Spherical Near Field offset measurements using downsampled acquisition and advanced NF/FF transformation algorithm

Abstract

Spherical Near Field (NF) measurements are widely used in order to accurately characterize the radiating performance of antennas. The main drawback of this type of measurement is the acquisition time that, depending on the electrical size of the Antenna Under Test (AUT), could be very long. This is due to the fact that, in order to correctly evaluate the Far Field (FF) with the NF/FF transformation [1]-[3], the NF has to be sampled over the full sphere with a sampling density that increases with the dimension of the so called AUT minimum sphere [3]. In many spherical NF measurement scenario, it is not possible to locate the AUT in the origin of the measurement sphere resulting in a larger minimum sphere, which implies a denser sampling and a longer acquisition time. An innovative NF/FF technique that allows to drastically reduce the samples density in offset spherical NF measurement is presented in this paper.

Reconstruction of the far field radiated by an offset mounted volumetric AUT from non‐redundant spherical spiral near‐field measurements

“…It must be pointed out that the number of the NF samples collected along the spiral is 36,502 (including the 704 extra samples acquired at reduced spacing nearby the poles) and, accordingly, significantly smaller than that (80,400) needed by the CS NTFFT in the considered case of offset mounting. It should be observed that such a number results to be also lower than the one required by the Foged's technique [36], that would employ an amount of NF data a bit greater than that (45,300) needed by the CS NTFFT when mounting the considered AUT in onset configuration.…”

“…As can be seen, in both the experimental sets, the number of NF samples to be gathered along the spiral results to be remarkably smaller with respect to the amount of NF data required by the CS NTFFT in the offset mounting case and is also lower than that would be needed when these AUTs are mounted in the onset configuration. Moreover, this number compares favourably also with the one needed by the Foged's technique [36], which, as already stressed, would use an NF data number a bit larger than that required by the CS NTFFT in the onset mounting case.…”

“…In this case, according to the minimum sphere rule, the number of the NF data required by the CS NTFFT [11] and, as a consequence, the related acquisition time can remarkably grow due to the increase of the radius of the smallest sphere enclosing the AUT and centred at the scanning sphere centre. To surmount this problem, a new spherical NTFFT, where the spherical wave functions are defined with reference to the centre of the AUT rather than to the scanning sphere one, has been proposed in [36]. It is worth noting that the related number of required NF data is much smaller than that set by the minimum sphere rule for such an offset mounting, but still results to be slightly larger than that relevant to the onset mounting.…”

Application of the TSWE algorithm to echo reduction of under‐sampled offset spherical near‐field measurements

“…The TSWE is an advanced NF/FF transformation technique to be used in case of SNF measurement with offset AUT. As already pointed out and demonstrated in [9], the advantage of using TSWE in case of offset AUT is the reduction of the number of NF samples to be measured w.r.t. the conventional Nyquist criteria.…”

“…An innovative NF/far-field (FF) transformation algorithm for spherical offset measurements named translated spherical wave expansion (TSWE) has been recently proposed [9,10]. The TSWE is based on the modification of the spherical wave functions, which are properly redefined in a new reference system located on the AUT rather than on the centre of the measurement sphere (as typically done) allowing to drastically reduce the number of samples with a consequent reduction of the testing time.…”