ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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The reference signal can be taken to be of the form V
ref
= V
0
cos t. The received signal, V
rec
, is then of the form:
V
rec
= V ' cos ( t + ) = (V ' cos )
cos t - (V ' sin ) sin t
(Eq 5)
Because it is the coefficient of the term that varies in phase with the reference signal, the quantity V ' cos is referred to
as the in-phase component, and the quantity V ' sin is termed the quadrature component. Standard electronic phase-
sensitive detectors are available that can detect each of these components separately. The transmission technique can have
three variations:
• Fixed-frequency continuous wave
• Variable-frequency continuous wave
• Pulse-modulated wave
Fixed-Frequency Continuous-Wave Transmission. In this technique, the frequency of the microwave generator
is constant. It is used either when the band of frequencies required for the desired interaction is very narrow or when the
band of frequencies is so broad that the changes of material properties with frequency are very small and therefore not
especially frequency sensitive. The fixed-frequency continuous-wave transmission technique is the only one of the
transmission techniques in which both components (in-phase and quadrature-phase) can be detected with little mutual
interference. When separation (the ability to separate the components) is important, this technique is generally used.
Swept-Frequency Continuous-Wave Transmission. Some microwave interactions are frequency sensitive in that
their resonant frequency shifts with changes in material properties. For others, the response as a function of frequency
over substantial bandwidth must be used. The swept-frequency continuous-wave transmission technique provides for a
transmission measurement over a selected range of frequencies. The fixed-frequency microwave generator shown in Fig.
6 is replaced with a swept-frequency generator whose frequency is programmed to vary automatically. With currently
available generators, a frequency band of one octave or more can be electronically swept (from 1 to 2 GHz, for example).
Broad band amplifiers of low noise and high gain also make it possible to detect transmitted signals through materials
having very high attenuation. Multioctave generators from 100 kHz to 4 GHz or 10 MHz to 40 GHz are available. Vector
network analyzers provide broadband amplitude and phase.
Pulse-Modulated Transmission. Although phase measurements can be made on the transmitted wave, they are only
relative to the reference wave. There is no simple method for tagging a particular sine wave crest relative to another to
measure transmission time. Therefore, when a measurement of the time of transmission is required, the pulse-modulated
technique is used. To produce the pulse modulation, the microwave generator is gated on and off. The phase-sensitive
detector in the receiver is usually replaced by a peak-value detector. Thus, the receiver output consists of pulses that are
delayed a finite time relative to the transmitted pulse. An oscilloscope with an accurate horizontal-sweep rate can be used
to display these pulses. Swept-frequency measurements give group delay information. The time domain features of vector
network analyzers can also be useful.
Reflection Techniques
The reflection techniques are of two types: single antenna and dual antenna. The single-antenna system, in which incident
and reflected waves are both transmitted down the waveguide between the microwave generator and the antenna, is
shown schematically in Fig. 7(a). The phase detector is set so that it compares the phase of the reflected wave with that of
the incident wave. This gives two output signals that are respectively proportional to the in-phase and quadrature
components in the reflected wave. Such a system works well only for normal or near-normal incidence.
Fig. 7 Diagram of the single-antenna and dual-antenna reflection systems used for microwave inspection
The dual-antenna reflection system (Fig. 7b) operates at any angle of incidence for which there is appreciable reflection.
A comparison of Fig. 7(b) with Fig. 6 shows that the dual-antenna reflection equipment is essentially the same as that
used for transmission measurements. In the transmission technique, the reflected wave is not used. In the reflection
technique, the transmitted wave is ordinarily not used except as a reference.
The boundary conditions must be complied with at the surface of the material. Therefore, the reflected wave, in principle,
has the same information about the bulk microwave properties of the material as the refracted wave. However, the wave
reflected from the first surface does not contain any information about the inhomogeneous properties of the material
within the sample being tested. There are further reflections from any internal discontinuities or boundaries, which
ultimately add to the surface-reflected wave when refracted at the surface. In this manner, properties beneath the surface
are sensed. If the component being inspected is a plate that is backed with a layer of conductive material, the wave
reflected from this metal face traverses the material twice, and it too adds to the surface-reflected wave to provide
information about the interior of the material.
Fixed-Frequency Continuous-Wave Reflection. The simplest microwave reflectometer is based on the fixed-
frequency technique. The microwave signal is incident on the material from an antenna; the reflected signal is picked up
by the same antenna. Both the in-phase and quadrature-phase components of the reflected wave can be determined. In
practice, most such techniques have used only the amplitude of the reflected signal. The dual-antenna reflection technique
(Fig. 7b) can also be used at a fixed frequency. The fixed-frequency continuous-wave technique has two limitations. First,
the depth of a flaw cannot be determined, and second, the frequency response of the material cannot be determined. For
these reasons, a swept-frequency technique may be more useful.
Swept-Frequency Continuous-Wave Reflection. When the interaction between a material and microwaves is
frequency sensitive, a display of reflection as a function of frequency may be valuable. Because phase-sensitive detection
over a wide range of frequencies is difficult, only the amplitude of the reflected signal is usually used as the output in
swept-frequency techniques. However, phase-sensitive detection over a wide range of frequencies has recently been
simplified with the use of vector network analyzers.
Depth can be measured with a swept-frequency technique if the reflected signal is mixed with the incident signal in a
nonlinear element that produces the difference signal. The difference frequency, then, is a measure of how much farther
the reflected signal has traveled than the incident signal. Thus, not only can the presence of an internal reflector be
determined but also its depth. Depth can also be measured on a vector network analyzer utilizing time domain techniques.
Another application of this technique employs a slow sweep of frequency to identify specific layers of several closely
spaced layers of material. The reflection at even multiples of one-fourth wavelength is larger than at odd multiples of one-
fourth wavelength. The reflected signal identifies specific frequencies for which the layer spacing is at even or odd
integral multiples of the quarter-wavelength. This same effect is used, for example, to reduce reflection from lenses by
coating them with layers of dielectric.
Pulse-Modulated Reflection. The depth of a reflection, in principle, can also be determined by pulse modulating the
incident wave. When the reflected time-delayed pulse is compared in time with the incident pulse and when the velocity
of the wave in the material is known, the depth to the site of the reflection can be determined. In both frequency and time
domain modulation, the nature of the reflector is determined by the strength of the reflected signal. The limitation of pulse
modulation is that the pulses required are very narrow if shallow depths are to be determined. For this reason, the use of
frequency modulation has been developed and used.
Standing Wave Techniques
A standing wave is obtained from the constructive interference of two waves of the same frequency traveling in opposite
directions. The result is a standing wave in space. If a small antenna is placed at a fixed point in space, a voltage of
constant amplitude and frequency is detected. Moving the antenna to another location would give a voltage of a different
constant amplitude with the same frequency. The graph of the amplitude of the voltage as a function of position (distance)
along a pure standing wave is shown in Fig. 8. One antenna is needed to produce the incident wave, which can interact
with the reflected wave to produce the standing wave. Another antenna or probe is needed to make measurements along
the standing wave. Thus, the dual-antenna system shown in Fig. 7(b) could be used to both make and measure microwave
standing waves. The receiving antenna must not interfere with the incident wave. A single antenna fed through a
circulator can also be used to separately transmit the incident wave and the reflected wave.
Fig. 8 Relation of electromagnetic wave amplitude (detector response) to the distance along a standing wave
Scattering Techniques
The dual-antenna system diagrammed in Fig. 7(b) must be used for scattering measurements because the angles of the
diffused or reradiated waves can be over a solid angle. To measure all of the scattered radiation, the entire sphere around
the irradiated object or material should be scanned and the detected signal graphed as a function of position.
Microwave Inspection
William L. Rollwitz, Southwest Research Institute
Microwave Holography
When two electromagnetic waves (Fig. 2) occupy the same space, the resulting electromagnetic wave is the vector sum of
the two. If the two waves have the same source, they will have the same frequency and will add or subtract as vectors
according to their relative phase angles. These additions and subtractions will cause an interference pattern, which is a
plane of interference composed of places where they add and places where they subtract. Wave front reconstruction is
discussed in Ref 4, 5, 6, 7, and 8.
When the electromagnetic waves are in the infrared through ultraviolet range of frequencies and when a photographic
film or plate is used at the plane of interference, a hologram is produced on the film. This hologram is a picture of the
interference pattern. When laser light passes through the hologram, the result is a three-dimensional projection of the
object from which the hologram was made. The image projected in this manner is sometimes called the hologram.
For electromagnetic waves of microwave frequencies (300 MHz to 300 GHz), the interference detector, instead of being a
photographic film, is a microwave receiving antenna feeding a receiver in which the peak amplitude of the received signal
is detected. This gives a voltage proportional to the magnitude of the vector sum of the wave reflected from the object and
a reference signal derived directly from the source of the incident microwave irradiating the object. The microwave
holographic system is illustrated in Fig. 9. When the antenna probe scans the plan of interference, the interference pattern
could be drawn out with the voltage from the detector. When the voltage is amplified to feed a lamp or an oscilloscope,
the interference pattern can be visually observed or photographed. The photograph is then a microwave hologram from
which a three-dimensional visual image of the object can be projected using light. Microwave holography is discussed in
more detail in the section "Microwave Holography Practice" in this article. Additional information is available in the
articles "Optical Holography" and "Acoustical Holography" in this Volume.
Fig. 9 Microwave holography with locally produced nonradiated reference wave
References cited in this section
4.
R.P. Dooley, X-Band Holography, Proc. IEEE, Vol 53 (No. 11), Nov 1965, p 1733-1735
5.
W.E. Kock, A Photographic Method for Displaying Sound Wave and Microwave Space Patterns,
Bell Syst.
Tech. J., Vol 30, July 1951, p 564-587
6.
E.N. Leith and J. Upatnieks, Photography by Laser, Sci. Am., Vol 212 (No. 6), June 1965, p 24
7.
G.W. Stroke, An Introduction to Current Optics and Holography, Academic Press, 1966
8.
G.A. Deschamps, Some Remarks on Radiofrequency Holography, Proc. IEEE,
Vol 55 (No. 4), April 1967, p
570
Microwave Inspection
William L. Rollwitz, Southwest Research Institute
Instrumentation
The high frequencies of microwaves do not allow the use of standard electronic circuits (which possess discrete
inductance, capacitance, and resistance). At microwave frequencies, these quantities become distributed along the
propagation path. Therefore, microwave circuit design and description are handled from a wave standpoint, rather than
from the conduction of electrons through wires and components.
Hollow metal tubes (typically, rectangular in cross section) called waveguides are normally used to convey microwaves
between two parts of a circuit. Sometimes the microwaves are conveyed, as a wave, along coaxial or parallel conductors
(Ref 3).
Microwave energy is conveyed through a waveguide in electromagnetic patterns called modes. The patterns consist of
repetitive distributions of the electrical and magnetic fields along the axis of the waveguide. A classic transverse
electromagnetic (TEM) wave (Fig. 2) cannot propagate in a waveguide. Modes are identified as either transverse electric
(TE) or transverse magnetic (TM), referring to whether the electric or the magnetic field is perpendicular to the
waveguide axis, or direction of propagation. High-order modes produce complicated patterns, while lower-order modes
produce relatively simple patterns. Practically all microwave nondestructive inspection (NDI) circuits use the lowest-
order transverse electric mode (TE
1,0
) (Ref 3).
The waveguide pattern for the TE
1,0
mode is shown in Fig. 10. The electric field is represented by the solid lines that are
vertical in the waveguide. The intensity of the electric field varies sinusoidally along and across the waveguide, with a
peak intensity in the middle of the waveguide and zero intensity at either sidewall (end view, Fig. 10). A uniform
(constant) electric field exists in the B (height) direction, and a sinusoidal variation occurs every one-half wavelength
along the length of the waveguide (front view, Fig. 10). A top view of the waveguide shows that the magnetic field is in
the form of loops, spaced at intervals of one-half wavelength.
Fig. 10 Diagrams showing the lowest-order transverse electric mode (TE
1,0
) in a waveguide
Each mode has a cutoff frequency below which it cannot be propagated down a given size of waveguide. For the
commonly used TE
1,0
mode, the cutoff frequency is equal to the free-space velocity divided by twice the A dimension.
This means that the A (width) dimension of the waveguide must be at least one-half the free-space wavelength for the
TE
1,0
mode to propagate. Therefore, large waveguides imply low frequencies, and small ones imply high frequencies.
In the past, microwaves were mainly generated by special vacuum tubes called reflex klystrons. Klystrons generate
microwave radiation by the synchronized acceleration and deceleration of electrons into bunches through a resonant
cavity. The microwave radiation, frequently emitted through a mica window, is virtually monochromatic and phase
coherent. Currently, microwaves are also generated over the range of 300 MHz to 300 GHz by solid-state or
semiconducting devices. The advantage of these sources is that their outputs are single frequencies that are coherent. The
frequency can also be varied over a wide range.
Microwave energy can be detected by devices such as special semiconductor diodes, bolometers (barretters), or
thermistors. Phase-sensitive detectors can also be used. The diode, the most common detector used for nondestructive
inspection, generates an electromotive force proportional to the microwave power level impinging on it. Another type of
detector operates on the principle of converting the microwave power to heat (barretters and thermistors). For many years,
the evapograph has been used to record an infrared image on a thermosensitive layer (Ref 9). The infrared radiation
causes localized heating on the heat-sensitive layer so that the infrared image is recorded. In 1967, this same technique
was reportedly being used to observe and record microwave electromagnetic wave interference patterns (Ref 10).
The experimental setup used is illustrated in Fig. 11. The microwave image is produced on the evaporated-oil membrane.
The membrane is composed of a thin plastic sheet with two coatings. The coating that receives the microwave energy is a
thin, vacuum-deposited layer of a low-conductivity metal such as bismuth or lead. This first layer absorbs a fraction (10 to
50%) of the incident microwave power pattern. This locally produced heat profile is transmitted through the plate sheet to
the second layer on the other side of the plastic membrane. The second layer is a thin film of hexadecane (C
16
H
34
, with
melting point of 20 °C, or 68 °F) or similar oil, which was deposited from a vapor and is held in the equilibrium state
between condensation and reevaporation. The local heat caused by the incident microwave power causes reevaporation of
the oil film. When the oil film is illuminated with light, as shown in Fig. 11, the oily film presents a uniform interference
color. Under the microwave heating, the oil coating presents a microwave interference pattern seen as a colored visual
image on the oil film. This colored visual image is also a microwave hologram. Good holograms have been obtained with
a microwave energy of 80 mW/cm
2
(520 mW/in.
2
) in a few minutes (Ref 9).
Fig. 11 Experimental setup used for microwave thermography
A newer microwave liquid crystal display (MLCD) has made it possible to obtain real-time two-dimensional color display
of microwaves having frequencies up to and above 300 GHz (Ref 10). Figure 12 illustrates the basic operating mechanism
of the MLCD. The display consists of an absorbing layer with a typical resistivity of 5 k /cm
2
and a liquid crystal layer,
both bonded to a flexible Mylar plastic sheet. The liquid crystal and resistive layers are 0.025 mm (0.001 in.) thick, while
the Mylar support is 0.075 mm (0.003 in.) thick. Absorbed power as low as 1 mW/cm
2
(6.5 mW/in.
2
) will form display
patterns. The absorbing layer converts electromagnetic energy into heat, and the liquid crystal layer selectively reflects
light according to its temperature. The above mechanism produces two-dimensional color displays, electromagnetic field
patterns, and interference field patterns with incident microwave radiation as low as 1 mW/cm
2
(6.5 mW/in.
2
).
Fig. 12 Schematic of a microwave liquid crystal display. A resistivity layer provides electromagnetic energy-to-
hea
t conversion, a liquid crystal layer provides pattern definition, and a Mylar section provides support and
protection from liquid crystal contamination.
Metal horns are usually used to radiate or pick up microwave beams. The directionality of the radiation pattern is a
function of its aperture size in terms of number of wavelengths across the aperture. Horns with aperture dimensions of
several wavelengths produce fairly good directionality. Most close-up NDI applications utilize even smaller horns,
regardless of their larger beam spread.
Most of the microwave equipment used in the past for nondestructive inspection operated at frequencies in the vicinity of
10 GHz (X band). The free-space wavelength at 10 GHz is 30 mm (1.18 in.). A few applications have used frequencies as
low as 1.0 GHz and as high as 100 GHz. The waveguides used for nondestructive inspection at 10 GHz are typically 25
mm (1 in.) wide and 13 mm ( in.) high, and horn sizes range from 25 × 25 mm (1 × 1 in.) in aperture to 127 mm (5 in.)
or more. The currently available solid-state sources from 1 to 300 GHz should make microwave NDE readily
accomplished in this range.
One of the developing areas of great interest to those wanting to use microwave frequencies for NDE is that in solid-state
and vacuum-tube sources and transmitters. The information in Fig. 13(a) and 13(b) gives the status of solid-state devices
as of October 1983. No more recent information in summary form was found up to October 1988. The solid-state sources
available in 1983 are listed by categories in Fig. 14. In Fig. 13(a) and 13(b), the numbers are the efficiencies (in percent)
of power output divided by the input dc power. Efficiency values as high as 30% are obtained with an output of 20 W at 5
GHz using a GaAs, impact avalanche transit time (IMPATT) diode. The IMPATT diode has a negative resistance region,
which comes from the effects of phase shift introduced by the solid-state sources, that closely follows the 1/f and the 1/f
2
slopes shown in Fig. 13(a) and 13(b). The transition from the 1/f to the 1/f
2
slope falls between 50 and 60 GHz. The
transition for silicon impact avalanche transit time (Si IMPATT) diodes is between 100 and 120 GHz. As shown in Fig.
13(b), in the 40 to 60 GHz region, the GaAs IMPATTs show higher power and efficiency, while the Si IMPATTs are
produced with higher reliability and yield (Ref 11).
Fig. 13 Plots of continuous wave power versus frequency to obtain efficiency (in %) of various solid-
state
devices. (a) Efficiency data for InP and GaAs Gunn diodes; and GaAs and Si IMPATT diodes. Data obtained f
rom
Hughes, MA/COM, Raytheon, TRW, Varian. (b) Efficiency data for power and low noise FETs. Data obtained from
Avantek, Hughes, MSC, Raytheon, Tl. Source: Ref 11
Fig. 14 Solid-state microwave sources by categories. HEMT, high electron-mobility transistor
The Gunn diode oscillators have better noise performance than the IMPATT's. They are used as local oscillators for
receivers and as primary source where continuous-wave (CW) powers of up to 100 mW are required. Indium phosphide
Gunn devices show higher power and efficiency than gallium arsenide Gunn devices.
The performance in power and low noise for GaAs field-effect transistors (FETs) is shown in Fig. 13(b). Again, the
number by the data points is the efficiency associated with that power. Higher powers can and will be achieved by using
optimized inpackage matching circuitry.
The noise figure of the low-noise GaAs-FET devices ranges from 14 to 7 dB over the frequency range of 3.5 to 60 GHz.
Peak powers 3 to 6 dB higher have been obtained from power GaAs FETs, while 5 to 10 dB increases in power are
possible with IMPATTs. The power combining of IMPATT devices was also demonstrated successfully by 1983. Peak
powers of several hundred watts were obtained in the X band (8 to 12.5 GHz, as shown in Table 2). Broadband CW
combiners in the U band (46 to 56 GHz) have been reported that deliver several watts of power (Ref 11).
The state-of-the-art performance for various microwave vacuum tubes is shown in Fig. 15(a) and 15(b). Again, the
numbers by the data points are the efficiency in percent. The parenthetical letter represents the company that supplied the
data and developed the device. The devices represented in Fig. 15(a) are the traveling wave tube (TWT), the helix TWT,
the backward wave oscillator (BWO) tube, the gyrotron, the orotron, the extended interaction amplifier (EIA), and the
extended interaction oscillator (EIO). The devices in Fig. 15(b) include the crossed-field amplifier (CFA), TWT, klystron,
and gyrotron. Power levels from these devices cover the range of 1 to 3 kW over the frequency range of 5 to 270 GHz.
The devices in Fig. 15(b) can supply almost 5 MW peak power at 1 GHz and 1.5 kW peak power at 300 GHz (Ref 11).
Fig. 15 Plots of power versus frequency to obtain efficiency (in %) of various microwave vacuum tubes.
(a)
Continuous-wave power. (b) Peak power. Source: Ref 12
Impressive power and voltage gain values are also available. Over 50 dB of gain is available in a 93 to 95 GHz TWT at an
average output level of 50 W (Ref 11). The most impressive power achievements have been made with the gyrotrons (Ref
11). Many researchers are examining the cyclotron resonance principle to build amplifiers (Ref 11). Recent developments
in the helix plane circuit promise to provide ultralow-cost, high-average-power TWTs (Ref 11). Increased activity from
1983 through 1987 by tube designers has resulted in TWTs exceeding 1 kW continuous wave and 100 W continuous
wave at 100 GHz (Ref 12). Gyrotrons in 1987 could generate close to 200 kW of continuous wave at 140 GHz (Ref 12).
Although these large powers are not usually used in NDE, developments such as these benefit the whole field of
microwave NDE. The sizes of these vacuum-tube amplifiers prevent their use for most NDE applications. All of the
continuing developments in solid-state sources and transmitters are usable for microwave NDE.
In the early development of microwaves, systems consisted of a large number of devices performing specific single
functions. The system engineer became a master of optimizing interfaces and writing device specifications with adequate
margin to overcome interactions. This led to reduced overall system performance. The need for improvement led to the
supercomponent concept. The microwave supercomponent was thought to be the answer to the problem of the complex
and costly device interfaces. The supercomponent was defined as a stand-alone device that contains multiple functions of
a generic nature (sources, amplifiers, mixers, and so on) and support functions such as switches, isolators, filters,
attenuators, couplers, control circuits, and supply circuits, all of which are tightly packaged and combined to meet a single
subsystem specification. One of the earliest microwave supercomponents was the combination of a TWT with a solid-
state power supply in mid-1960. This combination eliminated the need to adjust this interface after the device left the
factory (Ref 13).
The concept of supercomponents is best utilized when the system designer works actively with the manufacturer of the
supercomponent. The designer should keep in mind that he may be dealing with a supercomponent engineer who is more
skilled in individual devices than in system work. Therefore, simply writing a specification and submitting it will often
not produce the desired results. Good communication is necessary between the system designer and the supercomponent
engineer to ensure that both understand each other's needs so that they can jointly arrive at the best combination of
performance and cost. Most companies that are truly dedicated to supplying supercomponents are continuously training
engineers who have the necessary broad background to work successfully at producing supercomponents.
The supercomponent or subsystem concept has and is maturing rapidly. It has progressed beyond a risky, expensive
strategy to one that offers solid ground for improvement of microwave NDE equipment in terms of size, weight, cost
performance, reliability, and maintainability. For some NDE applications, the use of supercomponents may mean the
difference between successful and mediocre operation. Other information on supercomponents can be found in Ref 14
and 15.
Microwave instrumentation for nondestructive inspection can be set up for reflectometry through transmission and
scattering techniques. Usually, the single-frequency design is used, but swept-frequency systems have been constructed
for certain applications.
Some applications require the use of through transmission or wave-scattering systems involving a separate transmitter and
receiver. As with ultrasonics, through transmission is performed by placing the transmitter on one side of the material and
the receiver on the opposite side (see the article "Ultrasonic Inspection" in this Volume). Scattering setups orient the
receiver horn at some oblique angle to the transmitted beam. The receiver is sometimes placed against a side surface (for
example, the side of a block) so that its direction is oriented 90° with respect to the polarization of the transmitted beam.
If measurable scattering sites exist in the material, they can frequently be detected by scanning the obliquely oriented
transmitter and receiver.
Microwave flaw detectors, based on swept-frequency heterodyning principles, can be used to measure flaw depth. This
type of flaw detector transmits a signal whose frequency sweeps from a maximum to a minimum value at a specified
repetition rate. A discontinuity in the material under test reflects a portion of this energy back to the reflectometer. The
reflected energy interferes with the transmitted energy by a process known as heterodyning. The heterodyning action
produces a beat frequency equal to the instantaneous difference between the frequencies of the transmitted and delayed
received waves. This difference in frequency, caused by timing differences between the waves, is proportional to the
distance to the flaw.
Swept-frequency approaches are also used for microwave thickness gaging and density determinations. The frequency
that yields maximum power reflection is related to either density or thickness.
The pulse-echo system, as used in ultrasonic and many radar designs, cannot be applied to microwave flaw detection or
thickness gaging, because the wave velocities are too great and the distances too short. The necessary electronic resolving
powers, well into the picosecond (10
-12
s) range, are not available for accurately measuring flaw depth. This is one major
reason why frequency-modulated and standing wave designs are used. Another potential problem with pulse-echo
resolving methods is that the frequency of such short-duration pulses would be too high for desirable propagation
behavior in nonmetallic materials.