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J Electromagn Eng Sci > Volume 18(2); 2018 > Article
Kang, Kim, Kang, Yoon, and Park: Specular Reflectance Measurements of Dielectric Plates in Millimeter Frequency Range

Abstract

This paper describes specular reflectance measurements of dielectric plates in three waveguide frequency bands: D-band (110–170 GHz), G-band (140–220 GHz), and J-band (220–325 GHz). The transmit (Tx) part of the proposed specular reflectance measurement system is stationary, while the receive (Rx) part and the material under test (MUT) holder are concentric-rotating with a 2:1 speed ratio for specular reflectance measurements. In specular reflectance measurements, the first step measures the specular reflection coefficients of an MUT and a metal plate on the MUT holder located at the center of the Tx and Rx parts, and the second step calculates the specular reflectance defined by the specular reflection power (i.e., intensity) of the MUT normalized to that of the metal plate. Multiple reflection effects between the Tx and Rx antennas and the MUT on the measured specular reflectance are minimized by averaging out the multiple specular reflectances measured with changing the separation distance between the two antennas by λ/8 intervals. Measurement results of the perpendicular-polarized specular reflectance of commonly used dielectric plates are verified by comparing those with the analytic results and show that the results measured over the overlapped frequency range of the D-/G-bands and at the boundary frequency of the G-/J-bands agree well with the results for the other band, respectively.

I. Introduction

Recently, uses and applications of electromagnetic (EM) materials, devices, and systems have been becoming more popular in the fields of both the natural sciences, such as physics, chemistry, biology, earth science, and astronomy, and the application sciences, such as telecommunications, military, defense, security, transportation, and medicine. Their bandwidths have been broadening and operating frequency bands have been moving to the submillimeter frequency range. Measurements of the electrical properties of EM materials have gained considerable importance, particularly in the millimeter and submillimeter frequency ranges, as material parameters are fundamental parameters in the natural and application sciences fields [1, 2].
Various methods have been developed in millimeter and submillimeter frequency ranges to measure material properties (usually complex permittivity and permeability), e.g., the open resonator [3, 4], free space [510], and reflection ellipsometry methods [1113]. The open resonator method provides accurate material properties at discrete resonance frequencies. The free space method is based on the quasi-optic approach where the material under test (MUT) is coupled to transmit (Tx) and receive (Rx) antennas that are connected to the test ports of an S-parameter measuring instrument through two focused Gaussian beams generated by spot-focused horn lens antennas [5, 6], corrugated horn antennas with mirrors [79], and planar focusing arrays [10]. The reflection ellipsometry method is a well-established method for material characterization in optics and has been recently adapted to millimeter frequencies to determine material properties from the change of the polarization state of the wave reflected by an MUT that is illuminated by the linear-polarized wave radiated by a Tx antenna.
Recently, high speed and high data-rate communications have attracted considerable attention. The IEEE 802.15 THz Interest Group (IEEE 802.15 IGthz) has been performing a channel characteristics study for future millimeter and submillimeter indoor wireless communications in the frequency ranges of 75–110 GHz and 270–320 GHz [14].
As one of the important electrical parameters of indoor interior materials, specular reflectance, which is defined as the specular reflection power (i.e., intensity) of a dielectric plate normalized to that of a metal plate, is a prerequisite to the analysis of the channel characteristics for new millimeter and submillimeter indoor wireless communications. Specular reflection, as described by the law of reflection, is the direction of the incident wave and the direction of the reflected wave that make the same angle with respect to the surface normal of an MUT; thus, the angle of incidence is equal to that of reflection (θi =θr), as shown in Fig. 1.
This paper describes the specular reflectance measurements of dielectric plates in the 110–325 GHz frequency range. Preliminary results were given in [15], and in this paper, the work is extended by a detailed measurement procedure describing the calibration of a proposed specular reflectance measurement system, comparison of measurement results with those calculated from the analytic solution, and an in-depth analysis of measurement results. Section II briefly describes the analytic solution of the perpendicular-polarized specular reflection of a single-layer homogeneous isotropic dielectric plate, which is necessary to verify measured specular reflectances of MUTs. Sections III and IV describe a proposed specular reflectance measurement system and its measurement procedure, respectively. Section V shows measurement results of the perpendicular-polarized specular reflectance of commonly used dielectric plates and verifies the measured results. Finally, Section VI summarizes this work.

II. Specular Reflection of a Homogeneous Dielectric Plate

When a perpendicular-polarized plane wave is incident upon a single-layer homogeneous isotropic dielectric plate of thickness t, as shown in Fig. 1(a), the specular reflection coefficient of the dielectric plate is given by [11]
(1)
RSpecular=Er(θr=θi)Ei(θi)=1-exp(-2jβ)1-RInterface2exp(-2jβ)RInterface,
where β=kotɛr-sin2θi is the propagation factor through the dielectric plate, ko=2π/λo is the free-space wave-number, λo is the free-space wavelength, ɛr=ɛr-jɛr is the complex relative permittivity of the dielectric plate, θi and θr, respectively, are the incident and reflected angles, and RInterface is the perpendicular-polarized reflection coefficient at the air-dielectric interface, which is given by
(2)
RInterface=cos θi-ɛr-sin2θicos θi+ɛr-sin2θi.

III. Specular Reflectance Measurement System

A specular reflectance measurement system is designed to measure both the magnitude and phase of the specular reflection of dielectric plates in the incident angle range of 30°–70° from the surface normal of an MUT in the frequency range for 110–325 GHz. The specular reflectance measurement system consists of an S-parameter measuring instrument and a specular reflectance measurement apparatus, as shown in Fig. 2.
The S-parameter measuring instrument consists of a 67 GHz vector network analyzer used as the main frame and three frequency extenders which are operating at three waveguide frequency bands, respectively: D-band (110–170 GHz, WR-6.5), G-band (140–220 GHz, WR-5.1), and J-band (220–325 GHz, WR-3.4). The vector network analyzer and frequency extender are connected to each other via phase stable RF and LO cables.
The specular reflectance measurement apparatus, shown in Fig. 2(a), consists of a Tx part, a Rx part, and an MUT holder located at the center of the Tx and Rx parts, where the Tx and Rx parts consist of a horn antenna and a frequency extender. The Tx and Rx parts and the MUT holder are installed on manual/motorized stages and slides for aligning and fixing them. For adjusting the separation distance between the Tx and Rx antennas and the MUT, a manual Y slide is installed below each Tx and Rx part. Table 1 shows the configuration of the stages and slides and their degrees of freedom, where the Y and Z axes in Fig. 2(a) and Table 1 correspond to the line of sight from the MUT toward the Tx antenna and the rotation axis of the MUT holder, respectively.
Usually, during specular reflectance measurements, an MUT is kept fixed, and the angle of the incident wave radiated by the Tx part and that of the reflected wave received by the Rx part, with respect to the surface normal of the MUT, are simultaneously varying with the same speed [16], as depicted in Fig. 1. However, such a specular measurement scheme is not suitable for high frequency ranges, because the Tx and Rx parts, consisting of the horn antenna and frequency extender, are bulky. In this case, it is more efficient if one of the Tx and Rx parts is kept fixed, while the other and the MUT is rotated [17]. Here, the Tx part is kept fixed, while the Rx part and the MUT holder are concentric-rotating with a 2:1 speed ratio for allowing specular reflec-tance measurements, as depicted in Fig. 3, where the rotation angle of the MUT, equal to the angle of the incident wave, is half the rotation angle of the Rx part.
To minimize the edge diffraction effects of an MUT in free-space specular reflectance measurements without using EM absorbers, a Gaussian-beam-forming corrugated horn antenna, with a nominal power gain of 27 dB and an axially symmetrical beam with low side lobes, as given in Table 2, is used as Tx and Rx antennas.
The specular reflectance measurement apparatus is capable of being operated in the incident angle range of 30°–70° from the surface normal of an MUT. The lowest possible angle (30°) is restricted due to the size of the Tx and Rx parts. The highest angle (70°) is ascribed to the MUT size and the Gaussian-beam waste size of the Tx and Rx antennas, as the antenna beam size projected on the surface of an MUT increases with the incident angle.

IV. Measurement Procedure of Specular Reflectance

For minimizing the measurement errors inherent in the specular reflectance measurements of an MUT, a measurement procedure may consist of the following steps: 1) alignment of the Tx and Rx antennas and the MUT, 2) measurement of the specular reflection coefficients of the metal plate used as the reflection standard and the MUT, 3) calculation of the specular reflectance of the MUT, and 4) minimization of multiple reflection effects between the Tx and Rx antennas and the MUT on the measured specular reflectances.

1. Alignment of the Tx and Rx Antennas and the MUT

  • 1-1) Rotate the Rx part in the opposite direction of the Tx part with respect to the rotation axis of the MUT holder. Set the rotation angle of the Rx part to 180° under the assumption that the rotation angle of the Tx part is 0°.

  • 1-2) Install a Gaussian-beam-forming corrugated horn antenna and a frequency extender on both the Tx and Rx parts.

  • 1-3) Align the Tx and Rx antennas to make the two antennas face each other with respect to the rotation axis of the MUT holder.

  • 1-4) Vertically install a rectangular metal plate with a good planarity on the MUT holder, and align the center of the plate surface with the rotation axis of the MUT holder.

  • 1-5) Rotate the metal plate to make the plate surface parallel to the line of sight connecting the two antennas. Set the rotational angle of the MUT holder to 90°.

  • 1-6) Adjust the separation distance between the respective apertures of the Tx and Rx antennas and the rotation axis of the MUT holder to be the same.

  • 1-7) Before connecting the antenna to the frequency extender at both the Tx and Rx parts, perform a one-port calibration at the test port of the Tx and Rx frequency extenders under the measurement conditions of 100 MHz frequency step and 100 Hz IF bandwidth over an operating frequency range of the frequency extender. Connect the antenna to the test port of the calibrated frequency extender at both Tx and Rx parts.

  • 1-8) Rotate the MUT holder to the rotation angle of 0° to make the metal plate surface and the Tx antenna face each other.

  • 1-9) Search the rotation angle of the MUT holder where the magnitude of the reflection coefficient ( S11Tx) of the metal plate at the Tx part is the maximum in the vicinity of 0°. Set the rotation angle of the MUT holder to 0°.

  • 1-10) Rotate the MUT holder to the rotation angle of 180° to make the surface of the metal plate and the Rx antenna face each other.

  • 1-11) Search the rotation angle of the Rx part where the magnitude of the reflection coefficient ( S22Rx) of the metal plate at the Rx part is the maximum in the vicinity of 180°. Set the rotation angle of the Rx part to 180°.

2. Measurement of the Specular Reflection Coefficients of the Metal Plate and the MUT

  • 2-1) Measure the forward and backward directional transmission coefficients ( S21metal and S12metal) between the Tx and Rx antennas, which correspond to each directional specular reflection coefficient of the metal plate (theoretically assumed to be −1) in the incident angle range of 30°–70° with a 0.5° step by using the S-parameter measuring instrument. The incident angle in the 30°–70° range corresponds to the rotation angle of the MUT holder in the 30°–70° range and that of the Rx part in the 60°–140° range.

  • 2-2) Rotate the MUT holder and Rx part to the rotation angle of 90° and 180°, respectively.

  • 2-3) Replace the metal plate with an MUT (a rectangular dielectric plate under test). If the thickness of the MUT is different from that of the metal plate, align the center of the MUT surface with the rotation axis of the MUT holder.

  • 2-4) Measure the forward and backward directional transmission coefficients ( S21MUT and S12MUT) between the Tx and Rx antennas, which correspond to each directional specular reflection coefficient of the MUT in the incident angle range of 30°–70° with a 0.5° step by using the S-parameter measuring instrument.

3. Calculation of the Specular Reflectance of the MUT

  • 3-1) Calculate the forward and backward directional specular reflectance of an MUT, which is defined by the specular reflection power of an MUT normalized to that of the metal plate at each direction, i.e., |S21MUT/S21metal|2 and |S12MUT/S12metal|2.

  • 3-2) Calculate the specular reflectance (SRspecular) of an MUT, which is defined by the average of the forward and backward directional specular reflectance of an MUT:

    (3)
    SRSpecular=S21MUT/S21metal2+S12MUT/S12metal22.
  • 3-3) Repeat four times the specular reflectance measurement, changing the separation distances between the Tx and Rx antennas by λ/8 intervals (i.e., d=d0+/8, n= 0, 1, 2, 3, where d, d0, and λ are the separation distance, the initial separation distance, and the wavelength of the signal, respectively).

4. Minimization of Multiple Reflection Effects between the Tx and Rx Antennas and the MUT on the Measured Specular Reflectances

  • 4-1) Multiple reflection effects between the Tx and Rx antennas and the MUT can be minimized by averaging out the four measured specular reflectances as follows:

    (4)
    SRSpecularAverage=14n=03SRSpecular(d=d0+nλ8).

V. Measurement Results

Perpendicular-polarized specular reflectances of commonly used dielectric plates with a rectangular shape, as shown in Table 3, are measured by using the proposed specular reflectance measurement system in the incident angle of 30°–70° at nine frequencies that are equally selected from the three waveguide frequency bands. The selected nine measurement frequencies are 110 GHz, 140 GHz, and 170 GHz from the D-band; 140 GHz, 170 GHz, and 220 GHz from the G-band; and 220 GHz, 260 GHz, and 325 GHz from the J-band, where 140–170 GHz is the overlapped frequency range of the D-/G-bands, and 220 GHz is the boundary frequency of the G-/J-bands.
In the specular reflectance measurements, the separation distance between the respective apertures of the Tx and Rx antennas and the surface of an MUT and a metal plate is 110 mm in the D-/G-bands and 127 mm in the J-band, respectively, which means that these reflectance measurements are performed in the Fresnel (or radiating near-field) region of each antenna as shown in Table 2. The Fresnel region is defined by 0.62D3λ<R<2D2λ, where D and R are the largest dimension of an antenna aperture and the separation distance from the antenna, respectively [18].
For a rectangular acetal plate of 2.030 mm thickness and a 20 cm × 35 cm size, four specular reflectances, which were measured with changing the separation distance between the Tx and Rx antennas by λ/8 intervals, their averaged-out reflectance, and the analytic reflectance are shown at 140 GHz of the G-band and 220 GHz of the J-band as depicted in Fig. 4(a) and (b), respectively. Fig. 4 shows at the two frequencies that (1) multiple reflection effects between the Tx and Rx antennas and the MUT on the measured specular reflectances can be minimized by averaging out the multiple specular reflectances measured at the different separation distances by λ/8 intervals, and (2) the measured specular reflectances can be verified by the fact that the averaged-out results (green line with dot) and the theoretical ones (black dot) agree well with each other. This means that the specular reflectance measurement system can be effectively calibrated to minimize the measurement errors inherent in the measurement system in the Fresnel region of Tx and Rx antennas. The theoretical specular reflectances are calculated by using (1) and (2) above with the complex relative permittivity of acetal with 2.030 mm thickness, in this case ɛr =2.800 – j0.043, which is measured by using a quasi-optic free-space material measurement system [8, 9].
Measured specular reflectances of commonly used dielectric plates, as listed in Table 3, are shown in Figs. 514, whose upper, middle, and lower correspond to the specular reflectances measured in the D-/G-/J-bands, respectively. Regardless of the kind of MUTs, Figs. 514 show, in the Fresnel region of Tx and Rx antennas, that 1) multiple reflection effects between the two antennas and the MUT can be minimized by averaging out the multiple specular reflectances measured at the different separation distances by λ/8 intervals, 2) the multiple reflection effects in the D-band are weaker than in the G-/J-bands, and 3) the measured specular reflectances can be verified by the fact that the measured results over the two overlapped frequencies of the D-/G-band, i.e., 140 GHz (the center of the upper and the left of the middle in Figs. 514) and 170 GHz (the right of the upper and the center of the middle), and at the boundary frequency of the G-/J-band, i.e., 220 GHz (the right of the middle and the left of the lower), agree well with the results for the other band, respectively.

VI. Concluding Remarks

In this paper, specular reflectance measurements of dielectric plates are described in the 110–325 GHz frequency range, i.e., three waveguide frequency bands: D-band (110–170 GHz), G-band (140–220 GHz), and J-band (220–325 GHz).
The proposed specular reflectance measurement system consists of an S-parameter measuring instrument and a specular reflectance measurement apparatus. The Tx part of the specular reflectance measurement apparatus is kept fixed, while the Rx part and the MUT holder are concentric-rotating with a 2:1 speed ratio for allowing specular reflectance measurements.
The specular reflectance measurement system is effectively calibrated to minimize the measurement errors inherent in the measurement system in the following ways: 1) by using a Gaussian-beam-forming corrugated horn antenna with an axially symmetrical beam with low side lobes to minimize the edge diffraction effects of an MUT in free-space measurements without using EM absorbers, 2) by normalizing the specular reflection power of an MUT to that of a metal plate used as the reflection standard, and 3) by averaging out the multiple specular reflectances measured with changing the separation distance between the Tx and Rx antennas by λ/8 intervals to minimize the multiple reflection effects between the two antennas and an MUT on the measured specular reflectances.
Measurement results of the perpendicular-polarized specular reflectance of commonly used dielectric plates are verified in the Fresnel region of Tx and Rx antennas by the fact that 1), for acetal, the average of the multiple specular reflectances measured with changing the separation distance between the Tx and Rx antennas by λ/8 intervals agrees well with the analytic result, and 2) the results measured at the two overlapped frequencies of the D-/G-bands, i.e., 140 GHz and 170 GHz, and at the boundary frequency of the G-/J-bands, i.e., 220 GHz, agree well with the results at the other band for all of MUTs, respectively.

Fig. 1
Conventional specular reflection measurement configuration (MUT is kept fixed, while the Tx part and Rx part, with respect to the surface normal of the MUT, are simultaneously varying with the same speed).
jees-18-2-78f1.tif
Fig. 2
(a) Schematic diagram of the specular reflectance measurement apparatus. (b) Implemented specular reflectance measurement system.
jees-18-2-78f2.tif
Fig. 3
Proposed specular reflection measurement configuration (The Tx part is kept fixed, while the Rx part and MUT are concentric-rotating with a 2:1 speed ratio).
jees-18-2-78f3.tif
Fig. 4
Specular reflectance of an acetal plate of 2.030 mm thickness. (a) 140 GHz of the G-band. (b) 220 GHz of the J-band.
jees-18-2-78f4.tif
Fig. 5
Specular reflectance of an acetal plate of 2.030 mm thickness.
jees-18-2-78f5.tif
Fig. 6
Specular reflectance of an acetal plate of 5.644 mm thickness.
jees-18-2-78f6.tif
Fig. 7
Specular reflectance of an acryl plate of 2.010 mm thickness.
jees-18-2-78f7.tif
Fig. 8
Specular reflectance of an acryl plate of 5.055 mm thickness.
jees-18-2-78f8.tif
Fig. 9
Specular reflectance of a bakelite plate of 2.210 mm thickness.
jees-18-2-78f9.tif
Fig. 10
Specular reflectance of a bakelite plate of 5.163 mm thickness.
jees-18-2-78f10.tif
Fig. 11
Specular reflectance of a PC plate of 2.010 mm thickness.
jees-18-2-78f11.tif
Fig. 12
Specular reflectance of a PC plate of 4.955 mm thickness.
jees-18-2-78f12.tif
Fig. 13
Specular reflectance of a PVC plate of 1.730 mm thickness.
jees-18-2-78f13.tif
Fig. 14
Specular reflectance of a PVC plate of 5.138 mm thickness.
jees-18-2-78f14.tif
Table 1
Configuration of stages and slides and their degrees of freedom
Part name Detail Configuration and type of stage and slide Degree of freedom

Manual Motorized
Tx part Antenna fixture Z/X/θxy/rotation stage Y stage 6
Frequency extender fixture θxy/Z/Y slide - 3
Base stage Y slide - 1
MUT holder MUT fixture X/Y/θxy/rotation stage Z rotation stage 6
Rx part Antenna fixture Z/X/θxy/rotation stage Y stage 6
Frequency extender fixture θxy/Z/Y slide - 3
Base stage Y slide Z rotation stage 2

Y and Z axes correspond to the line of sight from the MUT toward the Tx antenna and the rotation axis of the MUT holder, respectively.

Table 2
Specifications of Tx and Rx antennas
Type Gaussian-beam-forming corrugated horn antenna
Features Axially symmetrical beams with low side lobes
Frequency band D-band G-band J-band
Operating frequency range (GHz) 110–170 140–220 220–325
Aperture diameter (D, mm) 28 23 17
Length (mm) 244 205 137
Waveguide connector WR-6.5 WR-5.1 WR-3.4
Gain (dB) 27 27 27
Fresnel region (0.62D3λ<R<2D2λ) 110 GHz 56–575 mm 140 GHz 47–494 mm 220 GHz 37–424 mm
140 GHz 63–732 mm 170 GHz 51–600 mm 260 GHz 40–501 mm
170 GHz 69–889 mm 220 GHz 59–776 mm 325 GHz 45–626 mm

D and R are the largest dimension of an antenna aperture and the separation distance from the antenna, respectively.

Table 3
MUT name, its dimension, and the figure number of its measured specular reflectance
MUT Dimension Figure number

Thickness (mm) Width × Height (cm2)
Acetal 2.030, 5.644 20 × 35 5, 6
Acryl 2.010, 5.055 20 × 35 7, 8
Bakelite 2.210, 5.163 20 × 35 9, 10
PC (polycarbonate) 2.010, 4.955 20 × 35 11, 12
PVC (polyvinyl chloride) 1.730, 5.138 20 × 35 13, 14

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Biography

Jin-Seob Kang received the B.S. degree in electronic engineering from Hanyang University, Seoul, Korea, in 1987, and the M.S. and Ph.D. degrees in electrical engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 1989 and 1994, respectively. He was a Postdoctoral Research Associate with the Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, in 1995 and an Assistant Professor with the School of Electrical and Electronic Engineering, Chungbuk National University, Korea, from 1996 to 1997. He joined the Korea Research Institute of Standards and Science, Daejeon, in 1998. His research interests include electromagnetics measurement standards (scattering parameters, antenna characteristics, field strength, material parameters) and (sub-)mm-wave measurements.
jees-18-2-78i1.tif

Biography

Jeong-Hwan Kim was born in Cheongju, Korea in 1954. He received the B.S. degree in electrical engineering from Seoul National University, Seoul, Korea in 1978, and the M.S. and the Ph.D. degrees from Korea Advanced Institute of Science and Technology, Seoul, Korea in 1980 and 2000, respectively, both in electrical and electronic engineering. He joined the Center for Electromagnetic Wave, the Korea Research Institute of Standards and Science, Daedeok Science Town, Korea in 1981. Since then, he has been working on the developments of the six-port ANA systems, standard antennas, and electromagnetics measurement standards.
jees-18-2-78i2.tif

Biography

Kwang-Yong Kang was born in Sacheon, Korea in 1951. He received the B.S. degree from Seoul National University, Seoul, Korea, in 1975, and the Ph.D. degree in solid-state physics from Pusan National University, Busan, Korea, in 1988. Since 1989, he has been a member of engineering staff of the ETRI, Daejeon, Korea. He was a member of the IEEE MTT and OSA of USA, JJAP of Japan, and KEES/KPS of Korea. He retired from the institute (ETRI), passed through the university, and he is working for a research institute of a small company, Panoptics Corp. He is now continuing his research work (THz field) at the Lab. of the Corporate Research Lab. His research interests include the HTS thin films and millimeter-wave device, metal-insulator transition (MIT) devices, and terahertz technologies.
jees-18-2-78i3.tif

Biography

Sung Won Park received the M.S. degree in information and communications engineering from Kyunghee University, Seoul, Korea, in 2003. He joined the National Radio Research Agency, Naju, Korea, in 2003 and in 2013, he was promoted to Deputy Director.
jees-18-2-78i4.tif

Biography

Dae-Hwan Yoon received the B.S. degree in electronic engineering from Chosun University, Gwangju, Korea, in 2007. He joined the Gwangju Radio Management Service, Naju, Korea, from 2008 to 2014. He joined the National Radio Research Agency, Naju, Korea, in 2015.
jees-18-2-78i5.tif

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