A novel, compact arrangement for Ka-band mode couplers, which convert a rectangular waveguide TE(1,0) to cylindrical waveguide TE(2,1) mode, has been designed, constructed, and tested. The design features a set of longitudinal slots, positioned in regions of negligible current flow for the TE(2,1) mode, allowing its propagation to be preferentially favored, by suppression of the fundamental TE(1,1) mode. Numerical simulations and experimental measurements display good agreement, showing transmission of the intended TE(2,1) mode at levels of better than -5 dB, from a frequency of ∼37.5 to 41 GHz. Subsequent farfield measurements confirm the presence of the TE(2,1) mode, demonstrating good agreement when compared with analytical expectations. Such a device would be an ideal candidate for an application where mode purity, bandwidth, and ease of construction are of primary importance and where the transmission efficiency is of limited concern.