Two deflecting mode RF systems can be used to separate secondary kaons from a momentum selected particle beam, primarily consisting of a mix of protons, pions and kaons, by a time of flight method. The principles of this methods and the choices of the parameters are explained. It is planned to use a 3.9 GHz 13-cell super-conducting cavity operated in a {pi}-dipole-mode. First, the passband structure of different cavity shapes, which mainly differ by the cell-to-cell coupling and the resulting dispersion slope, is investigated. The main concern is the frequency difference between the {pi}-mode and the next lowest mode in the same passband and the implications on the tuneability of the cavity. For three shapes the possible end-cell designs for a 7-cell and 13-cell cavity are presented. All numerical field calculations are performed by the MAFIA code. An equivalent circuit model, based on a two chain model, is applied to a 7-cell cavity. It is demonstrated that the dispersion diagram obtained by MAFIA calculations can be very well approximated by this equivalent circuit.