Hyeongdong Kim

Wavelet analysis of radar echo from finite-size targets

The wavelet analysis technique is applied to analyze the frequency-domain electromagnetic backscattered signal from finite-size targets. Since the frequency-domain radar echo consists of both small-scale natural resonances and large-scale scattering center information, the multiresolution property of the wavelet transform is well suited for analyzing such multiscale signals. Wavelet analysis examples of backscattered data from an open-ended waveguide cavity and a plasma cylinder are presented. Compared with the conventional short-time Fourier transform, the wavelet transform provides a more efficient representation of both the early-time scattering center data and the late-time resonances. The different scattering mechanisms are clearly resolved in the time-frequency representation. >

Electromagnetic scattering from an inhomogeneous object by ray tracing

A shooting and bouncing ray (SBR) formulation is presented for treating the electromagnetic scattering from electrically large, inhomogeneous objects. A dense grid of rays representing the incident plane wave is shot toward the inhomogeneous objects. At the scatterer boundary, reflected rays and refracted rays are generated due to the discontinuity of the medium parameters. The trajectory, amplitude, phase and polarization of the rays inside the inhomogeneous object are traced based on geometrical optics. Whenever the rays cross the scatterer surface, additional reflected/refracted rays are generated and are tracked. This is repeated until the intensities of the refracted/reflected rays become negligible. The contributions of the existing rays to the total scattered field are calculated using the equivalence principle in conjunction with a ray-tube integration scheme. The ray formulation is applied to calculate the backscattering from cylinders and spheres and good agreement with the exact series solutions is observed in the high-frequency range. In addition, the backscattering mechanisms in penetrable objects are interpreted in terms of simple ray pictures. >

Wavelet analysis of backscattering data from an open-ended waveguide cavity

The wavelet analysis technique is applied to analyze the electromagnetic backscattering data from an open-ended waveguide cavity. Compared to the conventional short-time Fourier transform, the wavelet transform results in a better representation of the scattering features in the time-frequency plane, due to its multiresolution property.<<ETX>>

Effect of an arcjet plume on satellite reflector performance

The effect of an arcjet plume on the performance of satellite reflector antennas is studied. The arcjet plume is modeled as a weakly ionized plasma. The spatial permittivity distribution of the plume is approximated using the measured electron density profile and a cold plasma model. Geometrical optics is applied to determine the ray paths as well as the transmitted fields through the inhomogeneous plume. The ray optics results are compared against several exact solutions for scattering from inhomogeneous dielectrics, and good agreement is observed for sufficiently large scatterer size. The far-field antenna patterns of the reflector in the presence of the plume are calculated from the transmitted ray fields using a ray-tube integration scheme. For arcjet prototypes in the 1-kW class, the plume effect on the antenna performance is small. As the electron density increases, the main beam and sidelobe level gradually degrade. The main beam also tends to squint away from the plume region. >

FDTD Dispersive Modeling of Human Tissues Based on Quadratic Complex Rational Function

We propose a dispersive finite-difference time domain (FDTD) suitable for the electromagnetic analysis of human tissues. The dispersion relation of biological tissues is characterized by a quadratic complex rational function (QCRF) that leads to an accurate FDTD algorithm in 400 MHz-3 GHz. QCRF coefficients are extracted by applying the complex-curve fitting technique, without initial guess. Numerical examples are used to illustrate the computational accuracy and stability of QCRF-based FDTD.

Decision of error tolerance in array element by the Monte Carlo method

In this paper, the tolerance of errors in each array el- ement that is required to satisfy the tolerance of beam pattern is decided by the Monte Carlo (MC) method. To examine the pass or fail of element errors to the desired beam pattern, instead of ex- ploring the whole -segmented element error as is usually done in deterministic methods, MC tests only for randomly selected er- rors and greatly reduce the computational costs with the desired numerical accuracy. The different effect of each element error on the resultant beam pattern is considered by calculating the vari- ance of error for each element, the tolerance of an element is de- cided when 95% of samples meet the acceptable beam pattern. To verify the proposed technique, the tolerance of each element in two-dimensional planar array is determined.

Mobile Antenna Using Multi-Resonance Feed Structure for Wideband Operation

A multi-resonance feed structure for exciting a mobile handset antenna in order to achieve multiband operation is presented. The feed structure adjusts its impedance and controls the coupling with the antenna structure. Based on the proposed feed structure, the dimensions allowed for the antenna design are only 10 × 55 × 2 mm 3 while the measured impedance bandwidth is from 694 to 1003 MHz in the lower band and from 1656 to 2772 MHz in the higher band under a voltage standing wave ratio of 3:1 with good measured realized efficiency. The proposed antenna is a competitive candidate that fully covers most 4th generation (4G) and existing wireless communication services.

Investigation on the characteristics of the envelope FDTD based on the alternating direction implicit scheme

This letter presents numerical characteristics of recently developed the envelope FDTD based on the alternating direction implicit scheme (envelope ADI-FDTD). Through numerical simulations, it is shown that the envelope ADI-FDTD is unconditionally stable and we can get better dispersion accuracy than the traditional ADI-FDTD by analyzing the envelope of the signal. This fact gives the opportunity to extend the temporal step size to the Nyquist limit in certain cases. Numerical results show that the envelope ADI-FDTD can be used as an efficient electromagnetic analysis tool especially in the single frequency or band limited systems.

A study of the numerical dispersion relation for the 2-D ADI-FDTD method

This letter presents a numerical dispersion relation for the two-dimensional (2-D) finite-difference time-domain method based on the alternating-direction implicit time-marching scheme (2-D ADI-FDTD). The proposed analytical relation for 2-D ADI-FDTD is compared with those relations in the previous works. Through numerical tests, the dispersion equation of this work was shown as correct one for 2-D ADI-FDTD.

Loop-Type Ground Radiation Antenna for Dual-Band WLAN Applications

A compact ground radiation method for 2.45 and 5.5 GHz wireless local area network (WLAN) applications is proposed. This method is based on two different loop-type current modes that are excited in the ground plane and separately operated at 2.45 and 5.5 GHz, producing a very small-sized antenna with good radiation efficiencies and wide impedance bandwidths. The proposed radiation method requires a small ground clearance of 8.5 mm × 4.5 mm for installing the antenna element. The experimental result shows that a good impedance matching performance is simultaneously achieved at the dual frequency bands and the -10 dB bandwidths are obtained as 120 MHz and 720 MHz at the 2.45 and 5.5 GHz frequency bands, respectively, and good radiation performance is achieved.