Twt theory pg one 6-23-20

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causes saturation, an actual decrease in power 
output results. (An increase in the beam current, of 
course, produces a corresponding increase in satura-
tion power and gain.) 
Because the gain of a TWT amplifier depends on 
beam current, the maximum gain that can be 
obtained from a given TWT is limited by the stability 
limit, that is, the beam current value at which the 
TWT begins to oscillate; the safe emission limit of the 
cathode; and the maximum current which can be 
focused through the helix without causing excessive 
current to be intercepted by the helix or other TWT 
elements and thereby producing overheating. For 
high power operation, the TWT must employ 
elements which can dissipate the heat created by the 
RF wave and intercepted beam current. In high 
power TWT amplifiers, therefore, a maximum value is 
often specified for helix and collector power dissipa-
tion.
The accuracy of the impedance match between the 
input and output couples and the helix of a TWT 
determines not only the RF power applied to and 
extracted from the helix, but also the power reflected 
from the input coupler back to the driving source, the 
power reflected to the helix by the output coupler, 
and the flatness of the gain of the TWT across a band 
of frequencies.
Normally, TWT coupler are capable of providing 
impedance matches with voltage-standing-wave 
ratios (VSWR) of less than 1.5 for a cold match and 
less than 2 for a hot match. A cold match is with no 
beam present in the helix, and a hot match is with a 
beam in the helix. Figure 4, is a chart of the hot and 
cold input and output VSWRs for a typical TWT appli-
cation.
Cold-match VSWR's of less than 1.5 over very wide 
frequency bands are achieved in most TWT's by the 
use of precision wound couplers and properly 
matched connecting section of coaxial lines or wave-
guides between the couplers and external RF 
connections.
Sufficient cooling must be available at the collector, 
electron gun, and body of a TWT to remove heat 
dissipated by these elements and thereby maintain 
the temperature at a safe operating level. Normally, a 
TWT is designed for either water or forced air or 
convection cooling. Most solenoids are designed for 
either water or forced-air cooling; however, some 
contain Teflon® or silicon insulation and are, there-
fore, cooled sufficiently by convection means.
In using water coolant, a closed circulating system is 
desirable to hold the content of oxygen and carbon 
dioxide to a low level. A flow interlock should be used 
in the closed system to remove all voltages from the 
TWT if the water coolant flow falls below a nominal 
pressure level.
Many low and medium power TWT's use cooling fins 
or radiators attached to the collector casing to absorb 
heat dissipated by the collector. Air is then forced 
across the surfaces of these cooling elements.
Effects of Output Conditions on TWTs
TWT Cooling
Figure 4