Material Detection and Sizing of Sub-Resonant Particles by mm-Wave Far-Field Scattering Measurements Using Orthomode Diplexers

Bi-static measurements of the scattering of sub-resonant, sub-wavelength-sized spheres and water drops at millimeter-wave frequencies are applied for simultaneous material classification and size estimation. Measurements at Ka-band (26.5–40 GHz) and W-band (75–110 GHz) frequencies are presented that reliably detect the correct material and size of the samples under test. In comparison to traditional resonance-based methods, the sub-resonant approach used here advantageously allows us to characterize smaller particles at a given frequency or to reduce the operating frequency for a given particle size. For the dual-polarized scattering measurements, a novel component (a resonant orthomode diplexer) is proposed. A multi-static measurement is used to additionally determine the position of the sample in the scattering plane. This allows for accurate material classification and size estimation even for a displaced particle. Dielectric and metal spheres with a diameter of 0.4–2 mm are investigated as well as water drops with a volume of 0.5–4 µL.