(TWT)

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.

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

high-power twt VTR 572B Russian low-power twt UV-1B (cyrillic: УВ-1Б)

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

Origen from thermionic valve or vacuum tube development

The tube itself was born out of the need for efficient high power microwave RF amplifiers in the Second World War.

TWT was actually invented and first developed by an engineer named by Rudolf Kompfner at a government based British radar research laboratory.

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.

The travelling wave tube, TWT, can be split into a number of separate major elements: Vacuum tube Electron gun Magnet and focusing structure RF input Helix RF output Collector

Cutaway view of a TWT. (1) Electron gun; (2) RF input; (3) Magnets; (4) Attenuator; (5) Helix coil; (6) RF output; (7) Vacuum tube; (8) Collector.

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.

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.

An octave means here that the upper cutoff frequency is the double value of the lower cutoff frequency. 

collector

electron beamattenuatingcover

helix

anode

electron gun

Coupling resonators

input output

Physical construction of a twt

RF- inputelectron-beambounching

influence of-attenuating cover

RF induced into Helix

The electron- beam bounching 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.

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.

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.

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.

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.

vacuum tube - vacuum that is required for the operation of the TWT.

first element-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.

electrons are made to travel as a tight or narrow beam along the length of the travelling wave tube

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.