Twt theory pg one 6-23-20

Characteristics of Ring Loop TWTs
Figure 1
Figure 2

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Traveling Wave Tube
Theory of Operation pg. 2
In normal operation, an RF signal introduced at the 
cathode end of the helix travels along the circum-
ference of the helix at approximately the velocity of 
light, although the velocity in the direction of the 
beam is reduced by the pitch-to-circumference 
ratio of the helix. When the voltage on the helix is 
properly adjusted, the velocity of the beam elec-
trons is made slightly greater than the velocity of 
the RF signal. Electrons emitted from the cathode 
transfer energy to the RF signal on the helix as the 
signal velocity modulates the beam on the way to 
the collector. This is known as forward-wave ampli-
fication in contrast to TWT's that operate as back-
ward-wave amplifiers. In backward-wave amplifica-
tion, the RF signal is introduced at the collector end 
of the helix and travels down the helix in the direc-
tion opposite to that of the beam.
Electrons intercepted by the collector of the TWT 
are returned to the cathode through an external 
collector supply. Resultant helix DC interception is 
returned to the cathode through an external over-
load relay and a regulated beam supply. The over-
load relay removes beam power when the helix 
current becomes excessive, which protects the 
TWT from self-destruction. Modulation in the Ring 
Loop TWT is accomplished by applying the carrier 
signal to the RF input element and applying the 
intelligence to the control anode. By this, the TWT 
is made to either amplify or phase or pulse modu-
late the RF carrier signal, depending on the applied 
signal and the TWT parameters. For pulse opera-
tion, or modulation, a TWT operates at the same 
helix voltage as the TWT would operate under cw 
conditions. For a given TWT type, therefore, the 
peak power output can be made larger than the cw 
power output only by an increase in the beam 
current. Different methods of modulating the beam 
current are available.