24-GHz Microstrip Grid Array Antenna for Automotive Radars Application

#Zihao Chen 1, Zhang Yue Ping1 1 School of Electrical and Electronic Engineering

Nanyang Technological University 50 Nanyang Avenue, 639798, Singapore

Email: zchen16@e.ntu.edu.sg; eypzhang@ntu.edu.sg

Abstract—A single-ended microstrip grid array antenna has been designed and measured for 24 GHz automotive radars application. It has been designed on Rogers 5880 substrate with dimensions of 60 60 0.787 mm3. The antenna exhibits 2.45 GHz impedance bandwidth, 19.26 dBi peak realized gain and broadside pencil-beam radiation pattern with low side lobe and weak cross-polarization.

Index Terms—Microstrip grid array antenna, 24 GHz, automotive radars.

I. INTRODUCTION The 24-GHz ISM band has been allocated for automotive

radars applications such as automotive speedometers, near obstacle detection, collision warning and vehicle guidance. All of these systems require high gain, compact and narrow beamwidth antennas and antenna arrays. For example, a slot array antenna based on substrate integrated waveguide in a Rogers 5880 substrate achieved the maximal gain of 22.5 dBi with 15 radiating elements at 24.15 GHz [1] and a patch array antenna fed by a T-junction network on Arlon 25 N laminate attained the maximal gain of 15.2 dBi with 8×4 radiating elements at 24 GHz [2].

However, the designs mentioned above are suffered from complex feeding network, sophisticated process techniques and narrow impedance bandwidth. To avoid these disadvantages, microstrip grid array antenna, which has all the usual benefits of conventional patch type radiators plus wide bandwidth, high gain and weak cross polarization [3]-[9], is adopted in our work. In this paper, we investigate how to design a high-gain microstrip grid array antenna with pencil beam at 24-GHz band for automotive radar application. The radiating elements are arranged specially into a square shape to achieve pencil beam and the parameters of the antenna are chosen theoretically to obtain high gain. The design procedure of the 24-GHz microstrip grid array antenna is described in Section II. To confirm the feasibility of the proposed design procedure, a 41-element microstrip grid array antenna with a broadside beam is fabricated. The measured antenna performance is reported in Section III. According to all the measured results, the developed microstrip grid array antenna can be considered as a good candidate for automotive radars systems.

II. 24 GHZ MICROSTRIP GRID ARRAY ANTENNA The configuration of 24-GHz microstrip grid array antenna

is exhibited in Fig. 1. It consists of one dielectric and two metallic layers. Rogers 5880 with dielectric constant of 2.2 and tangent loss of 0.0009 with a thickness of 0.787 mm is chosen as the substrate. The grid array radiator, which is on the top surface of the substrate, is composed of rectangular meshes whose long sides are l long and wl wide and whose short sides are w long and ws wide. It is fed by a coaxial cable with its outer conductor connected to the ground plane and its inner conductor connected to the junction of the mesh near the geometrical centre of the radiator.

The application of 24-GHz automotive radars to monitor blind spots requires the maximal directivity of the antenna to be larger than 20 dBi, the impedance and radiation bandwidth to be larger than 0.25 GHz and the single-shaped beam to be smaller than 15� [10]. Considering the specific maximal directivity and the required pencil-beam patterns, one can find that the required number of radiating elements to be 41, which leads to an estimation of the length a and width b of the substrate as 60×60 mm2. The resonance of the grid array radiator requires the length of long sides l = g 9.8mm, where g is guided wavelength at 24.15 GHz. The width of long sides wl controls the loss of transmission line and the radiation of the cross-polar components. According to [4], in order to obtain low transmission line loss and cross polarization, the same width wl for all short sides for wl/h < 1 with the characteristic impedance smaller than 200 is chosen. Considering realizability from fabrication and reliability for use in automobiles, we prefer wl = 0.4mm. Similarly, the resonance of the grid array radiator also requires the length w to be half of l, that is w = l/2 = 4.9 mm. It is more involved to determine the widths ws because ws sets the array radiation coefficients with the first order approximation of the coefficient being proportional to width [4]. For simplicity in fabrication, the short sides keep the same width with an optimized value ws = g/4 2.4mm. Although this antenna configuration was presented in [9], the far field performance of the antenna was not measured. Therefore, it was not sufficient to show the validity of the design. The measured return loss, antenna gain and radiation patterns will be included in the next section, which fully verified the design with good measured performance.

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Fig. 1. Configuration of 24-GHz microstrip grid array antenna.

Fig. 2. Photograph of the fabricated 24-GHz microstrip grid array antenna.

III. SIMULATED AND MEASURED RESULTS The designed 24-GHz microstrip grid array antenna is

fabricated for experiments. Fig. 2 shows the photo of the fabricated antenna with dimensions of 60×60×0.787 mm3. Four holes with diameter of 4 mm at the four corners of the substrate are reserved for stabilizing the radiator in order to flatten the whole antenna structure. The S-parameter measurements were carried out with a Rohde-Schwarz ZVA 67 four-port vector network analyzer up to 67 GHz.

Fig. 3 shows simulated and measured |S11| as a function of frequency. The simulated input impedance bandwidth is 1.99 GHz from 22.46 GHz to 24.45 GHz (or 8.24% at 24.15 GHz). The measured impedance bandwidth is 2.45 GHz from 22.23 GHz to 24.68 GHz (or 10.14% at 24.15 GHz). Both of the simulated and measured |S11| cover the 24-GHz band from 24 GHz to 24.15 GHz and they are in good agreement. Fig. 4 shows the simulated and measured peak realized gain as a function of frequency. The simulated 3-dB gain bandwdith is 1.54 GHz from 23.45 GHz to 24.99 GHz with a maximal gain of 20.55 dBi at 24.15 GHz. The measured 3dB bandwidth is 1.65 GHz from 23.54 GHz to 25.19 GHz with a maximal gain of 19.26 dBi at 24.32 GHz. The 3-dB gain bandwidths also cover the 24-GHz band. The radiation patterns at 24.15 GHz in E-plane and H-plane are shown in Fig. 5 and Fig. 6,

Fig. 3. Simulated and measured |S11|.

Fig. 4. Simulated and measured peak realized gain as a function of frequency.

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Fig. 5. Radiation pattern at 24.15 GHz in E-plane.

respectively. The simulated and measured 3-dB beamwidths are 16� in both E-plane and H-plane. They clearly demonstrate that the developed microstrip grid array antenna radiates pencil-beam patterns.

126 2015 IEEE 5th Asia-Pacific Conference on Synthetic Aperture Radar(APSAR)

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Fig. 6. Radiation pattern at 24.15 GHz in H-plane.

IV. CONCLUSION A 24-GHz microstrip grid array antenna on Rogers 5880

substrate has been designed and measured for automotive radars application. The results of the simulation and measurement show a good agreement. The measured results demonstrate 2.45 GHz impedance bandwidth, 19.26 dBi peak realized gain and broadside pencil-beam radiation pattern with low side lobe and weak cross-polarization.

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