A Novel Planar Circularly-Polarized Antenna Using Stepped-Width Cross-Dipole Yu Luo and Qing-Xin Chu
Lei Zhu
School of Electronic and Information Engineering South China University of Technology Guangzhou, Guangdong, China
[email protected] and
[email protected]
Department of Electrical and Computer Engineering Faculty of Science and Technology, University of Macau Macau SAR, China
[email protected]
Abstract—A novel planar circularly-polarized antenna using the stepped-width cross-dipole is presented in this paper. Firstly, the input impedance of a single stepped-width dipole is studied under different length and width ratios of two distinctive dipole sections. Then, two stepped-width dipoles with dissimilar dimensions are individually designed and orthogonally oriented in a crossed format, thus producing the circularly-polarized radiation. The entire antenna with a couple of dipoles is designed with a single feeding point. Our research work shows that the 10-dB bandwidth is about 25% in impedance and the axial ratio is less than 1 dB at the center frequency.
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INTRODUCTION
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In recent decades, circularly polarized (CP) antennas have been used widely for many wireless applications, such as global positioning systems (GPS), compass navigation satellite system (CNSS), wireless local area network, and radio frequency identification (RFID) system [1]. Cross dipole antenna (CDA) is a common terminal antenna for satellite communication for its good CP radiation. As usual, a CP CDA is realized by using the two orthogonally-oriented dipoles with different feeding elements in order to achieve equal amplitudes and 90° phase difference between them [2]. In this aspect, an additional phase shifting network needs to be designed in the backside plane of this antenna. Alternatively, a CDA can also radiate the CP wave by using a self-phase shifting structure with more simple and reliable structure [3]-[4], where the two dipoles have to be dissimilar in their lengths, i.e., they must be designed to resonate at two different frequencies. In this paper, a novel CP antenna composed of two steppedwidth dipoles is presented. These two stepped-width dipoles are designed with the same lengths but different stepped-strip widths. As such, the symmetrical property in two orthogonal planes of radiation pattern can be satisfactorily maintained. Our results exhibit that the CP axial ratio (AR) of this proposed antenna is less than 1 dB at central frequency in addition to its wide impedance bandwidth of about 25%.
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W_1=1mm,W_2=1mm W_1=3mm,W_2=1mm W_1=5mm,W_2=1mm W_1=1mm,W_2=3mm W_1=1mm,W_2=5mm
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Fig.2. Reflection Coefficient of the proposed dipole with stepped widths W_1=1mm,W_2=1mm W_1=3mm,W_2=1mm W_1=5mm,W_2=1mm W_1=1mm,W_2=3mm W_1=1mm,W_2=5mm
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Fig.1. Structure of a stepped-width dipole
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INPUT IMPEDANCE OF STEP-WIDTH DIPOLES
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Fig.1 depicts a stepped-width dipole radiator to be studied herein. The whole dipole structure is classified into the three distinctive strip sections, i.e., one central section and two identical sections in two sides. This symmetrical dipole is centrally fed as shown in Fig. 1. Using a fullwave simulator,
978-1-4799-7815-1/15/$31.00 ©2015 IEEE
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Fig.3. Phase of input impedance of the antenna in Fig. 2
AP-S 2015
TABLE I. PHASE OF INPUT IMPEDANCE AT CENTER FREQUENCY Size
W_1=1mm W_2=1mm
W_1=1mm W_2=3mm
W_1=1mm W_2=5mm
W_1=3mm W_2=1mm
W_1=5mm W_2=1mm
Phase (deg)
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41.8
50.8
-43.8
-55.5
its reflection coefficient can be numerically calculated, and Fig. 2 illustrates its frequency responses with respect to varied stepped-widths, i.e., different choices of W_1 and W_2. When W_1=W_2=1mm, this stepped-width dipole becomes its uniform one that is a classical dipole radiator. With reference to its resonant frequency at about 1.56GH, we can move it up or down by differently choosing W_1 and W_2. Under the fixed value of W_2=1mm, this resonant frequency gradually goes up as W_1 increases. Under the fixed W_1=1mm, it increasingly falls off as W_2 increases. With the above exhibited results, we can understand that the resonant frequency of this proposed dipole can be determined and adjusted by using the width ratio of W_1 and W_2 under the constant dipole length. Fig. 3 shows the phase of input impedance of stepped-width dipoles with different strip widths. Table I indicates different values of this input-impedance phase at center frequency. At center frequency, the phase angle is close to zero if the dipole has a uniform configuration, i.e., W_1=W_2=1mm. As W_1 is enlarged under W_2=1mm, this impedance phase becomes negative. Under the constant value of W_1=1mm, this phase is swapped to a positive value as W_2 is increased. As a result, the two crossed dipoles with positive and negative phases of input impedance can be used to general the two orthogonally linearly-polarized radiation waves with 900 difference in phase.
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Fig.5. Reflection Coefficient of the proposed CP antenna. 9 8 7
CP ANTENNA CONSIST WITH STEPPED-WIDTH DIPOLES
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Based on the data in Table I, we can find that the impedance phase angle of this stepped-width dipole is 41.8º in the case of W_1=3 mm and W_2=1 mm, whereas it becomes -43.8º in the case of W_1=1mm and W_2=3mm. By combining them as a pair of cross dipoles as shown in Fig. 4, the CP radiation can be expectedly produced. The reflection coefficient and axial ratio of this antenna are simulated, and their frequency responses are plotted in Fig. 5 and 6. Our results show that the reflection coefficient is less than -17 dB and axial ratio is less than 1 dB at the center frequency of 1.5GHz. Moreover, the impedance bandwidth achieves about 25% under the 10-dB definition. IV.
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CONCLUSION
In this paper, a CP antenna composed of two stepped-width dipoles is presented. The two stepped-width dipoles with the identical length are further studied to show good CP radiation. This CP antenna may be promising for satellite communication. ACKNOWLEDGEMENT This work was supported by the Research Grants (SRG2013-00043-FST and MYRG2014-00079-FST) in the University of Macau, and in part by the National Natural Science Foundation of China (61171029 and 61471172), Funds
Axial Ratio (dB)
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Fig.4. Detailed layout of the proposed CP antenna
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Fig.6. Axial ratio of the proposed CP antenna
for the Central Universities (2013ZP0018), and the State Key Laboratory of Millimeter Wave (K201406). REFERENCES [1] [2]
[3] [4]
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S. Gao, Q. Luo, and F. Zhu, Circularly Polarized Antennas, Wiley-IEEE Press, Nov. 2013. J.W. Baik, K.-J. Lee, W.-S. Yoon, et al., “Circularly polarised printed crossed dipole antennas with broadband axial ratio,” Electron. Lett., vol.44, no.13, pp.785-786, Jun. 2008. M. F. Bolster, “A new type of circular polarizer using crossed dipoles,” IRE Trans. Microw. Theory Tech., vol. 9, no. 5, pp. 385–388, Sep. 1961. Y. Wang, D. Lv, and L. Chang. "Research of broad-beam circularlypolarized cross dipole antenna with declined tails." General Assembly and Scientific Symposium (URSI GASS), 2014 XXXIth URSI. IEEE, 2014.
