OR

• When the applied RF signal propagates around the turn of helix it produces electric field at the center of helix. The RF field propagates with velocity of light. The axial electric field due to the RF signal travels with velocity of light multiplied by the ratio of helix pitch to helix circumference.
• When the velocity of electron beams, travelling through the helix approximates the rate of advance of axial field. The interaction takes place between them in such a way that on average the electron beam delivers energy to the RF field in helix. So the signal wave grows and amplified output is obtained at output of TWT. At a point where axial field is zero electron velocity is unaffected. A point where the axial field is positive, the electron coming against it is accelerated and tries to catch up with later electrons which encounter the RF axial field.
• A point where axial field is negative the electrons get velocity modulated. And the energy transfer from electron to RF field at axial and second wave is induced on helix. This produces an axial electric field that lags behind original electric field by λ/4. Bunching continues to take place. The electron in bunch encounter retarding field and deliver energy to way on helix. The output becomes larger than the input and then amplification results.

Applications of TWT:

• Low noise RF amplifier in broadband microwave receivers.
• Repeater amplifiers for long distance telephony.
• Used as power output tube in communication satellites.
• Continuous wave high power TWT‟s are used in troposcatter links.
• Airborne and ship borne pulsed high power radars, EM ground based radars.

Specifications of TWT:

1. Frequency operation: 3.1-3.5GHz
2. Output power: 150,000 watts
3. Duty cycle: greater than 15%
4. Gain: greater than 40dB
5. Pulse width: greater than 1 ms
6. Modulation: non-intercepting