is proposed. Finally, a $1 \times 8$ linear conformal array prototype is simulated, fabricated, and measured. The experimental results indicate that the antenna array achieves a fractional bandwidth of 22.1% with ±70° scanning along the E-plane. The measured results validate the performance of the proposed conformal array and the design.

The orthogonal basis functions (OBFs) method is a viable approach for magnetic target signal detection under the Gaussian white noise. However, its performance would degrade overtly under the Gaussian color noise with the power spectral density of $1/f^{\alpha }$ , which is the common environmental magnetic noise. In this letter, based on the linear constrained minimum variance criterion and the generalized likelihood ratio test, respectively, an alternative detection scheme is proposed through the optimization of the basis functions by considering both the signal and noise information. Experiment results using both the simulated data and the measured data show that the present approach can significantly improve the detection performance when compared to the original OBFs method.

Novel characteristic mode (CM) formulations for penetrable objects based on explicit separation of dissipation power from the total active power and use of single surface integral equation (SSIE) are proposed. Due to the separation of the dissipation power, the right-hand side of the generalized eigenvalue equation can be set to be exclusively related to the radiated power, which permits the far-field pattern by different CMs to be orthogonal whether the material is lossless or not. The use of SSIE allows both the equivalent electric and magnetic currents to be expressed by a single effective electric or magnetic current, so that the conventional inverse to express the equivalent electric current by the equivalent magnetic current or vice versa is avoided, which is favorable for frequency sweeping simulations. Numerical examples for verifications are provided by comparing with acknowledged results or measured data.

This paper is concerned with the electromagnetic theory of characteristic modes for general homogeneous material bodies using surface integral equations (SIEs), which are computationally more efficient than using volume integral equations (VIEs). However, the generalized eigenvalue equations (GEEs) based on SIEs are prone to generate spurious or extra modes, unlike their counterparts based on VIEs. In this work, several variant SIE-based GEEs are proposed to eliminate the spurious/extra modes and improve the numerical performances. These variant GEEs share a common point that the reactive power operator related to the material domain should be removed from the left-hand sides of GEEs by explicitly enforcing the boundary condition, because it seems sensitive to incur unwanted modes. These variant GEEs are diverse in choosing the active power operator related to the material domain on the right-hand sides to suit different medium parameters, which even can be dropped directly for lossless cases. Numerical results show that these variant GEEs do not generate spurious/extra resonant modes in observed frequency range, and at least one of the variants would have acceptable numerical accuracy.

Polyimide film is a practicable alternative to the conventional substrates utilized in conformal antennas. Due to its flexibility, the antennas based on the polyimide film can be easily conformed to the platform surface. An eight-element linear-phased array operating at the P-band with a wide scan angle is investigated in this letter, which is constructed by the dipoles printed on the polyimide film. The antenna surface is mounted on the front wing of an unmanned aerial vehicle (UAV). Strong coupling is observed between the array elements when the beam scans. Therefore, decoupling structures are loaded in the array elements to broaden the scan angle. To satisfy the requirement of the radar system, directors and reflectors are also employed on the surface of the front wing to narrow the main lobe beamwidth in the elevation plane. The proposed phased array achieves a fractional bandwidth of 16.7% when scanning up to ±60° in the E-plane. The validity of this design is verified by the simulation and measurements of a 1 × 8 array prototype.

Indium tin oxide (ITO) material is a practicable alternative to metal utilized in transparent antennas. However, the conductor loss of ITO material is the major obstacle to antenna design. In this communication, an optically transparent reflectarray based on glass substrate with ITO film operating at 28 GHz is presented. A novel sub-wavelength element operating in hybrid modes with $0.167\lambda _{0}$ ( $\lambda _{0}$ is the free-space wavelength at 28 GHz) spacing is proposed for transparent reflectarray design. The peak loss of the proposed element is below 3.4 dB and a phase variation of 420° is achieved at 28 GHz. A $54\times54$ -element reflectarray with a focus to diameter ratio (F/D) of 0.7 is designed and simulated for demonstration. It achieves radiation efficiency higher than 73% at the center operating frequency. The proposed reflectarray is verified by the measurements, which is reasonably consistent with the full-wave simulated results.

Mueller matrix imaging, as one of the polarimetric imaging modalities, is an effective tool to achieve imaging enhancement as well as explore fine details of the imaged sample at microwave and optical frequencies. However, little work has been reported at terahertz (THz) frequencies, which could provide an exclusive insight in exploring fine details of biological specimens for their sensitivity to THz absorption due to their various water contents. This article presents a 0.3-THz polarimetric system for Mueller matrix imaging measurements both in reflection and transmission modes. Two experiments are designed to demonstrate the capability of Mueller matrix theory and its polar decomposition in imaging enhancement and properties characterization. The structural similarity index (SSIM) is employed to serve as the metric for image quality assessment, by which the performance of the Mueller matrix imaging is evaluated. The SSIM values of the Mueller matrix could reach as high as 0.9–0.98 for the leaf sample and 0.9 for the indium tin oxide (ITO) target, respectively, whereas that of the raw polarization images could only stabilize at 0.7–0.9 and 0.2–0.4. Furthermore, the SSIM value of the decomposition parameters for the ITO target could reach nearly 1, the maximum value. The imaging results both in visual and analytical perspectives demonstrate successful system integration at THz band. The work paves the way for our exploration of biomedical imaging of tissues and cells at higher THz frequencies in the immediate future.

Partial-structure-oriented work-energy theorem (WET) governing the work-energy transformation process of Yagi-Uda array antennas is derived. Driving power as the source to sustain a steady work-energy transformation is introduced. Employing WET and driving power, the essential difference between the working mechanisms of scattering objects and Yagi-Uda array antennas is revealed. The difference exposes that the conventional characteristic mode theory (CMT) for scattering objects cannot be directly applied to Yagi-Uda array antennas. Under WET framework, this paper proposes a generalized CMT for Yagi-Uda antennas. By orthogonalizing driving power operator (DPO), the WET-based CMT can construct a set of energy-decoupled characteristic modes (CMs) for an objective Yagi-Uda antenna, and then can provide an effective modal analysis for the Yagi-Uda antenna. In addition, a uniform interpretation for the physical meaning of the characteristic values / modal significances (MSs) of metallic, material, and metal-material composite Yagi-Uda antennas is also obtained by employing the WET-based modal decomposition and the field-current interaction expression of driving power.

Work-energy principle (WEP) governing wireless power transfer (WPT) process is derived. Driving power as the source to sustain a steady WPT is obtained. Transferring coefficient (TC) used to quantify power transfer efficiency is introduced.WEP gives a clear physical picture to WPT process. The physical picture reveals the essential difference between transferring problem and scattering problem. The essential difference exposes the fact that the conventional characteristic mode theory (CMT) for scattering systems cannot be directly applied to transferring systems.Under WEP framework, this paper establishes a CMT for transferring systems. By orthogonalizing driving power operator (DPO), the CMT can construct a set of energy-decoupled characteristic modes (CMs) for any pre-selected objective transferring system. It is proved that the obtained CM set contains the optimally transferring mode, which can maximize TC.Employing the WEP-based CMT for transferring systems, this paper does the modal analysis for some typical two-coil transferring systems, and introduces the concepts of co-resonance and ci-resonance, and reveals some important differences and connections “between transferring problem and scattering problem”, “between co-resonance phenomenon of transferring systems and external resonance phenomenon of scattering systems”, and “between so-called magnetic resonance and classical electric-magnetic resonance”.

In this paper, we present a rigorous numerical approach for the electromagnetic (EM) analyses of a hypersonic object with plasma sheath. First, the effective EM parameters of the plasma flow are constructed by solving the Navier–Stokes fluid dynamics equation combined with proper thermochemical models. Then, the EM characteristics of the hypersonic complex are examined by the method of moments based on the volume–surface integral equations. Both the fluid field properties and the associated EM effects are analyzed for a real sphere–cone object that flies at a hypervelocity of 7650 m/s and at an altitude of 61 or 71 km. The impacts of the plasma sheath on the propagation, scattering, and radiation properties are discussed. The plasma sheath may block the transmission and radiation of radio waves for a wide range of frequencies, as the attenuation may exceed 20 dB. Also, the plasma sheath may influence the backscattering radar cross section to an extent of 20 dB. As the frequency increases, the effects tend to weaken and eventually may be ignored beyond 50 GHz. The simulation results are found in good agreement with available experimental data, which indicates that the proposed approach is reliable for predicting the level of radio blackout.

In this communication, we present a tailored multilevel fast multipole algorithm (MLFMA) for efficient analysis of scattering by multiple moving objects. There are two major issues with the conventional MLFMA for multiple moving targets. One is that the relative positions and attitudes of the targets change with time and we have to remesh the solution domain and refill the matrix elements at each simulation moment. The other is that we have to solve the matrix equations repeatedly for the moving targets. To alleviate these two burdens, we adopt the stationary grouping scheme in using the MLFMA for individual object, which is unchanged as the object is moving. With the grouping scheme, the near interactions are invariant, and most far interactions are reusable when the objects are in motions. The number of levels of MLFMA remains the same even if the overall dimension of the multiple targets is changing. The interactions between objects are expressed via coordinate transforms at the highest levels of their octrees. To accelerate the iterative solutions, the sparse approximate inverse preconditioner is incorporated. Numerical results show that the saving of CPU time is substantial for continuous simulations of a maneuvering process of a group of targets.

Electromagnetic inverse scattering (EMIS) is a noninvasive examination tool, which holds the promising potential in science, engineering, and military applications. In contrast to conventional tomography techniques, the inverse scattering is a quantitative superresolution imaging method since it is capable of accommodating more realistic interactions between the wavefield and the probed scene. In this paper, a full probabilistic formulation of the EMIS is presented for the first time, which is then solved by applying the well-known expectation maximization method. Afterward, the concept of the complex-valued alternating direction method of multipliers has been proposed as an alternative approach to solve the resulting nonlinear optimization problem. Finally, exemplary numerical and experimental results are provided to validate the proposed method.

This paper presents two kinds of novel elements to design single-layer dual-band reflectarray, with identical polarization in two closely separated bands. Several degrees of freedom of the proposed elements are tuned to match the desired phase compensations at two center frequencies simultaneously. It is noted that the dual-band characteristics are realized by a single integrated element rather than conventional dual-band elements with independent tunable components corresponding to two center frequencies. A 10 × 10-element offset-fed reflectarray operating at 9 and 13.5 GHz, with a ratio of the center frequencies 1.5, is designed and fabricated to validate the performance of the element, and the measured results show reasonable agreements with simulations. Due to the incompleteness of reflection phase distribution at the two center frequencies of the presented element, a complementary element with four resonances is introduced. Then, a 20 × 20-element reflectarray composed of both kinds of elements is also designed, and the measured results demonstrate a good availability of the proposed elements.

A novel single-layer reflectarray element with an I-shaped dipole encompassed by a circular ring, showing smooth phase-frequency response, is proposed in this letter. Based on this element, an optimization method is adopted to match the desired phase delays at two edge frequencies so as to get fixed phase difference between two adjacent elements in the optimized band. A low-profile X-band 6 × 10-element reflectarray with a scattering angle of 30° for normal incidence of plane waves is designed and fabricated. Simulations and measurements show that a bandwidth of 22.3% for 3 dB directivity dropping and 15.2% for -10 dB sidelobe level (SLL) can be achieved.

Prior theoretical studies and experience confirm that the stability of marching-on-in-time (MOT) solvers pertinent to the analysis of scattering from free-standing three-dimensional perfect electrically conducting surfaces hinges on the accurate evaluation of MOT matrix elements resulting from a Galerkin discretization of the underlying time domain integral equation (TDIE). Unfortunately, the accurate evaluation of the four-dimensional spatial integrals involved in the expressions for these matrix elements is prohibitively expensive when performed by computational means. Here, a method that permits the quasi-exact evaluation of MOT matrix elements is presented. Specifically, the proposed method permits the analytical evaluation of three out of the four spatial integrations, leaving only one integral to be evaluated numerically. Since the latter has finite range and a piecewise smooth integrand, it can be evaluated to very high accuracy using standard quadrature rules. As a result, the proposed method permits the fast evaluation of MOT matrix elements with arbitrary (user-specified) accuracy. Extensive numerical experiments show that an MOT solver for the electric field TDIE that uses the proposed quasi-exact method is stable for a very wide range of time step sizes and yields solutions that decay exponentially after the excitation vanishes.

The loss and cost introduced by the feeding network and the associated components, e.g., T/R modules or phase shifters and so on, of a large array antenna are the main issue limiting its efficiency and applications. A novel idea in order to build a sparse periodic patch array is presented while keeping similar aperture efficiency by taking advantage of mutual coupling between array elements, and then the total cost of the array can be obviously decreased. Operating principles are carefully addressed based on electric field distributions and impedance matrix. Simulations and measurements show that when the active element spacing is as large as 1.67λ0 (λ0 is the free-space wavelength at the operating frequency), approximately equal directivity can be achieved, compared to an ideal conventional array with a half-element spacing (0.835λ0) and an identical total size.

An augmented sparse-matrix canonical-grid (SMCG) approach for numerical analysis of scattering by dielectric random rough surfaces is developed. It is an extension of the previous modified SMCG scheme for perfect-electric-conducting (PEC) surfaces to dielectric cases, and is enhanced by adopting curved triangulation modeling. By using the curved modeling with curvilinear Rao-Wilton-Glisson basis functions (CRWG bases), the number of unknowns can be reduced substantially to extract results of the same accuracy, compared with using the planar triangular discretization with planar RWG bases. Numerical results for Gaussian and ocean-like dielectric rough surfaces are provided to confirm the validity and efficacy of the proposed method.

In this paper, we investigate both open- and short-ended half-wavelength microstrip resonators and their applications to bandpass filter designs. Using voltage-wave analyses, we examine the resonance and coupling properties of these resonators at their fundamental and harmonic frequencies. Loading them with a lumped-circuit element at the locations of zero-voltage point of the second harmonic allows control of fundamental frequency response without altering that of the second harmonic. A resistive loading can suppress the second harmonic while a p-i-n diode loading yield a switchable bandpass filter. The coupling regions between an open-ended resonator and a short-ended resonator of higher order bandpass filters can be chosen such that second-, third-, and fourth-harmonic suppression, as well as dual passband, can be achieved. Simulation and experimental results agree well, validating the proposed bandpass filter design methodology.

In this letter, a compact dual-band (DB) bandpass filter based on parallel short-ended feed scheme is presented. Two sets of quarter-wavelength resonators operating at diverse frequencies are adopted to generate DB responses. A special feed scheme is proposed, which consists of two parallel short-ended strip branches. Each of them acts as a feed line of the respective resonator at one passband and a loading element at the other one. This scheme provides two independent transmission paths for RF signals. It is convenient to control the center frequencies of one passband, while the other's remains unchanged. Source-load coupling is also employed to create four transmission zeros close to the passband edges, resulting in high skirt-selectivity. To verify the proposed idea, a topology is demonstrated. The measured results exhibit good agreement with the full-wave simulation results.