Investigation Of Extended Mixed-Mode S-Parameters And Application On SATCOM Isoflux Antenna Array

As a general method of characterizing the microwave network,the Scattering-Parameters(S-parameters)has been used widely in the theory and design of various microwave components due to their feasibilities in microwave frequency bands.Traditional S-parameters give the relations between different ports,while it's not satisfying for the analysis of multi-port cases.Developed from traditional S-parameters,mixed-mode S-parameters could be used to analyze the 2-port cases of differential circuits,and give the relations between differential and common modes.Mixed-mode S-parameters could give more intuitive physical meanings than traditional single-ended S-parameters in differential cases.However,there is no suitable theory of analyzing more general multi-port networks.In this way,how to use S-parameters to characterize the performance of the multi-port microwave network with more intuitive physical meanings is an issue worthy of study.Satellite communication is considered to be an indispensable technology of the sixth-generation wireless communications technology(6G)due to its advantages such as wide coverage and no geographic restrictions.With the increasing demand for low-earth-orbit satellite missions,miniature satellites have become more popular.As a kind of small satellite,Cube Sat has attracted wide attention due to its advantages in standardization and modularization.Isoflux beam is very important for SATCOM(satellite communications)antennas,because they are supposed to produce a uniform power coverage on the given area of the earth.At the same time,attributes such as CP(circular polarization)and light weight are also required for SATCOM antennas.However,most of the existing isoflux beam antennas are still too bulky and massive,and they do not fully meet the requirements of CP and light weight.Therefore,how to design an antenna with satisfactory performance within limited space is an issue worthy of study.In addition,it would introduce multiple ports to feed the radiator when using sequential rotation feeding technology to generate CP,which causes the single-ended S-parameters to lose the meaning of characterizing the matching performance of the whole structure.And the proposed extended S-parameters could be used to fix the problem.The main work is as follows:(1)Inspired by mixed-mode S-parameters designed to handle differential networks,more general S-parameters,called extended mixed-mode S-parameters,is proposed.N symmetric single-ended ports of a microwave/RF network,transmitting/receiving N correlated signals,are regarded as one extended mixed-mode port,supporting N orthonormal modes represented by N orthonormal basis vectors,respectively.By doing this,in antenna and RF designs,one can only focus on specific modes interested,while ignore others.In this way,a multiport network's characteristics could be represented and understood in a much simpler manner because fewer but more intuitive physical quantities are needed to be considered.Therefore,the proposed technique can significantly simplify the analysis,design,and optimization of some multiport microwave network.To bridge single-ended S-parameters and extended mixed-mode S-parameters,the conversion between them are also given.For better illustration,several examples are analyzed and designed by the proposed technique.As can be seen,complicated analyses in conventional microwave/RF network design using single-ended S-parameters,including radiation efficiency,amplitude and phase unbalance,could be converted into much simpler ones from the perspective of extended mixed-mode,and the results all prove the feasibility of proposed technique.(2)This paper presents an antenna array to produce circularly-polarized(CP)isoflux beam for Cube Sat earth coverage applications.Distinguished from conventional arrays formed by identical elements,the proposed one is based on sequential rotation-fed concentric annular aperture elements.By adjusting the excitation phase,magnitude and polarization of each annular aperture element,the radiation patterns of the array can be shaped to form isoflux beams.Radiation pattern modelling method is developed to obtain array radiation parameters.Besides,to effectively characterize the impedance matching of the radiators of the array with multiple excitations,an overall reflection coefficient is deduced from signal decomposition.A prototype operating at C band(5 GHz)is designed,fabricated and measured.It consists of a radiator and a feeding network.The radiator is formed by two annular aperture elements,of which the inner one is realized by a circular patch antenna while the outer one is formed by eight Planar Inverted-F Antennas(PIFAs).The feeding network drives annular aperture elements in the manner of sequential rotation.In this way,a CP isoflux beam array with a low profile(0.078?₀),light weight,and ease-of-integration for Cube Sat is realized.The proposed array can also be extended to realize other shaped beams.