Baofu Jia

Wide-band semivane and heavily dielectric loaded helix traveling-wave tubes

Theoretical simulation of the dispersion and interaction impedance characteristics of a semivane loaded negative dispersion helical slow-wave structure, which is a slight variant of a conventional vane-loaded helix of a wide-band traveling-wave tube (TWT), was validated for the nonresonant perturbation measurement. While the experimental and theoretical results agreed well with respect to interaction impedance for both the semivane structure and the structure without vanes, there was disagreement found with respect to the dispersion characteristics of the semivane structure, which was explained quantitatively by the deformation that had taken place in assembling the structure. The output performance of a TWT using the semivane structure fairly agreed with the prediction by an in-house one-dimensional nonlinear Lagrangian code, with respect to saturated output power (/spl sim/50 W), saturated gain (/spl sim/40 dB), and lower-band-edge second harmonic level (/spl sim/-5 dBc), in the operating frequency range of 6-18 GHz, except at the low frequency end of the operating band, where the saturated power was less and the second harmonic level higher than predicted, a finding that may be attributed to the departure of the deformed structure from exhibiting a negative dispersion at the lower band edge.

Dielectric effect on the rf characteristics of a helical groove travelling wave tube

A new type of partial-dielectric-loaded helical groove slow-wave structure (SWS) for millimetre wave travelling wave tube (TWT) is presented in this paper. The radio-frequency characteristics including the dispersion properties, the longitudinal electric field distribution and the beam-wave coupling impedance of this structure are analysed. The results show that the dispersion of the helical groove circuit is weakened, the phase velocity is reduced and the position of the maximum Ez is moved from the mouth to the inside of the groove after partially filling the dielectric materials in the helical groove SWS. Therefore, the dielectric-loaded helical groove SWS is suitable for a multi-beam TWT with broad band and high gain.

Theory of high-power wide-band traveling-wave tube using coaxial inverted helical groove slow-wave structure

A novel slow-wave structure (SWS), the coaxial inverted helical groove structure, is presented and those of its properties used for wide-band traveling-wave tube (TWT) are investigated. The first part of the paper concerns the wave properties of this structure in the case of a vacuum. The influence of the geometrical dimensions on dispersion characteristics and interaction impedance are investigated. The theoretical results reveal a very weak dispersion for the fundamental wave in the structure. The negative dispersion can be realized by a suitable selection of the structural parameters. The interaction impedance of the fundamental wave is about 10 /spl Omega/. The interaction impedance of the -1 space harmonic wave is much lower than that of the fundamental wave. Thus, the risk of backward wave oscillation is reduced. The software high frequency structure simulator (HFSS) is also used to calculate the dispersion property of the SWS. The simulation results from HFSS and the theoretical results agree well, which supports the theory. In the second part, a self-consistent linear theory of a coaxial inverted helical groove TWT is presented. The typical small signal gain per period is about 0.5 dB and the 3-dB small-signal gain bandwidth can exceed 25% with a 33-dB gain of tube.

Positive phase-velocity tapering of broadband helix traveling-wave tubes for efficiency enhancement

A positive phase-velocity tapering of 1.5 octave broadband helix traveling-wave tubes for efficiency enhancement, where the phase velocity is linearly increased in the output section, was studied by using the one-dimensional nonlinear theory. At high frequencies, the electromagnetic wave in the positively tapered section traps the fastest electrons in the decelerating electric field, extracting more energy from the electron beam. At low frequencies, a decreased velocity difference between the electron beam and the electromagnetic wave destroys the phase condition for second-harmonic generation, retaining fundamental wave efficiency as well as reducing its second-harmonic power.

Experimental demonstration of the effect of groove shape on the wave properties of the helical groove waveguide

This letter presents a simple and general fabrication method for helical groove waveguides (HGWs) with successful fabrication of a number of novel HGWs such as the hole-gap-shaped groove HGW, the swallow-tailed groove one, the trapezoid groove one and the ridge-loaded rectangular one. Also, measurements on these structures have been carried out to determine their dispersion properties. The experimental results agree well with the theoretical results and demonstrate the large influence of groove shape on the dispersion characteristics of the structures.

Dispersion characteristics of an asymmetric helical slow wave structure

In a practical traveling tube (TWT), helix slow wave structure (SWS) consists of a helix loaded by a number of dielectric supports arranged at fixed angular intervals, enclosed by a metal shell. In this paper we analysis the general asymmetric helical SWS estimates the dispersion characteristics through dispersion relation.

Ka-band folded waveguide traveling-wave tube experiment

The method to synthesize the main parameters of folded waveguide traveling tube (FWTWT) is studied and used for designing a Ka-band FWTWT. To estimate the synthesis, cold. and hot characteristics such as phase velocity, interaction impedance and growth rate, are calculated and compared with those obtained by the simulation using HFSS and MAGIC. A Ka-band folded waveguide TWT using a 12 kV, 120 mA electron beam is designed from the synthesis and fabricated. Theoretical and experimental details are presented. In addition, a 2/spl Pi/-mode stop band due to the fabrication error is studied and LIGA fabrication as an alternative method to overcome this phenomenon is considered.

Matching of the helix with coaxial coupler

Helical slow-wave structure has been widely used to obtain broad-band in TWTs. Its excellent wide-band characteristic is applied continuously to commercial communications and military radar sources as known as helix-TWTs, however, the impedance matching of the helix circuit with input or output transmission lines is still an important issue to be operated with low reflection. In this paper, the matching is calculated using an equivalent circuit theory to obtain broad-band, and the analysis is presented in detail with the theoretical methods on characteristic impedance of helix and matching of the circuit to the coaxial transmission line with a vacuum window.

The computation of the characteristic impedance for vane-loaded helix

The vane-loaded technology has been employed in TWTs to increase the bandwidth and output power. Here, we present a method to calculate the effect of the vane and supporting rod. We treat the vane-loaded helix as a series connection of the short section of the helical transmission line with different envelope radius and dielectric materials.

The design of the input and output transformer for wide-band helix TWT

The paper presents a design method for the input and output transformer placed inside the helix of a wide-band helix TWT. The transformer uses a two vane asymmetrically arrayed vane-loaded helix line to match the general helix line and the input and output connector. Because most or all of the transformer can be built inside the helix, the input and output devices of the TWT are smaller than before. The wide-band matching properties of the transformer are the same or better than the transformer consisting of all coaxial parts.