Research On Multi-Mode Low-Profile Terminal Antennas For Wireless Communication

In this thesis,three novel,low-profile terminal MIMO antennas with high performance and multi-function based on multiple modes for wireless communication are proposed,and their operating principles are deeply analyzed.The main contents are as follows:1.A wideband,low-profile pattern diversity antenna based on symmetric and antisymmetric feeding lines is proposed.TM₁₁ and TM₂₁ modes of the circular patch are excited by the two custom-designed feeding schemes,respectively.These two feeding lines can replace the power dividers under the ground and introduce non-radiative modes.The novel H-shaped grounded planes are introduced to realize the frequency reallocation of the feeding line modes and impedance matching at a lower profile,and shorting pins are utilized to merge the two radiative modes of the microstrip patch.Moreover,the two feeding schemes are orthogonally placed to realize pattern diversity,and the isolation is optimized by analyzing the current distributions.The overlapped impedance bandwidth of the antenna is greatly enhanced by this arrangement,and the reason for the bandwidth widening is analyzed by the electric field distribution and equivalent circuit.The antenna has been measured,and it can achieve wide overlapped impedance bandwidth(28.5%),high isolation(23d B)and high gain(7.0,9.9d Bi)at a low profile(the height of the feeding scheme is only 0.066 ?₀).2.A wideband,ultralow-profile pattern diversity antenna based on four radiative modes of a single patch is designed.TM₀₁,TM₀₂ and TM₁₁ modes are merged by shorting pins with appropriate number,position and size.Then,a slot and a novel metallic disk are introduced to reduce the resonant frequency of TM₁₂ mode to the corresponding band while guaranteeing the gain of TM₁₁ mode.The equivalent circuit is utilized to analyze each loading element,and the distributions of electric field vectors and equivalent magnetic currents are introduced to investigate the effects of the slot on the gain of odd-order modes.In addition,the feeding scheme of the ultralow-profile MIMO antenna is investigated,and the rat-race coupler is introduced to ensure the high isolation and realized gain.The antenna has been measured,and it can achieve wide overlapped impedance bandwidth(9%),high isolation(20d B)and relatively high gain(6.5,8.8d Bi)at an ultralow profile(0.033?₀).3.A miniaturized,dual-band dual-polarized filtering antenna based on the combination of CRLH-TL structure and microstrip patch is designed.By adjusting the position of the shorting pins,the negative order mode is excited at the low frequency.Moreover,the radiation pattern of TM₁₂ mode at the high frequency is reshaped by modifying equivalent magnetic currents,and the resonant frequency of TM₁₂ mode is reallocated to merge with that of antiphase TM₂₀ mode to achieve wideband property.By introducing a novel cross-shaped feeding scheme to provide filtering response for the antenna,and this structure has a great improvement in the cross-polarization level.The simulation results show that the miniaturized antenna(0.24?₀*0.24?₀*0.024?₀)achieves a wide beamwidth at the low frequency(HPBW of both planes are greater than 110°),and it exhibits wideband(12.9%)and high-gain properties(9.9d Bi)at the high frequency.There are two radiation nulls between the two bands,and a radiation null at the high frequency.Meanwhile,the isolation of the dual-polarized antenna is better than 48d B,and the cross-polarization level is very low.