Vol. 90

This paper discusses a circular loop frequency selective surface (FSS) using a 3-D (three dimensional) printed technique. The proposed FSS design consists of a metallic patch having a circular loop printed on one side of Acrylonitrile Butadiene Styrene (ABS) material. This design is used for harmonic radar applications at 5 GHz resonant frequency. Various FSS parameters are discussed to show the effect on the resonant frequency. To make fabrication process easier and cost-effective, transmitting and receiving antennas are also printed using a 3-D printing material. 3-D printing offers cost-effective fabrication technique compared with other conventional techniques and helps in rapid prototyping. The fabricated prototype is validated with the experimental results that show good agreement between simulated results and the measured ones.

A gradient index metamaterial (GIM) based conformal cloak is utilized to reduce the overall scattering of a dipole antenna and its blockage effect when being placed in close proximity of a horn antenna. The reduction in scattering is attributed to wave conversion properties of GIM cover, by virtue of which the propagating waves get converted to surface waves and vice versa, thus reducing the scattering signature of the dipole. The GIM cover also has the advantage of larger bandwidth than single metasurface based cloaks (mantle cloak). The proposed GIM based cloak proves to be effective in reducing the mutual interference between dipole and horn antenna without disrupting the performance of individual antennas in their respective frequency band of interest. The Ansys HFSS simulation results are presented to demonstrate the effectiveness of GIM based cover to reduce mutual blockage effect between a low band dipole and an S-band horn antenna.

In this paper, a dual-band 4-, 6- and 8-element multiple-input multiple-output (MIMO) antenna arrays operating at the sub-6-GHz (LTE 42/43 and 46) bands for the fifth-generation (5G) smartphones are proposed. To realize these three MIMO applications in two LTE bands, miniaturized spiral and meander line-shaped strips coupled-fed patch antenna elements are printed on the front side of an FR4 system circuit board and are able to excite two resonance modes. Polarization and spatial diversity techniques are applied to these elements so that the enhanced isolation and reduced coupling effects can be attained. The proposed single antenna element besides 8-element antenna array has been fabricated and experimentally measured. Desirable simulated and measured S-parameters (reflection and transmission coefficients) are obtained for the antenna arrays over the working dual frequency bands. The diversity performance, such as the envelope correlation coefficient (ECC) and diversity gain (DG), has also been simulated and analyzed. Moreover, the performance results, antenna gain and efficiency over the bands, and radiation patterns at the specified resonant frequencies are also presented.

A tri-band Cylindrical Dielectric Resonator Antenna (CDRA) array is proposed for WiFi, wireless LAN, and satellite applications in this paper. CDRA is massively demanded by various smart wireless devices. The claimed antenna array structure is developed and fabricated using an FR4 substrate having relative permittivity (ε_r) of 4.4. Microstrip power divider line is utilised for array excitation. The variation in return loss due to effect of varying micro strip line length, dielectric resonator height and ground plane height has been carefully recorded and presented using parametric study. The array structure is engineered for triple band operations working at 2.4 GHz, 4.1 GHz, and 5.4 GHz frequencies. To achieve adequate bandwidth accompanied by acceptable gain is a very inspiring task. The proposed structure shows a promising maximum impedance bandwidth of 1.14 GHz (40%) and a maximum gain of 9 dBi. The return loss and radiation pattern computed through CST software are verified by practical measurements using VNA device and anechoic chamber atmosphere.

This paper presents a miniaturized frequency selective rasorber (FSR) with both wide absorption and transmission bands. The proposed FSR consists of a resistive sheet and a band-pass frequency-selective surface (FSS) with non-resonant constituting elements. The unit cell structure of the resistive sheet is a meander line loaded with four lumped resistors, which generate a wide absorption band from 2.4 to 6.2 GHz, while the layer of FSS is coupled resonator spatial filter (CRSF) which generates a wide transmission band from 7.5 to 10.2 GHz. Furthermore, there is a 10.5 mm air spacer between the resistive and FSS layers. The period of the FSR structure, which maintains its passband and absorption band performance, is only 10 mm (0.08λL). Simulated and measured results are compared and found to show good agreement.

This paper presents the design of an 8-element linear array for Adaptive Antenna applications using the Least Mean Square (LMS) algorithm towards improving the directive gain, beam steering capabilities, half-power beamwidth, sidelobe level, and bandwidth of array. A conventional patch antenna is optimized to operate at 3.6 GHz (5G applications) with two symmetrical slots and Quarter Wave Transformer for feeding, and this design is extended up to 8 elements using CST Microwave Studio parameterization. The Return Loss (S₁₁), Directivity, HPBW and VSWR of the antenna array are observed for the 2, 4, and 8 element adaptive array. The inter-element spacing for resulting eight-element antenna array geometry is optimized to obtain maximum directive gain. This geometry appears promising in improving the directive gain from 7.6 dBi to 15.1 dBi for a single element to eight elements respectively. Further, the LMS algorithm is used to compute the optimal complex weights, considering different angles for the desired User (+45˚ and -45˚) and Interferer (+20˚ and -20˚) during MATLAB simulation, and then these optimal weights are fed to antenna elements using CST for beam steering in a different direction. Maximas in the direction of user and nulls in the direction of interferer are obtained using CST software and found closely matching with MATLAB results.

This paper presents the effect of gimbal geometry parameters on the electromagnetic performance of streamlined radome for airborne applications. The work demonstrates that the gimbal position significantly affects the boresight error performance. The optimization of gimbal position is performed, and the resultant boresight error is limited to 1.5 mrad while keeping the insertion loss below 0.25 dB over the entire antenna scan angle range. The analysis of the antenna-radome system is carried out using the 3D ray tracing method. This work shows that the gimbal geometry parameters provide additional degree of freedom for improving radome performance parameters and can be applied to both the gimbal mounted and electronically scanning antennas enclosed by streamlined radomes.

This paper introduces the design and analysis of a compact bandpass with sharp attenuation. The filter topology employs three different cells of a bisected-Π/Π configuration of a negative refractive index metamaterial transmission line. The filter centre frequency is 3.65 GHz, and its 3 dB cutoff frequencies are 2.55 GHz and 4.6 GHz (57% fractional bandwidth). The filter attenuation increases to 20 dB in only 100 MHz (at 4.7 GHz). Moreover, the filter has only 0.2 dB insertion loss within the passband. The filter stopband is characterized with typical flat with 0.2 dB return loss within the stopband (4.7 GHz-5.75 GHz) and very close to 20 dB insertion loss. Moreover, the filter has two frequency independent designed transmission zeros within this stopband. Along with previous specifications, the filter size is only 0.22λ_g × 0.20λ_g (12 × 11 mm²) at centre frequency. The filter performance has been validated through circuit model, electromagnetic simulation, and experimental measurements.

A fast direct solution of the electric current volume integral equation (JVIE) with the Sherman-Morrison-Woodbury (SMW) formula-based algorithm is presented to analyze electromagnetic scattering from inhomogeneous dielectric objects. The JVIE is discretized with the nonconformal face-based Schaubert-Wilton-Glisson (SWG) basis functions. Compared with conformal discretization that is advantageous to discrete homogeneous regions, the nonconformal discretization provides a more flexible and efficient scheme to separately handle the inhomogeneous subdomains depending on local parameters. Moreover, to take full use of both discretization methods, the mixture discretization is adopted. With the increase of object size, the impedance matrix equation arising from the JVIE becomes too large to solve and store for direct solution. In this paper, the SMW formula-based algorithm is adopted, leading to remarkable reduction on the computational complexity and memory requirement in contrast with conventional direct solution. This algorithm compresses the impedance matrix into a product of block diagonal submatrices, which can be inversed rapidly in direct way. Numerical results are given to demonstrate the efficiency and accuracy of the proposed method.

Passband flatness and band-edge selectivity in microwave filters with finite quality-factor resonators can be improved by the synthesis of lossy filters. This paper demonstrates the extension of this technique to a lossy diplexer by means of resistive coupling. A dual-mode stub-loaded resonator (SLR) junction and a fork-like feedline are used in the diplexer to address the challenge of independently controlling the external coupling from the common port to the two channel filters and therefore enable flexible realization of the channel bandwidth. The coupling matrices with resistive couplings for the lossy diplexer are generated. For verification, a microstrip lossy diplexer operating at 1.91 and 2.6 GHz was designed and tested. The flatness of the passband has been significantly improved, with a reduction of the passband insertion loss variation from 1.4/1.2 dB to 0.66/0.63 dB for the low/high band. The measured results are in good agreement with the simulations as well as the theoretical responses from the coupling matrix. This was also experimentally compared with a reference diplexer without resistive couplings.

Proximity Detection Systems (PDSs) are used in the mining industry for protecting mine workers from striking, pinning, and crushing injuries when they work in close proximity to heavy machines such as continuous mining machines (CMMs). Currently all PDSs approved by the Mine Safety and Health Administration (MSHA) are magnetic field based systems which can be influenced by the presence of steel wire mesh that is commonly used for supporting roof and ribs in underground coal mines. In this paper, researchers at the National Institute for Occupational Safety and Health (NIOSH) characterized the influence of the mesh on the performance of magnetic PDSs by measuring the magnetic field difference around a CMM caused by the presence of the mesh. The results show that the magnetic fields are generally enhanced by the mesh which causes PDS detection zones to be increased correspondingly. It was discovered that the fields around the joints of two mesh sections have the greatest enhancement and thus deserve more attention. In addition, it was found that the presence of mesh can also cause a variation in the generator current. The experimental results show that the generator current variation and thus the magnetic field change caused by the mesh can be significant (on the order of ten) when the mesh is extremely close to the generator (e.g, less than 1 cm) and is negligible when mesh is relatively far (greater than 0.15 m). The findings in this paper can be used to develop guidelines and best practices to mitigate the influence of mesh on PDSs.

This paper describes the measurement of a driver's instantaneous heart rate corresponding to R-R interval in electrocardiogram and heart-rate variability (HRV) using 24 GHz radar reflectometers. Elimination of the spurious component due to random movement of a driver has been the most difficult problem for microwave measurement. Auto-gain control of the receiver, template matching and cross-correlation technique among multiple reflectometers enable motion artifact elimination, signal peak detection, and data processing for various parameters. The measurement of vital signals is considered useful for predicting the change in a driver's state, such as a heart attack as well as detecting drowsy driving, drunk driving, and fatigue.

In this paper, a quadruple-band metamaterial polarization-insensitive absorber with low profile is proposed. The proposed unit cell is composed of three conformal modified rings with square patches at corners. 10*10 periodic unit cells constitute the proposed metamaterial absorber. The absorber offers low profile, and overall dimensions are 100 mm*100 mm. The surface current distribution and equivalent circuit model are presented to explain the mechanism. The proposed structure is fabricated, and experiments are carried out to validate the design principle. The simulated and measured results show that the proposed structure exhibites four absorption peaks of 98.87%, 95.11%, 93.97%, and 99.99% under normal incidence at 8.16-8.29 GHz, 10.275-10.38 GHz, 14.255-14.38 GHz, and 15.465-15.7 GHz which cover X- and Ku-bands, respectively. The designed structure is exactly symmetrical which makes it insensitive to polarization angle variations. Furthermore, the four operating bands of the absorber can be adjusted independently which makes the design suitable for absorbing electromagnetic energy and reducing the radar cross-section (RCS) of target.

This paper specifies optimization of a low active reflection coefficient (ARC) array element with a cavity-backed microstrip patch (CBMP) using a genetic algorithm (GA) at wide-band and 2-dimensional (2D) wide angle. Both the GA implemented with a user-defined MATLAB code and a 3-dimensional (3D) full-wave electromagnetic simulator CST MWS are simulated with a real-time direct link. An optimization method using not a traditional unit cell ora small array but a 15 × 15 finite array structure is proposed to apply to a large-scale array antenna. The CBMP array antenna to meet a design goal of a max ARC is optimally designed at equally divided 9 frequencies and 11374 beam angles for S-band 400 MHz operating frequency bandwidth and beam scan coverage (Az = -60° ~ +60°, El = -3° ~ +90°). Measurement results show that a prototype and a full-scale array antenna have low ARC below -8.1 dB and -6.9 dB respectively for required wide frequency bandwidth and beam scan coverage. It is confirmed that the proposed method is a good solution for optimizing a large-scale array antenna.

Metasheets are ultra-thin sheets built from sub-wavelength resonators designed to achieve certain frequency-dependent transmission behavior. A semianalytical approach based on an equivalent circuit representation is proposed to calculate the microwave transmission through metasheets consisting of two-dimensional periodic arrays of planar circular metal rings on a dielectric substrate. In the semianalytical approach, the impedances of the equivalent circuit are parameterized and fitted to match the values of transmission coefficients obtained by full-wave simulations at selected frequency points. As dimensional parameters, the outer radius and the width of the ring are considered. A metalens with four concentric zones is designed by using this semianalytical approach to correct the phase distortions due to a polypropylene hemispheric radome at frequencies around 28 GHz in the Ka band. It is shown that the designed metalens works well for 27 GHz, 28 GHz, 29 GHz and 29.5 GHz, implying the bandwidth of approximately 2.5 GHz. The field transmitted through the metalens and the radome is calculated by Physical Optics (PO). The electrically large integration area is divided into small square facets to calculate the PO integral. The calculated and measured results are shown to agree well.

The combination of low peak sidelobe level (PSLL) with wide sector nulling digital beamforming (DBF) is achieved for large planar array antennas. This combination is carried out using the iterative Fourier transform (IFT) method. The method is based on the iterative Fourier technique to derive element excitations from the prescribed array factor using successive forward and backward Fourier transforms. A 1024-element rectangular uniformly spaced array is used as an example to demonstrate the performance of the proposed method. Numerical examples show that the proposed method achieves very low PSLL with very wide nulling sectors up to the half plane of the far-field pattern. Moreover, numerical results show that the proposed method is effectively functional even when the mainbeam is steered to directions other than the broadside.

A linear polarization beam splitter operating in terahertz band is proposed and experimentally verified in this paper. The unit cell of beam splitter is composed of the top ``I'' type metal pattern, the middle dielectric layer, and the bottom metal layer. Each subarray structure of the device consists of four unit cells that rotate progressively at an angle of 45˚. The horizontal and vertical sub-arrays form the gradient metasurface of 4×4. The incident linear polarized terahertz wave is reflected by the device and divided into four beams with approximately equal power, while having different propagating directions in the 0.18-0.30 THz band. The proposed terahertz beam splitter has the advantages of small size, low cost, and easy processing, and it can be applied to terahertz stealth and terahertz imaging.

This paper reports a novel approach using an inductive loading to reduce the resonant frequency of a mushroom-shaped high impedance surface. The current path is extended on the mushroom-shaped structure's vias and additional traces, which introduces a three-dimensional inductor to the unit cell and leads to an increase in total inductance. As a result, the resonant frequency of the high impedance structure decreases, and a smaller unit cell size can be achieved at the low gigahertz frequency range. Finite element electromagnetic simulation, equivalent circuits modeling, and experimental measurements suggest the feasibility of the proposed approach.

In this paper, using quasi-conformal mapping, a bi-functional coating layer is designed with the intention of both cloaking and directivity enhancement of an omnidirectional antenna. For TM external waves coming from a certain direction, the proposed coating layer conceals the inner objects. In addition to the cloaking performance, the designed coating layer plays the role of a metamaterial-based lens that dramatically enhances the directivity level of inner omnidirectional loop-family antennas. To reach this goal, a proper coordinate transformation is elaborately utilized to transform the cylindrical wavefronts radiated from the antenna into semi-pure plane waves. With appropriate simplifications, the proposed coating layer turns into an isotropic meta-device, which is more suitable to be fabricated. To prove the feasibility of the implementation, an SRR-meander line meta-atom is designed to locally realize the required permittivity and permeability distribution of the bi-functional layer. Full-wave simulations are performed via COMSOL finite element solver to validate the cloaking effect and directivity enhancement of the proposed coating layer, at the same time.

The effect of measurement errors in the S-matrix of a reciprocal 2-port device is recognized in the (usually low) difference between S₁₂ and S₂₁, as the device were nonreciprocal. This ``false non-reciprocity'' is analyzed in the present paper, and it is verified that, for low loss device, the difference acts principally on the phases of S₁₂ and S₂₁. This anomaly can be removed if a numerical correction is applied to the experimental S-matrix. In doing so, it is proved that the residual measurement errors have comparable amplitudes on all scattering parameters.