This paper presents a sub-THz 3D computational imaging system utilizing compact wideband cavity-backed frequency-diverse antennas fabricated by silicon micromachining. The antennas are integrated with a novel two-section direct waveguide transition feed. A pair of fabricated Mills-Cross transmitter and receiver antennas, generating pseudo-random frequency-diverse patterns within an 18 cm × 18 cm scanning aperture, achieve high-resolution imaging in the 220-330 GHz frequency range, with range and cross-range resolutions of 1.36 mm and 3 mm, respectively. The image reconstruction is done with a Fast Iterative Shrinkage-Thresholding Algorithm (FISTA). The system performance is evaluated by emulated experiments using the measured radiation patterns of the frequency-diverse antennas in the forward model and investigated under various signal-to-noise conditions. The hardware/signal-processing combination performs excellently when the signal-to-noise ratio is 10 dB or better. Already, an image with a signal-to-noise ratio of 10 dB is not distinguishable from an image without noise.