The traveling wave tube is a form of thermionic valve or tube that is still used for high power microwave amplifier designs. The travelling wave tube can be used for wideband RF amplifier designs where even now it performs well against devices using newer technologies. TWTs are used in applications including broadcasting, radar and in satellite transponders. The TWT is still widely used despite the fact that semiconductor technology is advancing all the time.

The traveling wave tube, TWT, has a high bandwidth compared to many other designs and it can operate over bands of up to an octave, although it is possible to utilise narrow band designs when the applications require this. The TWT is essentially a microwave amplifier and TWT amplifiers may be used on frequencies ranging from around 300 MHz up to 40 or 50 GHz and providing gain levels of up to 40 dB in a single device.

TWT amplifiers, TWTAs may consist of two types. Broadband types generally use helix TWT amplifiers and they are typically capable of developing powers of up to 2.5 kW. For narrowband RF amplifier applications it is possible to use coupled cavity TWTs and these can deliver power levels of up to 15 kW.

TWT history and development

The travelling wave tube (traveling wave tube) can trace its origins back to the earliest days of thermionic valve or vacuum tube development. Without the development of these devices the travelling wave tube would not be possible. The tube itself was born out of the need for efficient high power microwave RF amplifiers in the Second World War.

The TWT was actually invented and first developed by an engineer named by Rudolf Kompfner at a government based British radar research laboratory. He developed the basic TWT theory and the practical device. Both the TWT theory and the tube itself were later refined by Kompfner and John Pierce at Bell Labs in the USA.

TWT Construction

The travelling wave tube, TWT, can be split into a number of separate major elements:

Vacuum tube Electron gun Magnet and focussing structure RF input Helix RF output Collector

Traveling wave tube operation

The travelling wave tube, TWT is contained within a glass vacuum tube. This obviously maintains the vacuum that is required for the operation of the TWT.

Within the travelling wave tube the first element is the electron gun comprising primarily of a heated cathode and grids. This produces and then accelerates a beam of electrons that travels along the length of the tube.

In order that the electrons are made to travel as a tight or narrow beam along the length of the travelling wave tube, a magnet and focussing structure is included. The field from the magnet keeps the beam as narrow as required and in this way ensures that the beam travels along the length of the TWT.

The RF input consists of a direction coupler which may either be in the form of a waveguide or an electromagnetic coil. This is positioned near the electron gun emitter and it induces current into the helix (see below).

A helix is an essential part of the traveling wave tube. It acts as a delay line, in which the RF signal travels at near the same speed along the tube as the electron beam. The electromagnetic field due to the current in the helix interacts with the electron beam, causing bunching of the electrons in an effect known as velocity modulation and the electromagnetic field resulting from the beam current then induces more current back into the helix. In this way the current builds up and the signal is therefore amplified.

The RF output from the traveling wave tube consists of a second directional coupler. Again this may either be an electromagnetic coil of a waveguide. This is positioned near the collector and it receives the amplified version of the signal from the far end of the helix from the electron gun or emitter.

An attenuator is included on the helix, usually between the input and output sections of the TWT helix. This is essential to prevent the reflected wave from travelling back to the cathode of the electron gun.

The collector finally collects and absorbs the electron beam. It is in this area that high levels of power may be dissipated and therefore this section of the travelling wave tube can become very hot and will require cooling.

Travelling wave tube applications

There are many areas in which TWT amplifiers are used. They are an ideal form of RF amplifier for satellites and as a result they are extensively used for satellite transponders where low levels signals are received and need to be retransmitted at much higher levels. In addition to this TWT amplifiers are used in microwave radar systems where they are able to produce the high levels of power required. Traveling wave tube, TWT technology is also used for electronic warfare

applications. In these applications the grid on the travelling wave tube may be used to pulse the transmission.

Traveling Wave Tube

Traveling wave tubes (twt) are wideband amplifiers. They take therefore a special position under the velocity-modulated tubes. On reason of the special low-noise characteristic often they are in use as an active RF amplifier element in receivers additional. There are two different groups of twt:

low-power twt for receivers occurs as a highly sensitive, low-noise and wideband amplifier in radar equipments

high-power twt for transmitters these are in use as a pre-amplifier for high-power transmitters.

Furthermore they are introduced to:

15 1/8 "

Figure 2. Russian low-power twt UV-1B (cyrillic: УВ-1Б)

15 1/8 "

Figure 2. Russian low-power twt UV-1B (cyrillic: УВ-1Б)

Physical construction and functional describing Characteristics of a twt

o twt's poweramplification o twt's bandwith

Physical construction and functional describing

The Traveling Wave Tube (twt) is a high-gain, low-noise, wide-bandwidth microwave amplifier. It is capable of gains greater than 40 dB with bandwidths exceeding an octave. (A bandwidth of 1 octave is one in which the upper frequency is twice the lower frequency.) Traveling-wave tubes have been designed for frequencies as low as 300 megahertz and as high as 50 gigahertz. The twt is primarily a voltage amplifier. The wide-bandwidth and low-noise characteristics make the twt ideal for use as an rf amplifier in microwave equipment.

couplingresonators

helix

attenuatingcover

Collector

electron gun

electron beam

input

output

Figure 3. - Physical construction of a twt

couplingresonators

helix

attenuatingcover

collector

anode

electron gun

electron beam

input

output

Figure 3. - Physical construction of a twt

RF- input

influence of-attenuating cover

RF induced into Helix

electron-beambunching

Figure 4. - Amplified helix signal

RF- input

influence of-attenuating cover

RF induced into Helix

electron-beambunching

Figure 4. - Amplified helix signal

The physical construction of a typical twt is shown in figure 3. The twt contains an electron gun which produces and then accelerates an electron beam along the axis of the tube. The surrounding magnet provides a magnetic field along the axis of the tube to focus the electrons into a tight beam. The helix, at the center of the tube, is a coiled wire that provides a low-impedance transmission line for the rf energy within the tube. The rf input and output are coupled onto and removed from the helix by waveguide directional couplers that have no physical connection to the helix. The attenuator prevents any reflected waves from traveling back down the helix.

The following figure shows the electric fields that are parallel to the electron beam inside the helical conductor.

Figure 5. - electron- beam bunching and a detail-foto of a helix (Measure detail for 20 windings)

Figure 5. - electron- beam bunching and a detail-foto of a helix (Measure detail for 20 windings)

The electron- beam bunching already starts at the beginning of the helix and reaches its highest expression on the end of the helix. If the electrons of the beam were accelerated to travel faster than the waves traveling on the wire, bunching would occur through the effect of velocity modulation. Velocity modulation would be caused by the interaction between the traveling-wave fields and the electron beam. Bunching would cause the electrons to give up energy to the traveling wave if the fields were of the correct polarity to slow down the bunches. The energy from the bunches would increase the amplitude of the traveling wave in a progressive action that would take place all along the length of the twt.

Characteristics of a twt

Figure 6: characteristic of a traveling wave tube

The attainable power-amplification are essentially dependent on the following factors:

constructive details (e.g. length of the helix) electron beam diameter (adjustable by the density of the focussing magnetic field) power input (see figure 6) voltage UA2 on the helix

As shown in the figure 6, the gain of the twt has got a linear characteristic of about 26 dB at small input power. If you increase the input power, the output power doesn't increase for the same gain. So you can prevent an oversteer of e.g the following mixer stage. The relatively low efficiency of the twt partially offsets the advantages of high gain and wide bandwidth.

Given that the gain of an twt effect by the electrons of the beam that interact with the electric fields on the delay structure, the frequency behaviour of the helix is responsible for the gain. The bandwidth of commonly used twt can achieve values of many gigahertzes. The noise figure of recently used twt is 3 ... 10 dB.

The helix may be replaced by some other slow wave structure such as a ring-bar, ring loop, or coupled cavity structure. The structure is chosen to give the characteristic appropriate to the desired gain/bandwidth and power characteristics.

Ring-Loop TWT

Figure 7: Ring-Loop slow wave structure

A Ring Loop TWT uses loops as slow wave structure to tie the rings together. These devices are capable of higher power levels than conventional helix TWTs, but have significantly less bandwidth of 5…15 percent and lower cut-off frequency of 18 GHz.

The feature of the ring-loop slow wave structure is high coupling impedance and low harmonic wave components. Therefore ring-loop traveling wave tube has advantages of high gain (40…60 Decibels), small dimension, higher operating voltage and less danger of the backward wave oscillation.

Figure 8: Ring-Bar slow wave structure

Ring-Bar TWT

The Ring-Bar TWT has got characteristics likely the Ring-Loop TWT. The slow wave structure can be made easier by cut-out the structure of a copper tube.

Figure 9: Coupled-cavity slow wave structure

Coupled-cavity TWT

The Coupled-cavity TWT uses a slow wave structure of a series of cavities coupled to one another. The resonant cavities are coupled together with a transmission line. The electron beam (shown in figure 9 as red beam) is velocity modulated by an RF input signal at the first resonant

cavity. This RF energy (displayed as blue arrow) travels along the cavities and induces RF voltages in each subsequent cavity.

If the spacing of the cavities is correctly adjusted, the voltages at each cavity induced by the modulated beam are in phase and travel along the transmission line to the output, with an additive effect, so that the output power is much greater than the power input.