Calorimeter analysis and test for symmetric circular waveguide mode output

Summary form only given, as follows. To measure the radiated power from a high power millimeter wave device it is preferable to perform calorimetry. A calorimeter has been designed and tested for measurements on a 35 GHz gyroklystron amplifier experiment at NRL. The output radiation is in the form of a TE/sub 01/ circular waveguide mode in a highly overmoded guide (operating frequency is more than three times cutoff). The final calorimeter design is an inverted cone structure with power absorption by octyl alcohol. Low radiation reflection, mode conversion, and extraneous heat dissipation are obtained. Three levels of analysis are performed. A simple ray tracing theory allows reflection coefficient and mode conversion to be found by replacing the symmetric circular mode with an equivalent rectangular one. A finite difference code based on a planar circuit analysis provides the same information with a more rigorous treatment of the rectangular boundary conditions. Finally, HFSS simulations are carried out for a restricted number of candidate designs using the actual circular structure. Cold test results will be described in additions to details of the calorimeter construction and calibration.