Ka-band measurement results of the irregular near-field scanning system PAMS

The portable antenna measurement system PAMS was developed for arbitrary and irregular near-field scanning. The system utilizes a crane for positioning of the near-field probe. Inherent positioning inaccuracies of the crane mechanics are handled with precise knowledge of the probe location within the transformation algorithm. The probe position and orientation is tracked by a laser while the near-field is being sampled. Far-field patterns are obtained by applying modern multi-level fast multipole techniques. The measurement process includes full probe pattern correction of both polarizations and takes into account channel imbalances. Because the system is designed for measuring large antennas the RF setup utilizes fiber optic links for all signals from the ground instrumentation up to the gondola, at which the probe is mounted. This paper presents results of the Ka-band test campaign in the scope of an ESA/ESTEC project. First, the new versatile approach of characterizing antennas in the near-field without precise positioning mechanics is briefly summarized. The setup inside the anechoic chamber at Airbus Ottobrunn, Germany is shown. Test object was a linearly polarized parabolic antenna with 33 dBi gain at 33 GHz. The near-fields were scanned on a plane with irregular variations of over a wavelength in wave propagation. Allowing these phase variations in combination with a non-equidistant sampling grid gives more degree of freedom in scanning with less demanding mechanics at the cost of more complex data processing. The setup and the way of on-the-fly scanning are explained with respect to the crane speed and the receiver measurement time. Far-fields contours are compared to compact range measurements for both polarizations to verify the test results. The methodology of gain determination is also described under the uncommon near-field constraint of coarse positioning accuracy. Finally, the error level assessment is outlined on the basis of the classic 18-term near-field budgets. The assessment differs in the way the impact of the field transformation on the far-field pattern is evaluated. Evaluation is done by testing the sensitivity of the transformation with a combination of measured and synthetic data.