The aim of the antenna diagnosis is the detection of errors in manufactured antennas. Since this diagnosis is quite hard to carry out by just observing field measurements, the diagnosis is done by using the reconstructed equivalent currents on a surface close to the antenna. This thesis describes different possibilities to perform this reconstruction on a flat surface from spherical near-field measurements. Specifically, the modal expansion techniques are extensively studied and applied to practical situations.
The main problem of the modal techniques is the limitation in the resolution of the reconstructed equivalent currents. The reason for this limitation is the small region available in the plane wave spectrum (whose Fourier transform are the equivalent currents). In this thesis this problem is studied and several examples are shown. Moreover, the possibilities to improve the resolution are described. Among these possibilities, the use of an extrapolation technique is proposed. By using this technique, the non-visible spectrum is estimated from the known region (the visible spectrum) and additional information about the antenna, e.g. the size of the antenna.
Among the different extrapolation techniques, the most commonly used techniques are described and compared. Firstly, the iterative Papoulis-Gerchberg algorithm is applied by using the size and shape of the antenna. Then, the direct versions of this algorithm, i.e., the extrapolation matrix by rows and columns, and the generalized extrapolation matrix, are described. Finally, the PDFT transformation is studied and compared to the previous algorithms. All these techniques are applied to real situations with a significant improvement of resolution.
The last chapter of this thesis deals with the probe calibration procedures. These procedures are especially important in antenna diagnosis since they allow to take into account the effect of the probe in the field measurement. Thus, the best diagnosis of the antenna under study may be carried out. In this thesis, the iterative probe calibration algorithm proposed by Hansen is described. In addition, several alternatives to this algorithm for three different situations are proposed. These techniques are verified in real situations with quite good results.