Sheath Helix Research Articles

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.

Slow Wave Couplers for Microwave Dielectric Heating Systems*

AbstractMicrowave heating of lossy dielectric materials in sheet form can be effectively accomplished with slow-wave transmission line couplers with a planar configuration. The attenuation perunit length in a traveling-wave system of this type partially filledwith dielectric is a measure of the heating produced and can be expressed in terms of an interaction impedance that is characteristic of a givenslow-wave structure. This interaction impedance is proportional to the ratio of phase velocity to group velocity which is increased by local reflections and standing waves in the structure . An analogous effect can be obtained by resonating the structure as a wholewith reflections at the ends of the structure. A comparison of resonant and traveling-wave couplers is given for an idealized structure, the developed sheath helix, which can be analyzed exactly.

Energy considerations in open and closed waveguides

Energy relations derived for a lossless closed uniform and periodic structure show that the total complex Poynting vector integrated over the cross section is identically zero for the case of an arbitrary complex wave. These results are also applicable to open structures provided certain conditions are satisfied. The application of these results in the field of leaky wave antennas is discussed. Two illustrative examples, those of the sinusoidally modulated reactance surface and the sheath helix, are worked out. It is shown that although the slope of the dispersion curve can no longer be associated with the direction of net power flow, the knowledge of the slope may still be helpful in selecting certain modes for representing a given source.

The scalar and vector potentials of the infinite sheath helical aerial

From consideration of the surface charge density and the linear current density on an infinitely long sheath helix, expressions are derived for the scalar and vector potentials, from which in turn the field quantities are evaluated. By considering one further boundary condition the characteristic equation for the helix is derived.

Series and Parallel-fed Linearly Polarized Helical Aerials

A comparison is made between series-fed and parallel-fed linearly polarized helical aerials. The series-fed aerial has been found to have a polarization which rotates significantly with frequency, and the magnitude of this rotation has been found to be in good agreement with the theoretical predictions of the sheath helix analysis. The parallel-fed aerial is useful over the same bandwidth as the single helix for spacings greater than half a wavelength.

Fundamental interaction impedance of a helix surrounded by a dielectric and a metal shield

The fundamental interaction impedance for a helix surrounded by a dielectric medium and a conducting shield is computed and the result is found to be in good agreement with measurements on several TWT structures. Results obtained in this paper indicate that the addition of a conducting shield around a helix reduces the interaction impedance; this impedance reduction becomes more severe as the shield is brought closer to the helix and as γa, the ratio of helix circumference to guide wavelength, is made smaller. A sheath helix surrounded by a dielectric medium and a conducting shield is used to obtain a factor representing the impedance reduction caused by the shield. This impedance reduction factor is then combined with Tien's factors to compute the helix impedance.

Theory and behaviour of helix structures for a high-power pulsed travelling-wave tube

The paper describes part of an investigation aimed at developing a multi-megawatt pulsed travelling-wave tube. To this end, a multistart helix structure which may be water cooled has been considered both theoretically and experimentally.Theoretical relationships have been determined for the axial series impedance of a sheath helix surrounded by a perfectly conducting cylinder, and for the phase velocity of the propagating modes. Results of experiments on four- and eight-start helix structures are found to give reasonable correspondence with the theoretical results for equivalent bounded sheath helices.

Space-Charge Wave Harmonics and Noise Propagation in Rotating Electron Beams

Higher-order space-charge waves on solid cylindrical electron beams produced by shielded or nearly shielded guns have only azimuthal periodicity, as in hollow beams. Because of beam rotation, they are members of a broad class of space-charge waves which can travel faster than the beams themselves, either forwards or backwards. The properties of such waves for the beam in a drift tube and in a concentric sheath helix are derived from a slow-wave, small-signal analysis and the appropriate boundary equations. Experimental observations of their interaction with harmonic fields of a helix, as well as of their role in noise propagation, tend to confirm the results of these computations.

Dispersion curves for a helix in a glass tube

The dispersion equations giving the variation of velocity of propagation with frequency for a sheath helix (a thin cylinder conducting only in a helical direction) inside a dielectric tube are obtained, and, from these, dispersion curves are plotted for various ranges of the parameters (tube diameter, permittivity, wall thickness, etc.). Simple approximate formulae are derived and are used to discuss the general characteristics of the curves. A structure is given which would be non-dispersive over a very large frequency range. Its dispersion is less than ?% for all values of ?a above 1.2; a free-space sheath helix has 21% dispersion over the same range of frequencies. Experimental results are presented. There are considerable effects due to wire shape and size, not predicted by the sheath helix analysis. However, it appears that, from a single measurement of the velocity reduction due to thick glass, the velocity reduction due to glass of any thickness may be calculated from the formulae and curves given.

General design procedure for high-efficiency helix-type traveling-wave amplifiers

Heretofore traveling-wave tubes have been designed using Pierce's small signal theory, and the design then modified according to previously obtained experimental results in order to achieve high saturation output and a reasonable efficiency. A procedure has been developed that makes use of the large-signal calculations on traveling-wave amplifiers to design tubes to operate at high saturution efficiency. The design procedure is particularly useful in the design of tubes for high power output and may be applied to the design of either cw or pulsed tubes. A procedure has been developed which enables the design of a traveling-wave tube to be completed with the use of a minimum number of curves and requires a minimum of calculation on the part of the designer. The design curves for helix-type tubes were calculated using the sheath helix impedance modified by the appropriate impedance reduction factors.

Helix Waveguide

Helix waveguide, composed of closely wound turns of insulated copper wire covered with a lossy jacket, shows great promise for use as a communication medium. The properties of this type of waveguide have been investigated using the sheath helix model. Modes whose wall currents follow the highly conducting helix have attenuation constants which are essentially the same as for copper pipe. The other modes have very large attenuation constants which depend upon the helix pitch angle and the electrical properties of the jacket. Approximate formulas are given for the propagation constants of the lossy modes. The circular electric mode important for longdistance communication has low loss for zero-pitch helices. The propagation constants of some of the lossy modes in helix waveguide of zero pitch have been calculated numerically, as functions of the jacket parameters and the guide size, in regions where the approximate formulas are no longer valid. Under certain conditions the attenuation constant of a particular mode may pass through a maximum as the jacket conductivity is varied.

Wave propagation on multifilar helices

The modes of the sheath helix are shown to be related closely to the space harmonics of real helices. Multiwire helices propagate many modes, the principal space-harmonic component of each approaching more closely the appropriate sheath helix mode as the number of wires is increased. Each mode can be excited by a certain phase sequence of exciting currents for the wires in one transverse plane. Experimental verification of the theory is reported using a bifilar helix tube. Phase velocities and impedances are measured, using Kompfner's null method and the start of backward-wave oscillations for the principal forward and backward components respectively.

Electromagnetic Wave Propagation on Helical Structures (A Review and Survey of Recent Progress)

The progress which has been made in recent years in analyzing and understanding the electromagnetic properties of helical structures is reviewed here. After a brief account of work prior to about 1950, the results obtained from an analysis of the sheath helix model are summarized. The more physically realistic tape helix model and its characteristics-forbidden regions, space harmonics, etc.-are discussed. Recent extensions to the analysis of the sheath helix model including the effect of radially stratified mediums, both isotropic and anisotropic, are noted. Also, additional work using the tape or finite wire helix model is reviewed. Finally, the recent investigations of filter helixes and contrawound helixes are discussed.

Wave Propagation Along a Magnetically-Focused Cylindrical Electron Beam

This paper analyses the nature of wave propagation along a cylindrical electron beam, focused in Brillouin flow by means of a finite axial magnetic field. Two different types of conducting boundaries external to the beam are treated: (1) the concentric cylindrical tube, forming a drift region; and (2) the sheath helix, forming a model of the helix traveling-wave tube. The field solution of the helix problem is used to evaluate the normal-mode parameters of an equivalent circuit seen by a thin beam, thereby permitting computation of the gain constant of growing waves. The gain constant of the cylindrical beam with Brillouin flow is found to exceed that of a similar beam with rectilinear flow, presumably because of the transverse component of electron motion in the former