|Theory of a Traveling Wave Tube|
|A TWT performs as a wideband
microwave amplifier. This wideband amplification feature is obtained
by the use of an interaction circuit which is essentially a
transmission line and does not usually contain any resonant
All TWT's incorporate the basic components shown in the figure below. These components include an electron gun (composed of a cathode, a control or modulating grid, and an accelerator), which produces an electron beam; an rf circuit (delay line), which propagates a microwave signal in a manner that permits interaction between the beam and the signal; an attenuator, which isolates the input and output sections to prevent oscillations; and a collector, which removes the unused beam energy. In addition, a magnetic field along the axis of the device (not shown) is used to focus the electron stream. Although TWT's may differ widely in size and construction, depending on the application, the basic theory of operation is the same for all types.
The principle of operation of a TWT is strikingly simple. As illustrated here, an electron beam is emitted from the cathode and accelerated toward the collector at the opposite end of the device. The electrons of the beam are surrounded by an rf wave with a strong field component in the direction of the beam travel. If the velocities of the beam and the wave are nearly the same, interaction takes place.
The delay line slows the rf wave down to the velocity of practical beams. This line is constructed so that the field components are primarily longitudinal in the vicinity of the beam. If a helix is used as the delay line, the rf wave actually travels along the path of the helix, but the beam sees only the much longer and slower wave components in the longitudinal direction; that is in traveling around one turn of the helix, the wave proceeds forward only by one pitch. The resultant velocity is a fraction of the original wave velocity.
|Characteristics of Ring Loop TWT's|
|The Ring Loop TWT's are best
known for an ability to amplify microwave signals simultaneously
over a radar band of frequencies. A Ring Loop TWT is illustrated
schematically in Figure 2, which also shows the power supplies to
operate the TWT. This TWT consists of an electron gun (composed of a
heater, cathode, and control anode), a helix (slow-wave structure),
an attenuator, a collector, and an input and an output coupling
Four sources of power are normally required to operate a Ring Loop TWT: heater power, regulated beam power, gain or modulation control power, and collector power. The electron beam in some TWT's is focused by electrostatic or electromagnetic methods, and in these situations, a power source is required to operate the beam focusing device.
In normal operation, an rf signal introduced at the cathode end of the helix travels along the circumference 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 electrons 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 amplification in contrast to TWT's that operate as backward-wave amplifiers. In backward-wave amplification, the rf signal is introduced at the collector end of the helix and travels down the helix in the direction 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 overload relay and a regulated beam supply. The overload 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 modulate the rf carrier signal, depending on the applied signal and the TWT parameters. For pulse operation, 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.
Ring Loop backward wave amplifiers have very narrow active bandwidths for which gain is possible but the active bandwidth is voltage tunable. Therefore such TWT's were used originally as tunable filters with gain. These had applications in electronic countermeasure systems.
Effects of Input Conditions on Traveling Wave Tubes
|Input conditions that affect TWT
operation include beam current, drive power, helix voltage, drive
frequency, and heater voltage. For a given beam current, the power
output of a TWT amplifier is a function of the rf drive power as
shown. The gain is essentially constant at a given
frequency for low rf drive levels, but decreases at higher levels.
When the rf electric field becomes too strong, as a result of either
amplification or input signal, the amount of energy which the beam
can deliver to the wave reaches a maximum limit. This condition,
known as the saturation point of the TWT, represents the maximum
power which can be delivered for a given condition of beam current.
If the input power is increased beyond the value which causes
saturation, an actual decrease in power output results. (An increase
in the beam current, of course, produces a corresponding increase in
saturation 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 dissipation.
Effects of Output Conditions on Traveling Wave Tubes
|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
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. Below is a chart of the hot and cold input and output VSWRs for a typical TWT application.
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 waveguides between the couplers and external rf connections.
Pulse Ring-Loop TWT
|The smallest and lightest TWT's
for pulsed radar and most advanced designs for commercial,
electronic warfare and radar systems use are included in this CPI
product line. TWT's in this group cover 1.0 kW to 10 kW peak power
(20 kW of peak power is attainable for a Ring Loop TWT), at
frequencies from 1.0 to 9.6 Ghz.
This unique ring-loop slow-wave structure used in these pulsed TWT's provides high gain at kilowatt peak rf power levels without oscillation. TWT's using this structure provide beam efficiencies as high as 25 percent without collector depression. TWT's with collector depression have achieved greater than 30% efficiency.
All high power pulse TWT's are metal-ceramic structures that provide exceptional reliability under the most adverse conditions encountered in aircraft, shipboard, and missile environments.
CPI's development of the UNIGRID« nonintercepting grid has solved many of the problems of close spaced shadow gridded TWT's in meeting temperature spaced shadow gridded TWT's in meeting temperature cycling, shock and vibration environmental encountered in military airborne systems. The CPI UNIGRID« attaches the shadow grid directly to the surface of the cathode of the TWT making an integral structure virtually immune to environmental problems.
A ring-loop TWT is a natural filter circuit and therefore features very low second harmonic emissions, low out-of-band emissions, and there are no backward wave oscillations.
List of TWT's (Type and Frequency Ranges)