Broad-band pulse performance of short helices

Helical antennas are popular and well characterized for CW frequency domain performance. Renewed interest in time-domain applications of electromagnetics, such as impulse radar, makes accurate time-domain data on broad-band antennas desirable. Although the principal endfire helix radiation mode has been extensively studied in the frequency domain, other modes important in pulse operation are poorly characterized, making a total Fourier transform approach difficult. We have performed impulse tests on helices with two to five turns, establishing novel features of the response and confirming some aspects of frequency domain data. Quick comparisons to time and frequency-domain modeling codes indicate good correspondence of gross features. Successful octave band-pulse operation was achieved, and a few features of helix pulse response invite further investigation     

Higher Order Modes in Coupled Helices (Correspondence)

The problem of electromagnetic wave propagation in a system consisting of the two infinitely long concentric sheath helices free space, shown in Fig. 1, may be approached as a boundary value problem. This has been done before for the case of the lowest mode, but the determinantal equation which gives the propagation constants the higher order modes has not been given before. This note describes the analysis which gives the propagation constants for all modes of propagation.     

Electromagnetic Instability of a Rotating Electron Layer in a Sheath Helix

A sheath helix supports slow electromagnetic modes with phase velocity considerably Iower than the velocity of light in a vacuum. In the presence of a rotating electron layer, the modes can be resonantly driven unstable via cyclotron maser interaction. Using the perturbation technique, the growth rate of the instability is obtained in the weak-beam approximation and is seen to decrease with the slowing down of the modes. For lower order modes, the growth rate is comparable to the one with a cocentric cylindrical waveguide. However, for higher order modes, the growth rate decreases rapidly, suggesting that a sheath helix may be used to suppress the higher order modes.     

Modal approach to scattering of electromagnetic waves by a conducting tape helix

Electromagnetic wave scattering by a tape helix of infinite extent is studied by using Floquet wave expansions for the guided modes and scattered fields. The solution reduces to earlier results as a special limiting case for normal incidence on a sheath helix. The current induced on an infinite helix computed by the presented technique bears close resemblance to the current induced on a long but finite helix as computed by Galerkin's method. The spatial frequency spectrum of the induced current is plotted to show the dominance of the spatial harmonics that are phase matched with the guided modes of the helix. Azimuthal patterns of the scattered field are included to illustrate that interference increases as the diameter of the helix is increased.     

Design formulas for helix dispersion shaping

A simple method of calculating effects of various boundaries around the helix on the dispersion relation and the interaction impedance is presented. The analytical technique is based on the use of equivalent circuit parameters derived from rigorous field analyses. The main advantage of this method is that effects of various perturbing objects (dielectrics, shields, etc.) can be calculated independently, regardless of the model of the unperturbed system. Equivalent circuit parameters are obtained for helices with several different forms of perturbing boundaries, including wedge-shaped dielectric rods, conducting boundaries, longitudinally conducting wires and fins, and combinations of these. The method of analysis is versatile so that the dispersion relation for any one particular form of the helix may be calculated from known characteristics of another using only three sets of universal curves.     

Electromagnetic Theory of the Loosely Braided Coaxial Cable: Part I

A solution to Maxwell's equations subject to boundary conditions on counterwound helical wires is achieved. The helices are contained in a cylindrical surface that is concentric to a perfectly conducting center conductor of circular cross section. The permittivity of the annular region may be different from that of the external region. The excitation is taken to be symmetrical about the cable which leads to a considerable simplification of the formulation. The key step is to recognize that the assumed form of the current on the thin helical wires is a spatial harmonic expansion that leads to a doubly infinite expansion, in such harmonics, for the resultant fields. The inherent complication of the problem results from the intercoupling between the spatial harmonics of the helix currents. Various generalizations of the theory are also indicated.     

Wave propagation in pyrolytic-graphite slow-wave circuits

The possibility of using an anisotropic resistive material to simulate sheath-type microwave slow-wave structures is investigated experimentally. The specific anisotropic material considered in the present study is pyrolytic graphite which has an anisotropic electrical conductivity. A sheath model of the Karp line is simulated by replacing wires and slots by a sheet of pyrolytic graphite. The mode of propagation in this waveguide structure is measured and compared with the theory. The Karp-line mode with a relatively high attenuation is found to be present in this structure.     

Impedance matching arrangement for quadrifilar helix antennas

A ring arrangement of four wires near to the excitation point of a quadrifilar helix antenna can be used to achieve an impedance match. The matching technique is applicable to resonant, fractional turn antennas, and to shaped conical beam backfire mode antennas     

Research of dynamic gesture recognition based on multi-instance learning of kinematics features

Compared to static gestures,dynamic gestures had some new characteristics.The problems of dynamic gestures recognition was spewed by using kinematics mode,such as divergence modes,curl modes,symmetric and ant-symmetric modes,the second and third principal invariant modes of the gradient tensor,the second and third principal invariant modes of the strain tensor and the third principal invariant modes of spin tensor; Further,a framework based on multi-instance learning was proposed,organize all these principle modes for each gesture were organized to a dynamic gestures bag-of-words,and the similarity between the mode of unknown type dynamic gestures and the all bag-of-words were calculated.Then,the nearest neighbor method was used to recognize the dynamic gestures.The experimental results show that the dynamic gestures recognition based on multi-instance kinematics features principal mode learning methods can obtain a higher recognition rate.     

A study of the sheath helix with a conducting core and its application to the helical antenna

A theoretical study of the sheath helix antenna with a conducting core is made via the complex solutions of an associated determinantal equation. The variation of the characteristics of operation with respect to core size and pitch angle is established. A comparison of the complex solutions for the sheath helix and tape helix indicates that a good understanding of the helical antenna can be gained from the sheath model. In particular, explaining broadside radiation in terms of complex solutions is shown to give results in agreement with previously reported empirical data. However, this comparison also points out the limitations of the sheath model in predicting the operational parameters accurately.     

Strapped bifilar helices for high-peak-power traveling-wave tubes

A method of increasing the peak-pulsed power output of broad-band traveling-wave tubes is described. The method involves the use of a modified bifilar helix for the slow-wave structure. The modification employs 1) special straps or 2) mode-selective attenuation to prevent backward-wave oscillation in the anti-symmetric mode. This results in the possibility of using helices in the symmetric mode at values of ka (circumference-to-free-space wavelength ratio) as large as 0.6 at the highest amplification frequency. This in turn makes possible an increase in peak-pulsed beam power of a factor of approximately sixteen times that possible with a single helix. Both analytical and experimental results regarding the behavior of the structures are presented showing the propagation characteristics. The experimental results include cold measurements to determine ω-β diagrams and measurements with an electron beam which yield experimental values of interaction impedance.     

Conducting Helical Structures from Celery Decorated with a Metallic Conjugated Polymer Give Resonances in the Terahertz Range

A method to decorate cellulose‐based helices retrieved from the plant celery with a conductive polymer is proposed. Using a layer‐by‐layer method, the decoration of the polyanionic conducting polymer poly(4‐(2,3‐dihydrothieno [3,4‐b]‐[1,4]dioxin‐2‐yl‐methoxy)‐1‐butanesulfonic acid (PEDOT‐S) is enhanced after coating the negatively charged cellulose helix with a polycationic polyethyleneimine. Microscopy techniques and two‐point probe are used to image the structure and measure the conductivity of the helix. Analysis of the optical and electrical properties of the coated helix in the terahertz (THz) frequency range shows a resonance close to 1 THz and a broad shoulder that extends to 3.5 THz, consistent with electromagnetic models. Moreover, as helical antennas, it is shown that both axial and normal modes are present, which are correlated to the orientation and antenna electrical lengths of the coated helices. This work opens the possibility of designing tunable terahertz antennas through simple control of their dimensions and orientation.     

Suppression of backward–wave oscillation by filter helix methods

An experimental study has been made of the filter helix properties of a periodically loaded helix, using a special traveling-wave tube. The filter helix is shown to possess filter-like frequency pass bands and stop bands and a phase velocity characteristic such that forward- and backward-wave phase velocities are separated, making the filter helix a useful circuit for traveling-wave amplification at high ka (ratio of helix circumference to free space wavelength) without danger of backward-wave oscillation. Certain difficulties associated with backward-wave oscillation frequency pushing were found to arise in filter helices, but useful gain was obtained at ka greater than 0.5 using filter helix techniques.     

Surface Waves on Plasma-Clad Metal Rods

Surface waves guided along a metal rod which is surrounded by a concentric isotropic plasma sheath are investigated for both the principal and the higher modes by employing a rigorous formulation. It is shown that all these modes exhibit a high frequency cutoff phenomenon; the two first modes propagate down to dc, whereas all the other modes possess a finite low frequency cut off and thus exhibit band-pass characteristics. Backward wave propagation is shown to exist when the plasma sheath is thin; dispersion curves are calculated and compared with previous data obtained by means of quasi-static approximations whose limitations and inaccuracies are also indicated. All the results are derived for the plasmaclad rod being placed either in free space or in a dielectric medium, and the dispersion features for both situations are represented in terms of universal curves.     

A Restatement of Mathematical Considerations of TEM Modes on an n-Wire Line

A restatement of mathematical considerations of TEM modes on an n-wire line is presented. An n-wire line inside a shielding conductor or over a ground plane supports n independent TEM modes which can be determined by obtaining eigenvectors on the n-wire line deduced from the characteristic admittance matrix. It is shown conclusively that the TEM modes are determined by the geometrical arrangement of the n wires as well as by the manner of excitation on the n-wire line. Power division ratios on each wire and terminating admittances for output ports of each wire are also discussed, and it is shown that one can excite a TEM mode similar to an even mode and n - 1 TEM modes, each of which resembles an odd mode, on the n-wire line.     

A heuristic analysis for an inhomogeneously loaded tape helix used in a practical travelling wave tube

The analysis of an inhomogeneously loaded helix in the tape model, reported elsewhere, considering the effect of space harmonics as well as the different radial propagation constants over the different structure regions, is quite tedious and rigorous. So, to avoid the rigorous analysis here, a heuristic analysis has been developed by combining the results of the tape model analysis of a helix in free space with those of the sheath helix model analysis of a loaded helix. The results presented here in terms of dispersion characteristics have been compared with those results obtained from a rigorous approach (both for homogeneously and inhomogeneously loaded structures) in the tape helix model. However, the results obtained from this approach agree closely with those of the rigorous approach for a practical situation in which the discrete supports are smoothed out into a single continuous region. The approach is valid for an inhomogeneously loaded helix for optimum conditions of structure parameters, namely, helix pitch angle, helix to barrel gap, support material and inhomogeneity. Moreover, the results predicted by this heuristic approach have also been validated with those experimental results published in the literature and are found to be in good agreement.     

Mode conversion in helix loaded waveguide

Forward coupling of a fast TE-like mode is an interesting and desirable option in the gyro-TWT amplifier. We consider in this paper a mode conversion at the abrupt junction of two infinitely long sheath-helix-loaded waveguides. A helix mode traveling from the left to the right side excites fast TE-like modes are numerically calculated by taking dominant terms in the into TE-like modes are numerically calculated by taking dominant terms in the eigenmode-expansion method. The bandwidth (3 dB) for TE-mode-like coupling in the second (right-hand-side) waveguide is found to be over 40$.     

宽频带毫米波脉冲行波管的研究

简要介绍采用双股螺旋线但波电路研制毫米波宽带脉冲行波管的进展，以及抑制多股螺旋线中q模振荡的方法。目前已得到最大脉冲输出功率360w的样管。     

Theory of the frequency responses of uniform and quasi-taper helical antennas

A theory is presented for the frequency responses of uniform and quasi-taper helical antennas radiating in the axial mode. Based on a model assuming a single traveling wave of current, expressions for the far-zone electric field are obtained by summing the contributions of the current elements from one end of the helix to another. These expressions are used to study the variation of the axial electric field with frequency, first assuming the Hansen-Woodyard (HW) condition and then assuming Sensiper's solution for the phase velocity. It is found that the HW condition leads to the result that the electric field increases monotonically with frequency over the entire axial mode range, which is in violation with the recent experimental measurements of King and Wong. On the other hand Sensiper's phase velocity yields frequency response curves that are consistent with King and Wong's measurements. Numerical results are given for uniform helices and quasi-taper helices consisting of two uniform sections of different diameters.