A Multi-mode Measurement Technique for Microwave Devices supporting Degenerate Modes

In Satellite Communications, the increasing demand for high data-rate is leading to innovative multi-beam and multi-band antenna architectures to be embarked in Geostationary Orbit (GEO) platforms. The ultimate goal is to increase the overall system capacity, while decreasing the system cost, so as to compete with terrestrial communication infrastructures. As in terrestrial networks, very high capacity can be implemented in GEO networks by exploiting spatial diversity, which, in turns, requires multi-spot coverages. In these system architectures, the geographical area of interest is divided into several small spots each one illuminated by an antenna beam operating at a giver “color”, defined as a specific combination of physical resources, namely frequency band and polarization. These systems operate the user links in K band (downlink) and in Ka band (uplink), while the links towards the gateways (feeder links) are moved to Q and V bands. To make the antenna systems compatible with the mass and envelope constraints of the platforms, dual-band antenna-feed chains can be used. These assemblies are designed to simultaneously provide the Tx downlink beams in K band and Rx uplink beams in Ka band. Finally, Additive Manufacturing (AM) enables the production of complex shaped RF sub-assemblies in single monolithic mechanical parts. Combining the novel asymmetric OMJ layouts with metal-based AM technologies results in an effective minimization of mass, envelope and interface of antenna-feed chains. At the same time, asymmetric OMJs suffer from the generation of higher-order modes that are above-cutoff in the upper Ka band. These modes can decrease the RF antenna performance. Although the excitation of higher-order modes can be controlled by design upto a certain extent, dimensional inaccuracies of 3D-printed parts can lead to a relevant excitation of spurious higher-order modes. In both cases, it is important to experimentally evaluate the level of the high-order modes generated by the OMJs and propagating towards the feed-horn. Although some multi-mode measurement techniques are already available, none can deal with the degenerate modes propagating in the common waveguide of a dual-polarization antenna-feed chain. The present works deals with the definition of a novel multi-mode measurement technique that can be applied also in the case of degenerate modes. The technique developed in this work is based on a simple multi-mode OMJ that can be calibrated by adopting four calibration standards connected to its common waveguide. Once the multi-mode OMJ is calibrated, it can be connected to the Device Under Test, and the full measurement of the DUT multi-mode scattering matrix can be carried out. The conceived technique has been developed for the specific case of a 3D-printed K/Ka-band feeding network, where five modes have to be considered at its common circular waveguide. The technique has been validated by emulating the measurement campaign and by evaluating the uncertainty in the de-embedded scattering parameters of the five-mode OMJ. Additionally, a user Graphic Unit Interface (GUI) has been developed. A DUT consisting of a two-port mode-coupler has also been designed to experimentally test the measurement technique in the case of a simpler and more reliable benchmark. Finally, a partial characterization of the DUT has been investigated with the goal of retrieving the reduced set of the most important scattering parameters, while reducing the number of measurements required.