For miniaturized vacuum klystron type devices utilization of a double- gap strip-line cavity with suspended ceramic substrate is very promising. 
The two-mode operating mode of such cavities practically isn't studied enough; so there is the great scientific and practical interest for research of electrodynamic characteristics of cavities during multi-mode operation that correspond to simultaneous excitement of anti-phase and co-phase modes in a double gap.  
In this article results of the 3-D numerical modeling by means of the Ansoft HFSS code are presented, and also the experimental results of the electrodynamic properties of the half-wave double- gap cavity with the suspended ceramic substrate are submitted. Optimum parameters of this design, allowing to carry out simultaneous excitement at multiple frequencies with frequency rate (K=2) are determined.
In such cavity high values of unloaded Q factors isn’t need because dielectric losses and conductor losses can be compensated at the expense of  the negative electronic conductivity that brought by modulated electronic beam. The operating mode with frequency rate   can be used for increasing of efficiency of generating and amplifying micro devices with filament cathode and matrix field emission cathodes.  

Keywords: double-gap cavity, electrodynamic parameters, mathematical simulation, resonance frequency, characteristic impedance, method of planned experiment