Generic EM Drive Information
This is a location for assorted information concerning EM Drives. Likely applies to all current experimental efforts.
Simulating With MEEP
MEEP is is a free finite-difference time-domain (FDTD) simulation software package developed at MIT to model electromagnetic systems. A number of people, including @aero, have been using it to simulate the evolution of the fields within the EM Drive.
Conservation of Energy and Momentum
An EM Drive is a cavity resonator that uses the Quality Factor Q to achieve greater response. There is definitely a peak resonant frequency to achieve the highest Q, though besides Q and frequency, mode shape must also be considered. The shape of the electric and magnetic fields inside the cavity at a given frequency.
The natural frequencies (and therefore the mode shapes) of a cavity are sensitive to the dimensions of the cavity.  A dielectric insert lowers the natural frequency at which a given mode shape takes place. Here is a plot of what a few mode shapes in a cylindrical cavity look like when viewed perpendicular to the circular cross section plane.