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Thursday, 21 February 2019

Travelling Wave Tube in Microwave Engineering


The travelling wave tube (TWT) is an amplifier which makes use of a distributed interface between an electron beam and a travelling wave. To extend the interface between an electron beam and an RF field, it is compulsory to make sure that they are equally moving in the similar way with almost the equal velocity. It differs from the multi-cavity klystron in which the electron beam travels, but the RF field stays motionless. The electron beam moves with a velocity preside over by the anode voltage which for a typical value of anode voltage can be as high as 0.1 vc where vc is the velocity of light in vaccum.

The RF field transmits with a velocity identical to the velocity of light vc. The slow-wave structures to retard RF field, either use a helix or a wave guide arrangement.

Helix Travelling Wave Tube

The construction of a usual TWT using a helix is shown in Figure. It has an electron gun to create a slight electron beam, which in turn is gone through the centre of a extensive axial helix. A magnetic focusing is given to stop the beam from scattering and to direct it through the centre of the helix.
The amplified signal comes out at the output or further end of the helix under suitable operating setting.
When the RF signal applied spread around the turns of the helix in which it generates an electric field at the middle of the helix. The RF field moves with a velocity of light, the axial electric field go forward with a velocity of light, multiplied by the ratio of helix pitch to helix boundary, So when the velocity of the electron beam moving through the helix equals the rate of progress of the axial field, then communication takes place between them. On an average, the electrons transport energy to the wave on the helix. Thus the signal wave develop and improved output is attained. The TWT can be thought of a limiting case of a multi-cavity klystron. Here all the electrons centred about the axis are velocity modulated.

To suppress the oscillations from being spontaneously generated in a TWT, it is necessary to prevent internal feedback arising from reflections due to impedance mismatch. This problem is solved by placing an attenuator in some convenient place in the tube. When the attenuator is placed at the input end of the tube, the bunching of tube remains unaffected because the attenuator attenuates both forward and reverse waves.

There are two types of TWTs. Lowpower, Low noise TWT and High power, Fairly noise TWT. These may be pulsed also. The noise level of low power is from 4 to 8 dB at about 0.5 to 16GHz. The power ranges from 5 to 25 W at frequencies upto 40 GHz. High power TWTs have narrower bandwidth. They operate between 0.5 to 95 GHz. The power output can be 25 kW, CW near 3 GHz or 10 MW pulsed at 3 GHz.

Applications  of TWT: 

The low noise TWTs are used as RF amplifiers in broadband microwave receivers and as repeater amplifiers in wide band communication links. They are also used in communication satellites with working lives in excess of 50,000 hours. Pulsed high power TWTs are used in airborne, shipborne RADAR and in high power ground based RADAR.

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