Researches On Mutual Coupling Characterization And Decoupling Methods For Antennas

For a long time, mutual coupling of antennas, as an important research topic in the antenna field, has attracted much attention. The mutual coupling has significant influences on various wireless communication and detection systems. For example, the mutual coupling can deteriorate the isolation between Rx/Tx antennas, degrade the system capacity, introduce the scanning blindness, and affect the design accuracy of the systems. Consequently, researching the mutual coupling problem and exploring its solutions have important significance and application value. The studies of this dissertation include the mutual coupling characterization of antennas and the decoupling methods. In this dissertation, a current distortion parameter based on the antenna current distribution is proposed for characterizing the mutual coupling, and the decoupling structures based on several methods are investigated and designed. The works and research achievements are as follows:(1) A current distortion parameter is proposed for characterizing the mutual coupling of antennas. This parameter shows the relative difference in the current distributions between with and without the mutual coupling, thereby characterizing the influence of the mutual coupling on the current distribution. Scattering parameters are usually used for the mutual coupling characterization, while they only show the port features of an antenna, and the features could not completely describe the mutual coupling effect on the antenna. Thus researching another parameter for the mutual coupling characterization is necessary. Since the current distribution dominates the radiation and port features of an antenna, the current distortion parameter can evaluate the mutual coupling effect on the antenna features. Several calculation examples, including the mutual coupling between dipole antennas, microstrip antennas, slot antennas, as well as the coupling between a dipole antenna and a metallic bar, exhibit the characteristics of the current distortion parameter. The proposed parameter and the scattering parameters both can describe the mutual coupling strength between antennas and evaluate the performances of a decoupling structure, but their emphases are different. The scattering parameters focus on the mutual coupling effect on the ports, while the current distortion parameter focuses on the mutual coupling influence on the whole current distribution. Under a certain case, it may be noted from the antenna port features that the mutual coupling is weak, but in fact, there is a great influence of the mutual coupling on the antenna, which even can lead to a large distortion of the radiation patterns. Thus the current distortion parameter under this case can more exactly characterize the mutual coupling influence. Also the current distortion parameter can characterize the sensitivity of an antenna to its non-free space environment. Consequently, the current distortion parameter, acting as a supplement to the scattering parameters, can be applied to investigate the mutual coupling.(2) Several decoupling structures based on the field cancellation method are designed, which focus on reducing the mutual coupling between closely spaced microstrip antennas. These cancellation-type decoupling structures can introduce an indirect coupling field between the antennas, which can cancel out the original direct coupling field to achieve the mutual coupling suppression. A simple microstrip decoupling structure is designed. The experiment result shows that the isolation improvement is 16.7 dB-22.8 dB, with an antenna edge-to-edge distance of just 0.023λ0. An asymmetric coplanar strip decoupling structure is designed, which not only effectively reduces the mutual coupling between compact microstrip antennas, but also solves the problem of the structure effect on the antenna radiation performances. A metallic wall with two open-ended slots is also designed, which has a hybrid decoupling capability. Except for the field cancellation effect, the metallic wall itself can partly block the mutual coupling. Finally, the microstrip decoupling structure is illustrated to exhibit that the cancellation-type structures can be applied for the decoupling of a multi-element array.(3) Two decoupling structures using the band-stop filtering method are designed, which have a filtering effect at the antenna coupling path, and both of them can be used for the decoupling between microstrip antennas with various antenna separations. A decoupling structure composed of half-wavelength ground slots is designed, which can create a band-stop ground plane between microstrip antennas to filter the ground current coupling, whose band-stop feature is only dominated by the slot resonance. The simulation and measurement indicate that this designed structure can achieve the decoupling with various antenna separations, and properly increasing the ground slots can broaden the decoupling bandwidth. A miniaturized structure for the half-wavelength ground slots is also designed, which is composed of the stepped impedance ground slots loaded with post walls. This structure effectively shortens the physical length of the half-wavelength slots, and comparing to the traditional stepped impedance structure, this loaded structure occupies less space and has a better decoupling effect. The designed half-wavelength ground slots and their miniaturized structure both can be conveniently applied for the decoupling of a multi-element array.(4) A mode transformation decoupling method is proposed, and an implementation structure of this method is designed. The proposed method, which is different from the field cancellation and the band-stop filtering, do not try to directly block the coupling between microstrip antennas, but transforms the coupling mode on the adjacent antenna into another mode that is orthogonal to the working mode of the antenna, and the antenna do not effectively work at the orthogonal mode, thereby improving the isolation between microstrip antennas. Based on this method, a decoupling structure composed of three microstrip interdigital lines is designed. The simulation analysis verifies that these microstrip interdigital lines have a mode transformation effect. The measurement indicates that the isolation improvement is 15.2 dB-24.7 dB, with an antenna edge-to-edge distance of only 0.07λ0. Moreover, this structure can be conveniently applied for the decoupling of a multi-element array.In this dissertation, the proposed current distortion parameter provides an approach to characterize the antenna mutual coupling from the perspective of the current distribution, and it is suitable for the researches of various mutual coupling problems, including the mutual coupling between antennas and the coupling between an antenna and its non-free space environment; the studied and designed decoupling structures not only can be used for the decoupling between two antennas, but also provide the solutions to the isolation problem of a multi-element array.