Microstrip Series Fed Antenna Array for Millimeter Wave Automotive Radar Applications YI Chong1,2 and DOU Wenbin2 1. Beijing Institute of Remote Sensing Equipment, Beijing 100854, P.R. China 2. State Key Laboratory of Millimeter Waves, Southeast University, Nanjing 210096, P.R. China Email:
[email protected] [email protected] Abstract-A 2D tapered microstrip antenna array is analyzed and designed in the paper by adopting the method of Modified Transmission Line Model. The pattern of the E-plane is determined by 20 linear series fed rectangular microstrip patches with tapered width, and 16 linear arrays compose the 2D array fed by four-stage T-junction power divider network. The excitation coefficients in both E and H plane follow the Taylor distribution. The antenna array achieves a pencil beam of 29dB gain at W band, and its half power beamwidths(HPBW) for E plane and H plane are both about 5e. The Side Lobe Level (SLL) of the pattern at the center frequency is lower than -17dB and 19dB at E and H plane respectively.
I.
INTRODUCTION
The collision avoidance radar is one of the most important components of the advanced vehicle control and safety system in the future Intelligent Transport System (ITS). The radar system operated at millimeter wave band has better immunity to bad weather condition than infrared wave system, and higher resolution ratio than microwave system. The operation frequency of automotive radar system is typically in 76GHz-77GHz band for frequency-modulated continuous wave (FMCW) or pulse-Doppler operation. The higher bandwidth allows for W band ultra-wideband (UWB) systems with improved distance resolution, which were previously the main target of 24GHz automotive radar development [1]. For automotive applications, microstrip antenna array is the most popular choice in many antenna types because of its lower profile and price. And the microstrip antenna array is also flexible to design and fabricate. Meanwhile the convenience to integrate with millimeter wave and microwave circuits is another important reason for its wide applications. In millimeter wave band, the loss of microstrip structure becomes considerable with frequency increasing [2]. In order to decrease the loss at high frequency, series fed method is often chosen to minimize and simplify the feed network for antenna array. In this paper, the series fed linear array is analyzed by using the Modified Transmission Line Model method, and an equivalent resonant circuit model of the array is presented. The linear array of 20 elements with -20dB side lobe is designed at the desired center frequency according to the resonant principle [3]. This presents in Section II in detail.
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Then in Section III, we designed the 2D array of low side lobe level by the combination of 16 linear sub-arrays with certain power distribution. The excitation distribution is realized by asymmetrical T-junction feed network. A pencil beam, with 29dB gain, side lobe -17dB and -19dB for E and H plane respectively, is achieved. It proves that the analysis and design method is available. And the reason of the degenerate side lobe and gain is analyzed in this section. Finally, the conclusions are drawn in Section IV. And the further working is under way. II. ANALYSIS AND SIMULATION OF THE LINEAR ARRAY In this section, a Modified Transmission Line Model is adopted to analyze and design the series fed linear array. The equivalent circuit model of the discontinuity and the optimized width-tapered linear array is presented as follows. A. The Modified Transmission Line Model The linear array can be considered to be composed of two components, the feedlines and the patches, shown in Fig.1. The slender parts are feedlines, and the wider parts are rectangular patches. The electrical length of the feedline and the patch are both NJg/2 (half wavelength in transmission line) at the center frequency, so that the array resonates at the design center frequency with radiation patch elements in phase or the slots in reverse phase. Many kinds of analytical methods have been proposed [4][5][6][7]. They consider the radiator element as two slots at first, but this method is inaccuracy to analyze longer array with lots of elements. Then a novel method is proposed, in which the feedline and the patch are considered as a unit. Actually, the wave in the array in Fig.1 forms a standing wave, which seems as electro-magnetic wave distributing in an open-end transmission line. To analyze the radiation characteristic visually and accurately, in this paper we choose the transmission line model proposed in [3] and [4].
Figure 1. Microstrip series fed linear array antenna schematic.
However, we find that the accuracy of the original model drops with both the number of the patches and frequency increasing, and the radiation pattern also distorts severely.
Furthermore, the main lobe of the pattern would be off the main direction or divide into two lobes with the peak turning to a valley. From the analysis and simulation of the antenna designed according to the original model, the reason of the distortion is made clear that the phase error accumulation disturbs the phase and amplitude distribution along the resonant array. To enhance the accuracy of the transmission line model, we must consider the width discontinuity between the feedline and the patch to modify and supplement the original model. In the modified model, the reactances X1 and X2 represent the effects caused by the discontinuities, and the patch is represented by a segment of transmission line of characteristic impedance Zpatch and electrical length NJpatch, shown in Fig.2.
Figure 2. The equivalent circuit of the patch and the discontinuity with the feedline derived by using the modified transmission line model.
To achieve the broadside radiation pattern, the elements must be in phase, that is to say, the phase difference between two elements must be 360°. The array would be resonant at the center frequency, so the equivalent circuit of the array can be simplified to n radiation conductances in parallel, or n radiation resistances in series (n is the number of patches.). The radiation power PR of a rectangular microstrip element fed at an edge can be determined by the equation (1) as follows,
1 Gi
V02 (2 PRi )
Figure 3. The model of 20 elements series-fed linear taper array in HFSS
For series fed patch array, there are mainly two methods to determine the excitation voltage distribution for the radiation pattern synthesis, tapering the width of both the feedline and patch, and tapering the width of the patch only [9]. For both the feedline and patch width taper, it is difficult to fabricate in W band, because the width value is so small that the width taper in feedline would cause design inaccuracy. As the method indicated in the Section II part A, the patch width taper is accurate enough for pattern synthesis with constant feedline width. To obtain the best side lobe level for a certain beamwidth, Taylor distribution is adopted in this design. For the optimized voltage excitation coefficients, we can deduce the suitable widths and lengths of patches. The optimized simulation result at 76GHz in HFSS is shown in Fig.4. The minimum side lobe level is -20dB, and the gain is 17dB, while the beamwidth of the main lobe is 5°, with a little wider than theoretical result far away the main direction.
(1)
where Ri and Gi are the radiation resistance and conductance of the ith patch element, respectively. And V0 is the excitation voltage of patches in equivalent circuit. According to the formula (1), we can learn that the radiation power is proportional to the conductance when the excitation voltage is constant. To obtain a certain excitation distribution, a series of conductances proportional to excitation coefficients are sufficient. For microstrip patch antenna with the relative dielectric constant İr of the substrate material lower than 5, the radiation resistance and conductance, which is mainly determined by the width of the patch, can be represented by a set of equations in [2] and [8]. When the parameters of the substrate are determined, we can design a desired excitation coefficients distribution antenna array with high accuracy simply by setting and adjusting the widths and lengths of the patch elements and feedlines according to the resonant principle.
HFSS Theoretical
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B. 20 Elements Series-Fed Linear Array with Taylor Distribution The array is printed on the substrate Rogers RT Duroid5880 (0.127mm thickness and İr=2.2), and the thickness of the copper cladding layer is 0.008mm. The model of the linear array is established in 3D EM simulation software High Frequency Structure Simulator (HFSS), which is based on Finite Element Method (FEM), shown in Fig.3.
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Figure 4. The normalized radiation pattern of the series fed linear array at 76GHz
The 2D planar array can be composed of the series fed linear array described in the previous section. The linear array is considered as an element of the planar array arranged along the horizontal axis, the structure of the planar array is illustrated in Fig.5. The aperture distribution along the horizontal axis is also Taylor distribution with -20dB SLL.
radiation effect of the feed network and excitation coefficients error. Eplane-HFSS Hpane-HFSS Hpane-Theoretical
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III. DESIGN AND SIMULATION OF 2D ARRAY
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Figure 7. The normalized radiation pattern of the 2D planar array at 76GHz.
IV. CONCLUSION
Figure 5. The arrangement of the 2D planar array in HFSS
According to the distribution, the feed network is realized by selecting asymmetrical T junctions of 4 stages as shown in Fig.5. The simulation result of the feed network is shown in Fig.6, in which the port number increases from top to bottom as shown in Fig.5, and its performance meets the ideal distribution very well.
A microstrip antenna array for automotive applications is designed in this paper. The linear series array with Taylor distribution is adopted as the element of 2D array to reduce the total length of feed line, that is to say, to reduce the loss caused by feed line. And a four-stage T-junction power divider with Taylor distribution is used as feed network. The array realizes a pencil beam with 29dB gain, -17dB and -19dB side lobe level in E and H plane respectively. The phase shifters for scanning and other further work are under researching. ACKNOWLEDGMENT This work is supported by NSFC under grant 60921063. REFERENCES
Simulation Ideal
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Figure 6. The ideal and simulation distribution of the feed network
And the simulation pattern of the planar array is presented in Fig.7, antenna achieves a radiation pattern, which has a 29dB gain, 5° beamwidth in both E and H plane, and -17dB and 19dB SLL in E and H plane respectively. The side lobe level is higher than the theoretical analysis, mainly because of the
[6] [7] [8] [9]
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