ieice communications express, vol.4, no.10, 310 316

Oblique incidencemeasurement setup formillimeter wave EMabsorbers

Shinichiro Yamamotoa) and Kenichi HatakeyamaGraduate School of Engineering, University of Hyogo,

2167 Shosha, Himeji-shi, Hyogo 671–2280, Japan

a) [email protected]

Abstract: Recently, the technologies using millimeter wave frequencies,

for example, automotive radar, high-speed wireless LAN, have been devel-

oped. 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 reflec-

tor. The absorption characteristics of millimeter wave EM absorbers for the

oblique incidence are evaluated.

Keywords: millimeter wave EM absorbers, oblique incidence measure-

ment setup, corner reflector, reflection coefficient

Classification: Electromagnetic Compatibility (EMC)

References

[1] K. Hatakeyama and H. Togawa, “Simplified measurement method of electro-magnetic wave shielding and absorbing characteristics in mm-waves,” IEICETrans. Commun., vol. J81-B-II, no. 6, pp. 651–656, June 1998.

[2] R. E. Hiatt, E. F. Knott, and T. B. A. Senior, “A study of VHF absorbers andanechoic rooms,” Dept. Elect. Eng., The Univ. of Michigan, 5391-1-F, Feb.1963.

[3] R. Ueno and N. Ogasawara, “Ferrites- or iron-oxides-impregnated plasticsserving as radio-wave scattering suppressors,” Proc. of ICF, pp. 890–893, 1980.

[4] P. Blacksmith, Jr., R. E. Hiatt, and R. B. Mack, “Introduction to radar cross-section measurements,” Proc. IEEE, pp. 901–919, Aug. 1965. DOI:10.1109/PROC.1965.4069

[5] S. Yamamoto, K. Hasegawa, T. Iwai, and K. Hatakeyama, “Complex reflectioncoefficient measurement setup at millimeter waves,” IEICE Trans. Commun.,vol. J90-B, no. 11, pp. 1216–1219, Nov. 2007.

[6] J. D. Kraus and R. J. Marhefka, Antennas for All Applications, 3rd ed.,pp. 36–37, McGraw-Hill, 2003.

[7] T. Kotera, S. Yamamoto, K. Hatakeyama, and T. Iwai, “Oblique incidenceevaluation of EM absorbers in millimeter waves from 40GHz to 75GHz,”IEICE Tech. Rep., vol. 112, no. 372, EMCJ2012-116, pp. 81–86, Jan. 2013.

© IEICE 2015DOI: 10.1587/comex.4.310Received July 23, 2015Accepted September 3, 2015Published October 5, 2015

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IEICE Communications Express, Vol.4, No.10, 310–316

http://dx.doi.org/10.1109/PROC.1965.4069





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 character-

istic, 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 (50–75GHz). 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 0.90m, 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 50Ω) is attached to© IEICE 2015DOI: 10.1587/comex.4.310Received July 23, 2015Accepted September 3, 2015Published October 5, 2015

311


the other port of the magic tee. The receiver (Spectrum Analyzer, Agilent Tech-

nologies 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 Pr at the receiving antenna can be expressed as following

Friis transmission formula [6]

Pr ¼ Pt þ Gt þ Gr þ 20 log�

4� � 2L� �

: ð1Þ

Here, Pt is the transmitted power, Gt and Gr 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 Pr is −40 dBm, which is the maximum received power in this

system, and λ is 4:8 � 10�3m (in the case of f ¼ 62:5GHz, the center frequency of

the measuring frequency range). In addition, Pt, Gt and Gr are 0 dBm, 23.6 dBi and

(a) Schematic configuration. (b) Millimeter circuit block diagram.

(c) Photo of the measurement setup.

Fig. 1. Oblique incidence measurement setup (f: 50–75GHz).


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23.6 dBi, respectively. By substituting above values in Eq. (1), the distance L of the

setup was estimated to be 1.5m. 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, Er is the reflected wave from the sample stage. Ed is the direct wave from

the transmitting antenna to the receiving antenna, and Eat is the propagating wave

through the attenuator. Ef is the sum of Ed and Eat. Er, Ed, Eat, Ef are given as

Eq. (2). ’r, ’d, ’at, ’f are the phases of Er, Ed, Eat, Ef, respectively.

Ef and Er 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, Ef can be

assumed the constant values. Then only phase ’r changes. The received wave Em at

the receiver is the sum of Ef and Er. The locus of the vector Em due to moving of

the sample draws the circle of radius Ers, establishing the center OB centering on the

vector Ef. Similarly, due to moving of the metal plate, the circle of radius Erm is

described. Thus, the reflection coefficient Γ is determined by the ratio of these two

radii jErmj, jErsj as Eq. (3). ’rm and ’rs are the phases of metal plate and sample.

Er ¼ jErjej’r ; Ed ¼ jEdjej’d ; Eat ¼ jEatjej’atEf ¼ Ed þ Eat ¼ jEfjej’f

)ð2Þ

� ¼ jErsjjErmj e

jð��ð’rm�’rsÞÞ ð3Þ

In this study, we defined jEfj so as to satisfy the condition jErmj < jEfj < 10jErmj.

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 absorb-

ers. 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


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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 10mm) 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 15mm).

First, the reflection characteristics of sample (A) when the incident waves hit

the sample at normal direction are measured. Measured jEmj values are shown in

Fig. 2(b) for the metal plate, sample, respectively. The measuring frequency is

62GHz. Both metal plate and sample jEmj values show the standing wave pattern.

jErmj, jErsj shown in Fig. 2(b) are approximately 1.68, 0.09mV, respectively. By

substituting these values in Eq. (3), j�j in 62GHz 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-ðAÞj of sample (A) from

50GHz to 75GHz. j�n-ðAÞj is less than −20 dB over the measuring frequency range.

Fig. 3(b) shows the TM and TE reflection coefficients j�TM-ðAÞj, j�TE-ðAÞj at variousangles in oblique incidence in the case of 62.5GHz. 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-ðAÞj 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 62.5GHz. The reflection coefficient of TM polarization is less than −20 dBbelow 70 degree except for 15 degree, and increase as incident angle increases

(a) Vector diagram of Em, Er, Ef. (b) |Erm|, |Ers| values of sample (A) ( f = 62 GHz).

Fig. 2. Vector diagram and measured results.


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above 60 degree. Furthermore, j�TM-ðBÞj almost agrees well to j�TE-ðBÞj below 40

degree. This indicates that the reflection characteristics show the isotropy. In

contrast, j�TM-ðBÞj is different from j�TE-ðBÞj above 40 degree.

(a) Reflection coefficient |Γn-(A)| under normal incidence of sample (A).

(b) Reflection coefficients |ΓTE-(A)|, |ΓTM-(A)| of sample (A) (in the case of 62.5 GHz).

(c) Reflection coefficients |ΓTM-(B)|, |ΓTE-(B)| of sample (B) (in the case of 62.5 GHz).

Fig. 3. Measured reflection coefficients.


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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 meas-

urements from 50GHz to 75GHz 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 22760292 (Grant-in-

Aid for Young Scientists (B)).


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