Hybrid-fed Broadband Circularly Polarized Antennas And Arrays

Owing to the features of transmitting and receiving polarization angle arbitrary,’Faraday rotation’ effect reduction and multipath interference suppression, circularly polarized antennas are widely used in the wireless communication systems such as satellites, navigation and radio frequency identification. Additionally, antennas with wide band and stable radiation performance within the band are highly required in the modern wireless communication systems with high speed and large capacity. Therefore, on the basis of the demand of wireless communication systems for wideband circularly polarized antennas, this dissertation is mainly concerned with band enhancement methods for circularly polarized antennas, wideband antenna feeding techniques and broadband circularly polarized patch arrays basing on mutual coupling. The author’s major innovative works are summarized as follows:1. Strip-probe hybrid feeding technique is proposed for broadband circularly polarized microstrip antennas. Single-fed corner-truncated patch with strip-probe hybrid feeding is investigated. The axial ratio bandwidth of the antenna is broadened by increasing the antenna height. The impedance of the single probe is well matched by the hybrid feeding. Dual-fed square patch with L-shaped strip-probe hybrid feeding is presented. The hybrid feeding exhibits the effects of power division, phase delay and impedance matching. Dual circularly polarized microstrip antenna with L-shaped strip-probe feeding is proposed. Due to the quasi-quarter wavelength transmission mode in L-shaped strip, good port isolation is achieved within the wide circular polarization band. The validity of strip-probe hybrid feeding technique has been verified by the fabricated antenna prototypes.2. Slot-based hybrid feeding technique is studied in the broadband circularly polarized antenna designs to broaden the antenna bandwidth and improve the radiation performance. The first two designs are cutting crossed slot in square patch and H-shaped slot in corner-truncated patch respectively. Symmetric radiation patterns with low cross-polarization are obtained by the slot-stripline hybrid feeding, which shows better performance than the traditional probe feeding. The second design is a wideband printed crossed dipole loaded with bent slot. The slot in the dipole arms provides capacitive 90° phase difference across a wide band. Thus no extra feeding network is required. The final designs are low-profile broadband circularly polarized antennas loaded with electromagnetic band gap structure. The 4×4 rectangular periodic structure, which utilizes mushroom structure and high-impedance-surface structure respectively, provides phase delay difference of 90° for two equivalent orthogonal radiation field from the 45° rotated coupling slot to achieve circular polarization. The measured results have good agreements with the simulation.3. Sequentially-rotated parasitic elements are employed in the broadband circularly polarized antennas. By loading rotating patches with cutting corners,’linear-circular-linear’ radiation mode of the four coupling patches is excited and wide axial ratio bandwidth is then obtained. The bandwidth of the corner-truncated patch is broadened by the four rotating strips, which bring new resonant frequency in the axial ratio curve. Also, antennas with slot-stripline hybrid feeding and probe-coupling feeding are compared to show their influences in radiation performances. Broadband omnidirectional circularly polarized antenna is designed by combining the two radiating modes from rotated slots and vertical strips. Another parasitic rotating slots can produce new resonant frequency to broaden the impedance matching bandwidth.4. Broadband circularly polarized microstrip arrays basing on coupling effect are studied. Corner-truncated patch array with the same direction and close arrangement is built. The relationship between array element spacing, element mutual coupling and axial ratio bandwidth, gain of the array is discussed. Arrays of 2×2 and 4×4 elements with strip-probe hybrid feeding are successfully established. Tightly-coupled array with sequentially rotated patch is investigated. The cross polarization in the diagonal plane is suppressed by decreasing the array element spacing. The active impedance of the rotated square patch and rotated corner-truncated patch is studied. The final rotating tightly-coupled array with two-order matching hybrid feeding exhibits wide gain bandwidth because of the two resonant modes in the corner-truncated patches. The measured results of 2×2 arrays with feeding network show that the coupling effects can improve the performance of circularly polarized patch arrays.