Initial Considerations Towards Hemispherical Near-Field Antenna Measurements

A new method for hemispherical near-field antenna measurements is proposed. The method allows the extraction of free-space antenna coefficients from field measurements of the antenna located above both ideal PEC and PMC image planes alternatively. To present the method, first the Image Rule for Vector Spherical Wave Sources is used to find an expression for the total radiated fields above ideal planes. This expression is validated analytically through the verification of boundary and symmetry conditions. The resulting field expression is analogous to the standard free-space expansion, but with a set of equivalent “over-plane” antenna coefficients that capture the effect of the plane through the definition of an image matrix. Thus, symmetry can be used to synthesize a full sphere from hemispherical measurements only. Therefore, standard spherical near-field algorithms can be used to extract the equivalent over-plane coefficients. Finally, a combined image matrix is proposed that can be directly inverted to extract the free-space coefficients once the equivalent PEC and PMC over-plane coefficients are known. Numerical examples are used to illustrate the method.