An ultra-wide band coaxial to rectangular waveguide converter for high power microwave application

Summary form only given. Many high power microwave (HPM) sources generate azimuthally symmetric output modes. As an example, the plasma Cherenkov maser has the ability to deliver hundreds of megawatt over an ultra-wide bandwidth in the coaxial transverse electromagnetic (TEM) mode. If radiated directly from the output windows of the coaxial, this mode will produce doughnut-shaped radiation pattern, with a boresight null. Furthermore, the wide bandwidth characteristic of the source involves that higher-order modes are able to propagate in the coaxial structure and to be excited by a discontinuity. The TE
₁₀
rectangular mode has the advantage to radiate a boresight pattern over a large frequency band. A coaxial to rectangular waveguide converter that transforms progressively the TEM mode of the output Cherenkov maser, into a system composed by two TE
₁₀
standard rectangular waveguide modes with high power-handling and wide bandwidth capability has been investigated. The converter has been designed, modeled and tested in the 2 GHz-6 GHz band using the electromagnetic code CST Microwave Studioreg (finite integral method). The reflection coefficient of each propagating mode (including higher-order modes) show, a total return loss less than -7 dB. In addition, the ability of the converter to transmit high power has been tested numerically. The total electric field in the structure over the 2 GHz-6 GHz band does not exceed the threshold that a waveguide can transmit, which is limited by the voltage breakdown of the metallic walls of the waveguide in vacuum (150 kV/cm)
²
. Finally, an experimental procedure has been defined in order to validate the numerical results. A low power test setup, including a conical transition between the low power source and the coaxial to rectangular waveguides converter, will provide measurements of S-parameters. The far field pattern will be measured for a horn antenna connected to one of the rectangular waveguide output. Experimental test using a large band HPM source will prove the capability of the structure to radiate efficiently the power, in a TE
₁₀
mode mainly