Hybrid Antenna Measurement

The thesis analyzes a method [1] to determine the electric far-field radiated by electrically large antennas (or antennas placed over structures) using few measured near field samples of the electric field and numerically constructed expansion functions. Full test of an antenna can be complex and time consuming. When working with classical expansion functions, namely vector spherical harmonics for classical spherical range measurement, an estimate on the number N of sampling points one needs to acquire is given by N = 4πr 2 (λ2 )2 (1) where r is the radius of the minimum sphere enclosing the radiating structure and λ the wavelength. When structures which are large in terms of wavelength are considered, as antennas placed on satellite or other platforms, this number is so large that makes the measurement easily impractical. The aim of this method is to use information about the antenna, the scattering structure and the far-field of the antenna in isolation to drastically reduce the number of sampling points needed to determine the radiated far-field of the antenna mounted on the platform. The classic reconstruction method consists in measuring spherical near field samples and then expand them through vector spherical wave functions [6], i.e. a set of basis functions that is general and can be used to reconstruct the electric field of any radiating structure. This method, instead, employs a numerical basis adapted to the structure under test. More precisely, the antenna is enclosed in a virtual surface B, where unknown electric and magnetic currents are placed. We determine these currents imposing matching between the field radiated by the currents and the near-field samples acquired, and then use them to evaluate the far-field. The radiation is computed with a Green’s function that takes into account the presence of the platform where the antenna is mounted. From a computational viewpoint, we construct triangular meshes of the surfaceB and of the platform; we express the unknown electric and magnetic currents defined on B as a linear combination of piecewise linear basis functions with limited support; we numerically compute the field radiated by each of these elementary basis function; we enforce matching between the measured samples of the field and the field due to a linear combination of the fields radiated by each basis function. Once appropriate coefficients are determined, we know the currents defined over B and are then able to evaluate the field radiated by these currents in any point of the space outside B. In particular, we are able to evaluate the far-field. The method requires a large computational effort to build the numerical basis, but there are two main advantages in the method. The economic cost of the simulations is much lower than the cost of acquiring many samples of the field radiated by the antenna; once computed, we can reuse the fields due to basis functions to express the field radiated by any antenna contained in the surface B working at the considered frequency, thus tests of different antennas placed over the same structure become particularly fast. We show results of reconstructions of simulated electric fields of • a dipole over a plane mock-up at frequency 3 GHz, • a reflector antenna at frequency 8 GHz, while in the last chapter we show results of a reconstruction with measured near field samples of the same reflector antenna.