High-frequency surface field excited by a magnetic line source on an impedance cylinder

The evaluation of the transverse electric (TE) surface field on an impedance boundary circular cylinder, excited by a magnetic current on the same surface is discussed. The source is of infinite axial extent. The integral which describes the field due to a single surface ray is evaluated numerically and asymptotically. The results compare well with each other as well as with the eigenfunction solution. The surface ray field has a uniform representation in the sense that it remains valid in the immediate vicinity of the infinitesimal source as well as in the deep shadow region. A comparison is made with a flat impedance plane to indicate the influence of curvature on the surface field.     

Evaluation of Hankel functions with complex argument and complexorder

The use of F.W.J. Olver's (1974) uniform asymptotic expansions to compute Hankel functions (of interest in EM scattering) of complex argument z and complex order v is examined. Emphasis is placed on how to choose the proper branches in evaluation of the complex functions in the asymptotic representations. Comparison is made with the nonuniform formulas of Debye and Watson. The Debye formulas are valid when z and v are far apart, and the Watson formulas are valid when z and v are close together. The fact that the Olver formulas are uniform is important from a numerical viewpoint, because a satisfactory criterion for deciding when to switch between the Debye and Watson (1958) formulas is not available. Validation by comparison with two nonasymptotic methods verifies that the Olver formulas are considerably more accurate than the Debye or Watson formulas     

Scattering from periodic array of grounded parallel strips, TEincidence

The field distribution between strips in a periodic array of parallel strips was modeled. The electric field integral equation (EFIE) together with method of moments (MoM) was employed to solve the problem. The incident plane wave was assumed to be transverse electric (TE). The effect of the incident angle on the field between strips was investigated. For comparison, an array of the same configuration but containing a finite number of parallel strips was also modeled using the EFIE and solved via MoM. Good agreement was found even when the number of strips was small. The results are of interest for mobile radio     

The near field of a wire grid model

The extreme near-field behavior of the wire grid model of a conducting surface is examined. Using a wire grid model of an infinite transverse magnetic circular cylinder, it is verified that the best accuracy for the E-field is obtained when the wire satisfies the `same surface area' rule of thumb. Two excitations are considered: a uniform surface current and plane wave incidence. In the first case, although the boundary value match between the wires is poor the extreme near field is still quite accurate. In the second case, the near field is also accurate, however, the largest errors occur not between the wires, but in the interior of the scatterer. In both cases, the boundary match between the wires as a check on the accuracy of the solution is misleading     

On the radiation from a large, open-ended waveguide

The expressions for the electric field radiated from a large, open-ended rectangular-waveguide aperture have been derived for the propagating H and E modes, since the available expressions are erroneous.     

Uniform asymptotic formulas for the creeping wave field on or off acylinder

This work develops a uniform asymptotic formula for the creeping wave field in the shadow region of a conducting circular cylinder, excited by a magnetic line source or line dipole on the surface. Unlike previous developments in terms of Fock integrals, the uniform formula does not require switching between on and off surface formulas. Furthermore, the uniform formula is more accurate in the transition region near the surface, where neither the on nor off surface formulas work very well. Validation is by comparison with an exact numerical solution for the field due to a single surface ray. An empirical switchover criterion for the nonuniform formulas is suggested, and limitations of both the nonuniform and uniform formulas are discussed     

The relation between creeping waves, leaky waves, and surface waves

In this paper, we examine the creeping wave propagation wavenumbers, modal impedance, and field behavior on a dielectric coated circular cylinder. The physical interpretation is assisted by comparing the cylinder's pole waves with the leaky waves and surface waves that occur on a flat, grounded dielectric slab. The propagation wavenumbers and modal impedance are computed in the complex wavenumber plane. The cylinder propagation wavenumbers come from a transcendental equation involving Hankel functions, which are entire functions of complex order, whereas for the slab, branch-point singularities are present. This difference is examined, so that one can better understand how a coated cylinder behaves like a flat slab, when the cylinder radius is large. It is found that for the cylinder, the Stokes line for the asymptotic expansion of the Hankel function plays a role that is similar to the planar slab branch cut.     

Antenna-Pattern Measurement Using Spectrum Analyzer for Systems with Frequency Translation [Measurements Corner]

Pattern measurements using a network analyzer are typically based on measuring the transmission coefficient, S₂₁. When the transmitting and receiving frequencies are different, it is not possible to directly measure the antenna pattern using a network analyzer. In this paper, an antenna-pattern measurement system using a spectrum analyzer, designed to measure the radiation pattern of an antenna with a microwave sampling beamformer (MSBF) structure, is presented. A synthesized oscillator was used as the transmitter in the measurement setup. The instruments were controlled through GPIB by a program specifically designed for the system. High-quality pattern measurements were obtained for several antenna types. The measurement results were verified using HFSS simulations.     

A technique for approximating the location of surface- and leaky-wave poles for a lossy dielectric slab

An approximation technique for locating the surface- and leaky-wave poles for a lossy dielectric slab is presented. The problem is reduced to the simultaneous solution of two transcendental equations (for each of the perfect magnetic conductor (PMC) and perfect electric conductor (PEC) cases) which is shown to yield a simple approximate solution for the poles, and which can subsequently be refined using numerical optimization. The technique yields both surface-wave and leaky-wave poles, and results are presented for a typical example to demonstrate the approach. The greatest approximation accuracy was observed for surface-wave and leaky-wave poles well removed from the spectral gap. For poles either within or in close proximity to the spectral gap, an alternative iterative technique is proposed. Expressions for the number of proper plus improper surface-wave poles in a given problem are also derived.     

On the accuracy of the UTD for the scattering by a cylinder

The UTD formulation for the scattering by a cylinder is valid for antennas that are removed from the cylinder surface. The usual guideline is that reliable results can be obtained for antennas that are about λ/4 or more away from the surface. By exploring a few cases, we show that λ/4 is unnecessarily large for the lit region and sometimes too small for the shadow region. In addition, we find that with a simple heuristic modification to the UTD, heights as small as λ/20 can be accommodated, with an accuracy that is sufficient for most engineering applications