DIELECTRIC PROPERTIES MEASUREMENT TECHNIQUES AND APPLICATIONS
S. O. Nelson
ABSTRACT. The reasons for interest in the dielectric properties of materials are discussed. Terms are defined and principles involving dielectric properties of materials are summarized. Measurement techniques for determining dielectric properties of agricultural materials in the radio-frequency (RF) and microwave ranges are identified with reference citations given for detailed information. Finally, a few new potential applications for dielectric properties of materials are discussed and a few precautions are given for reliable determination of such properties by RF and microwave measurements. Keywords. Dielectric properties, Permittivity, Measurements, Agricultural materials.
ielectric properties of agricultural materials and ovens for the home, the concepts of dielectric heating have products are finding increasing application as become much more widely appreciated. new technology is adapted for use in agriculture The use of dielectric properties for measuring moisture Dand related industries. The interest in dielectric content of products such as cereal grains has been properties of materials has historically been associated with recognized for at least 90 years and has been in common the design of electrical equipment, where various use for more than 50 years (Nelson, 1977, 1991). However, dielectrics are used for insulating conductors and other the first dielectric properties for grain were not reported components of electrical equipment. During much of the until 45 years ago (Nelson et al., 1953: Knipper, 1959). past century, materials research has provided many new Since then much data and information on the dielectric dielectric materials for application in electronics. As the properties of grain and other agricultural products have use of higher and higher frequencies came into practice, become available (Nelson, 1965, 1973a; ASAE Standards, new materials, suitable for use in the radio-frequency, 1998; Tinga and Nelson, 1973; Kent, 1987; Datta et al., microwave, and millimeter wave regions of the 1995), and the influence of important variables on these electromagnetic spectrum, have been developed. The dielectric properties has been evaluated (Nelson, 1981, dielectric properties of these materials are important in the 1991; Mudgett, 1995). design of electrical and electronics equipment, and suitable With the need for development of new sensing devices techniques for measuring these properties for various for the automation and control of various agricultural applications have been developed as they were needed. processes, there is a need for better understanding of the The interest in the dielectric properties of agricultural usefulness of dielectric properties of materials and methods materials and products has been principally for describing for measuring these properties. Therefore, it is the purpose the behavior of materials when subjected to high-frequency of this article to briefly summarize some of the or microwave electric fields for dielectric heating measurement techniques with helpful comments and applications and in their use for rapid methods of moisture provide information related to some of their applications. content determination. The influence of the dielectric Rather than explaining details of the various measurement properties on the heating of materials by absorption of techniques, readers are referred to appropriate sources for energy through radio-frequency dielectric heating, whether that information. at high frequencies or microwave frequencies, has been well known for a long time, and many potential applications have been investigated (Brown et al., 1947; DEFINITIONS AND PRINCIPLES Thuery, 1992; Roussy and Pearce, 1995; Metaxas and The fundamental electrical ac characteristics of Meredith, 1983). With the advent of commercial materials have been defined in detail previously in terms of microwave heating and the wide acceptance of microwave electromagnetic field concepts (Nelson, 1973a) and in terms of parallel-equivalent circuit concepts (Nelson, 1965). For practical use, the dielectric properties of usual ε′ Article was submitted for publication in July 1998; reviewed and interest are the dielectric constant and the dielectric loss approved for publication by the Information & Electrical Technologies factor ε′′, the real and imaginary parts, respectively, of the δ Division of ASAE in February 1999. Presented as ASAE Paper No. relative complex permittivity, ε = ε′ – jε′′ = εe–j , where 98-3067. δ is the loss angle of the dielectric. Hereafter in this article, The author is Stuart O. Nelson, ASAE Fellow Engineer, Research “permittivity” is understood to represent the relative Agricultural Engineer, U.S. Department of Agriculture, Agricultural Research Service, Richard B. Russell Agricultural Research Center, PO complex permittivity, i.e., the permittivity relative to free Box 5677, Athens, GA 30604-5677; voice: (706) 546-3101; fax: (706) space, or the absolute permittivity divided by the ε × –12 546-3607; e-mail: [email protected]. permittivity of free space, 0 = 8.854 10 F/m. Often,
Transactions of the ASAE
VOL. 42(2): 523-529 1999 American Society of Agricultural Engineers 523 the loss tangent, tan δ = ε′′/ε′, or dissipation factor, is also equation 1. Thus, the dielectric properties of the material used as a descriptive dielectric_____ parameter,_____ and sometimes are important. The frequency of the wave is also a factor, the power factor (tan δ/√ 1 + tan2δ) is used. The ac and the power absorption also depends on the square of the σ ωε ε′′ conductivity of the dielectric in S/m is = 0 , where electric field intensity. For a plane wave, the electric field ω = 2πf is the angular frequency, with frequency f in Hz. In intensity E, which has an ejωt dependence, can be given as: this article, ε′′ is interpreted to include the energy losses in jωt–γz the dielectric due to all operating dielectric relaxation E(z) = E0e (5) mechanisms and ionic conduction. The dielectric properties of materials dictate, to a large (von Hippel, 1954) where E0 is the rms electric field extent, the behavior of the materials when subjected to RF intensity at a point of reference, t is time, γ is the or microwave fields for purposes of heating or drying the propagation constant for the medium in which the wave is materials. The power dissipated per unit volume in the traveling, and z is the distance in the direction of travel. dielectric in W/m3 can be expressed as: The propagation constant is a complex quantity,
P = E2σ = 55.63fE2ε′′ × 10–12 (1) π γ =α + jβ = j 2 ε (6) λ0 where E represents the rms electric field intensity in V/m. The time rate of temperature increase, dT/dt in °C/s, in the dielectric material caused by the conversion of energy from where α is the attenuation constant, β is the phase constant, λ the electric field to heat in the material is: and 0 is the free-space wavelength. The attenuation constant α and phase constant β are related to the dielectric dT/dt = P/(cρ) (2) properties of the medium as follows (von Hippel, 1954): where c is the specific heat of the material in kJ/(kg·°C), π ε′ 2 ρ 3 α =2 1 + tan δ – 1 (7) and is its density in kg/m . In drying applications, the λ latent heat of vaporization for water removed from the 0 2 material must also be taken into account, because energy must be supplied for that purpose. π ε′ 2 β =2 1 + tan δ + 1 (8) From equation 1, it is obvious that the power dissipation λ is directly related to the dielectric loss factor ε′′; however, it 0 2 may also be dependent on the dielectric constant ε′ in that, depending on the geometry and the electric field As the wave travels through a material that has a configuration, the electric field intensity may be a function significant dielectric loss, its energy will be attenuated. For of ε′. Two simple examples illustrate this influence of a plane wave traversing a dielectric material, the electric relative ε′ values. In the RF dielectric heating of a layer of field intensity at the site of interest can be obtained by ε material of permittivity 1 of uniform thickness d1 between combining equations 5 and 6 as follows: ε parallel-plate electrodes, with an air gap of permittivity 2 –αz j(ωt–βz) and thickness d2 between the material and one electrode, E(z) = E0e e (9) the rms electric field intensity in the material is: where the first exponential term controls the magnitude of the electric field intensity at the point of interest, and it E1 = V (3) ε 1 should be noted that the magnitude of this term decreases d1 +d2 ε 2 as the wave propagates into the material. Since the power dissipated is proportional to E2,P ∝ e–2αz. The penetration depth, Dp, is defined as the distance at which the power where V is the rms potential difference between the drops to e–1 = 1/2.7183 of its value at the surface of the α electrodes. In a second example, a spherical inclusion of material. Thus, Dp = 1/2 . If attenuation is high in the ε permittivity 2 embedded in an infinite medium of material, the dielectric heating will taper off quickly as the ε permittivity 1 will have an electric field intensity in the wave penetrates the material. Attenuation is often spherical material of: expressed in decibels/m (dB/m). In terms of power densities and electric field intensity values, this can be
3ε 1 expressed as (von Hippel, 1954): E2 =E1 (4) 2ε 1 + ε 2 P0 E0 10 log 10 = 20 log 10 = 8.686αz (10) P z E z (Stratton, 1941), where E1 is the electric field intensity in the infinite medium. Thus, the value of the dielectric constants can have an important influence on the electric where P0 is the incident power, and P(z) is the power at field intensity in the material to be heated, which according distance z into the material. to equation 1 has an important influence on the power The dielectric properties of the materials are very dissipation in that material. important in evaluating the penetration of energy that can The absorption of microwave energy propagating be achieved. The attenuation in dB/m, combining through a material depends upon the variables of
524 TRANSACTIONS OF THE ASAE equations 7 and 10, can be expressed in terms of the and high-frequency ranges were reviewed by Field (1954), dielectric properties, when (ε′′)2 << (ε′)2, as follows: including the use of various bridges and resonant circuits. Dielectric properties of grain samples were determined πε′′ from measurements with a precision bridge for audio α ≅ 8.686 dB/m (11) λ ε′ frequencies from 250 Hz to 20 kHz with samples confined 0 in a coaxial sample holder (Corcoran et al., 1970). At very low frequencies, attention must also be paid to electrode A plane wave incident upon a material surface will have polarization phenomena which can invalidate measurement some of the power reflected, and the rest, Pt, will be data, and the frequency below which this affects transmitted into the material. The relationship is given by measurements depends on the nature and conductivity of the following expression: the materials being measured (Foster and Schwan, 1989; Kuang and Nelson, 1998). Γ2 Pt = P0(1 – ) (12) A large amount of dielectric properties data were obtained in the 1- to 50-MHz range on grain and seed where Γ is the reflection coefficient. For an air-material samples with a Q-Meter based on a series resonant circuit interface, the reflection coefficient can be expressed in (Nelson et al., 1953; Nelson, 1965, 1973a, 1979a). terms of the complex relative permittivity of the material as Techniques were developed for higher frequency ranges (Stratton, 1941). with coaxial sample holders modeled as transmission-line sections with lumped parameters and measured with an 1 – ε RX-Meter for the 50- to 250-MHz range (Jorgensen et al., Γ = (13) ε 1970) and for the 200- to 500-MHz, range measured with 1 + an Admittance Meter (Stetson and Nelson, 1970). Components of the sample holders used in the earlier The power density diminishes as an exponential studies with the RX Meter and Admittance Meter were function of the attenuation and distance traveled (eqs. 1 and assembled to provide a shielded open-circuit coaxial 9) as the wave propagates through the material: sample holder for grain, and a technique was developed for measurements from 100 kHz to 1 GHz with two impedance –2αz P = P t e (14) analyzers and use of proper calibrations and the invariance- of-the-cross-ratio technique (Lawrence et al., 1989). The shielded open-circuit coaxial sample holder was also used with α expressed in nepers/m. One neper is equivalent to by Bussey (1980) and by Jones et al. (1978) for 8.686 dB, so the number of dB/cm 0.08686 times the determining dielectric properties of grain with high- attenuation in nepers/m. frequency bridge measurements from 1 to 200 MHz. A coaxial sample holder, designed to accommodate flowing grain, was modeled and characterized by full two- MEASUREMENT PRINCIPLES AND port scattering parameter measurements, with the use of several alcohols of known permittivities, and signal flow TECHNIQUES graph analysis to provide dielectric properties of grain over The measurement techniques appropriate for any the range from 25 to 350 MHz (Lawrence et al., 1998). particular application depend on the frequency of interest, For measurements at frequencies above those just the nature of the dielectric material to be measured, both mentioned, a number of microwave measurement physically and electrically, and the degree of accuracy techniques are available. At microwave frequencies, required. Many different kinds of instruments can be used, generally about 1 GHz and higher, transmission-line, but any measurement instrument that provides reliable resonant cavity, and free-space techniques have been determinations of the required electrical parameters useful. Principles and techniques of microwave dielectric involving the unknown material in the frequency range of properties measurements have been discussed in several interest can be considered. reviews (Westphal, 1954; Altschuler, 1963; Redheffer, For the radio frequencies, a material can be modeled 1964; Bussey, 1967; Franceschetti, 1967; Baker-Jarvis, electrically at any given frequency as a series or parallel 1990). Microwave dielectric properties measurement equivalent circuit. Therefore, if one can measure the radio- techniques can be classified as reflection or transmission frequency (RF) circuit parameters appropriately, the measurements using resonant or nonresonant systems, with impedance or admittance for example, the dielectric open or closed structures for the sensing of the properties properties of that material at that particular frequency can of material samples (Kraszewski, 1980). Closed-structure be determined from equations that properly relate the way methods include waveguide and coaxial-line transmission in which the permittivity of the material affects those measurements and short-circuited waveguide or coaxial- circuit parameters. The challenge in making accurate line reflection measurements. Open-structure techniques dielectric properties or permittivity measurements is in include free-space transmission measurements and open- designing the material sample holder for those ended coaxial-line or open-ended waveguide measurements and adequately modeling the circuit for measurements. Resonant structures can include either reliable calculation of the permittivity from the electrical closed resonant cavities or open resonant structures measurements. operated as two-port devices for transmission Techniques for permittivity, or dielectric properties, measurements or as one-port devices for reflection measurements in the low-frequency medium-frequency, measurements.
VOL. 42(2): 523-529 525 With the development of suitable equipment for time- calibration methods have also been developed to eliminate domain measurements, techniques were developed for errors caused by unknown reflections in the coaxial-line measuring dielectric properties of materials over wide systems (Kraszewski et al., 1983; Lawrence et al., 1989). ranges of frequency (Fellner-Feldegg, 1969; Nicolson and Microwave dielectric properties of wheat and corn have Ross, 1970; van Gemert, 1973; Kent, 1975; Kwok et al., been measured at several frequencies by free-space 1979; Bellamy et al., 1985). Since modern microwave measurements with a network analyzer and dielectric network analyzers have become available, the methods of sample holders with rectangular cross-sections between obtaining dielectric properties over wide frequency ranges horn antennas and other types of radiating elements have become even more efficient. Extensive reviews have (Kraszewski and Nelson, 1990; Kraszewski et al., 1996, included methods for both frequency-domain and time- 1997; Trabelsi et al., 1997). Measurement of the complex domain techniques (Kaatze and Giese, 1980; Afsar et al., transmission coefficient, the components of which are 1986). attenuation and phase shift, permits the calculation of ε′ Dielectric sample holder design for the specific and ε′′. For free-space permittivity measurements, it is materials is an important aspect of the measurement important that an attenuation of about 10 dB through the technique. The Roberts and von Hippel (1946) short- sample layer be maintained to avoid disturbances resulting circuited line technique for dielectric properties from multiple reflections between the sample and the measurements provides a suitable method for many antennas, and the sample size, laterally, must be materials. For this method, the sample holder can be simply sufficiently large to avoid problems caused by diffraction at a short section of coaxial-line or rectangular-waveguide the edges of the sample (Trabelsi et al., 1998). with a shorting plate or other short-circuit termination at Open-ended coaxial-line probes have been used the end of the line against which the sample rests. This is successfully for convenient broad-band permittivity convenient for particulate samples, because the sample measurements (Grant et al., 1989; Blackham and Pollard, holder, and also the slotted line or slotted section to which 1997) on liquid and semisolid materials of relatively high the sample holder is connected can be mounted in a vertical loss, which includes most food materials. This technique orientation so the top surface of the sample can be has been used for permittivity measurements on fresh fruits maintained perpendicular to the axis of wave propagation and vegetables (Tran et al., 1984; Nelson et al., 1994a,b, as required for the measurement. The vertical orientation of 1995). The technique is subject to errors if there are the sample holder is also convenient for liquid or significant density variations in the material, or if there are particulate materials when the measurements are taken with air gaps or air bubbles between the end of the coaxial probe a network analyzer instead of a slotted line. and the sample, and the technique is not suitable for Dielectric properties of cereal grains, seed, and determining permittivities of very low-loss materials powdered or pulverized material have been determined (Nelson and Bartley, 1997), but it has been used to provide with various microwave measurement systems assembled broad-band information on granular and pulverized for such measurements. Twenty-one-mm, 50-Ω coaxial- materials when sample bulk densities were established by line systems were used for these measurements at auxiliary permittivity measurements (Nelson et al., 1997; frequencies from 1 to 5.5 GHz (Nelson, 1973b; Nelson et Nelson and Bartley, 1997; Nelson et al., 1998). al., 1980, 1989). A 54-mm, 50-Ω coaxial sample holder, The choices of measurement technique, equipment, and designed for minimal reflections from the transition, was sample holder design depend upon the dielectric materials used with this same system for measurements on larger- to be measured, and the frequency or frequency range of kernel cereals such as corn (Nelson, 1978, 1979b). A interest. Vector network analyzers are expensive but very rectangular-waveguide X-band system was used to versatile and useful if studies are extensive. Scalar network determine dielectric properties of grain and seed samples at analyzers and impedance analyzers are simpler and less 8 to 12 GHz (Nelson, 1972). A rectangular waveguide expensive and can be appropriate for some programs. For K-band system was used for measurements on fruit and limited studies, more commonly available microwave vegetable samples (Nelson, 1983) and on ground and laboratory measurement equipment can suffice if suitable pulverized materials for measurements at 22 GHz sample holders are constructed. When data are required at (Nelson et al., 1989; You and Nelson, 1988; Nelson and only one microwave frequency, a resonant cavity technique You, 1989, 1990). may be the logical choice (Bussey, 1967; Rzepecka, 1973). The Roberts and von Hippel method (1946) requires Such cavities can be easily constructed with rectangular measurements to determine the standing-wave ratios waveguide sections or from waveguide flanges and (SWRs) in the line with and without the sample inserted. waveguide stock (Kraszewski and Nelson, 1996). From the shift of the standing-wave node and changes in Construction of a cylindrical cavity (Risman and node widths related to SWRs, sample length, and Bengtsson, 1971; Li et al., 1981) may be advantageous, waveguide dimensions, etc., ε′ and ε′′ can be calculated depending on the needs. For temperature-dependent with suitable computer programs (Nelson et al., 1972, studies, a cavity with provision for alternate dielectric 1974). Similar determinations can be made with a network properties measurement and microwave heating of the analyzer or other instrumentation by measurement of the sample for temperature control may be advantageous complex reflection coefficient of the empty and filled (Bosisio et al., 1986). At common microwave frequencies, sample holder. a measurement system can often be assembled from Computer control of impedance analyzers (Lawrence et microwave laboratory components and using a short al., 1989) and network analyzers (Waters and Brodwin, waveguide section with a shorting plate as the sample 1988) has facilitated the automatic measurement of holder (Nelson, 1972) and an available general computer dielectric properties over wide frequency ranges. Special
526 TRANSACTIONS OF THE ASAE program (Nelson et al., 1972, 1974) for calculation of the obtained. An analysis of possible errors in determinations dielectric properties. resulting from uncertainties in the measurements is generally advisable. Often measurements can be checked by measurements on a few materials for which the APPLICATIONS FOR DIELECTRIC PROPERTIES dielectric properties are well known. However, such Many applications for dielectric properties data are materials are not as plentiful as often desired, especially evident from the references cited. As stated in the with dielectric constants and loss factors in the desired Introduction, the principal needs for information on the ranges similar to those of the materials being studied. dielectric properties of agricultural materials have been Confidence can sometimes be achieved by measurements their use in explaining the behavior of the materials when with two different systems or techniques and comparison exposed to dielectric or microwave heating and their use of resulting data. for rapid sensing of moisture content. These continue to Finally, the use of wide-frequency-range dielectric be the major interests. In studying the behavior of foods properties data is expected to offer promise for entirely new during microwave heating, the dielectric properties of the applications for which dielectric spectroscopy techniques food materials and their constituents is critical will provide the necessary tools for evaluation. The information (Mudget, 1995; Datta et al., 1995). The application of modern measurement techniques and dependence of these properties on temperature is also statistical and computational procedures along with sound very important in such applications. Dielectric properties principles of dielectric properties measurement may well over a wide range of frequencies are also useful in provide successful new applications that can be describing the dielectric relaxation phenomena in implemented. materials (Kuang and Nelson, 1997). Similarly, in the applications for sensing moisture content in grain and other materials, the dependence of the REFERENCES dielectric properties on other variables, such as temperature Afsar, M. N., J. R. Birch, R. N. Clarke, and G. W. Chantry. 1986. and bulk density is important (Nelson, 1981, 1991). Recent The measurement of the properties of materials. Proc. IEEE studies have revealed potential for determining grain 74(1): 183-199. moisture content independent of bulk density and also for Altschuler, H. M. 1963. Dielectric constant, Ch. IX. In Handbook determining bulk density from dielectric properties of Microwave Measurements, 495-546, eds. M. Sucher, and J. Fox. New York, N.Y.: Polytechnic Press of the Polytechnic Inst. measurements (Kraszewski et al., 1997; Trabelsi and Brooklyn. Nelson, 1998), with indications that temperature ASAE Standards, 45th Ed. 1998. D293.2. Dielectric properties of information may also be extracted from the measured grain and seed, 537-546. St. Joseph, Mich.: ASAE. dielectric properties of grain. These are mentioned as Baker-Jarvis, J. 1990. Transmission/reflection and short-circuit potentially new applications for dielectric properties data. line permittivity measurements. NIST Tech. Note 1341. U.S. Extension of dielectric properties sensing to the Dept. Commerce, Natl. Inst. Standards & Technol. monitoring of moisture content in moving grain (Lawrence Bellamy, W. L., S. O. Nelson, and R. G. Leffler. 1985. et al., 1998; Kraszewski et al., 1997; Trabelsi and Nelson, Development of a time-domain reflectometry system for 1998) promise applications important to improving yield dielectric properties measurements. Transactions of the ASAE determination on combines and other harvesters for 28(4): 1313-1318. Blackham, D. V., and R. D. Pollard. 1997. An improved technique precision farming operations. These techniques are also for permittivity measurements using a coaxial probe. IEEE applicable to control of grain dryers and other grain Trans. Instrum. Meas. 46(5): 1093-1099. processing equipment for improving energy use efficiency Bosisio, R. G., C. Akyel, R. C. Labelle, and W. Wang. 1986. and improvement of product quality. Computer-aided permittivity measurements in strong RF fields Other potential applications include the sensing of (Part II). IEEE Trans. Instrum. Meas. 35(4): 606-611. product maturity (Nelson et al., 1995). While such possible Brown, G. H., C. N. Hoyler, and R. A. Bierwirth. 1947. Theory applications have received little attention, research may and Application of Radio-Frequency Heating. New York, N.Y.: well identify other correlations between dielectric D. Van Nostrand Co., Inc. properties of products and their condition or suitabilities Bussey, H. E. 1967. Measurement of RF properties of materials — for particular uses, which permit rapid sensing for A survey. Proc. IEEE 55(6): 1046-1053. _____. 1980. Dielectric measurements in a shielded open circuit application in automated sorting systems (Schmilovitch et coaxial line. IEEE Trans. Instrum. Meas. 29(2): 120-124. al., 1996) or other processes. Corcoran, P. T., S. O. Nelson, L. E. Stetson, and C. W. Schlaphoff. The necessary dielectric properties data for evaluation of 1970. Determining dielectric properties of grain and seed in the potentially new applications can generally be obtained only audio-frequency range. Transactions of the ASAE 13(3): 348-351. by careful measurement. Research on some products has Datta, A. K., E. Sun, and A. Solis. 1995. Food dielectric property produced models that estimate the dielectric properties data and their composition-based prediction, Ch. 9. In reasonably well (Mudget, 1995; Datta et al., 1995; Nelson, Engineering Properties of Foods, 457-494, eds. M. A. Rao, and 1987), but, in general, the needed properties can only be S. S. H. Rizvi. New York, N.Y.: Marcel Dekker, Inc. determined accurately by direct measurement under the Fellner-Feldegg, H. 1969. The measurement of dielectrics in the specified conditions important for the application. time domain. J. Phys. Chem. 73: 616-623. Field, R. F. 1954. Sec. 1, Lumped circuits, A. Permittivity, Ch. II., Therefore, the best techniques and equipment for such Dielectric measuring techniques. In Dielectric Materials and measurements must be selected and developed. As with any Applications, ed. A. von Hippel. New York, N.Y.: John Wiley & measurements, a good understanding of the principles and Sons. techniques is important, and the measurements must always be questioned to be sure that reliable data are being
VOL. 42(2): 523-529 527 Foster, K. R., and H. P. Schwan. 1989. Dielectric properties of Mudgett, R. E. 1995. Electrical properties of foods, Ch. 8. In tissues and biological materials: A critical review. In Critical Engineering Properties of Foods, eds. M. A. Rao, and S. S. H. Reviews in Biomedical Engineering, ed. J. R. Bourne 17(1): 25- Rizvi, 389-455. New York, N.Y.: Marcel Dekker, Inc. 104. Boca Raton, Fla.: CRC Press, Inc. Nelson, S. O. 1965. Dielectric properties of grain and seed in the 1 Franceschetti, G. 1967. A complete analysis of the reflection and to 50-mc range. Transactions of the ASAE 8(1): 38-48. transmission methods for measuring the complex permittivity _____. 1972. A system for measuring dielectric properties at of materials at microwaves. Alta Frequenza 36(8): 757-764. frequencies from 8.2 to 12.4 GHz. Transactions of the ASAE Grant, J. P., R. N. Clarke, G. T. Symm, and N. M. Spyrou. 1989. A 15(6): 1094-1098. critical study of the open-ended coaxial line sensor technique _____. 1973a. Electrical properties of agricultural products—A for RF and microwave complex permittivity measurements. J. critical review. Transactions of the ASAE 16(2): 384-400. Phys. E: Sci. Instrum. 22: 757-770. _____. 1973b. Microwave dielectric properties of grain and seed. Jones, R. N., H. E. Bussey, W. E. Little, and R. F. Metzker. 1978. Transactions of the ASAE 16(5): 902-905. Electrical characteristics of corn, wheat, and soya in the 1-200 _____. 1977. Use of electrical properties for grain-moisture MHz range. NBSIR 78-897. U. S. Dept. Commerce, National measurement. J. Microwave Power 12(1): 67-72. Bureau of Standards. _____. 1978. Radiofrequency and microwave dielectric properties Jorgensen, J. L., A. R. Edison, S. O. Nelson, and L. E. Stetson. of shelled field corn. ARS-S-184. Agric. Res. Service, U. S. 1970. A bridge method for dielectric measurements of grain Dept. Agric. and seed in the 50- to 250-MHz range. Transactions of the _____. 1979a. Improved sample holder for Q-meter dielectric ASAE 13(1): 18-20, 24. measurements. Transactions of the ASAE 22(4): 950-954. Kaatze, U., and K. Giese. 1980. Dielectric relaxation spectroscopy _____. 1979b. RF and microwave dielectric properties of shelled, of liquids: Frequency domain and time domain experimental yellow-dent field corn. Transactions of the ASAE 22(6): 1451- methods. J. Physics E: Sci. Instrum. 13: 133-141. 1457. Kent, M. 1975. Time domain measurements of the dielectric _____. 1981. Review of factors influencing the dielectric properties of frozen fish. J. Microwave Power 10(1): 37-48. properties of cereal grains. Cereal Chem. 58(8): 487-492. _____. 1987. Electrical and Dielectric Properties of Food _____. 1983. Dielectric properties of some fresh fruits and Materials. Essex, England: Science & Technology Publishers. vegetables at frequencies of 2.45 to 22 GHz. Transactions of Knipper, N. V. 1959. Use of high-frequency currents for grain the ASAE 26(2): 613-616. drying. J. Agric. Eng. Res. 4: 349-360. (Translated from Nauch. _____. 1987. Models for the dielectric constants of cereal grains Trud. Elektrif. Selkhoz. 2: 185, 1956). and soybeans. J. Microwave Power 22(1): 35-39. Kraszewski, A. 1980. Microwave aquametry—A review. J. _____. 1991. Dielectric properties of agricultural products— Microwave Power 15(4): 209-220. Measurements and applications, Digest of Literature on Kraszewski, A., and S. O. Nelson. 1990. Study on grain Dielectrics, ed. A. de Reggie. IEEE Trans. Electr. Insul. 26(5): permittivity measurements in free space. J. Microwave Power 845-869. Electromagn. Energy 25(4): 202-210. Nelson, S. O., and P. G. Bartley Jr. 1997. Open-ended coaxial Kraszewski, A. W., and S. O. Nelson. 1996. Resonant cavity probe permittivity measurements on pulverized materials. IEEE perturbation—Some new applications of an old measuring IMTC Proc. 1: 653-657. technique. J. Microwave Power Electromagn. Energy 31(3): Nelson, S. O., and T.-S. You. 1989. Microwave dielectric 178-187. properties of corn and wheat kernels and soybeans. Kraszewski, A., M. A. Stuchly, and S. S. Stuchly. 1983. ANA Transactions of the ASAE 32(1): 242-249. calibration method for measurements of dielectric properties. Nelson, S. O., and T.-S You. 1990. Relationships between IEEE Trans. Instrum. Meas. 32(2): 385-386. microwave permittivities of solid and pulverized plastics. J. Kraszewski, A. W., S. Trabelsi, and S. O. Nelson. 1996. Wheat Phys D: Appl. Phys. 23: 346-353. permittivity measurements in free space. J. Microwave Power Nelson, S. O., P. G. Bartley Jr., and K. C. Lawrence. 1997. Electromagn. Energy 31(3): 135-139. Measuring RF and microwave permittivities of adult rice _____. 1997. Moisture content determination in grain by weevils. IEEE Trans. Instrum. Meas. 46(4): 941-946. measuring microwave parameters. Meas. Sci. Technol. 8: 857- Nelson, S. O., P. G. Bartley Jr., and K. C. Lawrence. 1998. RF and 863, addendum 9: 543-544, 1998. microwave dielectric properties of stored-grain insects and their Kuang, W., and S. O. Nelson. 1997. Dielectric relaxation implications for potential insect control. Transactions of the characteristics of fresh fruits and vegetables from 3 to 20 GHz. ASAE 41(3): 685-692. J. Microwave Power Electromagn. Energy 32(2): 114-122. Nelson, S. O., G. E. Fanslow, and D. D. Bluhm. 1980. Frequency Kuang, W., and S. O. Nelson. 1998. Low-frequency dielectric dependence of the dielectric properties of coal. J. Microwave properties of biological tissues: A review with some new Power 15(4): 277-282. insights. Transactions of the ASAE 41(1): 173-184. Nelson, S. O., W. R. Forbus Jr., and K. C. Lawrence. 1994a. Kwok, B. P., S. O. Nelson, and E. Bahar. 1979. Time-domain Microwave permittivities of fresh fruits and vegetables from measurements for determination of dielectric properties of 0.2 to 20 GHz. Transactions of the ASAE 37(1): 183-189. agricultural materials. IEEE Trans. Instrum. Meas. 28(20): 109- Nelson, S. O., W. R. Forbus Jr., and K. C. Lawrence. 1994b. 112. Permittivities of fresh fruits and vegetables at 0.2 to 20 GHz. J. Lawrence, K. C., S. O. Nelson, and P. G. Bartley Jr. 1998. Coaxial Microwave Power Electromagn. Energy 29(2): 81-93. dielectric sensor for cereal grains. IEEE IMTC Proc. 1: 541- Nelson, S. O., W. R. Forbus Jr., and K. C. Lawrence. 1995. 546. Assessment of microwave permittivity for sensing peach Lawrence, K. C., S. O. Nelson, and A. W. Kraszewski. 1989. maturity. Transactions of the ASAE 38(2): 579-585. Automatic system for dielectric properties measurements from Nelson, S. O., D. P. Lindroth, and R. L. Blake. 1989. Dielectric 100 kHz to 1 GHz. Transactions of the ASAE 32(1): 304-308. properties of selected and purified minerals at 1 to 22 GHz. J. Li, S., C. Akylel, and R. G. Bosisio. 1981. Precise calculations and Microwave Power Electromagn. Energy 24(4): 213-220. measurements on the complex dielectric constant of lossy Nelson, S. O., C. W. Schlaphoff, and L. E. Stetson. 1972. materials using TM010 cavity perturbation techniques. IEEE Computer program for calculating dielectric properties of low- Trans. Microwave Theory Tech. 29(10): 1041-1048. or high-loss materials from short-circuited waveguide Metaxas, A. C., and R. J. Meredith. 1983. Industrial Microwave measurements. ARS-NC-4. Agric. Res. Ser. U.S. Dept. Agric. Heating. London, England: Peter Peregrinus Ltd.
528 TRANSACTIONS OF THE ASAE Nelson, S. O., L. H. Soderholm, and F. D. Yung. 1953. Thuery, J. 1992. Microwaves: Industrial, Scientific and Medical Determining the dielectric properties of grain. Agricultural Applications. Norwood, Mass.: Artech House. Engineering 34(9): 608-610. Tinga, W. R., and S. O. Nelson. 1973. Dielectric properties of Nelson, S. O., L. E. Stetson, and C. W. Schlaphoff. 1974. A materials for microwave processing—Tabulated. J. Microwave general computer program for precise calculation of dielectric Power 8(1): 23-65. properties from short-circuited waveguide measurements. IEEE Trabelsi, S., and S. O. Nelson. 1998. Density-independent Trans. Instrum. Meas. 23(4): 455-460. functions for on-line microwave moisture meters: A general Nicolson, A. M., and G. R. Ross. 1970. Measurement of the discussion. Meas. Sci. Technol. 9: 570-578. intrinsic properties of materials by time-domain techniques. Trabelsi, S., A. W. Kraszewski, and S. O. Nelson. 1997. Microwave IEEE Trans. Instrum. Meas. 19(4): 377-382. dielectric properties of shelled, yellow-dent field corn. J. Redheffer, R. M. 1964. The measurement of dielectric constants. Microwave Power Electromagn. Energy 32(2): 188-194. In Technique of Microwave Measurements, ed. C. G. Trabelsi, S., A. W. Kraszewski, and S. O. Nelson. 1998. Montgomery, Vol. 11. MIT Radiation Laboratory Series, L. N. Nondestructive microwave characterization for determining the Ridenour, ed-in-chief. Boston, Mass.: Boston Technical bulk density and moisture content of shelled corn. Meas. Sci. Publishers, Inc. Technol. 9: 1548-1556. Risman P. O., and N. E. Bengtsson. 1971. Dielectric properties of Tran, V. N., S. S. Stuchly, and A. Kraszewski. 1984. Dielectric foods at 3 GHz as determined by cavity perturbation technique. properties of selected vegetables and fruits 0.1-10.0 GHz. J. J. Microwave Power 6(2): 101-106. Microwave Power 19(4): 251-258. Roberts, S., and A. von Hippel. 1946. A new method for measuring van Gemert, M. J. C. 1973. High-frequency time-domain methods dielectric constant and loss in the range of centimeter waves. J. in dielectric spectroscopy. Philips Res. Repts. 28: 530-572. Appl. Phys. 17(7): 610-616. von Hippel, A. 1954. Dielectric Materials and Applications. New Roussy, G., and J. A Pearce. 1995. Foundations and Industrial York, N.Y.: The Technology Press of M.I.T. and John Wiley & Applications of Microwaves and Radio Frequency Fields. West Sons. Sussex, England: John Wiley & Sons, Ltd. Waters, D. G., and M. E. Brodwin. 1988. Automatic material Rzepecka, M. A. 1973. A cavity perturbation method for routine characterization at microwave frequencies. IEEE Trans. permittivity measurement. J. Microwave Power 8(1): 3-11. Instrum. Meas. 37(2): 280-284. Schmilovitch, Z., S. O. Nelson, C. V. K. Kandala, and K. C. Westphal, W. B. 1954. Sec. 2, Distributed circuits, A. Permittivity, Lawrence. 1996. Implementation of dual-frequency RF Ch. II, Dielectric measuring techniques. In Dielectric Materials impedance technique for on-line moisture sensing in single in- and Applications, ed. A. von Hippel. New York, N.Y.: John shell pecans. Applied Engineering in Agriculture 12(4): 475-479. Wiley & Sons. Stetson, L. E., and S. O. Nelson. 1970. A method for determining You, T.-S., and S. O. Nelson. 1988. Microwave dielectric dielectric properties of grain and seed in the 200- to 500-MHz properties of rice kernels. J. Microwave Power 23(3): 150-159. range. Transactions of the ASAE 13(4): 491-495. Stratton, J. A. 1941. Electromagnetic Theory. New York, N.Y.: McGraw Hill Book Co.
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