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Evanescent-field microwave microscopy
for sub-surface diagnostics of artefacts
R. Olmi1, C. Riminesi2, S. Beni3, F. Micheletti1, S. Priori1
1
2
IFAC-CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino (Firenze), ITALY
ICVBC-CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino (Firenze), ITALY
3
ELab Scientific srl, Viale Donato Giannotti 61, 50126 Firenze, ITALY
Abstract – A system for microwave microscopy in the
frequency band 1 – 20 GHz is described. The system
is based on an evanescent-field resonant sensor,
allowing to obtain maps of dielectric contrast.
I.
INTRODUCTION
Dielectric spectroscopy measures the dielectric
contrast in composite materials, allowing to noninvasively detect the presence of dishomogeneity or
material discontinuities in the subsurface.
Dielectric sensors are being developed for detecting
and quantifying the presence of moisture in plaster,
wood, concrete etc. Actually, the information
contained in the frequency spectrum is redundant
for the purpose of this kind of diagnostics, where
primarily the shape and the position of the “defect”
are of interest. The acquisition of the full spectrum
is instead necessary for determining the physicalchemical nature of a dielectric, e.g. the presence of
polymeric chains or other dipolar material. A
specialization of dielectric spectroscopy, named
“dielectrometry”, consists in the measurement of
the dielectric properties in a narrow frequency
range.
Evanescent-field dielectrometry has been
successfully used in the last years for the
diagnostics of fresco and mural paintings [1-4], in
particular for quantifying the moisture content
inside a wall and for detecting the presence of
soluble salts, up to a depth of a couple of cm. In
that case, the measurement system consists of an
evanescent-field resonant sensor operating between
1 and 1.5 GHz. The depth of the investigated
region mainly depends on the size of the probe
head, as the measurement is conducted in nonradiative conditions (indeed, that is the meaning of
“evanescent” field). The geometrical size of the
head also affects the cross-section dimensions of
the measuring “spot”, that is comparable that size.
By reducing the probe size, the technique allows to
obtain information about smaller regions (thus
increasing the spatial resolution) to the expenses of
the penetration depth, that alike decreases.
II.
RESULTS
A dielectrometric system, i.e. a system
measuring the dielectric contrast, working in the
microwave region (0.3–300 GHz), employing a
small (non radiating) probe and equipped with a
motion system for automatically scanning the
surface of a sample, is conventionally called
“microwave microscope”, although it usually does
not investigates a material at microscopic lengths.
The microwave microscope described in this
communication works in the frequency range 1–20
GHz, uses probe heads of diameter between 1.5 and
5 mm, and allows to measure an area up to 20 x 20
cm. The resonant probe consists of a microstrip
resonator terminated by an open-ended coaxial line.
Fig. 1. The 3D scanner system of the microwave microscope,
using a VNA Agilent N5230A (10 MHz - 40 GHz).
The measuring system allows to produce 2D
maps of resonance frequency and/or quality factor
in various frequency sub-regions. Results on
wooden and plaster laboratory samples including
material dishomogeneities are presented.
REFERENCES
[1]. R.Olmi, M.Bini, A.Ignesti, S.Priori, C.Riminesi and
A.Felici, "Diagnostics and monitoring of frescoes using
evanescent-field dielectrometry", Measurement Science and
Technology, Vol. 17, No. 8 , pp. 2281-88, 2006.
[2]. R. Olmi and C. Riminesi, "Study of water mass transfer
dynamics in frescoes by dielectric spectroscopy", Il Nuovo
Cimento, Vol. 31 C, N.3, pp. 389-402, 2008.
[3]. V. Di Tullio, N. Proietti, M. Gobbino, D. Capitani, R. Olmi,
S. Priori, C. Riminesi, E. Giani, "Non-destructive mapping of
dampness and salts in degraded wall paintings in hypogeous
buildings: the case of St. Clement at mass fresco in St. Clement
Basilica, Rome", Anal. Bioanal. Chem., 396, pp. 1885-96, 2010.
[4]. R.Olmi, S.Priori, D.Capitani, N.Proietti, L.Capineri,
P.Falorni, R.Negrotti and C.Riminesi, “Innovative Techniques
for Sub-surface Investigations”, Materials Evaluation, vol.69,
No.1, pp. 89-96, 2011.

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