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A method of determining the moisture content of bulk wheat grain
Journal of Food Engineering 78 (2007) 1155–1158 www.elsevier.com/locate/jfoodeng
A method of determining the moisture content of bulk wheat grain Hong S. Chua, Graham Parkinson, Arthur D. Haigh, Andrew A.P. Gibson
*
Microwave Engineering Research Group, School of Electrical and Electronic Engineering, The University of Manchester, Sackville Street Building, P.O. Box 88, Manchester, M60 1QD, United Kingdom Received 11 June 2005; accepted 19 December 2005 Available online 24 March 2006
Abstract A reliable method of determining the moisture content of bulk wheat grain samples has been established. By compressing the grain seed into a self-supporting bulk sample, the determination of the complex permittivity has been simplified; waveguide windows and highQ cavities of previous techniques are not necessary. Measurement of the real and imaginary parts of the complex permittivity of the bulk grain samples, in a standard waveguide cell, showed that they are suitable indicators for determining the moisture content of the grain. Furthermore, the complex permittivity, both real and imaginary, is independent of the force applied to compress the grain and so the compression process control does not impinge on the accuracy of the measurement of the grain moisture content. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Wheat grain; Complex permittivity; Microwave frequency; Moisture content
1. Introduction The need to measure and control the moisture content of wheat grain is now recognised as a necessary requirement to maintain competitive advantage in the food processing industry. Furthermore, the measurement technique employed must be cost effective, requiring rapid and accurate evaluation of the moisture content. In particular, non-invasive microwave techniques have received considerable attention in the literature and by industry (Haigh, Thompson, Gibson, Campbell, & Fang, 2001; Kraszewski, 1996; Kupfer, Kraszewski, & Knochel, 2000). In general, the microwave techniques for moisture evaluation rely on the determination of the complex permittivity of the grain and this can be achieved by measuring either single grains, an aggregation of uncompressed grain samples or bulk grain samples where a block of grain is inserted into a microwave cavity or waveguide. The measurement of single wheat cells has been addressed previously (Chua, Haigh, Thompson, & Gibson, *
Corresponding author. Tel.: +44 161 3064810. E-mail address: [email protected] (A.A.P. Gibson).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.12.027
2004; Kraszewski & Nelson, 1992, 1994; Kraszewski, You, & Nelson, 1989; Thompson, Haigh, Dillon, & Gibson, 2003) but the need to determine the perturbation of a microwave field by a single grain requires the sample to be inserted into a sensitive, high-Q microwave cavity. This is a labour intensive process requiring careful positioning of the grain seed and to maintain accuracy, the environmental conditions such as temperature and pressure must be carefully controlled and/or accounted for in the calibration process. The measurement of loose grain samples in a waveguide with two dielectric windows to retain the grain has also been previously undertaken (Haigh et al., 2001). The presence of the two dielectric windows yields a four-boundary waveguide cell from which the grain data has to be de-embedded. This technique can suffer from measurement uncertainties due to the irregular formation of the grain seed at the interfaces with the dielectric windows. These interface problems can induce multiple scattering of the microwave field and this leads to inconsistencies in the reflection and transmission measurements which should, in principle, be symmetrical between the two ports.
1156
H.S. Chua et al. / Journal of Food Engineering 78 (2007) 1155–1158
This paper concentrates on the measurement of bulk grain samples, which overcomes the aforementioned difficulties. The grain is compressed into a ‘semi-solid’ block, which can be inserted into a standard waveguide section obviating the need for specialist high-Q cavities. Also, there is no requirement for waveguide windows to be inserted at the ends of the waveguide section since the grain block is self-supporting. The compact nature of the measurement sample also ensures that there is a more homogenous dielectric boundary interface, which will considerably reduce anomalous reflections compared to the loose grain case. The microwave scattering parameters of the block samples of compressed grain were measured in a waveguide cell and then used to derive the complex permittivity of the grain sample. The moisture content of the grain sample and the force used to compress it were both varied and the complex permittivity determined. The real and imaginary parts of the complex permittivity were found to have a strong dependence on the moisture content of the grain but showed little dependence on variations in the force applied to compress it. These results are therefore a starting point for a robust moisture measurement technique for bulk samples of wheat grain. 2. Complex permittivity of bulk wheat grain The complex permittivity of liquid and granular materials can be determined using a four-boundary waveguide cell (Haigh et al., 2001; Musil & Zacek, 1986; Nelson, Kraszewski, & You, 1991). Using this method, interface problems can introduce inconsistencies in measuring the real part of the permittivity although transmission measurements are more consistent than reflection ones. In the case of grain, the measurement problem can be reduced to two interfaces by compressing a bulk sample into a single semisolid block, which fits inside the waveguide. The standard WG 10 (1.5 GHz) cell was specially adapted to enable the grain to be crushed within it. A steel-reinforcing jacket was fitted to the outer wall of the copper waveguide to prevent distortion of the waveguide dimensions when force is applied. Two steel pistons were inserted into the waveguide cell with 40 g of loose grain between them. A hydraulic press was used to compress the grain into a block in situ. This ensured negligible air gaps between the block and the inner waveguide wall and also yielded flat, homogeneous boundaries at the measurement interfaces. Compressive forces from 100 kN to 180 kN in increments of 20 kN were used to produce 40 g blocks of grain for a range of moisture contents (5–18%) and densities. The Mallaca strain of wheat was used throughout the reported work. The waveguide measurement cell is illustrated in Fig. 1. The test cell in Fig. 1 illustrates a two boundary region for which Tischer (1960) developed closed form expressions relating the scattering parameters to the cell dimensions
Test cell Rectangular waveguide (WG 10)
Compressed grain seeds Plane of calibration (full 2 ports)
Steel reinforcing jacket
Fig. 1. Waveguide cell setup for the compressed grain bulk sample.
and waveguide propagation constant. The scattering parameters represent how much of the incident microwave signal is reflected, transmitted and hence absorbed by the grain sample. For a symmetrical and reciprocal measurement, the forward or reverse scattering parameter transmission coefficient (S21 or S12) is derived as S 12 ¼ S 21 ¼
2ðc20
þ
4jc0 bg 2 bg Þ sinh c0 L þ
ð1Þ
4jc0 bg cosh c0 L
where bg ¼ 2p , kg is the guide wavelength, and L the sample kg length. Similarly the reflection coefficient (S11 or S22) at either boundary is given by 2 c20 þ b2g sinh c0 L S 11 ¼ S 22 ¼ ð2Þ 2 c20 b2g sinh c0 L þ 4jc0 bg cosh c0 L Using the measured scattering parameters, the Newton– Raphson method can then be applied to extract the complex propagation constant, c0, from either Eqs. (1), or (2). Finally, the complex permittivity (e*) of the semi-solid block of grain is obtained from 2 2 c 0 k0 k0 e ¼ þ ð3Þ j2p kc where kc is the guide cut-off wavelength. In the following measurements the cell length (L) varied from 1.773 to 2.055 cm and the measurement frequency was 2.5 GHz which gives k0 = 12 cm. 2.1. Real permittivity The data in Table 1 relating real permittivity of the wheat grain to its moisture content and applied force is
Table 1 Moisture content and applied force versus real permittivity Measured MC (%)
Applied force (kN) 100
120
140
160
180
6.55 8.66 10.31 12.49 14.08 16.14
2.690 2.787 3.230 3.566 4.037 4.320
2.719 3.000 3.271 3.676 4.034 4.331
2.775 3.130 3.380 3.788 4.088 4.382
2.825 3.152 3.485 3.824 4.099 4.425
2.830 3.227 3.552 3.859 4.150 4.445
H.S. Chua et al. / Journal of Food Engineering 78 (2007) 1155–1158
Fig. 2. 3D Plot of moisture content and applied force versus real permittivity.
plotted in Fig. 2. It clearly demonstrates that the real permittivity of the wheat grain increases as the moisture content is increased. However, the real permittivity remains relatively constant as the force applied to compress the grain is varied. This is a significant finding with respect to the measurement technique because it confirms that the real permittivity is predominantly dependent upon the grain moisture content and not the applied force used to compress it. Thus, real permittivity is a reliable parameter in indirect measurements of moisture content at microwave frequencies. 2.2. Imaginary permittivity The data in Table 2 relating imaginary permittivity of the wheat grain to its moisture content and applied force is plotted in Fig. 3. In a similar manner to the real permittivity, the imaginary permittivity of the wheat grain is dependent on the moisture content, reducing with increasing moisture content, but remains relatively independent of the applied force. Hence, the imaginary part of the complex permittivity is also a reliable indicator of moisture content.
1157
Fig. 3. 3D Plot of moisture content and applied force versus imaginary permittivity.
3. Conclusions A two-boundary cell arrangement for the measurement of complex permittivity of bulk wheat grains has been described. Forty-gram wheat grain samples were compressed with forces ranging from 100 to 180 kN to form a semi-solid block. This compression technique is introduced to minimize the inconsistency in S-parameter measurements associated with interface problems in fourboundary cells. S-parameters of the grain block in a waveguide cell were measured and used to determine the complex permittivity of the block. The moisture content of the grain was varied between 5% and 18% and the resultant complex permittivity determined. Also, the relationship between the complex permittivity of the grain block and the force used to compress it was evaluated. Both the imaginary and real parts of the complex permittivity were found to be strongly dependent on the moisture content of the grain but relatively independent of the force used to compress it. The method is thus shown to be suitable as a quick and accurate means of measurement of the moisture content of grain and the method is readily adaptable to a production process.
Table 2 Moisture content and applied force versus imaginary permittivity
Acknowledgements
Measured MC (%)
Applied force (kN) 100
120
140
160
180
6.55 8.66 10.31 12.49 14.08 16.14
0.3969 0.6304 0.8507 1.2582 1.6776 1.9125
0.4700 0.6380 0.8801 1.3337 1.6626 1.9511
0.4956 0.6867 0.9546 1.3860 1.6691 1.9662
0.4989 0.7411 0.9776 1.3503 1.6920 1.9910
0.4967 0.7663 1.0472 1.3493 1.7506 2.0252
The authors would like to thank the Research Councils for their financial support through the Basic Technology Program GR/R88113/01 and EP/C009681/1. Mr. Chua would also like to thank the UK Universities for his Overseas Research Student (ORS) award. The Satake Corporation of Japan is gratefully acknowledged for its support of the activities of the Satake Centre for Grain Process Engineering.
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H.S. Chua et al. / Journal of Food Engineering 78 (2007) 1155–1158
References Chua, H. S., Haigh, A. D., Thompson, F., & Gibson, A. A. P. (2004). Measurement of the wet-mass, moisture content, volume and density of single wheat grain kernels. IEE Proceedings—Science Measurement and Technology, 151(5), 384–388. Haigh, A. D., Thompson, F., Gibson, A. A. P., Campbell, G. M., & Fang, C. (2001). Complex permittivity of liquid and granular materials using waveguide cells. Subsurface Sensing Technologies and Applications, 2(4), 425–434. Kraszewski, A. (1996). Microwave aquametry: Electromagnetic wave interaction with water-containing materials. New York: IEEE Press. Kraszewski, A. W., & Nelson, S. O. (1992). Observations on resonant cavity perturbation by dielectric objects. IEEE Transactions on Microwave Theory and Techniques, 40(1), 151–155. Kraszewski, A. W., & Nelson, S. O. (1994). Microwave resonator for sensing moisture-content and mass of single wheat kernels. Canadian Agricultural Engineering, 36(4), 231–238.
Kraszewski, A. W., You, T. S., & Nelson, S. O. (1989). Microwave resonator technique for moisture-content determination in single soybean seeds. IEEE Transactions on Instrumentation and Measurement, 38(1), 79–84. Kupfer, K., Kraszewski, A., & Knochel, R. (2000). RF and microwave sensing of moist materials, food and other dielectrics. Wiley-VCH. Musil, J., & Zacek, F. (1986). Microwave measurements of complex permittivity by free space methods and their applications. Elsevier. Nelson, S., Kraszewski, A., & You, T. (1991). Solid and particulate material permittivity relationships. Journal of Microwave Power and Electromagnetic Energy, 26(1), 45–51. Thompson, F., Haigh, A. D., Dillon, B. M., & Gibson, A. A. P. (2003). Analysis and design of a re-entrant microwave cavity for the characterisation of single wheat grain kernels. IEE Proceedings— Science Measurement and Technology, 150(3), 113–117. Tischer, F. J. (1960). Measurement of the wave propagation properties of plasma in the microwave region. IRE Transactions on Instrumentation, 2(1–9), 167–171.
A method of determining the moisture content of bulk wheat grain Hong S. Chua, Graham Parkinson, Arthur D. Haigh, Andrew A.P. Gibson
*
Microwave Engineering Research Group, School of Electrical and Electronic Engineering, The University of Manchester, Sackville Street Building, P.O. Box 88, Manchester, M60 1QD, United Kingdom Received 11 June 2005; accepted 19 December 2005 Available online 24 March 2006
Abstract A reliable method of determining the moisture content of bulk wheat grain samples has been established. By compressing the grain seed into a self-supporting bulk sample, the determination of the complex permittivity has been simplified; waveguide windows and highQ cavities of previous techniques are not necessary. Measurement of the real and imaginary parts of the complex permittivity of the bulk grain samples, in a standard waveguide cell, showed that they are suitable indicators for determining the moisture content of the grain. Furthermore, the complex permittivity, both real and imaginary, is independent of the force applied to compress the grain and so the compression process control does not impinge on the accuracy of the measurement of the grain moisture content. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Wheat grain; Complex permittivity; Microwave frequency; Moisture content
1. Introduction The need to measure and control the moisture content of wheat grain is now recognised as a necessary requirement to maintain competitive advantage in the food processing industry. Furthermore, the measurement technique employed must be cost effective, requiring rapid and accurate evaluation of the moisture content. In particular, non-invasive microwave techniques have received considerable attention in the literature and by industry (Haigh, Thompson, Gibson, Campbell, & Fang, 2001; Kraszewski, 1996; Kupfer, Kraszewski, & Knochel, 2000). In general, the microwave techniques for moisture evaluation rely on the determination of the complex permittivity of the grain and this can be achieved by measuring either single grains, an aggregation of uncompressed grain samples or bulk grain samples where a block of grain is inserted into a microwave cavity or waveguide. The measurement of single wheat cells has been addressed previously (Chua, Haigh, Thompson, & Gibson, *
Corresponding author. Tel.: +44 161 3064810. E-mail address: [email protected] (A.A.P. Gibson).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.12.027
2004; Kraszewski & Nelson, 1992, 1994; Kraszewski, You, & Nelson, 1989; Thompson, Haigh, Dillon, & Gibson, 2003) but the need to determine the perturbation of a microwave field by a single grain requires the sample to be inserted into a sensitive, high-Q microwave cavity. This is a labour intensive process requiring careful positioning of the grain seed and to maintain accuracy, the environmental conditions such as temperature and pressure must be carefully controlled and/or accounted for in the calibration process. The measurement of loose grain samples in a waveguide with two dielectric windows to retain the grain has also been previously undertaken (Haigh et al., 2001). The presence of the two dielectric windows yields a four-boundary waveguide cell from which the grain data has to be de-embedded. This technique can suffer from measurement uncertainties due to the irregular formation of the grain seed at the interfaces with the dielectric windows. These interface problems can induce multiple scattering of the microwave field and this leads to inconsistencies in the reflection and transmission measurements which should, in principle, be symmetrical between the two ports.
1156
H.S. Chua et al. / Journal of Food Engineering 78 (2007) 1155–1158
This paper concentrates on the measurement of bulk grain samples, which overcomes the aforementioned difficulties. The grain is compressed into a ‘semi-solid’ block, which can be inserted into a standard waveguide section obviating the need for specialist high-Q cavities. Also, there is no requirement for waveguide windows to be inserted at the ends of the waveguide section since the grain block is self-supporting. The compact nature of the measurement sample also ensures that there is a more homogenous dielectric boundary interface, which will considerably reduce anomalous reflections compared to the loose grain case. The microwave scattering parameters of the block samples of compressed grain were measured in a waveguide cell and then used to derive the complex permittivity of the grain sample. The moisture content of the grain sample and the force used to compress it were both varied and the complex permittivity determined. The real and imaginary parts of the complex permittivity were found to have a strong dependence on the moisture content of the grain but showed little dependence on variations in the force applied to compress it. These results are therefore a starting point for a robust moisture measurement technique for bulk samples of wheat grain. 2. Complex permittivity of bulk wheat grain The complex permittivity of liquid and granular materials can be determined using a four-boundary waveguide cell (Haigh et al., 2001; Musil & Zacek, 1986; Nelson, Kraszewski, & You, 1991). Using this method, interface problems can introduce inconsistencies in measuring the real part of the permittivity although transmission measurements are more consistent than reflection ones. In the case of grain, the measurement problem can be reduced to two interfaces by compressing a bulk sample into a single semisolid block, which fits inside the waveguide. The standard WG 10 (1.5 GHz) cell was specially adapted to enable the grain to be crushed within it. A steel-reinforcing jacket was fitted to the outer wall of the copper waveguide to prevent distortion of the waveguide dimensions when force is applied. Two steel pistons were inserted into the waveguide cell with 40 g of loose grain between them. A hydraulic press was used to compress the grain into a block in situ. This ensured negligible air gaps between the block and the inner waveguide wall and also yielded flat, homogeneous boundaries at the measurement interfaces. Compressive forces from 100 kN to 180 kN in increments of 20 kN were used to produce 40 g blocks of grain for a range of moisture contents (5–18%) and densities. The Mallaca strain of wheat was used throughout the reported work. The waveguide measurement cell is illustrated in Fig. 1. The test cell in Fig. 1 illustrates a two boundary region for which Tischer (1960) developed closed form expressions relating the scattering parameters to the cell dimensions
Test cell Rectangular waveguide (WG 10)
Compressed grain seeds Plane of calibration (full 2 ports)
Steel reinforcing jacket
Fig. 1. Waveguide cell setup for the compressed grain bulk sample.
and waveguide propagation constant. The scattering parameters represent how much of the incident microwave signal is reflected, transmitted and hence absorbed by the grain sample. For a symmetrical and reciprocal measurement, the forward or reverse scattering parameter transmission coefficient (S21 or S12) is derived as S 12 ¼ S 21 ¼
2ðc20
þ
4jc0 bg 2 bg Þ sinh c0 L þ
ð1Þ
4jc0 bg cosh c0 L
where bg ¼ 2p , kg is the guide wavelength, and L the sample kg length. Similarly the reflection coefficient (S11 or S22) at either boundary is given by 2 c20 þ b2g sinh c0 L S 11 ¼ S 22 ¼ ð2Þ 2 c20 b2g sinh c0 L þ 4jc0 bg cosh c0 L Using the measured scattering parameters, the Newton– Raphson method can then be applied to extract the complex propagation constant, c0, from either Eqs. (1), or (2). Finally, the complex permittivity (e*) of the semi-solid block of grain is obtained from 2 2 c 0 k0 k0 e ¼ þ ð3Þ j2p kc where kc is the guide cut-off wavelength. In the following measurements the cell length (L) varied from 1.773 to 2.055 cm and the measurement frequency was 2.5 GHz which gives k0 = 12 cm. 2.1. Real permittivity The data in Table 1 relating real permittivity of the wheat grain to its moisture content and applied force is
Table 1 Moisture content and applied force versus real permittivity Measured MC (%)
Applied force (kN) 100
120
140
160
180
6.55 8.66 10.31 12.49 14.08 16.14
2.690 2.787 3.230 3.566 4.037 4.320
2.719 3.000 3.271 3.676 4.034 4.331
2.775 3.130 3.380 3.788 4.088 4.382
2.825 3.152 3.485 3.824 4.099 4.425
2.830 3.227 3.552 3.859 4.150 4.445
H.S. Chua et al. / Journal of Food Engineering 78 (2007) 1155–1158
Fig. 2. 3D Plot of moisture content and applied force versus real permittivity.
plotted in Fig. 2. It clearly demonstrates that the real permittivity of the wheat grain increases as the moisture content is increased. However, the real permittivity remains relatively constant as the force applied to compress the grain is varied. This is a significant finding with respect to the measurement technique because it confirms that the real permittivity is predominantly dependent upon the grain moisture content and not the applied force used to compress it. Thus, real permittivity is a reliable parameter in indirect measurements of moisture content at microwave frequencies. 2.2. Imaginary permittivity The data in Table 2 relating imaginary permittivity of the wheat grain to its moisture content and applied force is plotted in Fig. 3. In a similar manner to the real permittivity, the imaginary permittivity of the wheat grain is dependent on the moisture content, reducing with increasing moisture content, but remains relatively independent of the applied force. Hence, the imaginary part of the complex permittivity is also a reliable indicator of moisture content.
1157
Fig. 3. 3D Plot of moisture content and applied force versus imaginary permittivity.
3. Conclusions A two-boundary cell arrangement for the measurement of complex permittivity of bulk wheat grains has been described. Forty-gram wheat grain samples were compressed with forces ranging from 100 to 180 kN to form a semi-solid block. This compression technique is introduced to minimize the inconsistency in S-parameter measurements associated with interface problems in fourboundary cells. S-parameters of the grain block in a waveguide cell were measured and used to determine the complex permittivity of the block. The moisture content of the grain was varied between 5% and 18% and the resultant complex permittivity determined. Also, the relationship between the complex permittivity of the grain block and the force used to compress it was evaluated. Both the imaginary and real parts of the complex permittivity were found to be strongly dependent on the moisture content of the grain but relatively independent of the force used to compress it. The method is thus shown to be suitable as a quick and accurate means of measurement of the moisture content of grain and the method is readily adaptable to a production process.
Table 2 Moisture content and applied force versus imaginary permittivity
Acknowledgements
Measured MC (%)
Applied force (kN) 100
120
140
160
180
6.55 8.66 10.31 12.49 14.08 16.14
0.3969 0.6304 0.8507 1.2582 1.6776 1.9125
0.4700 0.6380 0.8801 1.3337 1.6626 1.9511
0.4956 0.6867 0.9546 1.3860 1.6691 1.9662
0.4989 0.7411 0.9776 1.3503 1.6920 1.9910
0.4967 0.7663 1.0472 1.3493 1.7506 2.0252
The authors would like to thank the Research Councils for their financial support through the Basic Technology Program GR/R88113/01 and EP/C009681/1. Mr. Chua would also like to thank the UK Universities for his Overseas Research Student (ORS) award. The Satake Corporation of Japan is gratefully acknowledged for its support of the activities of the Satake Centre for Grain Process Engineering.
1158
H.S. Chua et al. / Journal of Food Engineering 78 (2007) 1155–1158
References Chua, H. S., Haigh, A. D., Thompson, F., & Gibson, A. A. P. (2004). Measurement of the wet-mass, moisture content, volume and density of single wheat grain kernels. IEE Proceedings—Science Measurement and Technology, 151(5), 384–388. Haigh, A. D., Thompson, F., Gibson, A. A. P., Campbell, G. M., & Fang, C. (2001). Complex permittivity of liquid and granular materials using waveguide cells. Subsurface Sensing Technologies and Applications, 2(4), 425–434. Kraszewski, A. (1996). Microwave aquametry: Electromagnetic wave interaction with water-containing materials. New York: IEEE Press. Kraszewski, A. W., & Nelson, S. O. (1992). Observations on resonant cavity perturbation by dielectric objects. IEEE Transactions on Microwave Theory and Techniques, 40(1), 151–155. Kraszewski, A. W., & Nelson, S. O. (1994). Microwave resonator for sensing moisture-content and mass of single wheat kernels. Canadian Agricultural Engineering, 36(4), 231–238.
Kraszewski, A. W., You, T. S., & Nelson, S. O. (1989). Microwave resonator technique for moisture-content determination in single soybean seeds. IEEE Transactions on Instrumentation and Measurement, 38(1), 79–84. Kupfer, K., Kraszewski, A., & Knochel, R. (2000). RF and microwave sensing of moist materials, food and other dielectrics. Wiley-VCH. Musil, J., & Zacek, F. (1986). Microwave measurements of complex permittivity by free space methods and their applications. Elsevier. Nelson, S., Kraszewski, A., & You, T. (1991). Solid and particulate material permittivity relationships. Journal of Microwave Power and Electromagnetic Energy, 26(1), 45–51. Thompson, F., Haigh, A. D., Dillon, B. M., & Gibson, A. A. P. (2003). Analysis and design of a re-entrant microwave cavity for the characterisation of single wheat grain kernels. IEE Proceedings— Science Measurement and Technology, 150(3), 113–117. Tischer, F. J. (1960). Measurement of the wave propagation properties of plasma in the microwave region. IRE Transactions on Instrumentation, 2(1–9), 167–171.