Investigation of the Dielectric-Loaded Folded Waveguide Traveling-Wave Tube Amplifier

The cold-test characteristics of the dielectric-loaded folded waveguide traveling-wave tube (FWTWT) amplifier are investigated theoretically and the Pierce small-signal theory is employed to confirm the results. For the purpose of analysis, the discussions are separated into symmetric case and unsymmetrical case according to the loading form of the dielectrics. The calculation results indicate that both of them, especially the unsymmetrical form, can significantly reduce the phase velocity and flatten the dispersion curve. However, loading dielectric in an unsymmetrical form will lead to a large decrease in the on-axis interaction impedance, thus it is recommended to use the symmetrical form. It is further found that when the thickness of the dielectric is small, i.e. h ₁/a ≤ 0.1, the interaction impedance is slightly affected by the variations on dielectric constant. The Pierce linear theory for a Q-band dielectric-loaded FWTWT amplifier shows that the bandwidth is increased from 15.08% to 27.03% and the operating voltage is decreased from 20.5 kv to 18.6 kv.