Eugene F. Knott

Comparison of three high-frequency diffraction techniques

Three high-frequency methods of calculating the scattering from metallic edged bodies are compared. The first two are the physical and geometrical theories of diffraction, which have been well established since the late 1950s, and the third is the method of equivalent currents. It is shown that the three share remarkably similar features, although each has its particular virtues and limitations in practical applications.

Equivalent currents for a ring discontinuity

Based on the geometrical theory of diffraction, equivalent filamentary currents are derived for a linear element of a surface singularity for general incidence and polarization. The currents are used to obtain an analytical expression for the bistatic field diffracted by a ring singularity, valid within the caustic region as well as at wide angles.

Plates and edges

The backscattering from a rectangular plate at edge-on incidence is due essentially to the individual contributions from the front and trailing edges. The contribution of the front edge is analogous to that of a wire illuminated by a plane wave, but the rear edge excitation is almost wholly determined by current waves along the side edges of the plate.

Suppression of edge scattering with impedance strings

The mass and bulk of conventional absorbing materials make it difficult to use them to reduce the radar scattering from objects at low radar frequencies such as in the VHF and UHF bands. A more attractive alternative is the impedance line or string, which is a two-dimensional (2-D) version of the one-dimensional (1-D) Salisbury screen. Using a method-of-moments (MoM) computational scheme, we explore the performance of the string as an echo-reduction device for a few elementary shapes. The study shows that the optimum spacing between the string and the body is always less than that demanded of a 1-D impedance sheet and that bandwidths of 80% or better can be achieved.

How far is far

Recent comparisons of computed and measured backscattering cross sections of cylindrical bodies revealed unexpectedly large discrepancies which were apparently due to the finite range used in the measurements. This led to a reexamination of the farfield criterion for a variety of two-dimensional scatterers and one three-dimensional object. It was found that large errors can be incurred in measurements at the standard far-field distance, especially of bodies characterized by strong edge scattering. Errors as great as 6 dB can occur and it may be necessary to exceed the usual farfield distance by a factor of 5 or more in order to reduce the error to 1 dB.

Cross polarization diagnostics

The cross polarized radar cross section of simple axially symmetric objects is shown to be related to transverse body dimensions and the severity of any edges.

A truncated cone model support for anechoic chambers

Theoretical and experimental data are given for a truncated cone which can be used as a model support in anechoic chamber work. It is concluded that such a support when fabricated from a low-density plastic foam is superior to that of a cylindrical column.

A surface field measurement facility

The direct measurement of surface fields using probes is a valuable diagnostic tool in scattering studies. The history of these measurements is reviewed, and a new facility, constructed specifically for routine surface field probing, is described.

Comments on "Phase properties of backscattered fields from thin rods"

In his analysis of the phase characteristics of fields backscattered by thin rods (or wires) in the above communication,’ Mack explores the influence of wire length and diameter, and investigates the variation of phase with aspect angle. His results show that in the end-on region the phase steadily decreases with increasing length which, with eiwl t.ime dependence, agrees with the accepted concept that the scattering wises primarily from the far end (see, e.g., Peters [I]). At broadside incidence, by cont,rast, the phase oscillates with damped excursions about a steady value determined by the precise wire diameter as length increases. Mack gives only the formula for, and a short description of, the broadside behavior and we would like to supplement his discussion gpith the experimental data shown in Fig. 1. The measurements were carried out at a frequency of 9.97 GHz by means of a cancellation scheme wherein a reference signal of controllable phase and amplitude was sampled from the hansmitted signal and used to null the received signal. A wire 0.062 in (L59 mm) in diameter was the subject of the measurements and was init.ially 7 wavelengths long. After each measurement an end of the wire was clipped off and the new length became the basis of a subsequent measurement. Since frequency and diameter were held fixed, the length-to-diameter ratio varies from 132 to 7.7, b0t.h of which are much smaller than the values assumed by Mack in the analysis. The phase data were not calibrated with respect to a standard scatterer and, as such, must be regarded as relative only. The dashed line in the figure is t,he phase prediction given by equation (5) of Mack, and for the purposes of comparison we adjust.ed all measured values by an additive constant to produce a good visual fit. Although the formula should be used for a lengthtodiameter ratio exceeding 900; the agreement between measured and predict.ed values is very good, the disparity being at worst 0.08 rad (5”) near Z/X = 0.75. In this respect it appears that the thickness of the wire is less influential on the phase of the backscattering than on t,he amplitude.

Simulation of reentry vehicle motion during laboratory measurements of radar cross section

Describes a simple mechanical way of moving a model target during laboratory measurements of radar cross section to simulate the precession motion of a satellite or reentry vehicle. The value of the simulation lies in the speed with which it can be accomplished and in its use as a diagnostic tool to aid the analysis of typical signature data