Radiation and Scattering from Conformal Array Antennas

University dissertation from Stockholm : Alfvénlaboratoriet

Abstract: This thesis deals with radiation and scattering fromconformal array antennas. A hybrid technique between the Methodof Moments (MoM) and different asymptotic methods is used forthe analysis, combining the accuracy of the MoM and theefficiency of the asymptotic methods.In the first part of the thesis, radiation and scatteringproperties of waveguide aperture antennas mounted on perfectlyconducting circular cylinders are analysed. The theoreticalresults are verified against measurements on an experimentalantenna built at Ericsson Microwave Systems AB in Mölndal,Sweden. A trade-off between gain and radar cross section (RCS)is presented for this type of antenna and it is shown that itis possible in some cases to maximize the radiation andminimize the scattering properties with a suitable choice ofimpedance load.The second part of the thesis deals with the analysis ofdielectric coated circular cylinders. A high frequency methodis developed to calculate the radiated far field from magneticsources placed on a coated circular cylinder. The method isverified against the classical eigenfunction solution andagainst measurements. A very good accuracy is achieved withinthe expected region of validity. When analysing the performanceof array antennas the mutual coupling among the antennaelements is an important parameter to consider. For the coatedcylinder an asymptotic technique is used to calculate themutual coupling in the non-paraxial region. For closely spacedelements and for elements in the paraxial region a planarapproximation is used with a corresponding spectral domaintechnique. The scattering properties of a cylindrical arrayantenna coated with a dielectric layer is investigated and theeffect of the coating is illustrated. Finally, the radiationcharacteristics and the matching properties of a finiteconformal waveguide aperture array antenna are analysed as afunction of scan angle and electrical thickness of the coatingin the frequency range where only the fundamental modepropagates. It is shown that the matching properties can beimproved by placing reactive elements in the form of thinirises inside the waveguides.

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