TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 1287 



plated system application made it necessary to investigate in detail the 

 problems associated with band flatness, matching, noise output, certain 

 signal distortions, reproducibility, and long life. 



The solution of some of these problems required the development of a 

 precisely constructed helix assembly in which the helix winding is bonded 

 to ceramic support rods by glaze. Others required the initiation of a life 

 test program. Early results indicate that life exceeding 10,000 hours 

 can be obtained. This, in no small measure, is a result of a dc potential 

 profile which minimizes the ion bombardment of the cathode. Since 

 power consumed by focusing solenoids seriously degrades the o\'erall 

 efficiency of a traveling-wave amplifier, permanent magnet focusing cir- 

 cuits such as the one shown in Fig. 1 have been designed. Finally, to 

 further improve efficiency, a collector which can be operated at abcut 

 half the helix voltage was developed. 



The major difficulties encountered in the course of the MI789 develop- 

 ment were: excessive noise output, ripples in the gain-frequency char- 

 acteristic, and lack of reproducibility of gain. There is evidence that a 

 growing noise current wave on the electron stream was the source of the 

 high noise output. This phenomenon has been observed by a number of 

 experimenters but is not yet fully explained. By allowing a small amount 

 of the magnetic focusing flux to link the cathode, the growing noise wave 

 was eliminated, and the noise reduced to a reasonable level for a power 

 amplifier. Reflections caused by slight non-uniformities in the helix pitch 

 were the source of the gain ripples. Precise helix winding techniques re- 

 duced these reflections so that the ripples are now less than ±0.1 db. 

 The lack of reproducibility in gain was caused by variations in helix 

 attenuation. Here, too, careful construction techniques alleviated the 

 problem so that in a recent group of tubes the range of gain variation at 

 five watts output was ±2 db. 



We have divided this paper into four main parts. The next section 

 discusses some of the factors affecting the design of the traveling-w-ave 

 tube. (We will henceforth use the abbreviation TWT.) Section III 

 describes the tube itself. Certain performance data are included there 

 when closely related to a particular portion of the tube. Section IV 

 considers the rf performance in detail. There comparisons are made 

 lietween the performance predicted from TWT theory and that actually 

 observed. Finally Section V summarizes our life test experience. 



This paper is written primarily for workers in the vacuum tube field 

 and assumes knowledge of TWT theory. However, we believe that 

 readers interested in TWT's from an application standpoint may also 

 benefit from the discussion of the rf performance in Section IV. Much of 

 that section can be understood w'ithout detailed knowledge of TWT's. 



