An additional high power RF system has been built for Nimrod to provide an 8th harmonic accelerating field i.e. the 2nd harmonic of the main RF. Its purpose is to reduce the bunching factor of the proton beam thus increasing beam acceptance and accelerated intensity. In the accelerating cavity a drift tube is tuned over a frequency range from 2.8 MHz to 16 MHz by two ferrite loaded resonators. The configuration adopted proved to have considerable advantages over a self contained tuned cavity particularly for assembly and access, permitting economy in the use of ferrite. The problems discussed include that of covering a wide tuning range while restricting the magnetic flux in the ferrite, suppressing unwanted modes in the cavity and maintaining accurately the phase and amplitude of the harmonic field relative to the main RF.