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. >

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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. >

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

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

Microwave and millimetre-wave non-destructive testing and evaluation (NDTE Zoughi 2000 Micr...

https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.2019.0585

Review of advances in microwave and millimetre-wave NDT&E: principles and applications.

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.

W. Taylor

Papers

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