A Broadband Circularly Polarized Cross-dipole ...

2015 Loughborough Antennas & Propagation Conference (LAPC)

A Broadband Circularly Polarized Cross-dipole Antenna for GNSS Applications Chaoyun Song, Yi Huang*, Qian Xu, and Umniyyah Ulfa Hussine Department of Electrical Engineering and Electronics University of Liverpool Liverpool, L69 3GJ, United Kingdom [email protected], Yi. [email protected] Abstract— A broadband circularly polarized (CP) cross-dipole antenna is proposed for GNSS applications. The antenna has a very simple structure and a small size but has a very broad impedance bandwidth from 1.15 to 1.75 GHz (a fractional bandwidth of 41.4%) for S11 < -10 dB. The CP bandwidth is from 1.37 to 1.5 GHz for axial ratio < 3 dB. A computer simulation is conducted to optimize its dimensions and predict its performance. The optimized antenna is fabricated and measured. The simulated and measured results are in good agreement and have demonstrated that the proposed antenna is indeed a very good candidate for GNSS applications. Index Terms— Cross dipole, broadband antenna, CP antenna, GNSS.

I.

INTRODUCTION

CP antennas are now used in a range of wireless systems such as the Global Position System (GPS), satellite communication systems, and some radio frequency identification (RFID) systems. A growing number of literatures on CP antennas have emerged owing to their ability to deal with multipath or Faraday rotation effects and to improve the flexibility in antenna orientation constrains. In addition to the well-developed GPS, there are some other Global Navigation Satellite Systems (GNSS) which are not yet fully developed or implemented but will become major competitors of the GPS soon, such as the European Galileo, Russian GLONASS, and Chinese Beiduo (Compass). They all use CP waves in order to combat the Faraday rotation effect of the ionosphere. Thus, there is an increasing demand for CP antennas to cover not only the GPS frequency bands, but also Galileo or GLONASS or Beidou frequency bands. Traditionally, to generate a CP radiation, two orthogonal currents with the same amplitude and a 90-degree phase difference should be generated on the antenna. This can be achieved by using two orthogonal feeding ports or a feeding network in a conventional antenna system [1], [2]. In addition, a single-feed system can also generate two orthogonal currents with the same amplitude but a 90-degree phase difference in such as [3] and [4]. Some antennas are particularly suitable for generating circularly polarized waves such as the circular patch antennas, helical antennas and spiral antennas [5]–[7]. Recently, there have been some designs in such as [8] and [9]

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using the broadband cross-dipole antenna and a 90-degree phase delay line between the dipoles to produce a broadband CP antenna. Both the impedance bandwidth and CP bandwidth of these cross dipole antennas are quite broad. In this paper, we propose a broadband antenna to cove the GPS L1–L5 bands as well as the Galileo E5a, E5b, E6 and E1 bands (1164 MHz – 1591 MHz). The structure of the antenna is very simple, which is a planar micro-strip line cross-dipole antenna, and the size is relatively small. A printed vacantquarter ring is used as a 90-degree phase delay line between the dipole pair to produce the CP radiation. The detailed dimensions are presented, and good results are obtained. The CST Microwave Studio is employed for the numerical simulation. The antenna is also fabricated and measured, good agreements between the simulation and measurement are obtained. II.

GEOMETRY OF THE ANTENNA

Fig. 1. Geometry of the proposed antenna.

The geometry of the proposed antenna is shown in Fig. 1. The critical part is the feeding structure which is relatively new and the dimensions should be optimised. The crossdipole antenna is produced on both sides of the PCB board. The inner conductor of the coaxial cable is fed to the top metal of the antenna while the outer conductor is connected with the bottom metal. The pair of dipole is linked by a printed vacant-quarter ring structure in order to generate a 90degree phase delay and produce the right hand circular


polarization (RHCP) radiation field at the front side of the antenna and the left hand circular polarization (LHCP) radiation field at the back side. The reasons of choosing the cross dipole in this design are that the impedance bandwidth of the cross dipole is much boarder than a conventional microstrip path antenna and it is much easier to generate the CP radiation and obtain a broader CP bandwidth by using the special feeding mechanism as mentioned above. The substrate has relative permittivity of 10.2 and a thickness of 1.28 mm. The antenna is modelled and optimized by using the CST software. III.

ANTENNA PERFORMANCE

strip widths. It covers from 1.37 to 1.5 GHz which involves the GPS L3 and L4 bands. For the different values of the dipole length (parameter L), the simulated S11 and axial ratio of the proposed antenna are depicted in Figs. 4 and 5. It is shown that the lowest resonate frequency of the antenna changes from 1.2 GHz to 1.1 GHz by increasing the value of L from 31 mm to 35 mm. However, as shown in Fig. 4, it is noted that the impedance bandwidth of the antenna becomes narrower if the length of the dipole is too large. The optimal value of L is 33 mm in order to cove the desired bandwidth. In addition, it can be seen that the effects on the axial ratio of the antenna is very significant if the parameter L is at the different values. This is because the effective current length/path of the antenna can be changed by modifying the length of dipole. Thus, as shown in Fig. 5, when L = 35 mm, the CP bandwidth for AR < 3 dB is from 1.3 to 1.45 GHz while the CP bandwidth is from 1.45 to 1.55 GHz if the value of L is 31 mm.

Fig. 2. The simulated S11 with different values of S.

Fig. 4. The simulated S11 with different values of L.

Fig. 3. The simulated axial ratio with different values of S.

The simulated S11 and axial ratio of the antenna with different values of the strip/dipole width (parameter S) are depicted in Figs. 2 and 3. It can be seen that the bandwidth of the antenna (for S11 < -10 dB) is improved by increasing the width of the micro-strip line. When S = 5.6 mm, the results are the optimal. The impedance bandwidth of the antenna covers from 1.15 to 1.75 GHz, which is good enough to cover the entire GPS (L1: 1575 MHz, L2: 1227 MHz, L3: 1381 MHz, L4: 1379.9 MHz, and L5: 1176 MHz) and Galileo (E5: 1191 MHz, E6: 1279 MHz, and E1: 1575 MHz) bands. The CP bandwidth (AR < 3 dB) is insensitive to the variation of the

Fig. 5. The simulated axial ratio with different values of L.

The simulated 3D radiation pattern of the antenna is depicted in Fig. 6. It can be seen that the radiation pattern is shown to be bi-directional with a very broad half power beamwidth. The maximum gain of the antenna is of around 2 dBi over the broadband. For GNSS applications, the desired


unidirectional radiation pattern can be achieved by placing the antenna over a PEC reflector or cavity at a quarter-wavelength (ɉ ) distance. The simulated total efficiency of the antenna is shown in Fig. 7. It can be seen that the efficiency is greater than 90% for the desired bandwidth between 1.15 and 1.75 GHz. (a)

(b)

Fig. 8. The fabricated prototype antenna. (a) Front view. (b) Back view.

Fig. 6. The simulated 3D radiation pattern of the antenna.

Fig. 9. The simulated and measured S11 of the antenna.

Fig. 7. The simulated total efficiency of the antenna.

IV.

ANTENNA MEASUREMENT

The optimized antenna dimensions in mm are: W = 66, S = 5.6, and L = 33. Fig. 8 shows the front and back views of the fabricated prototype antenna. The antenna was measured in an anechoic chamber using a vector network analyzer. The simulated and measured reflection coefficients of the antenna are depicted in Fig. 9. It can be seen that the results agreed reasonably well within the bandwidth between 1.1 and 2.1 GHz but the measured S11 is greater than -10 dB for the frequencies between 1.2 GHz and 1.7 GHz. This is probably due to the unexpected errors caused during the antenna fabrication which is to be further investigated. Fig. 10 shows the simulated and measured axial ratio of the proposed antenna. It can be seen that a very good agreement was obtained between the simulation and measurement. The CP bandwidth for AR < 3 dB is from 1.37 to 1.5 GHz as expected.

Fig. 10. The simulated and measured axial ratio of the antenna.

V.

CONCLUSION

In this paper, a broadband cross-dipole antenna has been proposed and designed for the GNSS applications. The impedance bandwidth of the antenna is very broad to cover the entire GPS and Galileo bands. The structure of the antenna is very simple. The dimension of the antenna is 66 mm ൈ66 mmൈ1.28 mm which is small and compact. To validate the predicted performance, the antenna has been made and


measured. All the results have demonstrated that the antenna is an excellent candidate for the GNSS applications. ACKNOWLEDGMENT

[4]

[5]

The financial support from BAE Systems and Innovate UK is acknowledged.

[6]

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