| Horn antennas are widely used in the fields of modern microwave engineering,due to high power capacity,low transmission loss,high performance stability,etc.Single-mode horns,such as pyramidal or conical horns,generally have simple structures and regular aperture field distribution,whose radiation field components can be calculated in approximately closed form.Referring to feed antennas with complex interior structures,the analysis and designs have to reply on efficient numerical methods.With the development of modern communication and remote sensing technologies,narrowband high-gain antenna systems are no longer of attractiveness compared to those featuring multi-band functions,which makes the designs of feed antennas more challenging.Improvements on design methodologies,numerical simulation techniques and optimization procedures have to be carried out simultaneously,to complete complex multiobjective optimization tasks within limited amount of time.After extensive analysis of the application backgrounds,design methodologies and analysis methods of feed antennas in multi-band satellite communication systems,which are in great demands in both military and civil applications,aiming at high computation efficiency and high optimization cap acity,complete modal-analysis-based numerical solutions for multi-band feeds are proposed in this paper,including multi-mode equivalent circuit modeling of homogeneous or inhomogeneous cavities inside the horns with symmetries of revolution,radiation and impedance characteristic analysis of feeds in free space,and coupling characteristic analysis of horns in reflector antenna systems.Meanwhile,full-wave simulation techniques and experiments are used to demonstrate the computation efficiency and accuracy of the proposed numerical models.Various discontinuities between inhomogeneous waveguides or between inhomogeneous and homogeneous waveguides,which may be included in the interior cavities of multi-band feeds,are analyzed initially.The Generalized Scattering Matrix(GSM)of each kind of discontinuity is calculated by the Mode-Matching Method(MMM),while the analytical expressions of power coupling coefficients between interfering waveguides are obtained by rigorous mathematical derivations.In combination with the well-discussed homogeneous waveguide discontinuities and matrices cascading techniques,multi-mode equivalent circuit models of typical feed horn interior cavities can be constructed rapidly,making the complex numerical models of horn antennas simplified to the cascading between the circuit models abovementioned and the radi ation aperture discontinuities.Thereafter,the Boundary Contour Mode-Matching Method(BCMM)is utilized to calculate the GSMs of radiation apertures,in combination with the GSMs of horn interior cavities by MMM,leading to the overall GSM description of antenna radiation problems.Once the excitation mode at the input port is determined,the radiation field components in any regions can be calculated analytically,as we ll as port characteristics such as reflection coefficients and isolation.To validate the hybrid modal analysis method,with respect to the rapidly-developing Ku/Ka dual-band satellite communications,two Ku/Ka multi-band feed antenna designs are proposed,both employing the dielectric/corrugation hybrid loading technique to optimize their radiation characteristics in each band.The first feed is in the form of a small-aperture open-ended waveguide,featuring a wide main radiation beam,which can be used in vehicle-borne satellite communication terminals and other platforms where the dimensions of reflector antennas are highly restricted,helping realize high aperture efficiency over all relevant bands.The second one can provide frequency-independent radiation patterns in multi bands,including stable phase centers,main beamwidths and radiation gains,acting as an ideal Gaussian beam to primarily excite the reflector in multiple bands.Meanwhile,low cross-polarization levels,axially symmetrical radiation patterns,stable phase centers,as well as excellent impedance matching at all input ports and adequate isolation between different ports,are also realized by both feeds.The hybrid modal analysis is combined with multiobjective optimization procedures based on Genetic Algorithm to optimize the multi-band feeds abovementioned globally.Comparison among the numerical results by the hybrid method and those by full-wave simulation tools and experimental measurements demonstrates its significantly high computa tional efficiency and numerical accuracy.The hybrid method can be widely used in various optimization designs of feeds with symmetries of revolution,particularly those loaded with dielectric.It can provide all kinds of relevant waveguide discontinuities equivalent circuit models,to establish modal-analysis-based numerical models which are in rigorous accordance with the real feed structures,so adequate computation accuracy and high computation efficiency can be guaranteed.Meanwhile,to construct complete Ku/Ka dual-band feed systems,with respect to the coaxial-type feeding networks adopted by the two feed horns,a Ku-band turnstile-junction-based coaxial waveguide orthomode transducer(OMT)for orthogonally linear-polarized signal discriminations and a Ka-band dual-band polarizer for orthogonally circular-polarized signal excitations are proposed in this paper.The OMT can achieve excellent impedance matching and high isolation between orthogonal ports over the entire Ku-band;better characteristics within a narrower bandwidth can be obtained by simply adjusting the dimensions of the inner scatterer.The Ka-band polarizer can provide circular-polarization excitations of any type in both bands.Finally,a rigorous analysis method based on generalized net work theory is proposed to analyze the coupling and radiation problems with respect to feeds in reflector antenna systems.GSM-based equivalent circuit models of each radiator and scatterer are established initially.After the constructions of equivalent free-space transmission lines with the help of spherical wave Addition Theorems,the independent sub-domains can be connected effectively,leading to a global GSM description of the coupling-radiation problem,and the computation efficiency can be improved significantly.Two rigorous methods are proposed to analyze the GSMs of radiation aperture discontinuities.Metal scatterers with smooth walls can also be analyzed by BCMM,leading to their one-port-network equivalent circuit models with respect to the incident and reflected waves in the form of spherical wave expansions.Furthermore,referring to two feed antenna system configurations where couplings exist significantly: feed arrays and dual-reflector antennas,the generalized network theory is used to analyze the couplings between feeds and those between feeds and sub-reflectors,for accurate calculation of their radiation characteristics.To improve the computation efficiency with respect to multi-band feed optimization designs and multiple feed antennas/scatterers coupling and radiation by conventional full-wave simulation techniques,a complete modal-analysis-based numerical calculation method is proposed in this paper,and the efficiency advantage has been demonstrated sufficiently by applications in various designs and analysis with practical backgrounds. |
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