A Near Field 3D Scanner for Millimetre Wavelengths

Typical millimetre wave astronomical receiver systems make use of quasi-optical (QO) components such as lenses, interference filters, polarisers, or polarisation modulators. Each of these may be located in the near field of additional components. Combinations of these components lead to standing waves and reflections, which degrade the overall RF performance. Therefore, it is important to characterise the near field of QO components and QO systems, as well as the evolution of the near field along the propagation axis. Additionally, it is not always feasible to measure the far-field of large systems. This limitation may be overcome by using analytic methods to transform near field measurements into far field radiation patterns. We have developed a 3D near field scanner for millimetre wavelengths. The scanner has a working volume of 50 cm ×50 cm ×50 cm and is coupled to a vector network analyser (VNA) operated in the W-band (75-110 GHz). Volumetric scanning provides the advantage of being able to directly measure field parameters at all positions in the optical path and to map the evolution of the near field. We present the design and performance of this facility together with the initial near field characterisation of some QO components that are also compared to RF simulations.