Electromagnetic scattering from a flat plate with rim loading andRAM saving

The problem of electromagnetic scattering from a plate with a rim loading for transverse electric (TE) and transverse magnetic (TM) polarizations is investigated. The analysis is based on the work of R. Tiberio et al. (ibid., vol.AP-33, no.8, p.867-73, 1985) on the uniform geometrical theory of diffraction analysis of a wedge with two face impedances. The two-dimensional field is converted into three-dimensional field using Siegel's 2-D to 3-D conversion formula. The corner effects are neglected. The width of the coating around the edges which gives the same result as a uniformly coated plate of the same size is estimated. Hence, the percentage of saving of radar-absorbing material (RAM) can also be estimated for practical application in radar cross-section (RCS) reduction problems     

Comments on "Wide-band diffraction improved dual-shaped reflectors"

It is pointed out that the procedure mentioned in the above paper need not be followed for dual-reflector systems using small subreflectors. In addition, a uniform procedure suitable for both large and small subreflector systems, is presented.     

Comments on "Comparison of three high-frequency diffraction techniques"

It is shown that the method of edge waves (MEW) [1] (or "physical theory of diffraction") describes the secondary edge waves correctly. Opposing statement found in [2]-[4] results from some misunderstanding. It is noted that both MEW and the geometrical theory of diffraction may predict, at best, only the first two terms in an asymptotic expansion (with k → ∞) of the sum of the edge waves for real three-dimensional bodies. Some formulas dealing with MEW in [1] are defined more precisely.     

Comments on "AC impedance of an isolated flat conductor"

For original paper see Z. Zhihua and M. Weiming, IEEE Trans. Electromagn. Compat., vol.44, pp.482-486, (2002). Employing the moments method, and a triangular patch modeling technique, this paper gives an analysis of the AC impedance of an isolated flat conductor, taking into consideration capacitive effects. It is observed that, for the flat conductor under consideration, the capacitance has a significant effect on the impedance beyond a frequency of about 20 MHz. Thus, it can be concluded that the capacitive effects should also be considered in evaluating the AC impedance of an isolated conducting strip.     

Remarks on "Comments on the limitations of the Thevenin and Norton equivalent circuits for a receiving antenna"

Collin (see IEEE Antennas and Propagation Magazine, no.45, p.119-124, 2003) remarks that Love (see IEEE Antennas and Propagation Magazine, vol.45, no.4, p.98-99, 2003) has misinterpreted his. In the paper under discussion, he did not discuss the efficiency of a receiving antenna or give any comment on the maximum efficiency a parabolic receiving antenna could have. Love's statement that it appears that Collin believes most receiving antennas cannot have an efficiency greater than 50% is speculation on his part.     

Effects of multiple scattering and Fresnel diffraction on random volume scattering

A wave incident on a volume of random fluctuations of medium parameters will be scattered in all directions. Two kinds of effects on random volume scattering are considered: 1) multiple scattering effect, and 2) Fresnel diffraction effect. The cumulative forward scattering is known to be responsible for the scintillation phenomenon. When strong, such cumulative or multiple scattering can drive the signal statistics into the saturated regime in which the complex amplitudes behave as a complex Gaussian process. These cumulative scattering effects on signals of a bistatic radar are investigated. Additionally, to allow for the possibility that the size of the irregularity is comparable or larger than the first Fresnel zone, Fresnel diffraction instead of Fraunhofer diffraction is considered. A generalized formula for calculating average scattered power, taking both multiple scattering and Fresnel diffraction into account, is derived. It shows that the Fresnel diffraction pattern appearing around the forward direction tends to be smeared out by multiple scattering. Also, both multiple scattering and Fresnel diffraction effects weaken and broaden the forward-scattered beam, and reduce the backscattered power when compared with the single scattering and Fraunhofer diffraction cases respectively. Under certain conditions, the enhancement effect of the backward scattered power is confirmed.     

High-frequency scattering from the edges of impedance discontinuities on a flat plane

A high-frequency solution is presented for the scattering of a plane wave at the edges of surface impedance discontinuities on a fiat ground plane. Arbitrary uniform isotropic boundary conditions and a direction of incidence perpendicular to the edges of the discontinuities are considered for both the transverse electric (TE) and transverse magnetic (TM) cases. An asymptotic approximation of the exact solution given by Maliuzhinets and a spectral extension of the geometrical theory of diffraction (GTD) are used. Uniform expressions for the scattered field received at a point on the surface are given, including surface wave contributions. Numerical results are shown and in some examples they are compared with those obtained from a moment method (MM) solution.     

Radiation and scattering from structures involving finite-sizedielectric regions

A full-wave approach is presented for calculating the scattered fields produced by structures that involve finite-size dielectric regions. The dielectric is first approximated by an array of interlocking thin-wall sections; the electric field boundary conditions are then applied through the use of appropriate surface impedances. Rooftop basis functions, chosen to represent the surface current, are appropriately placed on the thin-wall sections in such a way as to accurately represent the polarization current while preventing fictitious charge within the dielectric. Rooftop currents are also used to represent the current on any conductor that may be present. The matrix elements are calculated, depending upon the distance between the source and field locations, through a scheme that employs Taylor series expansions and point source approximations. The technique is applied to scattering from dielectric cubes and composite dielectric-conductor structures, and to radiation from microstrip structures. Numerical convergence and agreement with the literature are demonstrated     

Comments on "A uniform geometrical theory of diffraction for an edge in a perfectly conducting surface"

The edge diffraction coefficients of Kouyoumjian and Pathak, when applied to edges in curved screens produce discontinuities in the diffracted fields which do not coincide with those of the specular field on the concave side of the screen. In regions where coincidence occurs the discontinuities add, rather than cancel. The field calculated from the diffraction coefficient violates the principle of reciprocity.     

Comments on “Radiation and scattering from thin wires inchiral media”

Comments that the above paper (see Jaggard, Liu, Grot and Pelet, IEEE Trans. Antennas Propagat., vol.40, p.1273-1282, Nov. 1992) lacks the generality of the paper published by Monzon (see Antennas Propagat., vol.38, p.227-235, Feb. 1990). The latter paper contains an analysis for thin wires, including an explicit integral equation and method for numerical solution (summing impedance matrices), which renders the problem of trivial numerical implementation. Further, the analysis applies to full biisotropic regions, and is therefore more general than chiral since chiral is just a special case of biisotropic. It appears that Jaggard et. al. use the integral equation of Monzon and follow the numerical scheme therein literally     

Application of geometric diffraction theory to scattering from cones and disks

The ability of the geometrical theory of diffraction to predict the radar cross section (RCS) of a perfectly conducting, right circular cone as a function of viewing angle is evaluated by comparison of computed and measured values of RCS. Both vertical and horizontal polarization have been considered for cones ranging from 0.98 to 2.87 wavelengths in diameter at the base and having half angles of 4°, 15°, and 90°; the latter case corresponds to a disk. It is shown that for cones having normalized base circumference (ka) of 8 or 9 the predicted and measured RCS agree very well except when the cone is observed within about 30° of nose-on with vertical polarization, in which case large errors occur for some as yet unknown reason. For smaller cones having diameters about equal to the wavelength (ka around 3), the computed RCS is generally predicted within 5 dB, but the form of the RCS pattern is not predicted very accurately. Backscattering from the base of the cone is very nearly the same as backscattering from a disk of the same diameter for viewing angles within 60° of the normal to the base.     

Comments on "Realization of an all-pass transfer function using the second generation current conveyor"

It is pointed out that an error exists in the circuit given in the above letter [1]. In correcting the circuit, it becomes identical to the general configuration previously described in the literature [1]. The realization of a first-order all-pass transfer function should not be compared with that of a second-order all-pass based on the number of circuit components.