Reflection Reduction Through Modal Filtering for Integrated Antenna Measurements Above 100 GHz

Feeding integrated antennas during a measurement requires special feeding structures. Due to the small antenna dimensions, these feeding structures are often much bigger than the antenna under test (AUT) itself and with chip sizes of around 1 mm2, the achievable separation between antenna and feed is limited. Wafer probes have to be used to feed the AUT during passive antenna measurements and present a large reflective surface in close proximity to the AUT. Reflections from the wafer probe cause interference on the measurement surface and distort the results. The same is true for active antenna measurements, where bondwires and the package can have a significant effect on the radiated fields. The fragility and size of the components do not allow to reduce reflections with absorbers, which is why modal filtering was used in this paper to mitigate undesired reflections and improve the measurement result through postprocessing. Two issues that limit the performance of the algorithm are discussed, namely, phase center inaccuracies of the AUT and a limited measurement surface. It is shown that modal filtering is applicable to integrated antenna measurements at frequencies over 100 GHz and that a significant improvement in the measured radiation pattern can be achieved. Furthermore, it is shown that the postprocessed results make it possible to measure the directivity of integrated antennas, despite strong probe reflections.