Passive spatial sampling of mmWave and sub-THz propagation channels using omnidirectional lens antennas

6G wireless systems promise extreme data rates and dense spatial multiplexing, yet accurate characterization of propagation channels at mmWave and sub-THz frequencies remains challenging due to severe path loss and reliance on mechanically scanned directional antennas. Here, we demonstrate a wideband omnidirectional lens architecture enabling uniform near-horizon spatial sampling across the 75-100 GHz and 220-300 GHz bands. The design combines symmetric waveguide excitation, a conical field redistribution stage, and a dielectric lens to impose a transverse phase profile, producing stable azimuthal radiation with minimal angular bias. Functioning as a passive spatial sampling interface, the antenna captures multipath components across the full azimuth without mechanical scanning. Additively manufactured prototypes exhibit broadband impedance matching and realized gains of 7-8 dBi, with measurements matching full-wave simulations up to 300 GHz. Integrated into a wideband channel sounder in a SISO configuration, the platform enables the extraction of channel parameters in indoor environments, providing a scalable approach to channel characterization and future MIMO-relevant propagation modeling for 6G systems.