NEAR-FIELD MICROWAVE AND EMBEDDED MODULATED
SCATTERING TECHNIQUE (MST) FOR DIELECTRIC
CHARACTERIZATION OF MATERIALS
D. Hughes1 and R. Zoughi1
Applied Microwave Nondestructive Testing Laboratory (amntl), Electrical and Computer
Engineering Department, University of Missouri - Rolla, Rolla, MO 65409
ABSTRACT. Several approaches to using combined near-field microwave NDT techniques, utilizing openended rectangular waveguides, and embedded modulated scatterer technique (MST) for the determination of
the dielectric properties of a material have been investigated in the past. A technique currently under
investigation involves using the ratio of dynamic forward- and reverse-biased reflection coefficients,
measured at the aperture of the waveguide. One important aspect of this method is that the ratio of reflection
coefficients is not a function of the relative location or polarization of MST probe to the waveguide,
Formulation of the ratio of dynamic reflection coefficients is presented, as well as preliminary measurements
showing the coherent ratio of reflection coefficients to be relatively constant as a function of location in freespace.
INTRODUCTION
The use of microwave nondestructive testing (NDT) for determination of dielectric
properties of materials has many applications, particularly when evaluating composite and
cement-based structures. In these areas, microwave NDT has been shown to be effective for
the following:
•
•
•
cure monitoring of cement and resin [ 1,2],
detection and evaluation of chloride ingress in cement-based structures [3-5],
determination of water-to-cement (w/c), sand-to-cement (s/c) and course aggrigate-tocement (ca/c) ratios of cement-based materials [6-8],
• detection of delamination, disbonds, and impact damage in complex composite
structures, including the spatial extent and severity of the damage [9-12],
• etc.
For these applications, the measured dielectric properties of a material (at microwave
frequencies) are correlated to the presence of a defect, the state of cure, or the presence of a
foreign material. Mixing models and models for multilayered structures have been
developed in the past to model cement-based materials and complex sandwich composites
[13-15].
Modulated scatterer technique (MST) involves the use of a small probe whose scattering
properties change as a function of time between a high and low state. Previous applications
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
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of this technique include antenna pattern measurements, nondestructive testing and
biomedical imaging [16]. The combination of tradition microwave NDT measurements and
embedding an MST probe in a material has been shown in the past as an novel means of
determining the local dielectric properties of a material, and monitoring changes in the
dielectric constant of a material [17]. Furthermore, multiple measurements can be recorded
rapidly using this technique, and coherent averaging of the modulated signals can be used to
increase the signal-to-noise ratio of the measurements. This can be used to increase the
sensitivity to small changes in dielectric properties of a material.
The dielectric constant of a material can be determined in a variety of ways using this
technique. Previous investigations have shown the capability to detect with high sensitivity
slight variations in the dielectric properties of a material, and to determine the dielectric
constant of a material through a variety of approaches [17-19]. The previous results of these
have shown mixed level of success for determining the dielectric properties of a material.
This paper investigates the potential for making sensitive dielectric property measurements
based on the ratio of measured high and low reflection coefficients. Using the ratio of
measured dynamic reflection coefficients in this way, many parameters necessary for the
above approaches may be rendered unnecessary.
APPROACH
Figure 1 shows the schematic of the approach used for this technique. A dipole antenna,
centrally loaded with a PIN diode, is embedded in a generally lossy material, with a
complex dielectric constant. The PIN diode is modulated between a forward and reverse
biased state using a low frequency square wave. This changes the impedance of the PIN
diode between a high and low state, which effectively alters the impedance of the MST
probe as a whole between two states. The probe is illuminated by an open-ended
rectangular waveguide placed at the surface of the material over the probe. The incident
electric field is then reflected off of the probe, and the reflected signal alters between a
forward and reverse biased state with the modulated waveform. Thus, the reflection
coefficient measured at the aperture of the waveguide consists of a static reflection due to
mismatches at the surface of the material, and a dynamic, modulated reflection coefficient
due to the MST probe.
FIGURE 1. Schematic of embedded MST probe, illuminated by an open ended rectangular waveguide at the
surface of the material.
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The dynamic reflected electric field for either the high state or low state of the MST
probe can be expressed qualitatively as:
fay.z)
(1)
for the high state of the MST probe, and
Erhw = C0(radiatorJ,£r)-e^^" •r,ow(Zlow,Zliipole(l,f ,£,))• E,(X,y,z)
(2)
for the low state of the MST probe. In both equations, Co represents the parameters of the
radiator (i.e., the waveguide), e"2yd is the transfer function of the material, and E; is the
incident electric field on the MST probe. rhigh and FIOW are the overall reflection coefficient
of the MST probe in either the higji or low state. This reflection coefficient is a function of
the impedance of the diode, and the impedance of the dipole. Because the dipole impedance
changes as a function of the surrounding material, while the diode impedance does not, the
dielectric properties of the material should be able to be determined from this reflection
coefficient.
Taking the ratio of equations (1) and (2), the common terms cancel, leaving the ratio of
the reflection coefficient of the MST probe, i.e.,
Eyhigh _ ^high (Zftigh 9 % dipole {'' J '£r ) )
Eriaw
,~\
r/ow (Zlow , Zdipole (/,/,£,.))
This equation is now only a function of the high and low state of the diode, and the
impedance of the dipole which, for a given length of the dipole, is correlated only to the
dielectric constant of the surrounding media. One important aspect of equation (3) is that
the ratio of measured dynamic reflection coefficients will be constant for a given dielectric
constant. Thus, the dielectric properties of a material can be determined using an embedded
MST approach regardless of the location of the probe, or the relative polarization between
the probe and the waveguide.
RESULTS
In order to verify this concept, dynamic reflection coefficient measurements were made
in free-space, with the MST probe location varying as a function of distance from the
aperture of the waveguide. The MST probe consisted of a half-wave dipole at 10 GHz
centrally loaded with a commercially available glass-package PIN diode. Calibrated
measurements were made using X-Band waveguide, and static reflection coefficients were
coherently removed.
Figure 2 shows the magnitude and phase of dynamic reflection coefficient as a function
of distant for the free-space measurements. Figure 3 shows the coherent ratio of these two
values (i.e., the ratio of magnitude and difference in phase). As can be seen in Figure 3, the
ratio of the magnitude of reflection coefficient is relatively constant. However, some
variation can be seen. This average ratio is 1.22, with a standard deviation of 0.08. The
difference in phase of reflection coefficient is also relatively constant, with a similar
variation to that of the ratio of the magnitude. The average difference in phase is 37.7 , \rith
a standard deviation of 3 .
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Distance (mm)
Distance (mm)
a
b
FIGURE 2. Forward and reverse biased magnitude (a) and phase (b) of dynamic reflection coefficient as a
function of distance between the probe and the aperture.
a
b
FIGURE 3. a) Ratio of magnitude and b) difference in phase of forward and reverse bised reflection
coefficients as a function of distance between the probe and the aperture.
The coherent ratio of the forward and reverse biased reflection coefficients produces a
relatively constant value, however, there does exist some small variation as a function of
distance. The variation in these two is though to be partially due to multiple reflections
between the MST probe and the waveguide. This effect decreases as distance between the
waveguide and the probe increases, and is reduced in a lossy material. Also, this effect will
be seen if the value of static reflection coherently removed from measured values is not
exact.
CONCLUSION
The use of combined embedded modulated scatterer technique and near-field microwave
nondestructive testing techniques requires an accurate means of determining the dielectric
properties of the material in which the MST probe is embedded. Using the coherent ratio of
dynamic reflection coefficients, it should be possible to determine the input impedance, and
hence the dielectric constant of the material. For a given material and probe, this ratio
should be independent of the relative probe location and polarization. Free-space
measurements agree well with this criterion, however, some effect due to multiple
reflections is still seen. This effect decreases as the distance increases, or in a lossy
material. More accurate modeling of the interaction between the waveguide and the MST
probe can also account for this. Also, inaccurate static reflection coefficient values can
cause this effect.
The results for free-space shown here agree well with the qualitative aspects of the
approach given for determining the dielectric properties of the material. This approach still
needs to be tested for an MST probe embedded in complex dielectric materials. It is
expected that the same observations may be made for a homogeneous dielectric material.
Based on the results of this, further development of an inverse routine for determining the
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dielectric constant of a material based upon dynamic reflection coefficient measurements
will be performed.
ACKNOWLEDGEMENT
Partial funding for this was has been provided by The Graduate Assistantships in Areas
of National Need (GAANN) Fellowship.
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