Microwave power amplifiers containing ‘vacuum electronic devices’ specified above and having all of the following: (a) Operating frequencies above 3 GHz; (b) An average output power to mass ratio exceeding 80 W/kg; and (c) A volume of less than 400 cm 3 ; Note : This category does not apply to equipment designed or rated for operation in any frequency band which is ‘allocated by the International Telecommunication Union (ITU)’ for radio communications services, but not for radio determination.