The characterization of integrated antennas is challenging as they usually do not provide a suitable direct interface for the measurement equipment. Adding a connector or using on-wafer probes to interconnect to the measurement system often results in a change in antenna characteristics and requires a separate prototype resulting in additional complexity and associated costs. In order to measure the input impedance and realized gain of an integrated antenna, a contactless characterization method (CCM) can be used. In this article, the CCM is applied to an open-ended waveguide (OEWG). The OEWG operates in the K_a-band and has a gain similar to typical integrated antennas. A good match between the reference measurements and the CCM is achieved, making this method potentially useful for the characterization of integrated mm-wave antennas.