Design_of_an_X-band_dual_circularly_polarized_offset_reflector_antenna

Design of an X-band dual circularly polarized

offset reflector antenna

Yumeng Zhang, Yong Liu, Yifan Jiang,Wei Hou , Xin Lv

(Beijing Key Laboratory of Millimeter Wave and Terahertz Technology,

Beijing Institute of Technology Beijing, China 100081)

Abstract: A design of compact X-band dual circularly-polarized offset reflector antenna is described for the radar satellite system. The antenna is composed of a metallic reflector and a horn feed. By introducing the septum polarizer in the horn feed, it makes a more compact size for reduced space. Meanwhile, it is easier for the horn feed to reach the far field boundary condition. The offset reflector has obtained the circularly polarized radiation pattern of above 30dB gain, which the side lobe is less than -24dB; The axial ratio of the direction of the maximum radiation is below 1dB. And the two ports isolation is under -20dB at the center of the frequency of 8GHz. This configuration meets the reliability and consistency requirements for the space.

Key words: offset reflector, dual circularly polarization, Altair FEKO, septum polarizer

1 Introduction

Reflector antennas[1] which operate at millimeter and sub millimeter wavelengths are widely used in the telecommunication and high-resolution radars as its high gain and narrow pencil-beam. Meanwhile, with the development of satellite communication and telemetry technology, a single form of polarization cannot meet the requirements of microwave satellites or remote sensors for the demand of tracking and measuring the target in various polarizations. So it urges to utilize the circularly polarized antennas. In the radar communication, circularly polarized antennas can counter the interference of the rain and clouds. In the electronic countermeasures, they radiate right-hand or left-hand circularly polarized electromagnetic waves and the waves can be received by the antennas with any polarization direction so as to catch wide information even in the extreme environments or climates. With the growing number of the remote sensors in the satellite and the enhancement of the resolution, the high speed data transmission in the Satellite-Earth communication has gained an increasing concern. The most challenging

technology is to solve the limitation of the band of the data transmission.one effective solution is introducing the dual-circularly polarized antenna[2].

The essential purpose of the reflectors is to transform the medium gain radiated by the feeders to a high gain with pencil beams to improve the resolution. Consequently, the dual-circular polarized reflector antennas are more promising in the satellite radars.

In this paper, we present an X-band dual circularly polarized offset reflector antenna with a more compact structure. The offset reflector antenna consists of a single metallic reflector and a circularly polarized horn feed. The design of the horn feed is a key factor which can influence the performance of the reflector antennas. And the design is based on the septum polarizer[3]-[7] that can achieve the dual-circularly polarization.

2 Geometry description and Design

2.1 Feeder Design

Feeders of the reflector antenna are essential components. The commonly type of the feeder is the horn antenna. We here present a novel circularly polarized horn feed with the septum polarizer for its smaller size and excellent dual circularly polarization performance. The main feature of the septum polarizer is can achieve the right-handed or left-handed circularly polarization waves by two ports respectively. As shown in Figure 1, the schematic drawings of the horn feed with the septum polarizer are provided. The critical specifications for the septum polarizer is to obtain simultaneously, a quadrature phase shift between the two orthogonal signal components, excessively low insertion loss

as well as a good return loss and isolation at the input ports.

Figure1. The schematic drawing of the horn feed

2.2 Reflector design

In the circularly polarized single offset reflector antenna [8]-[9], Figure 3 shows that an offset parabolic reflector is used. The focus of the reflector is located at the phase center

of feed antenna. The parabolic reflector is described by the diameter D which is objected circular aperture, focal length F, offset height H and the edge angle θ.

Figure2. The schematic drawing of the horn feed

This offset configuration eliminate the block of the horn feed and reduce the side lobe generated by the block and improve the VSWR of the whole antenna. Furthermore, the axial symmetry of the feed makes it suitable for any polarization.

3 Results and discussion

3.1 Horn feed performance

Figure 4 and 5 show the symmetrical normalized radiation pattern at 8GHz without any obvious side lobes which are suitable for the feed of the reflector antennas.

Figure3. The axial ratio of horn feed at 8GHz Figure4. The normalized radiation pattern

of horn feed at 8GHz

3.2 Reflector performance

Figure 5. The modeling of the offset reflector.

The modeling of the offset reflector is shown in Figure5. We focus the bandwidths ranging from 8GHz to 8.5GHz, which the axial ratio and the gain of right-hand and

left-hand at operating band is demonstrated in Figure 6. It shows that in the working band the circularly polarized gain is above 30dB which the antenna efficiency is about 71%. And

the axial ratio of the direction of the maximum radiation is below 1dB.

Figure6. The return loss and isolation between two ports.

Figure7. The radiation pattern at 8 GHz.

Figure8. The axial ratio at 8GHz.

Figure 7 and 8 shows the radiation pattern and the axial ratio at the frequency of 8GHz. The offset reflector has obtained the right-hand circularly polarized gain of 30.1dB, which a low side lobe of less than -24dB.The angle range of -4dB axial ratio is from-8.5°to10°. The measurements show that the dual-circularly polarized offset reflector is working well. The structure is being processed and we will measure this type of antenna subsequently.

4 Conclusions

A novel dual circularly polarized offset reflector with the septum polarizer has been designed by using FEKO of the company of Altair. By applying the septum polarizer, good performance is obtained as predicted in the simulation results. The whole reflector

antenna achieved a high circularly polarized gain of over 30dB and the axial ratio of the direction of the maximum radiation is below 1dB in operating frequency band. Meanwhile, the design makes a more compact size for reduced space of space-borne environment.

5 References

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[2]Farooqi A R, Mughal M J, Mehmood M Q. Focusing of electromagnetic field by a circular reflector coated with chiral medium[J]. International Journal of Applied Electromagnetics and Mechanics, 2012, 38(4): 181-193.

[3]Zhong W, Li B, Fan Q, et al. X-band compact septum polarizer design[C]//Microwave Technology & Computational Electromagnetics (ICMTCE), 2011 IEEE International Conference on. IEEE, 2011: 167-170.

[4]Bornemann J, Labay V A. Ridge waveguide polarizer with finite and stepped-thickness septum[J]. Microwave Theory and Techniques, IEEE Transactions on, 1995, 43(8): 1782-1787.

[5]Albertsen N C, Skov-Madsen P. A compact septum polarizer[J]. IEEE transactions on microwave theory and techniques, 1983, 83(8): 654-660.

[6]Kroupa G D, Wong M N. Horn radiator assembly with stepped septum polarizer: U.S. Patent 5,305,001[P]. 1994-4-19.

[7]Scrank H E. Polarization measurements using the septum polarizer[C]//Antennas and Propagation Society International Symposium, 1982. IEEE, 1982, 20: 227-230.

[8]Park Y J, Sobahan K M A, Hwangbo C K. Wideband circular polarization reflector fabricated by glancing angle deposition[J]. Optics express, 2008, 16(8): 5186-5192.

[9]Oguzer T. Analysis of circular reflector antenna covered by concentric dielectric radome[J]. Antennas and Propagation, IEEE Transactions on, 2001, 49(3): 458-463.

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