Electrical and mechanical uncertainty study in cylindrical near field antenna measurement systems

Abstract

IntroductionIn order to evaluate how mechanical or electrical errors may affect in the final results (i.e. radiation patterns, directivity, side lobe levels (SLL), beam width, maximum and null position…), an error simulator based on virtual acquisitions of the measurement of the radiation characteristics in a cylindrical near-field facility has been implemented [1], [2]. In this case, the Antenna Under Test (AUT) is modelled as an array of vertical dipoles and the probe is assumed to be a corrugated horn antenna. This tool allows simulating an acquisition containing mechanical errors -deterministic and random errors in the x-, yand z-position -and also electrical inaccuracies -such as phase errors or noise -. Then, after a near-to-far-field transformation [3], by comparing the results obtained in the ideal case and when including errors, the deviation produced can be estimated. As a result, through virtual simulations, it is possible to determine if the measurement accuracy requirements can be satisfied or not and the effect of the errors on the measurement results can be checked. This paper describes the error simulator implemented and the results achieved for some of the error sources considered for an L-band RADAR antennas in a 15 meters cylindrical near field system. Description of the error simulator for the inaccuracies evaluation In this case, since the system where this work is applied is an outdoor system, there are some error sources more relevant than the others. Actually, the effects of the wind for the probe positioning and the temperature changes that affect the phase response of the cables are the ones to be considered. Thus, the strategy adopted to evaluate the sources of error is to simulate these deviations and to examine the influence that they have in the final results. This procedure starts with the modelling of the transmitting and receiving antennas. Then, the near-to-far-field transformation is applied to obtain the far-field radiation patterns. So, to evaluate how errors could affect the final results, a model of the antennas and a simulation of the acquisition process including errors has been performed. Finally, the simulator compares the outcomes achieved from the reference data (i.e. the array infinite far-field) with the ones including the deviations.The received field in each point of the grid was calculated taking into account the field radiated by all the dipoles modified by the probe pattern. The field from a dipole in each point of the grid is given by the sum of three spherical waves [4]. The probe is an ideal conical corrugated horn characterized by the calculated radiation pattern of the main planes. For this investigation two antennas are considered. In the first analysis, the AUT is 1.44 meters long and 1.04 meters high (antenna 1). In the second study, the AUT evaluated is bigger: 5.3 meters by 2.1 meters (antenna 2). In both cases, the probe was modelled as an ideal horn (keeping µ=±1). Besides, the AUT radiating elements considered are vertical λ/2 dipoles over a ground...