Numerical and experimental results of a microwave noncontact, nondestructive detection and evaluation of disbonds and thickness variations in stratified composite media are presented. The aperture admittance characteristics of a flange mounted rectangular waveguide radiating into a layered, generally lossy dielectric media backed or unbacked by a conducting sheet is modeled. The theoretical implementation is based on a Fourier transform boundary matching technique to construct the field components in each medium, coupled with a stationary form of the terminating aperture admittance of the waveguide. The model can serve as a reliable test bed for real-time examination of layered composite media. Experimental results for several cases are presented which show good agreement with the theoretical findings. This is a versatile technique for near-field in situ interrogation of stratified composite media which provides for high resolution measurements. >

/pdf/microwave-noncontact-examination-of-disbond-and-thickness-4fdqjh5ovw.pdf

Microwave noncontact examination of disbond and thickness variation in stratified composite media

Solutions for fields inside a slab of a generally lossy dielectric medium backed by a conducting plate, placed outside a waveguide-fed rectangular aperture, are used for the microwave nondestructive thickness measurement of such dielectric slabs. Upon construction of the waveguide terminating admittance expression from its variational form, an inverse problem is solved to extract the slab thickness form the conductance and susceptance in a recursive manner. A comparison between the experimental and theoretical results showed that the significance of higher order modes is minimal; hence, the dominant mode assumption is, in general, valid for describing the aperture field distribution. The validity of this assumption has led to the construction of a simple integral solution which is fast converging for generally lossy dielectric slabs, and may easily be implemented for real-time applications. Good agreement was obtained between the theoretical and experimental results. Multiple thicknesses of two different dielectric samples were estimated in this way. >

/pdf/open-ended-rectangular-waveguide-for-nondestructive-14pzys0ae0.pdf

Open-ended rectangular waveguide for nondestructive thickness measurement and variation detection of lossy dielectric slabs backed by a conducting plate

General coordinate transformations which are commonly encountered in many antenna applications are presented. Neither the feed coordinates nor the far-field pattern coordinates in general coincide with the antenna coordinates. Transformations discussed allow one to relate the spherical and Cartesian components of one system to the spherical and Cartesian compoents of the other system. In particular, attempts are made to use unified notations to assist the reader in a straightforward application of the transformations.

Useful coordinate transformations for antenna applications

Comparing results of reentry plasma diagnostic experiment at 25,000 ft/sec with flow-field values at four locations on spacecraft

/pdf/flight-results-of-a-25000-foot-per-second-reentry-experiment-1dq4j4pw9k.pdf

Flight results of a 25000-foot-per-second reentry experiment using microwave reflectometers to measure plasma electron density and standoff distance

The measurement of reflection from an open-ended waveguide is a simple and nondestructive technique for determining the dielectric properties of materials. A flange-mounted waveguide is considered, the flange being pressed on an unknown material which may be of finite or infinite thickness. The relationship linking the reflection coefficient to the dielectric properties is obtained from a theoretical analysis of the electromagnetic field in the vicinity of the aperture. The theory includes the effects of both cross polarization and higher order modes. An integral equation is obtained, the kernel of which is the dyadic Green function in each medium. The method of characteristic modes is used for the numerical computation. The theoretical results are in good agreement with experimental measurements. Futhermore, a simple and handy technique for data inversion is provided.

The Reflection from an Open-Ended Rectangular Waveguide Terminated by a Layered Dielectric Medium

The variational analysis of Compton is modified to account for the surface-wave pole contribution to the admittance of a waveguide-fed slot on a ground plane covered with a low-loss dielectric slab. The results of an extensive experimental study to determine the effects of ground-plane and dielectric slab size upon waveguide admittance are compared with calculations using the dominant waveguide mode field as a trial solution. It is found that a waveguide terminated in a flat flange is of sufficient size to approximate the infinite ground plane assumed in the theory. A disagreement of 20 percent was noted for some thin sheet cases. Based upon a thorough theoretical and experimental investigation, it was concluded that tolerances do not explain the error; however, a measurement of the aperture field for cases where maximum error occurs reveals significant distortions. Hence, it is concluded that induced higher-order modes are the probable cause of disagreement.

The admittance of a rectangular waveguide radiating into a dielectric slab

The admittance of a waveguide-fed rectangular aperture under an inhomogeneous plasma is derived and experimentally verified. For both flowing and stationary inhomogeneoas plasmas, the agreement is at least as accurate as presently available plasma diagnostic techniques will allow. It is shown that there is a clear separation of the dependence of aperture-admittance properties upon plasma properties, such as electron density, profile shape and collision frequency, over two to three decades of peak electron density, change. The collision frequency primarily affects the aperture conductance, particularly for critical and overdense plasmas. The profile shape, i.e., a boundary layer, primarily affects the aperture susceptance.

The Input Admittance of a Rectangular Waveguide-Fed Aperture Under an Inhomogeneous Plasma: Theory and Experiment

The development and application of a numerical technique for the rapid calculation of the far-field radiation patterns of a reflector antenna from either a measured or computed feed pattern are reported. The reflector is defined by the intersection of a cone with any surface of revolution or an offset sector of any surface of revolution. The feed is assumed to be linearly polarized and can have an arbitrary location. Both the copolarized and the cross polarized reflector radiation patterns are computed. Calculations using the technique compare closely with measured radiation patterns of a waveguide-fed offset parabolic reflector. The unique features of this technique are the freedom from restrictive feed assumptions and the numerical methods used in preparing the aperture plane electric field data for integration.

Analysis of the radiation patterns of reflector antennas

The results of experiments with dielectric obstacles of various geometries placed directly upon the apertures of circular and rectangular waveguides are presented. It is found that dielectric spheres, and in some instances dielectric cubes, two to four wavelengths in dimension produce directive patterns with low sidelobe levels. For some cross sections, the measured gain of these antennas is greater (6 dB in some cases) than that produced by optimum horns having apertures of identical cross section. The linearity of the polarization of these antennas is essentially the same as that of an open-end waveguide antenna.

Waveguide excited dielectric spheres as feeds

The results of an experimental and theoretical study of the effects of placing a dielectric sphere over the aperture of a circular waveguide are presented. A dielectric sphere excited by a Huygens' source is used us the theoretical model to interpret the experimental results. Calculations based on this model show good overall agreement with experimental patterns and directivity measurements. It is concluded that dielectric loaded antennas such as the dielectric sphere excited by a waveguide can produce directivities in excess of that obtained by a uniformly illuminated aperture of the same cross section, particularly for dielectric objects with dimensions of two wavelengths or less. Measurements of the coupling between these antennas show that they, in general, are not as closely coupled as plain apertures without dielectric object loading.

W. Croswell

Papers

The admittance of a rectangular waveguide radiating into a dielectric slab

The Input Admittance of a Rectangular Waveguide-Fed Aperture Under an Inhomogeneous Plasma: Theory and Experiment

Analysis of the radiation patterns of reflector antennas

Waveguide excited dielectric spheres as feeds

Radiation from a homogeneous sphere mounted on a waveguide aperture