Observations of the flux density ratio of Cas A to Cyg A at 13.5, 15.5 and 16.5 GHz made in 1994 and 1995 with the Cosmic Anisotropy Telescope are presented. These observations are compared with the predictions of Baars et al. and lower frequency observations, in order to understand better the frequency dependence of the secular decrease in the flux density of Cas A. When compared with measurements at other frequencies, these observations imply that there is a frequency dependence to the secular decrease in the flux density of Cas A, but that it is not as strongly frequency dependent as was thought by Baars et al. when they constructed their absolute flux density spectrum of Cas A.