Multiple Reflection Reduction in mm-Wave Antenna Characterization Using Scattering Caps

Multiple reflections (MRs), particularly between the probe and the antenna under test (AUT), degrade measurement accuracy. Conventional absorber-based mitigation techniques lose effectiveness above 100 GHz, and their large size causes them to extend undesirably close to the antenna aperture. Perturbing impedance and radiation pattern measurements. In this study, we analyze the contributions of these MRs and reintroduce a metal flange cover, e.g., “scattering cap,” to mitigate the errors introduced by such reflections and, with it, improve the measurement system. The design methodology is presented and validated across three frequency bands within 26–330 GHz using simulations, for probe geometries, including open-ended waveguides (OEWGs) and horn antennas. Its effectiveness is experimentally evaluated for near-field (NF) and far-field (FF) measurements with an OEWG probe antenna alongside various types of electromagnetic-absorbing materials in the 90–140-GHz frequency band. However, while carbon-based absorbers typically degrade and inconsistently affect the probe antenna behavior, the scattering cap does not. The scattering cap reduces standing-wave ripples in NF measurements from 1.3- to 0.5-dB peak-to-peak, representing a reduction of more than 60%. The proposed scattering cap provides an effective, cheap, and easy-to-manufacture solution with wideband applicability to reduce MRs, making it highly suitable for accurate and consistent antenna measurements.