Ion cyclotron heating (ICH) is applied to TMX-U to improve the thermal barrier performance by reducing the passing ion collisionality. During its development, measurements of the antenna loading resistance, Rp, and the absorption efficiency, η, were compared with calculations with the antenna design code ANTENA over a wide range of densities and frequencies. Good agreement in Rp was obtained in the short wavelength slow wave regime but not for long wavelength fast waves because the experimental magnetic field gradients are not modelled in ANTENA. Similarly, η is much larger experimentally (40%) than in ANTENA (10%) due to the magnetic beach in TMX-U. In its application, ICH successfully decreased the passing ion collisionality tenfold but did not extend thermal barrier plugging to higher density, indicating that collisional barrier filling is not currently limiting TMX-U performance.