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Spherical near field antenna measurements

Kılıç, Hüsnü Doğan
Accurate determination of the antenna characteristics is one of the key subjects where different methods have been developed for this purpose. When advantages and facilities of these methods are considered, near field measurements have come forward especially in recent years. In this thesis, one of these methods which is referred to as spherical near field measurement method, has been studied. First, wave expansion in the spherical coordinates is introduced. This enables expressing the field as summation of modes which are composed of vector wave functions and coefficients of these functions. It is shown that by obtaining coefficients for measurement distance, field can be obtained at any other distance with these coefficients. However, test setup of an antenna is composed of receiver and transmitter such that one of them is antenna under test and other is measurement antenna which is usually called the probe. Hence, probe effects on the measurement must also be included for accurate results. Initially, Lorentz’s Reciprocity Theorem had been used for this purpose. Then scattering matrix theory has been more commonly used. In this thesis study, scattering matrix theory is explained and generation of transmission formula with the probe compensation is presented. Different numerical integration techniques are used in order to calculate wave coefficients and comparisons are given. Eventually, tests with the theoretical data and results of measured antennas are performed and results are discussed.