A passive millimeter-wave imager BHU-2D-U based on synthetic aperture interferometric radiometer (SAIR) technique has been developed by Beihang University. The imager is designed for detecting concealed weapons on human body and operated under the near-field condition of the antenna array, thus the conventional Fourier imaging theory does not apply. In this paper, an accurate numerical image reconstruction algorithm using regularization theory is proposed. By means of adding a prior information of desired brightness temperature image, the influences of measurement noise and focusing error on the reconstructed image have been reduced. Numerical simulations and experiments on BHU-2D-U have been performed to verify the superiorities of the proposed algorithm over the corrected Fourier method and the Moore-Penrose pseudo inverse method. The results demonstrate that the proposed method is an advantageous imaging algorithm for near-field millimeter-wave SAIR.

In this paper, the synthesis of sparse time-modulated linear arrays with minimum number of elements and controlled harmonic radiations is investigated. The proposed iterative approach based on a particle swarm optimization is aimed at finding the array configuration with the minimum number of elements and the optimal pulse sequence that affords a beam pattern with the same features of a reference one also limiting the amount of sideband radiations under a specific threshold. A set of representative numerical examples are discussed to assess the effectiveness and the reliability of the proposed approach.

A broad-band transmission line spatial power combiner (SPC) is proposed in this paper, which uses a square coaxial (Squarax) transmission line (TL). This structure has some advantages over the traditional circular coaxial spatial power combiner, which have been described in this paper. Fin-Line to microstrip transitions are inserted into the Squarax TL, in order to allow an easy integration of Monolithic Microwave Integrated Circuit (MMIC) Solid State Power Amplifier (SSPA). The Squarax SPC geometry allows the feeding of a higher number of MMIC than in a Waveguide SPC, so that this structure ensures high power outputs and small sizes, together with theoretical DC frequency cut-off. In this work, the design and simulation of a passive 4-18 GHz Squarax SPC are reported.

In this paper, an ultra-wideband (UWB) patch antenna integrated with a dielectric resonator is proposed for cognitive radio applications. The patch antenna is fed by a coplanar waveguide (CPW) line, and it consists of a rectangular monopole having an elliptical base, and operates from 2.44 to 12 GHz, this UWB antenna is intended to collect the information. Moreover, the proposed structure integrates a narrow-band rectangular dielectric resonator antenna (RDRA) for operation, with very good isolation between the two ports (transmission coefficient S₂₁ less than -20 dB). The RDRA provides a bandwidth from 5.23 GHz to 6.11 GHz. The electromagnetic analysis is carried out using tow commercial software tools. Furthermore, to validate the proposed concept, experimental measurements are also performed.

A planar inverted F antenna is combined with passive (reactively loaded) elements in order to implement a multi-frequency configuration appropriate for biomedical applications in microwave frequencies. A case study of an electronically Reconfigurable PIFA (R-PIFA) is pursued for the detection of temperature abnormalities in human tissue phantom using microwave radiometry, where the performance of the structure is optimized with respect to input impedance matching in multiple frequencies. The optimization of the array is performed using a Genetic Algorithm (GA) tool as a method of choice. Due to its limited physical size, the proposed R-PIFA can also be used as a portable antenna system for deployment in mobile medical applications.

Taking into account long signal propagation time, curved orbit and ``near-far-near'' slant range histories at apogee, a refined slant range model (RSRM) is presented for geosynchronous earth orbit synthetic aperture radar (GEO SAR) in this paper. Additional linear component and high order components are introduced into straight orbit assumption (SOA) model to describe relative motion during long signal propagation and curved orbit respectively. And the special slant range histories at apogee are considered through adding terms changing with the sign of Doppler rate. Then, based on RSRM under an ideal acquisition and ignoring nonideal factors (such as depolarization and attenuation effects), a refined two-dimensional nonlinear chirp scaling algorithm (RTNCSA) is proposed. Space-variant range cell migration (RCM) caused by range-variant effective velocities is corrected by refined range nonlinear chirp scaling algorithm, and the variable Doppler parameters in azimuth direction are equalized through refined azimuth nonlinear chirp scaling algorithm. Finally, RSRM is verified by 600-second direct signal received by a stationary receiver on a tall building from BeiDou navigation satellite, and RTNCSA is validated through simulated point array targets with resolution of 5 m and scene size of 150 km.

Applications of copper (Cu) nanorod arrays, produced by glancing angle deposition (GLAD) technique, which extends the function of conventional microstrip antennas to encompass passive wireless gas sensors at microwave frequencies are presented. The proposed microstrip antenna consists of Cu nanorod arrays grown on silicon wafers which were coated with thin films of Cu of 50 nm in thickness. To study the effect of the length of Cu nanorods on antenna performance, Cu nanorods of different lengths (400, 700, and 1000 nm) were fabricated. The effects of Cu nanorods morphologies (Cu thin film, closely-spaced Cu nanorods, and well-separated Cu nanorods), were investigated too. Conventional microstrip antennas based on sputtered Cu thin film were prepared for comparison. It was found that as the length of Cu nanorods increases, the antennas exhibit a wider bandwidth and lower frequency resonance than those of the conventional antennas based on Cu thin film. Furthermore, moving from flat surface to well-separated nanorods results in a decrease in the resonant frequency, while there was no observable effect on the bandwidth. These enhancements are attributed to the mutual coupling occurring among Cu nanorods. Based on the antenna characterization, the 1000 nm long Cu nanorods sample was selected for gas detection measurements due to its observed sharp resonance and narrow bandwidth. The detection mechanism is based on the change of in the magnitude of the reflection coefficient as well as the resonant frequency due to the introductions of different gases. The proposed sensor based on Cu nanorods shows a significant response in response to the introduction of different gases such as oxygen, nitrogen, and nitrogen, while the conventional antenna shows no measurable response. It is believed that the proposed sensor is applicable to the other gases based on the suggested sensing mechanism.

A novel approach for designing planar dual-mode bandpass filters using symmetrical T-shaped stub-loaded stepped-impedance resonators (TSLSIRs) with high frequencies selectivity is presented. The proposed symmetrical TSLSIR structure is mainly composed of three elements, i.e. a stepped-impedance resonator (SIR), and two T-shaped stubs which symmetrically loaded at the middle sections of the SIR. Then, two dual-mode bandpass filters based on TSLSIRs with high frequencies selectivity are proposed for experimental verification. Firstly, a dual-mode single passband filter with two transmission zeros located at the both sides of passband is presented, whose passband is centered at 5.23 GHz with the fractional bandwidth of 10.1%, and a wide upper-stopband with harmonic suppression better than 20 dB in range of 5.9 GHz to 12.9 GHz is achieved. Secondly, a dual-mode dual-band bandpass filter with four transmission zeros located at the both sides of the two passbands is presented, whose two passbands are centered at 3.4 GHz and 4.54 GHz with the corresponding fractional bandwidth of 8.4% and 7.5% respectively, and the spurious frequencies from 4.9 GHz to 8.45 GHz are successfully suppressed to the level lower than -20 dB. Both filters have been designed, fabricated and measured. The measured results show good agreements with those of the simulation.

Depending on the aperture extension (AE), a high performance three-dimensional (3D) near-field (NF) source localization algorithm is proposed with the nonuniform linear array (NLA). The proposed algorithm first generates some fictitious sensors to extend the array aperture by constructing a new Toeplitz matrix, and then obtains a two-dimensional (2D) covariance matrix which only contains the elevation angle and range parameters, and another 3D covariance matrix which contains the elevation/azimuth angle and range parameters. Then based on the 2D covariance matrix, both the elevation angle and range parameters are estimated by using the NLA along the Z axis. With the estimates of both the elevation angle and range parameters and combining the 3D covariance matrix, the estimates of the azimuth angle parameters are obtained using the NLA along the Y axis. The proposed algorithm has four main merits: i) unlike some classical NF source localization algorithms, the quarter-wavelength sensor spacing constraint is not required and more sources can be located simultaneously by the proposed algorithm; ii) the 3D parameters of the proposed algorithm are paired automatically; iii) the 3D search required in conventional 3D multiple signal classification (MUSIC) algorithm is replaced with only one-dimensional (1D) search, and thus the computational burden is reduced; iv) the proposed algorithm gains superior parameter estimation accuracy and resolution.

This paper presents an electrical model of aperture and mutually coupled three-elements cylindrical dielectric resonator antenna (CDRA) array designed for 802.11a system applications. In electrical model, each antenna component is represented by its equivalent RLC circuit. The advanced design system (ADS) software is used to build the electrical model and predict the behavior of return loss, while the antenna structure is simulated using CST microwave studio before fabrication. The first and last radiating elements of the proposed array are excited through the aperture slots while the middle element is excited through the mutual coupling of its neighboring elements. The slot length and inter-slot distance effects on bandwidth are comprehensively analyzed and presented. The maximum gain of the proposed array for 5.0 GHz band is about 10.8 dBi, and the achieved simulated (CST, ADS) and measured impedance bandwidths are 1.076 GHz, 1.0 GHz, and 1.2 GHz respectively. The proposed CDRA array antenna exhibits an enhancement of the gain (7.4%) and bandwidth (93.3%) as compared to a literature work with aperture slots. In this study, it is also observed that by using the mutual coupling instead of third slot to excite the middle CDRA, side lobe levels are also reduced significantly over the entire 5.0 GHz band.

A method to increase the bandwidth of a series-fed dipole pair (SDP) antenna by using a parasitic strip pair director is presented. A conventional SDP antenna consists of two dipole elements having different lengths and a ground reflector, which are serially connected with a transmission line. In the proposed antenna, a parasitic strip pair director is appended to the conventional SDP antenna. Initially, a conventional SDP antenna that operates in a frequency range of 1.7-2.7 GHz is designed by optimizing the lengths of the elements (two dipoles and ground reflector) and the distances between these elements. Subsequently, a parasitic director containing a pair of strips is placed near the top dipole element to improve the bandwidth and gain of the conventional SDP antenna. Then, the effects of the location and size of the director on the impedance bandwidth and realized gain are investigated. A prototype of the proposed antenna is fabricated on an FR4 substrate, and its performance is compared with that of the conventional SDP antenna. The experimental results show that the proposed antenna has an enhanced bandwidth of 1.63-2.97 GHz (58.26%) and an increased gain of 5.6-6.8 dBi.

A scalable and highly accurate RF symmetrical inductor model (with model error of less than 5%) has been developed from more than 100 test structures, enabling device performance versus layout size trade-offs and optimization up to 10 GHz. Large conductor width designs are found to yield good performance for inductors with small inductance values. However, as inductance or frequency increases, interactions between metallization resistive and substrate losses render the use of large widths unfavorable as they consume silicon area and degrade device performance. These findings are particularly important when exploiting the cost-effective silicon-based RF technologies for applications with operating frequencies greater than 2.5 GHz.

Circuit and multipolar approaches are presented to investigate the correlation between absorption and scattering processes in 2D problems. This investigation was inspired by earlier works of Prof.R.E. Collin, which pointed out deficiencies of the Th'evenin/Norton circuit models to evaluate the scattered and absorbed powers associated with receiving antennas and, thus, encouraged research on new analytical tools to address these problems. Power balance results are obtained with both circuit and multipolar approaches that are fully consistent. This analysis serves to illustrate how the correlation between absorption and scattering processes results in upper bounds for their power magnitudes, as well as stringent design trade-offs in both far-field and near-field source and scattering technologies.

This paper presents a class of polarization-agile arrays with controlled sidelobes. The architecture is based on the interleaving of two independently polarized sub-arrays through a deterministic strategy derived from Almost Difference Sets (ADSs). The efficiency, flexibility and reliability of the proposed design technique is assessed by means of a set of numerical simulations. Moreover, selected experiments aimed at comparing the performances of the presented approach with state-of-the-art design are provided. Finally, mutual coupling effects are numerically analyzed and discussed.

In this paper, two kinds of dual-band bandpass filter (BPF) based on a single square ring resonator are presented, which are realized by loading capacitance or inductive stubs, respectively. Even-odd-mode analysis is applied to explain their model characteristics. One filter is implemented by loading a pair of capacitance stubs, and both in-band couplings can be tuned in a reasonable range. Another filter is completed by loading a pair of inductive stubs, and the first and second passband center frequencies can be controlled independently. For demonstration, two filters operating at 2.45/5.25 GHz and 2.4/5.25 GHz are designed and fabricated. The simulated and measured results show good agreements.

This paper presents method of moments based analysis of high-gain broad-band waveguide broad-wall longitudinal slot array antenna. Initially a basic two-element slot array antenna has been analyzed, and corresponding scattering parameter data have been obtained. The theoretical data have been compared with experimental data and Ansoft HFSS's simulated data to validate the proposed method. The excellent agreement obtained between the results validates the analysis. After validating the methodology attempts have been made to design a broad-band high-gain slot array antenna. An 18-element slot array antenna has been designed, fabricated and tested. The fabricated antenna provides a high gain over large band width.

Complementary split ring resonators (CSRRs) are applied on a UWB Vivaldi antenna to eliminate some unwanted narrow band services. Based on the sensitivity of band rejection, we successfully separate the whole radiating patch of a Vivaldi antenna into three subareas: The feeding area, where the Vivaldi antenna demonstrates a highly sensitive response to CSRRs with a narrow notching band; The transition-area, where CSRRs transfer a ultra wideband (UWB) Vivaldi into a narrow band antenna; and the rest area, where CSRRs are proved to have little effects on the antenna bandwidth property. A band notch Vivaldi antenna with 4.8 GHz to 5.4 GHz rejection band is demonstrated to verify our study from both simulated and measured results.

In a time domain Marching-on-in-degree (MOD) solver based on a Galerkin implementation of the Method of Moments (MoM), it is observed that the matrix elements for the matrix to be inverted contain integrals that are similar to the ones encountered in a frequency domain MoM solver using the piecewise triangular patch basis functions. It is also observed that the error in the evaluation of the matrix elements involving these integrals are larger in the time domain than those involved in the frequency domain MoM solvers. The objective of this paper is to explain this dichotomy and how to improve upon them when using the triangular patch basis functions for both the time and the frequency domain techniques. When the distance between the two triangular patches involved in the evaluation of the matrix elements, are close to each other or when the degree of the Laguerre polynomial in a MOD method is high, the integral accuracy will be compromised and the number of sampling points to evaluate the integrals need to be increased. Numerical results are presented to illustrate this point.

The frequency dependent characteristics of grounding system buried in half homogenous earth model have been discussed before; however, the importance of mutual inductive and capacitive effects on the grounding problems in both frequency and time domains is unclear. In order to study the importance of the mutual inductive, capacitative and conductive effects on the grounding problem in both frequency and time domains, hybrid method, which is a hybrid of Galerkin's method of moment and conventional nodal analysis method, has been used to study the dominant factor among the mutual inductive, capacitive and conductive effects in the paper.

In through-the-wall radar imaging (TWRI), wall returns are often stronger than target returns, which make the targets behind walls invisible in the radar image. Spatial filtering that relies on the removal of the spatial zero-frequency components is a useful way for wall-clutter mitigation. Unfortunately, it applies to through-the-wall radar (TWR) with synthetic aperture array only. In this paper, a method based on spatial signature is proposed to suppress the wall-clutter in multi-input and multi-output (MIMO) TWRI. Firstly, the traditional spatial filtering method is discussed, as well as the reasons for the inapplicability for MIMO TWR. Secondly, the wall and target spatial signatures based on MIMO array are analyzed, respectively. The results indicate that the former has stability and symmetry, whereas the latter not. Thirdly, according to the above differences, a new method, symmetry subtraction, is applied to describe the wall-clutter suppression procedure. Finally, simulation results demonstrate that the proposed method is efficient in mitigating the wall returns and highlighting the targets.