Design of Wideband Circularly-polarized Millimeter-wave Antennas


Student thesis: Doctoral Thesis

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Award date8 Oct 2018


Millimetre-wave (mmw) antennas have recently drawn the attention of wireless industries and researchers for its wide bandwidth (57-64 GHz), and support for instant massive data transmission. Meanwhile, circularly polarized (CP) radiation has been applied to millimetre-wave antennas due to the orientation freedom between transmitting and receiving antennas, better mobility, and reduction in multipath reflections. In this thesis, several wideband CP millimetre-wave antennas are investigated. There are five chapters in this thesis.

The existing wideband CP millimeter wave antennas are covered in the first chapter. Two wideband CP millimeter wave antennas with novel 3D-printed dielectric polarizers are presented in the second chapter. The first polarizer which consists of several air and dielectric slabs is used to transform the polarization of the antenna radiation from linear into circular. The proposed polarizer is placed above a horn antenna operating at the centre frequency of 60 GHz. An electric field, E, radiated from the horn generates two component electric fields, Ex and Ey. After passing through the polarizer, both fields Ex and Ey can be degenerated with an orthogonal phase difference which results in having a wide axial ratio bandwidth. The phase difference between Ex and Ey is determined by the thickness of the dielectric slabs. The proposed polarizer yields a wide axial ratio bandwidth of 50% from 52 to 94 GHz for an axial ratio < 3 dB. However, the profile of the polarizer is too high. To reduce the thickness of the polarizer, two methods are proposed. The first one is by using a dielectric with high permittivity. The second one is by increasing the length of the rectangular slot. Then, to enhance the gain of the antenna, a cylindrical polarizer with small radius is adopted. With refraction of the polarizer, the directivity of the radiation pattern is increased to devote high-gain and wideband characteristics to the antenna. To verify our concept, an intensive parametric study and an experiment were carried out. Three antenna sources including dipole, patch, and aperture antennas were investigated with the proposed 3D-printed polarizer. All measured results agree with the theoretical analysis. The proposed antenna with the polarizer achieves a wide impedance bandwidth of 50% from 45 to 75 GHz for a reflection coefficient < -10 dB, and it yields an overlapped axial ratio bandwidth of 30% from 49 to 67 GHz for an axial ratio < 3 dB. The maximum gain of the antenna reaches 15 dBic. 

Although the cylindrical polarizer and tapered polarizer can enhance the gain up to 15 dBic, the gain is not high enough. Therefore, the polarizer is integrated with the extended hemispherical dielectric lens in the third chapter, which can enhance the gain up to 22dBic. This design is different from the cascaded design of a polarizer with an extended hemispherical dielectric lens. In this design, only one device is required to achieve dual functions. Therefore, size reduction and loss reduction can be obtained. To suppress the sidelobe level, a rectangular groove is etched around the antenna source. The groove can stop the surface wave and therefore suppress the sidelobe level lower than -20 dB. Finally, a thin CP lens antenna is proposed to replace the CP extended hemispherical lens antenna. The gain can be further enhanced up to 30 dBic.

In the fourth chapter, two multibeam antennas are presented. The fist multibeam antenna consists of 18 polarizer elements. Different shapes of the polarizer are investigated in this chapter to obtain a low sidelobe level of the antenna. It is found that a tapered polarizer has a lower sidelobe level. Therefore, it is adopted in the multibeam antenna design. The multibeam antenna has 18 radiation beams with a coverage from -180° to 180°. Moreover, the sidelobe levels of each beam can be lower than -19 dB. To achieve a 2D scan beams, a thin lens is adopted. The lens can achieve a wide CP bandwidth and high gain performance. By placing nine antenna sources on the focal plane of the lens, a 2D steering beam CP millimeter wave antenna can be achieved. Finally, a conclusion and some future works are presents in Chapter 5.

In addition, thanks to the low-cost three-dimensional (3D) printed technology, all the antennas in this thesis can be fabricated in an accurate and convenient way. The proposed method of this design can apply to applications related to millimeter-wave wireless communication systems. The ultimate goal of this work is to develop wideband, high-gain and low-cost antennas for the millimeter-wave frequency band.

    Research areas

  • Millimeter-wave, Antennas, lens antennas, Polarizers