Dispersion shaping is the most important concern in ensuring the stability of a Traveling Wave Tube (TWT). Dispersion with respect to variation in helix parameters: (a) pitch, (b) tape width, (c) tape thickness and (d) relative permittivity of the dielectric support rods have been studied. The helix structure for the Ku Band helix TWT was simulated using the Ansoft High Frequency Structure Simulator (HFSS) simulator and the dispersion graphs were plotted. The sensitivity of the propagation constant and interaction impedance with respect to change in the helix parameters have been predicted.
The
Traveling Wave Tube (TWT) [1-2] is a high gain and broadband
microwave amplifier, which is commonly used in a communication
system. It works on the principle of continuous interaction
between the electric field and the electron beam. The major
components of a TWT are electron gun, Slow Wave Structure
(SWS), PPM focusing system, I/O couplers and the collector
for collecting the spent beam. A special type of RF circuit
known as the SWS is employed for this purpose. Helix, made
from the metallic tape or wire is a commonly used SWS for
extremely wide-band applications. The broadband width of helix
results from its low dispersion (variation of phase velocity
with frequency). Ideally, the helix structure is thought of
as being derived from a single wire transmission line for
which the dispersion is zero. However, the practical helix
structure tends to be dispersive [3] due to the following
reasons:
As
the frequency decreases, number of helix per wavelength increases.
So the coupling between electric and magnetic fields between
turns changes. As a result, the flux generated per unit current
(inductance) decreases as wavelength (no. of turns per wavelength)
increases. So if frequency decreases, inductance decreases,
the wave velocity increases and hence the phase velocity. |
