Ciersiang Chua

Planar Helix With Straight-Edge Connections in the Presence of Multilayer Dielectric Substrates

A planar slow-wave structure consisting of a planar helix with straight-edge connections has been studied in the context of application in traveling-wave tubes. The effects of several practical modifications to the basic structure are examined. These modifications comprise a vacuum tunnel, metal shield, and multilayer dielectric substrates. A modified effective dielectric constant method is proposed to obtain the dispersion characteristics for different possible configurations. Furthermore, coupling impedance for the different configurations has been calculated using the corresponding 2-D approximations. It is shown that, far from cutoff, the phase velocity and coupling impedance values calculated in this manner match very well with the simulation results obtained from CST Microwave Studio. The effects of variations in aspect ratio, metal shield distance, and dielectric constant of the substrates on phase velocity and coupling impedance are studied. A coplanar waveguide feed has been designed for one of the possible configurations. The measured S-parameters and phase velocity values for this proof-of-concept configuration agree well with the simulated results and confirm the ease of fabrication, low loss, and the wideband potential of the planar helix with straight-edge connections.

Microfabrication and Characterization of W-Band Planar Helix Slow-Wave Structure With Straight-Edge Connections

A slow-wave structure (SWS) consisting of a planar helix with straight-edge connections and incorporating a coplanar waveguide feed has been designed for operation at W-band and has been fabricated using microfabrication technique. On-wafer cold measurements have been carried out on a number of fabricated SWSs, and the results are reported here for the first time. The parameters measured are return loss, attenuation, and phase velocity, and the results cover a frequency range of 70-100 GHz. Cold-test parameters of the SWS have been also obtained using simulations, and the effects of fabrication, such as surface roughness, have been accounted for by estimating effective conductivity of different parts of the microfabricated structures. The measured and simulated results match well. The effects of silicon wafer resistivity have been also discussed. Planar helical SWSs fabricated in this manner have application in traveling-wave tubes operating at millimeter wave and higher frequencies.

Effective Dielectric Constant Method for a Planar Helix With Straight-Edge Connections

A new planar slow-wave structure consisting of a planar helix with straight-edge connections is presented. It is shown that the phase velocity of the proposed structure can be much slower than that for a rectangular helix with identical cross-sectional dimensions. Further, the dispersion characteristics are obtained simply by using the effective dielectric constant method; it is shown that the method yields results which, far-from-cutoff, match very well with simulation results obtained from CST Microwave Studio. The proposed structure has the potential of easier fabrication using microfabrication techniques.

PIC simulation for W-band planar helix with straight-edge connections

A square cross section planar helix with straightedge connections has been simulated with a cylindrical electron beam for traveling-wave tube amplifier application. The saturated output peak power varies from 0.65 W to 2.58 W over the frequency range of 85-110 GHz. The maximum electronic efficiency is 7.3% at 97.5 GHz for a beam of 5.8 kV and 8 mA.

Vane-Loaded Planar Helix Slow-Wave Structure for Application in Broadband Traveling-Wave Tubes

Dispersion control of a planar helix slow-wave structure (SWS) using vane loading for applications in traveling-wave tubes has been studied. The addition of metal vanes and coplanar ground planes to the planar helix structure has been investigated with the help of simulations aimed at achieving low or negative dispersion. It is shown that, similar to the case of circular helix, the addition of metallic vanes to the planar helix can produce a flatter dispersion curve or negative dispersion characteristics. Furthermore, it is shown that even stronger dispersion control can be achieved by the use of metal vanes together with extended coplanar ground planes on the dielectric substrates that support the planar helix. As a proof of concept, one of the designs of the planar helix SWS including metal vanes and operating at S-band frequencies has been fabricated and tested; the measured phase velocity results match the simulation results very well.

Symmetric planar helix slow-wave structure with straight-edge connections for application in TWTs

A symmetric configuration for planar helix slow-wave structure (SWS) with straight-edge connections (PH-SEC) is proposed in this paper. This structure is compared with a previously proposed PH-SEC SWS on a thick substrate. The results at Ka-band are presented to show that the new structure has higher and more uniform coupling impedance and thus is more suitable for TWT applications.

Connected Pair of Planar Helices With Straight-Edge Connections for Application in TWTs

A novel planar slow wave structure (SWS) consisting of a connected pair of planar helices with straight-edge connections (PH-SEC) has been proposed in this paper. It is shown with the help of dispersion and coupling impedance characteristics that this structure offers a larger electron-beam tunnel and lower risk of backward-wave oscillation compared with a single PH-SEC. Fabrication using printed-circuit techniques is reported and measured S-parameters over 1-7 GHz are presented to demonstrate the ease of feed-design and wideband characteristics for the new SWS. Using particle-in-cell simulations, hot-test parameters such as the output power, gain, and efficiency are also obtained and reported. It is shown that the connected pair of PH-SECs has a higher gain growth rate than that for the single PH-SEC. The proposed SWS is also compared at frequencies around 60 GHz with a recently reported structure which is derived from meander-line SWS; the comparison shows that the connected pair of PH-SECs offers a significantly higher coupling impedance. With a good potential for fabrication using microfabrication techniques, the connected pair of PH-SECs can be an attractive candidate for application in mm-wave traveling wave tubes.

A 3-D U-Shaped Meander-Line Slow-Wave Structure for Traveling-Wave-Tube Applications

A novel 3-D U-shaped meander-line (ML) slow-wave structure (SWS) is proposed for traveling-wave-tube applications. This 3-D structure has the potential to have a better performance than the corresponding 2-D ML SWSs proposed in the literature. Simulation results at S-band obtained using CST Microwave Studio are presented to compare the phase velocity, interaction impedance, and circuit attenuation of the proposed structure with those of a recently reported symmetric double V-shaped microstrip ML SWS, showing advantages with respect to circuit attenuation, bandwidth, and feed design. Particle-in-cell simulations are also carried out for the proposed structure for a cylindrical electron beam using CST Particle Studio. The saturated gain and electronic efficiency of the 3-D U-shaped ML SWS is significantly higher than that of the symmetric double V-shaped ML SWS. The proposed structure has been designed and fabricated with a microstrip-line feed at S-band. The measured return loss, phase velocity, and circuit attenuation match well with the simulation results. By using microfabrication techniques, the proposed SWS has the potential to operate at millimeter-wave and higher frequencies.

Design of a planar helix with straight-edge connections for traveling-wave tube applications

A printed planar helix with straight-edge connections incorporating a coplanar waveguide feed and an air tunnel has been designed for traveling-wave tube applications. Cold parameters have been simulated using CST Microwave Studio. The simulated S-parameters and phase velocity values agree well with the measured results.

A Microfabricated Planar Helix Slow-Wave Structure to Avoid Dielectric Charging in TWTs

Modifications to a slow-wave structure (SWS) consisting of a planar helix with straight-edge connections are proposed so as to avoid dielectric charging problem when it is used in a traveling-wave tube (TWT). The planar helix SWS is suitable for microfabrication using materials such as silicon (Si) and silicon dioxide (SiO2). First, through a simple dielectric slab model, it is shown that the phenomenon of dielectric charging can be accurately simulated using Computer Simulation Technology Particle Studio. Next, a more realistic double-dielectric model is considered to obtain guidelines to avoid dielectric charging. Based on these guidelines, two simple modifications are suggested for the planar helix SWS when it is microfabricated using Si wafers and a layer of SiO2. The modifications consist of partial removal of the SiO2 layer and a careful enhancement of the conductivity of the Si layer. The simulation results are presented for a Ka-band planar helix SWS to demonstrate a very significant reduction in dielectric charging while maintaining a low insertion loss when these modifications are incorporated. The proposed technique is compatible with microfabrication and is expected to be useful in microfabricated millimeter-wave TWTs.