Straight-edge Connections Research Articles

Investigation of Half Rectangular-Ring Helix Slow Wave Structure for W-Band Wide Bandwidth High-Efficiency TWTs

The half rectangular-ring helix (HRRH) slow wave structure (SWS), which evolves from the rectangular helix (RH), is proposed to develop a high-efficiency wide bandwidth millimeter-wave traveling-wave tube (TWT). The comprehensive advantages of the HRRH SWS are lower phase velocity and higher interaction impedance in comparison with the RH and the planar helix with straight-edge connections (PH-SECs) SWS with the same dimensions. The particle-in-cell (PIC) simulation shows that, at a beam voltage of 6450 V, a beam current of 0.03 A, the maximum output power of 42.2 W, corresponding to a differential small-signal gain of 1.43 dB/mm, and an electron efficiency of 21.8% can be obtained for the HRRH TWT at 95 GHz. The output power, differential small-signal gain, and electron efficiency of the HRRH TWT are considerably enhanced compared with the RH TWT in a wide frequency range of 60–115 GHz under the same electrical parameters.

A Broadband Circularly Polarized Endfire Loop Antenna for Millimeter-Wave Applications

In this letter, a broadband circularly polarized (CP) endfire loop antenna is proposed for millimeter-wave (mmW) applications. The loop consists of two arms constructed by printed strips with straight edge connections implemented by metalized via-holes, and is fed by substrate integrated waveguide differentially. The quasi-traveling wave currents flowing along strips and via-holes, which generate horizontal and vertical polarizations, respectively. By using the offset strip, comparable amplitudes of two polarizations are realized to achieve a good CP radiation. Furthermore, a dielectric rod is applied for higher gain. The experimental prototype reported that the impedance bandwidth of 28.3% from 23.75 to 31.4 GHz, the 3 dB axial-ratio bandwidth of 25% from 24.25 to 31 GHz, and the gain no less than 9 dBic for endfire radiation. In addition, the whole antenna is curved on one substrate with a low profile of 0.14 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the free-space wavelength at 27 GHz). Thus, the antenna is attractive for fifth-generation (5G) applications in mmW integrated circuits.

Design and Evaluation of Planar Bifilar Helical Antennas for Radio Frequency Identification Tags

In this letter, planar bifilar helical antennas with varied turns for ultrahigh-frequency radio frequency identification (RFID) tags are proposed. The bifilar helix, formed using printed strips with straight-edge connections implemented by plated via-holes, is employed to match the complex impedance of the chip. The structures of the two-turn, one-turn, half-turn, and quarter-turn helical antennas are simulated and optimized. The simulation results show that the gain of the antenna is improved as the turn number increases. The gain of the quarter-turn helical antenna is 3 dB lower than that of the two-turn helical antenna, but the size of the quarter-turn helical antenna is only 3.1% of that of the two-turn helical antenna. Thereafter, the impedance, efficiency, and mutual coupling of these fabricated antennas are investigated in a reverberation chamber. The simulated and measured results show that the coupling between two stacked quarter-turn helical antennas is 15 dB lower than that of the commercial tags Alien 9662 in the operating frequency band. The proposed tags can achieve a maximum read range of over 40 cm in free space and 36 cm in dense environment. The radiation efficiencies of the tags are more than 55% at 920–925 MHz (China RFID band).

A &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${W}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Band Backward-Wave Oscillator Based on Planar Helix Slow Wave Structure

A backward-wave oscillator (BWO) operating at W-band is presented. The BWO is based on a microfabrication-compatible planar helix slow wave structure with straight-edge connections (PH-SECs). The oscillator is designed to operate with a beam current of 20 mA and a beam voltage varying from 7 kV to 11 kV. Dispersion characteristics and their sensitivity to some of the geometrical parameters are presented. The particle-in-cell simulation results show that the oscillator frequency tunes from 86.9 GHz to 100.07 GHz with a tunable bandwidth of 14%. The oscillator provides a maximum peak output power of 2.3 W and a peak efficiency of 1.62%. Results of oscillator performance with a beam current of 18 mA and 16 mA are also presented. To validate the simulation results of the PH-SEC, a scaled version of the PH-SEC operating at ${X}$ -band is fabricated. The measured S-parameters and the phase velocity for the fabricated structure match very well with the simulation results.

Simplified Tape-Helix Analysis of the Planar Helix Slow Wave Structure With Straight-Edge Connections

Tape-helix analysis for determining the dispersion characteristics as well as the interaction impedance of a planar helix slow-wave structure with straight edge connections (PH-SEC) is presented. The analysis is simplified by using the characteristic equation for an infinitely wide planar helix (PH) in free space and incorporating the effect of transverse confinement by straight-edge connections by applying the effective dielectric constant (EDC) method. It is shown that the results calculated from analytical expressions derived in this manner match well the simulation results obtained from CST in the frequency range far from cutoff. The EDC method is known to be inaccurate over the frequency range below cutoff. The simplified analysis is also used to determine the dispersion characteristics of a rectangular helix. The results based on the simplified analysis are shown to be more accurate than those from a previously reported complex tape-helix analysis of the rectangular helix.

Design and Fabrication of a Planar Helix Slow-Wave Structure for $C/X$ -Band TWT

This paper presents the design and fabrication of a planar helix slow-wave structure (SWS) with straight-edge connections at ${C}/{X}$ -band frequencies. Besides RF performance, various issues such as vacuum compatibility of materials as well as ease and accuracy of assembly techniques are specially emphasized. The basic SWS is fabricated on a pair of alumina substrates using printed-circuit techniques and is fixed in a nonmagnetic stainless steel housing. The SWS is fed with coaxial couplers. Simulated and measured cold-test parameters of the fabricated structure are presented, showing a wide −10-dB bandwidth in the 6–9-GHz frequency range. The measured phase velocity and circuit loss match the simulated values well. The coupling impedance and hot-test parameters are also studied using simulations. It is shown that the proposed structure is able to provide a saturation output power of 100 W in the frequency range of 6–8 GHz. Such a structure is an attractive candidate for low-cost ${C}$ - and ${X}$ -band traveling-wave tubes fabricated using printed-circuit techniques.

Wideband Power Combining of Four Microfabricated W-Band Traveling-Wave Tubes

Wideband power combining of four W-band microfabricated traveling-wave tubes (TWTs) is presented. The proposed TWTs are based on a planar helix slow-wave structure (SWS) with straight-edge connections (PH-SEC) that can be microfabricated with stripline input–output feed. A novel 1:4 WR-10 waveguide-to-stripline power divider–combiner is designed that covers the frequency range of 92–104 GHz. The simulation results show that ${S}_{{11}}$ is less than −20 dB and the magnitude and phase differences among the four output signals are less than 0.01 dB and 0.41°, respectively, indicating a power combining efficiency as high as 99.9%. The power divider, four PH-SEC SWSs, and the power combiner are assembled and the performance of the overall assembly is checked by simulation. The overall ${S}_{{11}}$ is better than −15 dB in the frequency range of 91.7–100.7 GHz and ${S}_{{21}}$ is better than −12.3 dB. Effects of power and phase variation of individual TWTs have also been considered. With four 5-kV and 10-mA sheet electron beams, particle-in-cell simulations show that the combined TWTs can give 25-W saturation peak power at 94 GHz with a gain of 18 dB.

Design of a Sheet-Beam Electron-Optical System for a Microfabricated &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$W$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Band Traveling-Wave Tube Using a Cold Cathode

A design of a sheet-beam electron-optical system (EOS) for a microfabricated $W$ -band traveling-wave tube (TWT) is presented. The proposed TWT is based on a rectangular field-emitter array (FEA) and a planar helix slow-wave structure with straight-edge connections (PH-SEC). The PH-SEC is designed to work with a 5-kV and 10-mA sheet electron beam. The particle-in-cell simulation results show that the TWT can give 3.7-W peak power at 110 GHz with a 3-dB bandwidth of 60%. The EOS consists of a novel beam-forming electrode, a novel anode, a magnetic circuit to provide solenoidal focusing magnetic field, and a depressed collector. Except for the magnetic circuit, all components are compatible with microfabrication. Moreover, the anode and the collector can be fabricated together with the PH-SEC using the same process steps. Using an FEA with the dimensions of $1000 \times 500\mu \text{m}^{2}$ , a beam of cross section $160 \times 40\mu \text{m}^{2}$ is generated and under ideal conditions 100% transmission is achieved through a 25-mm-long drift tunnel of cross section $240 \times 100\mu \text{m}^{2}$ . The effects of misalignments of the FEA, anode, and the magnetic circuit are investigated, and the limits on misalignments are determined to achieve satisfactory beam transmission.

Planar Helical Antenna of Circular Polarization

A planar helical antenna is presented for achieving wideband end-fire radiation of circular polarization while maintaining a very low profile. The helix is formed using printed strips with straight-edge connections implemented by plated via-holes. The currents flowing on the strips and along via-holes of the helix contribute to the horizontal and vertical polarizations, respectively. Besides, the current on the ground plane is utilized to weaken the strong amplitude of the horizontal electric field generated by the one on the strips. Thus, a good circular polarization can be achieved. Furthermore, a tapered helix and conducting side-walls are employed to broaden the axial ratio (AR) bandwidth as well as to improve the end-fire radiation pattern. The designed antenna operates at the center frequency of 10 GHz. Simulated results show that the planar helical antenna achieves wide-impedance bandwidth ( $\vert S_{11} \vert ) from 7.4 to 12.8 GHz (54%) and 3-dB AR bandwidth from 8.2 to 11.6 GHz (34%), while retaining a thickness of only $0.11{{{\lambda }}_0}$ at the center frequency. A prototype of the proposed antenna is fabricated and tested. Measured results are in good agreement with simulated ones.

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.

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.

Analysis of Coupled Planar Helices with Straight-Edge Connections for Application in Millimeter-Wave TWTs

Two different types of coupled planar helix structures with straight-edge connections (PH-SECs), namely, an unconnected pair and a coaxial pair of planar helices, are proposed and analyzed in the context of application in millimeter-wave traveling-wave tubes. It is shown that the nature of the propagating modes, dispersion characteristics, field distribution, and coupling impedance for the proposed structures can be obtained as special cases of a generic structure that consists of four parallel infinite unidirectionally conducting screens. For both the coupled planar helix structures, the results based on simple analytical approximations of the generic structure match well with the simulation results. As an application of these structures, simulation results are presented to show that the unconnected pair of planar helices has a reduced interaction with backward waves compared with that for a corresponding single planar helix. The proposed PH-SECs offer easier fabrication using printed-circuit or microfabrication techniques compared with their circular counterparts.

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.

Microfabrication of a Planar Helix with Straight-Edge Connections Slow-Wave Structure

A 100GHz planar helix with straight-edge connections slow-wave structure incorporating coplanar waveguide feed has been designed and fabricated. The proposed fabrication process can produce high-aspect-ratio planar helical structures which is essential for sheet electron beam traveling-wave tube application. The photoresists used in this process can be easily stripped after electroplating of three-dimensional structures.

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.

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.