Oblique incidence measurement setup for millimeter wave EM absorbers

Transcription

1 Oblique incidence measurement setup for millimeter wave EM absorbers Shinichiro Yamamoto a) and Kenichi Hatakeyama Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji-shi, Hyogo , Japan a) Abstract: Recently, the technologies using millimeter wave frequencies, for example, automotive radar, high-speed wireless LAN, have been developed. Electromagnetic (EM) wave absorbers are often used to prevent EM wave interferences by undesired EM waves. In this study, the authors are proposed a novel oblique incidence measurement setup using corner reflector. The absorption characteristics of millimeter wave EM absorbers for the oblique incidence are evaluated. Keywords: millimeter wave EM absorbers, oblique incidence measurement setup, corner reflector, reflection coefficient Classification: Electromagnetic Compatibility (EMC) References [1] K. Hatakeyama and H. Togawa, Simplified measurement method of electromagnetic wave shielding and absorbing characteristics in mm-waves, IEICE Trans. Commun., vol. J81-B-II, no. 6, pp , June [2] R. E. Hiatt, E. F. Knott, and T. B. A. Senior, A study of VHF absorbers and anechoic rooms, Dept. Elect. Eng., The Univ. of Michigan, F, Feb [3] R. Ueno and N. Ogasawara, Ferrites- or iron-oxides-impregnated plastics serving as radio-wave scattering suppressors, Proc. of ICF, pp , [4] P. Blacksmith, Jr., R. E. Hiatt, and R. B. Mack, Introduction to radar crosssection measurements, Proc. IEEE, pp , Aug DOI: / PROC [5] S. Yamamoto, K. Hasegawa, T. Iwai, and K. Hatakeyama, Complex reflection coefficient measurement setup at millimeter waves, IEICE Trans. Commun., vol. J90-B, no. 11, pp , Nov [6] J. D. Kraus and R. J. Marhefka, Antennas for All Applications, 3rd ed., pp , McGraw-Hill, [7] T. Kotera, S. Yamamoto, K. Hatakeyama, and T. Iwai, Oblique incidence evaluation of EM absorbers in millimeter waves from 40 GHz to 75 GHz, IEICE Tech. Rep., vol. 112, no. 372, EMCJ , pp , Jan

2 1 Introduction The application using millimeter waves, for example, automotive radar, high-speed wireless LAN, etc., have been rapidly increased. Electromagnetic (EM) wave absorbers are widely used to prevent EM interferences between electronic devices and equipment. The absorption characteristics of EM absorbers can be evaluated as the ratio of the sample reflection to the metal plate reflection. The authors proposed the millimeter wave EM absorber evaluation methods, for example, a setup using compact range techniques [1]. However, the evaluation is limited for the normal incidence. Most of EM absorbers in outdoor use are attached under oblique incidence condition. To put EM absorbers into practical use, it is necessary to evaluate EM absorbers under oblique incidence. The NRL method [2] is the initial proposal of arch type. In order to improve the measurement accuracy of reflection characteristic, the direct coupling between transmitting and receiving antennas must be eliminated. This method has no direct wave removal function. On the other hand, the arch type free space measurement setup [3] can be eliminated direct wave component by displacing the distance between the sample and the antennas. However, a large-scale measurement space is required to use above setups, and the accuracy of the evaluation at incident angle almost above 60 decreases. Therefore, we propose a novel millimeter wave measurement setup using a corner reflector. In this paper, the construction of proposed oblique incidence measurement setup and TM, TE reflection characteristics for two kinds of EM absorbers are investigated at the automotive radar frequency range (50 75 GHz). In the method of [3], the amplitude and the phase of receiving wave measured by Vector Network Analyzer. In our proposed setup, the receiving system simplification is achieved by using Spectrum Analyzer. 2 Oblique incidence measurement setup 2.1 Construction of the measurement setup Fig. 1(a) shows the schematic configuration of the measurement setup. The height and width and depth of this setup are 1.65 and 1.10 and 0.90 m, respectively. The transmitting and receiving horn antennas (HUGHES 45824H, aperture size: 3.6 cm 2.8 cm) are separately placed (the distance between antennas is 10 cm). The incident EM waves are radiated from the transmitting antenna toward the sample stage. The reflecting power from the stage is received by the receiving antenna. The millimeter circuit based on the radar cross-section measurement setup reported by [4] is placed on the top of the setup. Components of the circuit are shown in the block diagram of Fig. 1(b). The transmitting antenna is attached to the branch arms of a directional coupler; a isolator and a frequency multiplier (at quadruple frequency) are attached to one collinear arm and a signal generator (Hewlett Packard 83751A) is attached to the frequency multiplier. The receiving antenna is attached to the waveguide magic tee; a isolator and a mixer are attached to one collinear arm and a matched load (¼ 50 Ω) is attached to 311

3 (a) Schematic configuration. (b) Millimeter circuit block diagram. (c) Photo of the measurement setup. Fig. 1. Oblique incidence measurement setup (f: GHz). the other port of the magic tee. The receiver (Spectrum Analyzer, Agilent Technologies E4407B) is attached to the mixer to measure the reflected wave. In this system, the attenuator is inserted between a directional coupler and a magic tee so as to control the receiving signal amplitude [5]. The noise level of this system is 85 dbm. To evaluate EM absorber, the measurement range of the system must be at least 35 db [5]. Furthermore, assume that the millimeter circuit loss component is 10 db, then the maximum received power level (the receiving level from metal plate) must be greater than 40 dbm. The received power P r at the receiving antenna can be expressed as following Friis transmission formula [6] P r ¼ P t þ G t þ G r þ 20 log 4 2L : ð1þ Here, P t is the transmitted power, G t and G r are the gains of transmitting and receiving antennas, respectively. λ is the wavelength, and L is the distance between the antennas and the sample stage. The distance L is determined as follows. The received power P r is 40 dbm, which is the maximum received power in this system, and λ is 4: m (in the case of f ¼ 62:5 GHz, the center frequency of the measuring frequency range). In addition, P t, G t and G r are 0 dbm, 23.6 dbi and 312

4 23.6 dbi, respectively. By substituting above values in Eq. (1), the distance L of the setup was estimated to be 1.5 m. In this setup, a 20 cm square sample is placed on the measurement sample stage so as to satisfy the far field region measurement. Fig. 1(c) shows the photo of the oblique incidence measurement setup. 2.2 Measurement principle The reflection coefficient measurement techniques, in particular that for radio-wave scatter suppressor, have been troubled with the extraneous direct wave such as the direct coupling between antennas, the scattering waves from the floor and the wall, etc. In order to compensate the above-mentioned EM waves, the technique that slightly displaces vertical position is adopted to the sample stage of this system. Since the direct wave and the scattering waves do not vary with the displacement of the stage, only the reflected wave from the sample can be obtained [3]. To simplify the reflected power measurements, the rise and fall behaviors of the sample stage are automatically controlled by the external computer using LabVIEW8.5 (National Instruments). Here, E r is the reflected wave from the sample stage. E d is the direct wave from the transmitting antenna to the receiving antenna, and E at is the propagating wave through the attenuator. E f is the sum of E d and E at. E r, E d, E at, E f are given as Eq. (2). r, d, at, f are the phases of E r, E d, E at, E f, respectively. E f and E r can be represented by vector diagram as shown in Fig. 2(a). Since the sample stage displacement d is much shorter than the distance L, E f can be assumed the constant values. Then only phase r changes. The received wave E m at the receiver is the sum of E f and E r. The locus of the vector E m due to moving of the sample draws the circle of radius E rs, establishing the center OB centering on the vector E f. Similarly, due to moving of the metal plate, the circle of radius E rm is described. Thus, the reflection coefficient Γ is determined by the ratio of these two radii je rm j, je rs j as Eq. (3). rm and rs are the phases of metal plate and sample. ) E r ¼jE r je j r ; E d ¼jE d je j d ; E at ¼jE at je j at ð2þ E f ¼ E d þ E at ¼jE f je j f ¼ je rsj je rm j ejð ð rm rs ÞÞ ð3þ In this study, we defined je f j so as to satisfy the condition je rm j < je f j < 10jE rm j. 2.3 Corner reflector To evaluate the reflection characteristics of EM absorbers under oblique incidence, a corner reflector shown in Fig. 1 is placed on the sample stage. A corner reflector used here consists of two metal plates and both plates attach to the edges at 90 angles each other [7]. The two intersecting surfaces have square shapes. As the typical characteristic of the corner reflector, the reflected waves coming from the reflector propagate in opposite direction parallel to the incident waves. In this system, the side beam levels become rather high around measuring frequency range. This undesired side beam radiation is eliminated by EM absorbers. The sheets of EM absorbing material made of carbon-impregnated urethane foam (the carbon amount is 5 g per 1 l) are placed around the sample. In the 313

5 (a) Vector diagram of E m, E r, E f. (b) E rm, E rs values of sample (A) ( f = 62 GHz). Fig. 2. Vector diagram and measured results. proposed measurement setup, the oblique incidence characteristics can be correctly evaluated from 15 to 75 in incident angle by using corner reflector [7]. 3 Measurement results The oblique incidence characteristics of two samples (A) and (B) are measured. The sample (A) is a carbon impregnated urethane foam (the carbon amount is 5 g/l and the thickness is 10 mm) that is used as EM absorbers for anechoic chamber walls, etc. The sample (B) is a carbon coated vinylidene chloride fibrous absorber (Touhoku Chemical Industries product, the thickness is 15 mm). First, the reflection characteristics of sample (A) when the incident waves hit the sample at normal direction are measured. Measured je m j values are shown in Fig. 2(b) for the metal plate, sample, respectively. The measuring frequency is 62 GHz. Both metal plate and sample je m j values show the standing wave pattern. je rm j, je rs j shown in Fig. 2(b) are approximately 1.68, 0.09 mv, respectively. By substituting these values in Eq. (3), j j in 62 GHz can be obtained to be 25.4 db. By same procedure, the reflection coefficients at other frequencies were obtained. Fig. 3(a) shows the measured reflection coefficient j n-ðaþ j of sample (A) from 50 GHz to 75 GHz. j n-ðaþ j is less than 20 db over the measuring frequency range. Fig. 3(b) shows the TM and TE reflection coefficients j TM-ðAÞ j, j TE-ðAÞ j at various angles in oblique incidence in the case of 62.5 GHz. The both reflection coefficients increase as incident angle increases above around 40 degree. In addition, the matching condition (j j 20 db) can be obtained at 20 to 55 degree in the case of the TM wave reflection coefficients. Next, in order to investigate the reflection characteristic measurement repeatability, we measured j TE-ðAÞ j three times (1st, 2nd, 3rd) as shown in Fig. 3(b). From the results, the repeatability can be achieved within 1:5 db. This variation is not considered to give significant effect of the EM absorption in practical use. The reflection coefficient of sample (B) under the normal incidence is less than 20 db over the measuring frequency range as well as the result of sample (A). Fig. 3(c) shows the incident angle dependency of reflection characteristics in the case of 62.5 GHz. The reflection coefficient of TM polarization is less than 20 db below 70 degree except for 15 degree, and increase as incident angle increases 314

6 (a) Reflection coefficient Γ n-(a) under normal incidence of sample (A). (b) Reflection coefficients Γ TE-(A), Γ TM-(A) of sample (A) (in the case of 62.5 GHz). (c) Reflection coefficients Γ TM-(B), Γ TE-(B) of sample (B) (in the case of 62.5 GHz). Fig. 3. Measured reflection coefficients. above 60 degree. Furthermore, j TM-ðBÞ j almost agrees well to j TE-ðBÞ j below 40 degree. This indicates that the reflection characteristics show the isotropy. In contrast, j TM-ðBÞ j is different from j TE-ðBÞ j above 40 degree. 315

7 4 Conclusion A novel oblique incidence measurement setup for millimeter-wave EM absorbers was proposed. In this setup, a corner reflector was placed on the sample stage so as to evaluate EM absorptions under oblique incidence. Reflection coefficient measurements from 50 GHz to 75 GHz can be made by the new setup described here. Both TM and TE reflection coefficients for two millimeter wave EM absorbers were measured. For further investigations, measurements of other kinds of millimeter wave EM absorbers and absorption property calculations by transmission line theory are now in progress. Acknowledgments This work was supported by JSPS KAKENHI Grant Number (Grant-in- Aid for Young Scientists (B)). 316