TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 



1303 



(1) The dielectric loading and intrinsic attenuation of the helix are 

 comparatively low because the amount of supporting structure in the rf 

 fields is small. 



(2) High loss per unit length in the helix attenuator is made possible. 

 The reason for this will be discussed further below. 



(3) The heat dissipation capability of the helix is greatly increased 

 because the glaze provides an intimate thermal contact between winding 

 and support rods. This is illustrated by Fig. 16 which compares the heat 

 dissipation properties of glazed and non-glazed helices. 



(4) Mechanical rigidity is realized and therefore the helix can be 

 handled without risk of disturbing the pitch or diameter of the winding. 



On the other hand, use of the ceramic rods in the j\II789 has a signifi- 

 cant disadvantage in that it makes the outside radius of the vacuum 

 envelope large compared to the helix radius, thus making coupled helix 

 matching out of the question. However, since the MI789 is required to 

 match over less than a 10 per cent band, this is not particularly serious. 



To obtain reproducibihty of performance in the MI789, the helix 

 must be precisely constructed. Together, the pitch of the helix and the 

 amount of dielectric loading determine the synchronous voltage. A 

 pitch variation of ±1 per cent results in a voltage variation of about 

 ±50 volts, and a loading variation of it 1 per cent results in a variation 



»- 



ir -■ 



m 

 U 



LU 

 CL 



2.5 5.0 7.5 10.0 12.5 15.0 



POWER OUTPUT IN WATTS 



17.5 20.0 



Fig. 13 — Per cent interception on the helix a.s a function of rf power output. 

 These measurements were made u.sing permanent magnet circuit.s charged to 

 different field strengths. The magnetic field variation as a function of distance 

 from the cathode was as shown in Fig. 10. The component of magnetic field per- 

 pendicular to the tube axis in these circuits was less than 0.2 per cent of the longi- 

 tudinal field. All measurements were taken with a beam current of 40 ma and with 

 the helix voltage adjusted to maximize the power output. 



