Investigation Of Antenna In-band And Out-of-band Decoupling Techniques

Multiple bands and multiple antenna elements have been one of the important trends of the development of mobile communications system.However,when multiple antennas are placed closely,it exists strong mutual coupling between antenna elements.According to the different operating frequencies,the antenna mutual coupling can be divided into in-band and out-of-band coupling.The in-band mutual coupling is very common in a MIMO?Multiple-Input Multiple-Output?system,which uses multiple antenna elements.When antenna elements couple strongly to each other,it causes large pattern correlation and then degrades the channel capacity.As for out-of-band mutual coupling,it is a widespread question in the multi-band base station antenna.When closely spaced multiple antennas at different operating frequencies share the same aperture,a strong out-of-band coupling can distort the antenna radiation pattern.Therefore,it is a research subject with a bright practical prospect and great challenges to solve the in-band and out-of-band mutual coupling question in a closely spaced antenna array.This thesis has investigated the in-band and out-of-band decoupling techniques in detail and the main work can be divided into two parts as follows:1.MIMO antenna using hybrid electric and magnetic coupling for isolation enhancement.This paper presents a novel decoupling structure by using hybrid magnetic and electric coupling to improve isolation of MIMO antenna elements.It is formed by a split ring,which consists of an inductive line and a capacitive gap to realize magnetic and electric coupling,respectively.By introducing the electric coupling in addition to the magnetic coupling,more degrees of freedom are obtained and the original coupling between antennas can be cancelled out within a wide frequency band.Meanwhile,the performance of each antenna element is maintained,such as impedance bandwidth and radiation characteristics.Theoretical analysis and experimental verification are carried out.When the decoupling structure is placed between these two monopole antennas with edge-to-edge space of only 5.0 mm?0.04?₀ at 2.5GHz?,more than 30 dB over the frequency band of 2.3-2.8 GHz can be obtained.In addition,by flexibly controlling the electric and magnetic coupling between antennas,the isolation level and bandwidth can be controlled in different application scenarios.To verify it,a MIMO antenna with 0.01?₀ spacing is designed in this paper.The measured results show that about50 dB isolation is obtained.2.Dual-band dual-polarized wideband co-aperture base station antenna.The co-aperture base station antenna consists of a lower band and four high band elements.And the four higher band elements are placed around the lower band one in a close distance.Due to more than two times difference between the minimum and maximum frequency,the antenna sizes of higher band and lower band are very different.Firstly,the problem that the higher band element is shielded by the larger size lower band element is analyzed,thereby designing a lower band element with slight impact on higher band one.Moreover,by suppressing the higher band harmonics,the out-of-band coupling at higher band can be reduced.As for higher band element design,the impact of low-band common harmonics of the higher band element on the lower band antenna gain is analyzed in detail.Therefore,a higher band element with common harmonics suppression is designed,thereby avoiding the decrease of lower band gain.Moreover,by suppressing the lower band harmonics in port,the port-to port coupling between the higher band and lower band element can be reduced.Though optimizing and adjusting,the designed higher band and lower band elements are put together for fabrication and measurement.The measured results indicate a low coupling between the two antenna elements at different frequencies.Therefore,a stable radiation pattern is maintained at higher and lower band.