Houjun Sun

A Compact, Planar, and CPW-Fed Metamaterial-Inspired Dual-Band Antenna

A novel compact, planar, and coplanar waveguide (CPW)-fed dual-band antenna is designed and proposed using composite metamaterial. Such composite metamaterial consists of an inner split-ring resonator (SRR) and an outer closed-ring resonator (CRR). The composite metamaterial can provide dual-band operation at 2.595-2.654 and 3.185-4.245 GHz with reflection coefficient better than -10 dB by the two resonant modes of SRR and CRR, respectively. A CPW-fed line with trapeziform ground plane and tapered impedance transformer line is employed to improve the impedance matching of the antenna. The uniqueness of this design is that the inner SRR with size much smaller than the resonant wavelength is used for obtaining the lower narrow frequency band, which makes the dual-band antenna very compact. Antenna parameters, including reflection coefficient, radiation pattern, radiation efficiency, and gain, are analyzed with numerical simulation and experimental measurement. Good agreement between the simulation and measurement is observed.

Two-Dimensional NUFFT-Based Algorithm for Fast Near-Field Imaging

A fast and accurate algorithm for near-field imaging is presented. First, a two-dimensional (2-D) nonuniform fast Fourier transformation (NUFFT) is directly applied to the near-field scattered data. Second, the circular wavefront curvature is compensated by mapping the NUFFT images to a new rectified grid. The 2-D NUFFT-based algorithm directly exploits the data collected over a polar grid and accordingly avoids the polar to rectangular interpolation. Through numerical simulations, we show that the 2-D NUFFT-based algorithm can provide accurate images with lower sidelobes and is much faster than the near-field tomographic method.

NUFFT-Based Near-Field Imaging Technique for Far-Field Radar Cross Section Calculation

Near-field images can be applied to determine the far-field radar cross section (RCS) of the target overcoming the need for long-range measurement setups. This letter presents a far-field RCS calculation technique based on the nonuniform fast Fourier transformation (NUFFT). The NUFFT is first incorporated in the near-field imaging procedure, and then is used for the subsequent RCS calculation. Comparisons of the NUFFT-based technique and the Cartesian tomography-based technique are demonstrated by means of numerical simulations as well as by measured data of objects in the outdoor range.

A Compact Single-Feed Circularly Polarized Microstrip Antenna with Symmetric and Wide-Beamwidth Radiation Pattern

A compact single-feed circularly polarized microstrip antenna is proposed to achieve symmetric radiation pattern over a wide range of observation angles. In order to reduce the radiation aperture and consequently broaden the circular polarization (CP) and the half power beamwidth (HPBW) of the antenna, a partially etched superstrate and a conducting cavity are employed in the design. Further, reasonable axial ratio (AR) and impedance bandwidths are realized within the compact structure by using a simple series crossed-slot aperture coupled feeding. As a consequence, the overall dimension of the fabricated prototype is 0.32λ0 × 0.32λ0 × 0.12λ0 at the center operating frequency of 1.56 GHz, and a 3.0% overlapped bandwidth of 10 dB return loss (RL) and 3 dB AR is obtained. Within the bandwidth, symmetric CP radiation pattern over almost the entire upper hemisphere is observed and the HPBW is also increased from 60° to 106°.

Experimental Realization of High Transmittance THz 90

We present a high efficiency terahertz (THz) 90°-bend electromagnetic crystal (EMXT) waveguide using Si-based microelectromechanical systems technology. The 2-D EMXT consists of a periodical array of square-shaped gold rods (13 × 13 cells) deposited on a silicon substrate, and a 200-nm gold layer is further sputter-coated on the gold rod array. The dimension of each rod is 48 × 48 × 241 μm3, and the total size of the EMXT is 1756 × 1756 ×787 μm3. The 90°-bend waveguide is fabricated by removing three rows of rods from the EMXT. Through rigorous numerical simulation, it is found that transmittance higher than 90% are found in the whole range from 0.365 to 0.578 THz. The bending performance of the waveguide is further experimentally measured, where transmittance as high as 91.2% is observed at 0.5 THz. The proposed 90°-bend EMXT waveguide can be widely used for applications in THz circuit integrations and THz communications.

^{\circ}

Millimeter-wave (MMW) imaging techniques have been used for the detection of concealed weapons and contraband carried on personnel at airports and other secure locations. The combination of frequency-modulated continuous-wave (FMCW) technology and MMW imaging techniques should lead to compact, light-weight, and low-cost systems which are especially suitable for security and detection application. However, the long signal duration time leads to the failure of the conventional stop-and-go approximation of the pulsed system. Therefore, the motion within the signal duration time needs to be taken into account. Analytical three- dimensional (3-D) backscattered signal model, without using the stop-and-go approximation, is developed in this paper. Then, a wavenumber domain algorithm, with motion compensation, is presented. In addition, conventional wavenumber domain methods use Stolt interpolation to obtain uniform wavenumber samples and compute the fast Fourier transform (FFT). This paper uses the 3- D nonuniform fast Fourier transform (NUFFT) instead of the Stolt interpolation and FFT. The NUFFT-based method is much faster than the Stolt interpolation-based method. Finally, point target simulations are performed to verify the algorithm.

-Bend Waveguide Using EMXT Structure

A novel two-dimensional (2-D) compressive sensing (CS) based method is presented for near-field radar imaging. First, an accurate near-field approximation is proposed, based on which the circular wavefront curvature of spherical waves can be compensated by mapping the images to a rectified new grid. More importantly, the near-field approximation makes the two dimensions of the scattered data separable for the range and cross-range directions, which makes it possible to solve the 2-D reflectivity matrix for the image reconstruction directly. Then, a 2-D proximal subgradient algorithm for near-field radar imaging based on a fast iterative shrinkage/thresholding algorithm (FISTA) is introduced to resolve the memory usage and computation time issues. Simulation and experimental results are provided to demonstrate the performance of the proposed method with comparisons to the traditional Fourier-based method and to the conjugate gradient (CG) based method, which proves that the proposed method is an effective way to solve the near-field radar imaging problem.

MODIFIED WAVENUMBER DOMAIN ALGORITHM FOR THREE-DIMENSIONAL MILLIMETER-WAVE IMAGING

A small-size broadband circularly polarized U-slot patch antenna with dual-feed is proposed. By introducing an additional feeding probe near the vertical slot of the conventional singly fed square U-slot patch antenna printed on a high-permittivity substrate, two series resonances in close proximity are excited. The two resonant frequencies are found to be independent of the orientation of U-slot with respect to the patch, and broadband circular polarization is achieved by introducing a nonquadrature phase difference between two feeding ports. Experimental results show that the overlapped bandwidth of VSWR ≤ 1.5 and AR ≤ 3 dB is over 20% with a small overall size of 0.33λ₀ × 0.33λ₀ × 0.068λ₀, where λ₀ is the free-space wavelength at the center frequency within the operating band.

Near-Field Radar Imaging via Compressive Sensing

A dual-band multifunction hybrid antenna for carborne satellite communication relay system is presented in this paper. As a consequence of the radiation requirements, the proposed antenna consists of a left-hand circularly polarized (LHCP) microstrip patch and an omnidirectional biconical antenna with a conductor tube holding and radome. The LHCP microstrip antenna is used for satellite signal reception, and biconical antenna is used for relay communication transmission. A novel overmoded coaxial waveguide feed structure eliminates the interference between two feed ports. The proposed antenna has the advantages of robustness, low cost, and easy fabrication with conventional materials and printed circuit technology. An antenna prototype is fabricated to validate the design. Both the simulated and measured results are obtained with reasonable agreement.

A Small-Size Dual-Feed Broadband Circularly Polarized U-Slot Patch Antenna

In this paper, a novel W-band waveguide power divider based on H-plane slot bridge is presented. The divider, relying on the H-plane waveguide multi-step couplers, consists of a coupling gap in the narrow common wall between two WR10 waveguides and two arc debosses in H-plane of the two waveguides. The power dividing is realized by adjusting the length of the coupling gap and radius of the arc deboss. The designed divider has a good performance over 80-90GHz. The amplitude balance of two output ports is within ±0.3dB; the reflection coefficient is less than -22dB and the isolation is greater than 22dB. The proposed structure allows the mechanical tolerance within 0.2mm.