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Novel sensors for ultrashort-distance measurements, with choke flanges applied at the open ends of both a WR10 waveguide and a circular waveguide, featuring a radius of 1.42 mm, are proposed. Field distributions between the sensors and a target are analyzed and explained on the basis of higher order modes of a radial line formed by the sensors and a planar reflective target. A robust equivalent circuit using a cascade of lossy waveguides is proposed and a new calibration/correction method is derived. Properties of both sensors are verified experimentally for target distances of 0–6 mm (more than one and half of wavelength). The new circular waveguide sensor operates in a frequency band of 75-76 GHz with a maximum error of measured distance $19~\mu \text{m}$. The new rectangular waveguide sensor enables to use an outstanding applicable frequency bandwidth from 76 up to 86 GHz, with best properties around 80 GHz, where the maximum error of the measured distance is 12 $\mu \text{m}$. The new sensors make possible to achieve about ten times smaller distance measurement errors in comparison with the state-of-the-art sensors. [ABSTRACT FROM AUTHOR]