There are other works, who did study theoretical background of EMDrive experimentally, like the Juday-White's interferometers

The Eagleworks Lab's warp-field interferometer (WFI) has been using an aluminum 1.48 GHz pancake resonant cavity since the first time I talked about it at NSF.com during the spring of 2015, and was included in Dr. Rodal's NSF.com summary of that time period, see attached slide. In short while in-air, using 25W-to-40W of 1.48 GHz RF in an non-frustum aluminum resonant cavity with a loaded Q-factor of ~3,000, the EW lab folks documented spacetime compression LIKE effects with signal to noise rations well above the system's noise platform, see attached PRELIMINARY 2015 & 2016 test result slides. The better WFI signals in the 2016 tests were attributed to higher RF power delivered to the cavity via better RF tuning and RF loss management in the RF amplifier system driving it. (And yes, the generated spacetime compression effect magnitude is VERY non-linear with respect to RF input power to the cavity appearing to scale with the 4th power of the driven peak E-fields.

However, the EW folks will have to redo this test series in a hard vacuum (~5x10^-6 Torr) before the professional doubter will acknowledge these preliminary test results as a real spacetime compression effect, instead of just another experimental error. The EW lab folks will also have to perform a time-of-flight test series in the same pancake resonant cavity to further buttress these initial in-air WFI tests. In other words, when NASA management considers this activity import enough, it will get done, but only when the financial and labor support shows up to do it.

The EW lab used the aluminum pancake resonant cavity in the WFI instead of the frustum cavity because they didn't want to generate thrust with this system, but just a spacetime compression effect that could be used to envelope a vehicle in a warp drive bubble. However the same spacetime compression effects should be occurring in the frustums as well but combined with a way to force rectify the spacetime compression effects into a unidirectional force.