Microwave Testing

Microwave testing (MW) and millimeter wave NDT are most often applied to electrically insulating, dielectric materials, such as rubber, many composites, and ceramics. However, they may also be applied to conductive metals. These techniques are useful for (1) measuring and controlling the distance from or geometry of a specimen, involving microwave imaging or holography; (2) evaluation or spectroscopic analysis of moisture content or chemistry; and (3) detection and sizing of discontinuities in metals.
Microwave measurement setups may be contact or non contact  and are divided into three categories: reflection, transmission and scattering.
Reflection measurements generally use one antenna for transmitting and receiving signals. Transmission measurements place an antenna on each side of the specimen, and scattering measurements are obtained using severa transmitters or several receivers positioned at key locations.

The interrogating microwave energy is emitted from the transducer, and signals of interest may be caused by localized changes in the dielectric constant of the sample, including delaminations, surface-breaking discontinuities, moisture content and impurities, or by polarization of the signal due to 300MHz and 300GHz, so wavelengths range from 1m down to 1mm. Microwaves in the frequency range below about 40GHz are generally referred to as millimeter waves because that is the length scale of their wavelengths in free space. Microwaves reflect almost completely- that is, they do to not penetrate-when they encounter conductive materials. However, these interrogating waves can penetrate dielectric test objects. The depth of penetration is governed by the material`s ability to absorb microwave energy.