TRAVELING WAVE TUBE FOR G,000-MC RADIO RELAY 1317 



The maximum output at saturation is obtained at a higher helix voltage 

 as is common in TWT's. The helix voltage also affects the shape of the 

 input-output curves — linear operation being maintained to higher 

 output levels at higher helix voltages. 



As a measure of the efficiency of electronic interaction in a TWT, we 

 use an "electronic efficiency" which is defined as the ratio of the rf out- 

 put power to the beam power (product of helix voltage and beam cur- 

 rent). The "over-all efficiency" we define as the ratio of the rf output 

 power to the total dc power (exclusive of heater power) delivered to 

 the tube. With the collector operated at 1,200 volts, it is about twice the 

 electronic efficiency. For the M1789, maximum efficiency occurs at the 

 saturation level with a helix voltage of 2,600 volts. The electronic and 

 over-all efficiencies there are equal to about 14 per cent and 28 per cent, 

 respectively. 



The curves of Figs. 23(a) and (b) were taken with sufficient time al- 

 lowed for the tube to stabilize at each power level. If the TWT is driven 

 to a high output level after having been operated for several minutes 

 ' with no input signal, the output will be somewhat greater than is shown 

 ' in the curves. It will gradually decrease until it reaches a stable level in a 

 period of about two minutes. This "fade" is caused by an increase in the 

 intrinsic attenuation of the helix near the output end. The increase is a 

 result of heating from rf power dissipation. At maximum output the 

 fade is about 0.6 db (about 15 per cent decrease in output power). At 



the five-watt output level the fade is about 0.1 db (about 2 per cent 



.( — — ■ ' 



Fig. 33 — See opposite page 



j -c: (a) Output power as a function of input power. Both ordinate and abscissa are 

 in dbm (db with respect to a reference level of one milliwatt). A straight line at 

 45° represents a constant gain. A gain scale is included along the top of the figure. 

 For this tube a helix voltage of 2,400 volts gives maximum gain at low signal levels 

 and a voltage of about 2,600 gives maximum output at saturation. 



(b) Gain as a function of output power. This is an alternate way of presenting 

 the information shown in (a). 



(c) Compression as a function of input power. Three regions are shown in the 

 figure. The "compression" region is that in which there is less than one db change 

 in output level for a db change in input level. The "expansion" region is that in 

 which there is more than one db change in output level for a db change in input 

 level. The "inversion" region is that in which the output level decreases when 

 the input level increases (or vice versa). It occurs for input levels greater than 

 that necessarj^ to drive the TWT to saturation. In this region the change in out- 

 put is of opposite sign to the change in input. Using the definition in the text this 

 gives rise to compression values in excess of 100 per cent. 



(d) Compression as a function of output power. 



(e) Conversion of amplitude modulation to phase modulation as a function of 

 input power. This conversion arises because the electrical length of the TWT is a 

 function of the input level. The effect can cause rather serious difficulties in cer- 

 tain types of low index FM systems. 



(f) Conversion of amplitude modulation to phase modulation as a function of 

 output power. 



