Tsenchieh Chiu

Electromagnetic scattering interaction between a dielectric cylinder and a slightly rough surface

An electromagnetic scattering solution for the interaction between a dielectric cylinder and a slightly rough surface is presented in this paper. Taking the advantage of a newly developed technique that utilizes the reciprocity theorem, the difficulty in formulating the secondary scattered fields from the composite target reduces to the evaluation of integrals involving the scattered fields from the cylinder and polarization currents of the rough surface induced by a plane wave. Basically, only the current distribution of isolated scatterers are needed to evaluate the interaction in the far-field region. The scattered field from the cylinder is evaluated in the near-field region using a stationary phase approximation along the cylinder axis. Also, the expressions for the polarization current induced within the top rough layer of the rough surface derived from the iterative solution of an integral equation are employed in this paper. A sensitivity analysis is performed for determining the dependency of the scattering interaction on the target parameters such as surface root mean square (RMS) height, dielectric constant, cylinder diameter, and length. It is shown that for nearly vertical cylinders, which is of interest for modeling of vegetation, the cross-polarized backscatter is mainly dominated by the scattering interaction between the cylinder and the rough surface. The accuracy of the theoretical formulation is verified by conducting polarimetric backscatter measurements from a lossy dielectric cylinder above a slightly rough surface. Excellent agreement between the theoretical prediction and experimental results is obtained.

Compact Dual-Band Slot Antenna at the Corner of the Ground Plane

A compact design of a dual-band (2.4-2.485 GHz and 5.15-5.825 GHz) slot antenna is proposed for WLAN applications. To achieve compact and practical design, the proposed antenna is composed of narrow slots, and placed in the 10 times 10 mm2 corner region on a typical FR4 printed circuit board for WLAN personal-digital-assistance (PDA) phones. The detailed design considerations are described in this communication. Also, simulation and measurement regarding the reflection coefficient and radiation pattern are conducted. Good agreement between both results is obtained.

Electromagnetic scattering from short branching vegetation

A polarimetric coherent electromagnetic scattering model for short branching vegetation is developed. With the realistic structures that reasonably describe the relative positions of the particles, this model is able to consider the coherent effect due to the phase difference between the scattered fields from different particles, and account for the second-order, near-field interaction between particles, to which the relative positions and orientation of the particles are essential. The model validation with measurements is also presented, and excellent agreement is obtained. The polarimetric radar backscatter measurements for soybean plants using truck-mounted scatterometers were conducted at L-band and C-band under different soil-moisture conditions. Through an extensive ground truth, the important plant and rough surface parameters such as the soil moisture and surface roughness, vegetation dielectric constant, and geometry of the soybean plants, were characterized for model verification. It is found that the second-order near-field scattering is significant at C-band for fully grown soybeans due to the high vegetation particle density, and at L-band, the contribution from the second-order near field is negligible. The coherence effect is shown to be important at L-band and to a much lesser extent at C-band. This model is then used to demonstrate its ability for estimating the physical parameters of a soybean field, including soil moisture from a polarimetric set of AIRSAR images.

Electromagnetic scattering from slightly rough surfaces with inhomogeneous dielectric profiles

Remote sensing of soil moisture using microwave sensors require accurate and realistic scattering models for rough soil surfaces. In the past, much effort has been devoted to the development of scattering models for either perfectly conducting or homogeneous rough surfaces. In practice, however, the permittivity of most soil surfaces is nonuniform, particularly in depth, for which analytical solution does not exist. The variations in the permittivity of a soil medium can easily be related to its soil moisture profile and soil type using the existing empirical models. In this paper, analytical expressions for the bistatic scattering coefficients of soil surfaces with slightly rough interface and stratified permittivity profile are derived. The scattering formulation is based on a new approach where the perturbation expansion of the volumetric polarization current instead of the tangential fields is used to obtain the scattered field. Basically, the top rough layer is replaced with an equivalent polarization current and, using the volumetric integral equation in conjunction with the dyadic Greens function of the remaining stratified half-space medium, the scattering problem is formulated. Closed-form analytical expressions for the induced polarization currents to any desired order are derived, which are then used to evaluate the bistatic scattered fields up to and including the third order. The analytical solutions for the scattered fields are used to derive the complete second-order expressions for the backscattering coefficients as well as the statistics of phase difference between the scattering matrix elements. The theoretical results are shown to agree well with the backscatter measurements of rough surfaces with known dielectric profiles and roughness statistics.

A broadband transition between microstrip and coplanar stripline

A design for a broadband microstrip-to-coplanar-stripline transition using broadside-coupled strips is proposed in this letter. Two transitions are designed in back-to-back configuration and the insertion loss is optimized in X-band (8.2 GHz/spl sim/12.4 GHz). Both simulation and measurement are in good agreement. The measured bandwidth for 1 dB insertion loss and 20 dB return loss is from 6.9 GHz to 12.4 GHz.

LTCC Wideband Filter Design With Selectivity Enhancement

A new design scheme for the wideband filters with the low temperature co-fired ceramic (LTCC) technology is proposed in this letter. Based on the traditional second-order combline filter, an extra shunt capacitor is added in the middle of the coupled line, and a new transmission zero is thus obtained. The proposed design not only possesses excellent selectivity at both sides of the passband, the fractional bandwidth can also be as wide as 86%. To validate the design scheme, the proposed technique is applied to the third-order combline filter and realized by using the LTCC technology. The wide-band filter with passband of 3.1-5.1 GHz is obtained. With four transmission-zeros, the selectivity and out-band rejection of the third-order filter is further improved from those of the second-order designs.

Dual-Band Reconfigurable Antenna Design Using Slot-Line With Branch Edge

A novel design of dual-band reconfigurable slot antenna is proposed in the paper. The configuration of the proposed antenna is based on a folded slot with a branch edge formed by multiple strips loaded with circuit components. In the design, the antenna is decomposed into networks, and the corresponding equivalent circuits are deduced. By applying resonant condition on the equivalent circuit at the desired center frequencies, the circuit components can be determined. The proposed design is later transformed into a reconfigurable antenna by using varactors. The antenna configuration is also modified for applying biasing voltages to varactors. Dual-band reconfigurable operation of antenna is studied and demonstrated for S-band applications at 2.45 GHz and GPS at 1.227/1.381/1.575 GHz. The design approach is described in details. Also, antenna measurements are conducted for design validation.

NCURO Data-Retrieval Algorithm in FORMOSAT-3 GPS Radio-Occultation Mission

Radio-occultation (RO) technique, which has been used in planetary science, is a method to obtain the profiles of atmosphere and the global atmospheric data. In 2006, Taiwan launched six low-Earth-orbit satellites as a RO constellation mission, known as FORMOSAT-3. In order to thoroughly understand the process of the RO data retrieval and obtain as much as possible the information for the weather prediction, a National Central University Radio Occultation (NCURO) retrieval algorithm has been developed. The focus of the algorithm development is on the correction of the excess phase of the signal received with open-loop (OL) technique and the criteria for assessment of the data quality. When the OL is activated, the excess phase of the Global Positioning System (GPS) signal is modulated with navigation messages of satellites. In our algorithms, two methods are incorporated to recover the excess phase. Moreover, as the altitude of the received signal decreases, the quality of the GPS signal generally deteriorates, and eventually, the signal is too noisy to be processed. In order to assess the quality of the signal, instead of the signal-to-noise ratio, the degree of unclearness is defined and used in the algorithm. In this paper, the algorithm including the phase-correction methods and the criteria for the quality assessment will be described. The data retrieval using the algorithm will be compared with those obtained from the COSMIC Data Analysis and Archive Center at the University Corporation for Atmosphere Research and Pingtung radiosonde measurement. Some intermediate results of the NCURO algorithm will also be demonstrated.

Dielectric constant measurement technique for a dielectric strip using a rectangular waveguide

A dielectric constant measurement technique for dielectric strips is presented in this paper. In the measurement, the strip is placed parallel to the broad walls of a rectangular waveguide, and it is found that the measured reflection coefficient is insensitive to the position of the strip when it is placed around the middle of the waveguide cross section. The new sample placement scheme becomes very convenient, especially when a large number of strips have to be measured. To develop the forward scattering formulation, the reflection coefficient of the strip placed in the waveguide is evaluated using the method of moments. With this forward model, a genetic algorithm is developed to retrieve the dielectric constant of the strip from the measured reflection coefficient. The validity of the calculated reflection coefficient is verified by measuring a Teflon strip in a WR187 waveguide, and the dielectric constant of the Teflon is successfully retrieved from the measurement. For the measurement of many strips, a special sample holder is made to ensure insensitivity of the measured reflection coefficient to the position of the strip.

Design of Dual-Band Millimeter-Wave Antenna-in-Package Using Flip-Chip Assembly

A dual-band antenna-in-package for millimeter-wave (mmW) applications is presented in the paper. The proposed antenna, which consists of a radiating slot and an air-filled cavity, is fed by a microstrip loaded with two tuning open-circuited stubs through a coupling C-shape aperture to achieve dual-band characteristics. The air-filled cavity, which is formed by the space between CMOS chip and integrated passive device substrate after flip-chip assembly process, can reduce loss and improve antenna gain. Simulation and measurement regarding antenna reflection coefficient, radiation pattern, and peak gain are conducted for design validation. The measured results show that the antenna can operate in V-band and E-band, and the impedance bandwidths with the reflection coefficient less than -10 dB are 6.1% and 5.8%, respectively. The measured gains are -2 dBi at 58 GHz and 0.3 dBi at 77 GHz, respectively. The proposed antenna is well suited for dual-band mmW high-data-rate wireless communication systems.