Novel microwave imaging systems require flexible forward solvers capable of incorporating arbitrary boundary conditions and inhomogeneous background constitutive parameters. In this work we focus on the implementation of a time-harmonic Discontinuous Galerkin Method (DGM) forward solver with a number of features that aim to benefit tomographic microwave imaging algorithms: locally varying high-order polynomial field expansions, locally varying high-order representations of the complex constitutive parameters, and exact radiating boundary conditions. The DGM formulated directly from Maxwell's curl equations facilitates including both electric and magnetic contrast functions, the latter being important when considering quantitative imaging with magnetic contrast agents. To improve forward solver performance we formulate the DGM for time-harmonic electric and magnetic vector wave equations driven by both electric and magnetic sources. Sufficient implementation details are provided to permit existing DGM codes based on nodal expansions of Maxwell's curl equations to be converted to the wave equation formulations. Results are shown to validate the DGM forward solver framework for transverse magnetic problems that might typically be found in tomographic imaging systems, illustrating how high-order expansions of the constitutive parameters can be used to improve forward solver performance.

We propose a novel design of broadband plasmonic nanoantenna that is suitable for fluorescence and Raman enhancement. The structure consists of a gold nanoring and bowties at the center. We numerically investigate the near field and far field performance by employing the finite-difference time-domain method. High Purcell enhancement and large SERS are demonstrated in a record wide spectral bandwidth of 700 nm based on a single emitter-antenna configuration. Moreover, unlike a traditional antenna design, the proposed nanoantenna has low heat generation and high field enhancement at the gap simultaneously, when operating off resonance.

A broadband substrate to substrate microwave circuit interconnection is proposed using bond wires and defected ground structure (DGS). The proposed square-shaped DGS etched under compensated microstrip open stubs not only expands its operating bandwidth, but also increases the characteristic impedance of microstrip line without narrowing its width, which breaks the PCB fabrication limitation of narrow stubs. The proposed structure can make the impedance of the microstrip line much larger than that without DGS. A 250 Ω characteristic impedance is easily achieved using 0.6 mm microstrip line with the proposed DGS. Measured S₂₁ and S₁₁ of the proposed interconnection are better than -0.8 and -15 dB from DC to 38 GHz, respectively. And a bandwidth increment of more than 1200% is achieved compared with the conventional one.

In this paper, we demonstrate a device that is capable of generating an ultrashort (sub-nanosecond) high power microwave pulse by means of passive pulse compression in a compact reverberant cavity. The long duration input pulse into the cavity is created using time-reversal techniques, which allows the waveform to contain the inverse profile of the cavity phase distortion. When fed back into the cavity, the wave focusing at the output port results in a com-pressed ultrashort pulse with enhanced peak amplitude. We experimentally demonstrate a pulse compressor consisting of a 0.0074 m³ cavity capable of generating a 130 picosecond pulse from an input waveform of 300 nanosecond duration with the peak gain of up to 19 dB.

We derive and compare several finite-difference frequency-domain (FD-FD) stencils for points on or near a planar dielectric interface. They are based on interface conditions or from modifying Helmholtz equation. We present a highly accurate formulation based on local plane wave expansion (LPWE). LPWE-based compact stencil is an extension of the analytically obtained LFE-9 stencil as used by the method of connected local fields [Chang and Mu, PIER 109, 399 (2010)]. We report that merely using five points per wavelength spatial sampling, LPWE coefficients achieve better than 0.01% local error near a planar interface. We numerically determine that we have fourth to eighth-order accuracy in the local errors for LPWE stencils.

The increasing sophisticated power and communication demands have motivated a variety of research on simultaneous wireless information and power transfer system, aiming to provide higher power transfer efficiency and improved communication rate. This letter demonstrates that resonant wireless power transfer system with relays can be a candidate to reach these aims. Based on coupled resonator filter theory, mathematical equations for transmission efficiency and bandwidth are derived for arbitrary number of relays. Improved efficiency and bandwidth are verified by equations, simulation and experiments. Experimental results show that under the distance of two times the diameter of the resonator, system efficiency increases from 5.43% (no relay) to 29.47% (one relay) and 38.02% (two relays), with the fractional bandwidth broadened from 1.33% (no relay) to 3.31% (one relay) and 4.47% (two relays) at operation frequency of 42.55 MHz, providing available channel for simultaneous power and data transfer. The procedure for the design of relays is also listed in detail.

In this paper, the localized pseudo-skeleton approximation (LPSA) method for electromagnetic analysis on electrically large structures is presented. The proposed method seeks the low rank representations of far-field coupling matrices by using pseudo-skeleton approximations (PSA). By using PSA, only part of the original matrix is needed to be calculated and stored which is very similar to the adaptive cross approximation (ACA). Moreover, rank approximation and index finding schemes are given to improve the performance of the method in this paper. Several numerical results are given to demonstrate that the proposed method performs better than the randomized pseudo-skeleton approximation (RPSA) and ACA.

A novel compact differential microstrip antenna is presented. Owing to the introduction of slots in the patch and ground plane, the size of the proposed differential antenna is about 0.45 times that of the traditional microstrip antennas. The measured results show that the proposed antenna can work at 2.45 GHz. The gain is about 5.54 dB and the impedance bandwidth about 150 MHz.

A low-profile broadband omnidirectional MIMO antenna with dual-polarization is proposed for 4G LTE applications. It consists of two orthogonally polarized radiating elements with separate ports. The horizontally polarized element consists of four printed arc dipoles, a broadband balun feeding network, four arc parasitical strips and four double L-shaped parasitical strips. It exhibits a 48.9% fractional bandwidth (return loss > 10 dB) from 1.70 GHz to 2.80 GHz with a cross-sectional size less than 100×100 mm². The vertically polarized element consists of a discone, a round sleeve, and a top-loading ring shorted to the ground-plane with three equally-spaced pins. It provides a 47.3% fractional bandwidth (return loss > 10 dB) from 1.68 to 2.72 GHz with an overall volume less than 84×84×22 mm³. In addition, an isolation of 25 dB is achieved in the overlapping band of two elements and the cross-polarization levels for both radiating elements are lower than 20 dB.

In this letter, a compact stable bandpass frequency selective surface (FSS) operating at 3.14 GHz is proposed by using a novel Y-type element. The measured and numerical results are in good agreement, except a little deviation of resonant frequency and a little change of bandwidth, which show that the proposed FSS has good angle and polarization stability. Numerical results show that the dimension of the element is only 0.042λ₀×0.042λ₀, where λ₀ represents the wavelength at the resonant frequency 3.14 GHz. Thus, the FSS is suitable for practical application in limited space.

This paper is aimed at analyzing the electromagnetic (EM) scattering from the two-dimensional (2-D) Gaussian rough surfaces characterized by textures. Visual appearances of the stripe texture can be generated through the angle rotating in Fourier transform when the ratio of the correlation lengths in two directions is large enough. The scattering field is derived in Cartesian coordinate system through the small-slope approximation (SSA) method with plane incident wave. The normalized co-polarized radar cross section (NRCS) from 2-D Gaussian rough surface characterized by textures are calculated. In particular, several numerical results show the influences of incident angle, texture angle, correlation length, and root-mean-square height on the scattering from the textured rough surface. Finally, the validity of the SSA method is verified by comparisons of theoretical value and measured data.

In this paper, a new reconfigurable antenna is introduced. This antenna is a printed spiral-shaped monopole antenna with a compact structure. By embedding microwave switches in the structure of the antenna, different resonance frequencies can be achieved in different states of the switches. The introduced antenna is capable to cover two standard frequency bands for biomedical applications, i.e. Medical Implant Communication Service (MICS) and Industrial, Scientific and Medicine (ISM) bands. MICS band covers 402 MHz to 406 MHz and ISM covers 2.4 GHz to 2.5 GHz frequency range. The proposed antenna has a compact size of 32 mm×50.3 mm×1.8 mm, and it is fabricated on an FR4 substrate. The measurement results are in a good agreement with the simulations.

In this work, characteristics of an arc-monopole loaded with a DRA is analyzed. It is observed that the arc-monopole can be used to effectively couple to multiple DRA modes and generate dual/triple/wideband topologies. The structure is easy to fabricate with no additional substrate or matching slot/vias required to excite the multiple DRA modes. In addition, both broadside and monopole like patterns are obtained for the dual/triple band configurations which are suitable for communication with satellite or airborne targets and for surface-to-surface communication. The enhanced radiation in the source plane for the monopole like pattern can be effectively used to communicate with preferred targets or enhance the range in the direction of interest. In addition, the arc-monopole can be suitably located to couple the source mode to the DRA modes to generate broadband behavior.

A compact magnetic Yagi antenna and a four-element array with vertically polarized radiation are presented using the substrate integrated waveguide (SIW) technology. The SIW functions as driven element to generate vertically polarized wave. Microstrip patches are connected to ground plane function as magnetic dipole directors. With this arrangement, a compact magnetic Yagi antenna with vertically polarized radiation is designed. The total area is only 1.58λ₁×0.95λ₁ (λ₁ represents the wavelength at 9.5 GHz) and reduced by 63.1% compared to previous magnetic Yagi antenna. The relative bandwidth is 16.15% and peak gain 7.31~8.82 dBi. Then the four-element linear array is analyzed, fabricated and measured. Simulated and experimental results demonstrate that the array antenna still preserves vertical polarization, and the peak gain is 14.06~14.78 dBi in the relative bandwidth of 14.43%.

In this paper, a novel compact band pass filter (BPF) is proposed for Global Positioning System (GPS) receivers. The proposed BPF configuration is composed of a low pass filter (LPF) section formed by the coupled line transformer connected with a radial stub and two short circuited stubs embedded within the 50 Ω microstrip line connecting the input/output (I/O) port of LPF. The lumped equivalent model of proposed BPF is also presented and analyzed. Simulation as well as experimental results shows very good in-band (pass-band) and out-of-band (≈ 7f_c (centre frequency)) characteristics. The 3 dB fractional bandwidth (FBW) is 3.2 % of f_c, thus satisfying the GPS receiver requirement and the minimum insertion loss (IL) in pass-band is 1.28 dB.

Transformation optics is a convenient way to control the pattern of electromagnetic fields. In this paper, using a novel transformation, we propose the design procedure of a horn antenna having low backlobe and sidelobe levels in its E-plane. By applying conformal transformation, the rectangular horn proposed in this paper can be realized with isotropic materials. This proposed antenna can be easily implemented by both ordinary dielectric materials and isotropic graded refractive index (GRIN) materials. In the rst proposed design, in addition to the isotropy, homogeneity is furthermore introduced into the horn, and only four kinds of isotropic materials are required throughout. In the second design, it is demonstrated that the designed structure can also be implemented by graded photonic crystals (GPCs) operating in metamaterial regime. They have low loss as well as broad frequency band and are easy to implement. Simulation results are presented to validate the design approach.

An accurate and efficient computational approach is presented for analyzing radiation characteristics of large antenna arrays with radome. This approach is based on the hybrid finite element-boundary integral-multilevel fast multipole algorithm (FE-BI-MLFMA). Unlike the conventional single-domain FE-BI-MLFMA, the whole domain of the antenna array with radome is separated into many disconnected domains. A large free space area unavoidable in the single-domain FE-BI-MLFMA is eliminated in this multi-domain FE-BI-MLFMA formulation, thus the number of unknowns is greatly reduced in the presented multi-domain FE-BI-MLFMA approach. Different from the single-domain FE-BI-MLFMA, many integral equations are required in this multi-domain FE-BI-MLFMA. The numerical experiment shows that the presented multi-domain FE-BI-MLFMA is more efficient than the single-domain one while maintaining the same accuracy. A whole complicated system of a slotted-waveguide array with radome mounted on an aircraft is analyzed to further demonstrate the generality and capability of the presented multi-domain FE-BI-MLFMA.

This study discusses the operating characteristics of a large-orbit electron gun and a corresponding permanent magnet system of a 3rd harmonic peniotron. After optimization, a novel axis-encircling electron beam with axial velocity spread 4.48%, guiding centre deviation ratio 6.97% and high velocity ratio 2.03 is obtained. Driven by the electron gun, an output power of 35.4 kW is obtained, and the device efficiency is up to 56.0%, which is an attractive result in laboratories. The main advantages of such a peniotron are its compact size and low cost, which can meet the needs of vehicle, airborne and other mobile devices. The numerical analysis reveals that the relative axial position between the electrode system and magnet system has a great influence on the device performance, which needs careful control and precise adjustment.

In this paper, a theoretical analysis of numerical dispersion of the three-dimensional (3-D) high-order finite-difference time-domain (FDTD) method with weighted Laguerre polynomials (WLPs) is presented. The phase velocity of numerical wave modes is relevant to the direction of wave propagation, grid discretization and time-scale factor. The formula to determine a suitable time-scale factor is derived. By a theoretical evaluation, the dispersion errors for the 3-D high-order WLP-FDTD scheme with different time-scale factors are obtained. Finally, one numerical example is included to validate the effectiveness of the theoretical solution of the time-scale factor.

A novel method for a dual-band filter and quad-channel diplexer design is presented in this paper. This method, by altering the gap between resonators, realizes a transformation from bandpass to dual-band for the filter and diplexer. At first, a high selectivity bandpass filter (BPF) with four controllable transmission zeros (TZs) is designed. Then altering the gap between resonators, a band gap is generated and utilized to split the passband of the proposed BPF into two bands, which transforms the BPF to a dual-band filter with narrow passband separation. The center frequency and bandwidth of the new dual-band filter are controllable by adjusting the frequency and width of band gap. Based on the dual-band filter, a quad-channel diplexer with stepped impedance T-junction is designed, and it can be transformed to a wideband diplexer. For demonstration, the dual-band filter and quad-channel diplexer are fabricated and measured.