Measured Performance of Millimeter-Wave Micro Strip Comb-Line Antenna

8/6/2019 Measured Performance of Millimeter-Wave Micro Strip Comb-Line Antenna

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Measured Performance of Millimeter-Wave

Microstrip Comb-Line Antenna

using Reflection-Canceling Slit StructureYuki HAYASHI, Yuichi KASHINO, Kunio SAKAKIBARA,

Nobuyoshi KIKUMA and Hiroshi HIRAYAMANagoya Institute of Technology

Gokiso-cho, Showa-ku, Nagoya, 466-8555, Japan

E-mail [email protected]

1. Introduction

Millimeter-wave antennas have been developed for various applications such as broadband high-

speed wireless communication systems and automotive radar systems. Microstrip antenna is more ad-

vantageous than other millimeter-wave antennas at the viewpoints of low-profile and low-cost. A comb-

line feeding structure is eff ective for relatively low loss compared with other ordinary microstrip patch

array antennas since feeding loss is smaller [1]. When element spacing is just one guided wavelength

for broadside beam of traveling-wave excitation, reflections from all the radiating elements are in phase.

Therefore, return loss increases significantly. Furthermore, radiation due to the reflection wave from the

element degrades the design accuracy for required radiation pattern. Reflection characteristic is improved

in the conventional design by beam-tilting of several degrees, where reflections from all the elements are

canceled out of phase [2]. Admitted beam-directions are limited by some specific phase diff erences in

which reflections are canceled. We propose the way to improve the reflection characteristic of the antennawith arbitrary beam directions including just broadside direction. In order to suppress the reflection, we

propose reflection-canceling slit structure installed on the feeding line around each radiating element. A

27-element linear array antenna with broadside beam is developed at 76.5 GHz. Measured performance

is evaluated in the millimeter-wave band.

2. Configuration

A microstrip comb-line antenna is composed of several rectangular radiating elements directly at-

tached to a straight feeding line printed on a dielectric substrate (Teflon, thickness t = 0.127 mm, relative

dielectric constant εr = 2.2 and loss tangent tan δ = 0.001) with a backed ground plane as shown in

Fig. 1. The radiating elements are inclined 45 degrees from the feeding microstrip line for polarization

requirement of automotive radar systems. The radiating elements are arranged on the both sides of the

feeding line, which forms interleave arrangement. Element spacing d n is approximately a half guided

wavelength so that all the elements are excited in phase. Reflection-canceling slit is installed on the

feeding line and is closely located at each radiating element. An ordinary patch antenna is connected for

a matching element at the termination of the feeding line in order to radiate all the residual power in the

microstrip line.

3. Design

Figure 2 shows the analytical structure of a radiating element with a slit. The distance between the

slit and the radiating element is Ds. Length and width of the slit are defined as Ls and W s, respectively.

These three parameters are optimized to suppress the reflection lower than−

30 dB by using the electro-magnetic simulator. Reflection characteristic is shown in Fig. 3 for the element with slit whose coupling

power is 7.4 %. Reflection coefficient of the element with slit is −42.3 dB at 76.5 GHz. Therefore, it is

Proceedings of ISAP2007, Niigata, Japan

ISBN: 978-4-88552-223-9 C3055©IEICE 1118

POS2-5



confirmed that the reflection from radiating element is canceled sufficiently with the reflection from the

slit.

Microstrip comb-line antenna is designed to operate in traveling wave excitation. The input power

is gradually radiated from all the radiating elements during transmission from the input port toward the

termination. Array design is implemented for Taylor distribution with sidelobe lower than −20 dB. Re-

quired coupling power is designed to be small near the input port and to increase toward the termination.The required variety of coupling is 2.0 ∼ 49.8 %. Radiation from the elements is controlled by variation

of the width W n of the elements where Ln is the resonant length. A 27-element linear array antenna with

broadside beam is designed for experiments.

4. Experiments

Figures 4 (a) and (b) show photographs of the fabricated antenna. Width of radiating element is

small near the input port and it is large near the termination. Slit is cut on the feeding line at the opposite

side of each radiating element. Figure 5 shows measured reflection characteristics of fabricated anten-

nas. Reflection-canceling slit is not used in the conventional antenna. Instead, return loss is reduced by

ordinary beam-tilting technique. Reflection coefficient of the proposed antenna is −17.2 dB at the design

frequency 76.5 GHz and is almost the same level with the conventional antenna although the proposedantenna is a broadside array. Thus, the eff ect of reflection-canceling slit is confirmed. Figure 6 shows

measured radiation pattern and array factor of design in yz-plane at 76.5 GHz. The measured radiation

pattern almost agrees well with the array factor. However, some errors are observed in the sidelobe level

and the beam direction. To clarify this cause, the aperture amplitude and phase distribution are shown

in Figs. 7 (a) and (b), respectively. Growing sidelobe is due to error of amplitude distribution. Figure

7 (b) shows the measured phase distribution and the calculated phase distribution for beam direction of

−2 degrees. These phase distributions almost agree with each other. Therefore, error of beam direction

is due to phase distribution. We have already confirmed by using electromagnetic simulator that the fab-

rication error of the etching process aff ects the phase perturbation in transmission through the radiating

element. Figure 8 shows frequency characteristics of gain and antenna efficiency. Since aperture lengths

of proposed and conventional antennas are slightly diff erent depending on the design, directivities arediff erent. The maximum gain is obtained at the design frequency for both antennas. Efficiency of pro-

posed antenna is almost the same level with that of conventional antenna. Consequently, it is confirmed

that the loss due to the slit structure is extremely small.

5. Conclusion

To suppress the reflection from each radiating element, we propose the reflection-canceling slit

structure. A 27-element linear array antenna with broadside beam is designed and fabricated. Reflection

coefficient of proposed antenna is almost the same level with the conventional antenna with beam-tilting

technique although broadside array is designed for the proposed antenna. The measured radiation pattern

almost agrees well with array factor. However, beam direction of measured antenna is −2 degrees. It is

confirmed that the fabrication error of the etching process aff ects the beam direction. We estimate gain

and antenna efficiency. It is confirmed that the loss due to slit structure is extremely small. As a result,

microstrip comb-line antenna with arbitrary beam direction can be designed without increasing return

loss by using reflection-canceling slit structure.

References

[1] H.Iizuka, T.Watanabe, K.Sato and K.Nishikawa, ”Millimeter-wave Microstrip Array Antenna For

Automotive Radar,” IEICE Trans. Commun., Vol.E86-B, No.9 pp.2728-2738, Sept. 2003.

[2] Y.Owa, K.Sakakibara, Y.Tanaka, N.Kikuma and H,Hirayama, ”Low Sidelobe Millimeter-wave

Microstrip Array Antenna Radiation-controlled by Modification of Feeding-line Width,” Proc.ISAP2005, pp.1153-1156, Aug. 2005.

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Measured Performance of Millimeter-Wave Micro Strip Comb-Line Antenna

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