[IEEE Infrared and Millimeter Waves, Conference Digest of the 2004 Joint 29th International Conference on 2004 and 12th International Conference on Terahertz Electronics, 2004. - Karlsruhe, Germany (Sept. 27 - Oct. 1, 2004)] Infrared and Millimeter Waves, Conference Digest of the 2004 Joint 29th International Conference on 2004 and 12th International Conference on Terahertz Electronics, 2004. - Wide range frequency tuning in volume free electron lasers with rectangular resonator