Bifilar Helix Research Articles

Characteristics of Bifilar Helix Circuit

The characteristics of two kinds of bifilar helix circuit are discussed in this paper. One is alike wound bifilar helix and another is contrawound bifilar helix. Alike wound bifilar helix is often used in O-type backward wave tube (BWO). Owing to wider operating bandwidth, higher interaction impedance and large dimensions, the contrawound helix is suitable for broadband millimeter wave traveling wave tube (MMW TWT).

Improved form of backfire bifilar helix conical beam antenna

The nonresonant-length backfire bifilar helix is often used as a vehicle satcom antenna, as it has a circularly polarized conical beam with azimuthal symmetry. Some antenna configurations, however, exhibit poor back-lobe levels. It is shown that radiation patterns can be significantly improved by correct termination of the helix. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 14:278–280, 1997.

End effects and focusing ability of a double bifilar helix linear wiggler

The end effects and focusing ability of a double bifilar helix linearly polarized wiggler are analyzed and demonstrated. The period of the wiggler used in experiments is 3.2 cm. On EPA-74, with 600 keV electron energy, the transported beam, with no external focusing, is 30 A at 23 wiggler periods. The results show that the fields at both ends of the wiggler, with no tapering, can be reasonably smooth and helically symmetric, and the wiggler is capable of focusing an intense relativistic electron beam.

Improvement in front-to-back ratio of a bifilar backfire helix by a flared open end

The current distribution of a bifilar helical antenna which radiates a circularly polarized wave in the backward endfire direction is decomposed into forward and reflected currents, and the contribution of each current to the radiation is evaluated numerically. Calculations show that the presence of the reflected current deteriorates the front-to-back (F/B) ratio of radiation. To reduce the reflected current and to improve the F/B ratio, flaring of the open end is proposed and investigated. The flared configuration leads to an improved F/B ratio, with an almost constant input impedance.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The K u-band ubitron experiment at the Naval Research Laboratory

Operation of a ubitron/FEL amplifier with a helical wiggler/axial guide field configuration in the frequency band from approximately 13 to 18 GHz is reported. The experiment employs a 190–250 keV electron beam with currents up to 37 A, and has been operated using the fundamental wiggler harmonic interaction with the TE 11 circular waveguide mode. The wiggler is a bifilar helix with a 2.54 cm period and a total length of 50 cm, and is capable of achieving an amplitude of 575 G or more in pulsed mode. The wiggler is flared both on entry and exit in order to obtain smooth transitions in the beam trajectory with 12 periods of uniform amplitude. Small signal gains as high as 17–19 dB have been observed, corresponding to a gain per free-space wavelength approaching 1.25 dB/λ. The experimental results are in reasonable agreement with nonlinear simulation of the interaction in the Raman regime.

NRL ubitron amplifier cold test results

The Naval Research Laboratory ubitron (a low voltage free electron laser) amplifier experiment is nearing operation. The experiment has been designed to explore the potential of the ubitron as an efficient, high power radiation source. The initial experimental configuration consists of a modified SLAC klystron gun, “turnstile” rf input coupler, double taper bifilar helix wiggler, rf output monitor, and an rf calorimeter/load, and has been designed to operate in the collective FEL mode. The results of component cold testing will be presented along with the calculated performance of the amplifier. In particular, a new rf input coupler design has demonstrated reasonable coupling efficiency over the design bandwidth with the ability to control the polarization of the input wave. Detailed comparison of experimental results with a recently developed 3-D ubitron theory will allow an accurate assessment of the ubitron as a millimeter- and submillimeter-wave source.

Axial mode helical antennas

The radiation characteristics of helical antennas operating in the axial mode are evaluated on the basis of the theoretical current distributions and are verified by experimental work. A comparison between monofilar helix and bifilar helix antennas is made, and a helical antenna with a parasitic helix (HAP) is proposed in order to enhance the power gain. It is found that the gain of the HAP in which the parasitic helix is wound from a point diametrically opposite to that of one and one-half turns of the driven helix is about 1 dB higher than that of the monofilar helix. The ratio of the frequency band in which the HAP radiates a circularly polarized wave within an axial ratio of 3 dB is calculated to be 1:1.8.

High-resolution electron microscopic analysis of the amyloid fibril in Alzheimer's disease.

Examination of brain biopsies from Alzheimer's patients by electron microscopy revealed two types of filaments in and around the argyrophile plaques. The first neurofibrillary tangles were in degenerating neuronal processes, and each twisted tubule was made up of paired helical filaments. The second type stained for amyloid and had the ultrastructural appearance of amyloid. Amyloid fibril material found in these plaques was studied by means of tilt-stage electron microscopy and compared with X-ray images of scale models of bifilar helix. The model fulfilled the structural criteria established by electron microscopy. These observations confirmed that each amyloid profile was composed of a pair of twisted tubules, each of which measured about 60 A in diameter.

Neurofibrillary tangles of paired helical filaments

The abnormal aggregates of fibrillar material found in neurons in Alzheimer's disease, senile dementia, the Guam Parkinsonism-Dementia complex, Down's syndrome, and postencephalitic Parkinsonism were studied by means of tilt-stage electron microscopy and with X-ray images of scale models of a bifilar helix. The model fulfills the structural criteria established by electron microscopy. These studies showed that the “twisted tubule” which makes up the neurofibrillary tangle in many pathological situations is a bifilar helix made up of 130 Å filaments.

Microtubule-Macrotubule Transitions: Intermediates after Exposure to the Mitotic Inhibitor Vinblastine

Vinblastine treatment of microtubule protein or intact microtubules assembled in vitro produced bifilar rings and bifilar helices. Suspentsions of rings and helices were demonstrated to bind [(3)H]colchicine, a diagnostic property of microtubule protein. Macrotubules are suggested to consist of tightly coiled helices formed by longitudinal compacting of loosely coiled protofilament pair intermediates.

Multielement, Fractional Turn Helices

Two new resonant antennas are discussed: 1) A bifilar helix with antiphase feed that radiates a \sin \theta shaped, circularly polarized pattern with the null perpendicular to the helical axis; and 2) a quadrafilar helix with feed currents in quadrature that radiates a cardiod shaped, circularly polarized pattern.

An investigation of the magnetic transverse waves on an electron beam

An electron beam in the presence of an axial magnetic field supports four transverse waves; two cyclotron waves, and two synchronous waves. A general coupling theory is developed from an electronic equation and a circuit equation to describe how these waves can be coupled to traveling-wave circuits. The polarization and power flow characteristics of the waves are derived. The theory is applied to. the bifilar helix, a circuit which can couple selectively to each of the four modes. A bifilar helix traveling-wave tube was used to investigate experimentally the four beam modes and provide a quantitative check of the coupling theory.

Electrostatic periodically focused backward-wave oscillator

A low voltage electrostatic periodically focused BWO has been developed which operates over most of X-band and delivers a maximum output power of 20 m.w. This device employs a hollow beam and a bifilar helix circuit. This is a remarkable result as the performance of such a tube is known to be extremely critical to the quality of the focusing. It experimentally demonstrates the rigid focusing predicted theoretically for the bifilar arrangement. Theoretical and experimental investigations have been conducted to determine the feasibility of using a modified focusing scheme which employs a uniform radial electric field in addition to the periodic fields of the bifilar helix. An improved S-band model under construction is expected to give about 50 m.w. output over a 2 to 1 bandwidth with maximum beam voltage less than 1 KV. The test results from this tube will be presented.

An electrostatically focused high power traveling-wave tube

Previous work on periodic electrostatic focusing of the electron beam in traveling-wave tubes was confined primarily to bifilar helices. Most structures of the helical type, however, are severely limited in their power-handling capabilities. This paper discusses the application of periodic electrostatic focusing to high-power slow-wave structures to allow the development of extremely light-weight, high power amplifiers. It has been shown recently that a solid beam of high perveance can be focused by the use of a prescribed potential distribution between alternating focusing electrodes. Two slow-wave structures, a crossed-bar type and a folded-line type, were developed which incorporate the required focusing-electrode arrangement without impairing the rf properties of the basic slow-wave circuit. Hot-test results are presented for a crossed-bar structure designed for operation at X-band with a peak power output of five kilowatts. To date, a relatively short structure without attenuator, tested in a continuously pumped bell jar system, has given a peak power of six kilowatts with an efficiency of 17 percent. The tests proved the feasibility of incorporating electrostatic focusing into high power, periodic, slow-wave structures.

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.

Traveling-wave amplifiers and backward-wave oscillators for VHF

An important aspect of the design of traveling-wave amplifiers and backward-wave oscillators for frequencies below 500 mc is the problem of obtaining a tube of reasonably small physical dimensions. Hollow beams of greater perveance than is obtainable with solid beams offer one method of reducing the size of such tubes by permitting operation at a lower voltage and greater gain per wavelength, for a specified beam power, than is possible in a solid beam tube. Some aspects of the design of minimum size hollow-beam forward-wave amplifiers using single helix circuits and backward-wave oscillators using bifilar helix circuits are presented. Several tubes of these types for operation below 500 mc have been built. Amplifier bandwidths and oscillator tuning ranges in excess of four to one in frequency have been obtained experimentally. Amplifier efficiencies in excess of 20 per cent and oscillator efficiencies in excess of 10 per cent have been achieved.

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.

Focusing of a Long Cylindrical Electron Stream by Means of Periodic Electrostatic Fields

This paper presents a theory of focusing long cylindrical electron streams by means of electric fields which vary periodically along the stream. The use of a bifilar helix or a series of annular rings to produce periodic electric fields is considered. The potential distribution at the position of the electrons is expanded in a power series, and an equation of electron motion is written. The solutions obtained indicate that the flow of electrons is essentially parallel, provided that the electrons enter the focusing structure practically without transverse velocities and are distributed across the stream such that the space charge fields decrease radially toward the axis in a manner similar to that of the periodic electric fields. The method of periodic electrostatic focusing is of particular interest in connection with traveling wave tubes. The possibility of using a bifilar helix as both the slow wave and the focusing structure is discussed.

Bifilar Helix for Backward-Wave Oscillators

A bifilar helix used as the slow wave circuit for the backward-wave oscillator is investigated in this paper. Calculations show that the bifilar helix has a higher impedance than that of the ordinary single-wire helix. The propagating characteristics of the bifilar helix are analyzed and some experimental results of cold measurements are presented. It is also shown that with a bifilar helix, a periodic electrostatic field which can be used to focus the electron stream is readily obtained by applying a proper dc potential difference between two helical wires. The same bifilar structure may thus be used both for the propagating and for the focusing circuit. A tube of this kind has been constructed and the experimental results are described.

The Helix as a Backward-Wave Circuit Structure

The impedance Em2/2βm2P of the backward wave on helical structures is considered from the point of view of the design of backward-wave oscillators. Curves of impedance versus ka (circumference to free-space wavelength ratio) for single and bifilar helices are presented. Both the case of thin tape and that of round wire are treated. The impedance curves are calculated on the basis of an approximation to the actual distribution of rf current over the surface of the conductors; the use of this approximation is justified by comparison with an exact solution at specific values of ka. A proof is presented which demonstrates that the backward-wave impedances of two tape helices are identical if the gap width of one is equal to the tape width of the other. Expressions are derived for effects of dielectric supports upon the velocity and impedance of the backward wave.