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Radio-frequency (RF) scattering parameters (S-parameters) play an important role to characterise RF signal transmission and reflection of active and passive devices such as transmission lines, components, and small-signal amplifiers. Vector network analysers (VNAs) are employed as instrumentation for such measurements. During the last years, the upper frequency limit of this instrumentation has been extended up to several hundreds of GHz for waveguide measurements. Calibration and verification procedures are obligatory prior to the VNA measurement to achieve accurate results and/or to obtain traceability to the International System of Units (SI). Usually, verification is performed by measuring well-matched devices with known S-parameters such as attenuators or short precision waveguide sections (shims). In waveguides, especially above 110 GHz, such devices may not exist and/or are not traceably calibrated. In some cases, e.g. filter networks, the devices under test (DUT) are partly highly reflective. This paper describes the dependency of the S-parameters a) on the calibration procedure, b) on the applied torque to the flange screws during the mating process of the single waveguide elements. It describes further c) how highly-reflective devices (HRD) can be used to verify a calibrated VNA, and d) how a measured attenuation at several hundreds of GHz can be substituted by a well-known coaxial attenuation at 279 MHz, the intermediate frequency (IF) of the VNA, to verify the linearity. This work is a contribution towards traceability and to obtain knowledge about the measurement uncertainty of VNA instrumentation in the millimetre-wave range.

Increasing resolution and accuracy is an important issue in almost any type of radar sensor application. However, both resolution and accuracy are strongly related to the available signal bandwidth and energy that can be used. Nowadays, often several sensors operating in different frequency bands become available on a sensor platform. It is an attractive goal to use the potential of advanced signal modelling and optimization procedures by making proper use of information stemming from different frequency bands at the RF signal level. An important prerequisite for optimal use of signal energy is coherence between all contributing sensors. Coherent multi-sensor platforms are greatly expensive and are thus not available in general. This paper presents an approach for accurately estimating object radar responses using subband measurements at different RF frequencies. An exponential model approach allows to compensate for the lack of mutual coherence between independently operating sensors. Mutual coherence is recovered from the a-priori information that both sensors have common scattering centers in view. Minimizing the total squared deviation between measured data and a full-range exponential signal model leads to more accurate pole angles and pole magnitudes compared to single-band optimization. The model parameters (range and magnitude of point scatterers) after this full-range optimization process are also more accurate than the parameters obtained from a commonly used super-resolution procedure (root-MUSIC) applied to the non-coherent subband data.