Abstract: A frequency agile magnetron employs an altitude compensation vacuum envelope external to the vacuum envelope of the magnetron. The tuning assembly of the magnetron includes a tuning plunger coupled through a first bellows assembly to one end of a tuning shaft. The tuning shaft is coupled at its opposite end through a second bellows assembly to the compensation vacuum envelope. When the two bellows assemblies are identical, atmospheric forces acting upon the tuning shaft are balanced at any altitude. Translation of the tuning assembly is provided by an actuator such as a linear motor coupled to the tuning shaft. Improved transfer of heat from the resonant cavity is provided.