Backward-wave Amplifiers Research Articles

Plasmon modes supported by metamaterial-filled monolayer graphene cylindrical waveguides

A theoretical analysis of the characteristics of the hybrid surface plasmon of a monolayer graphene-wrapped metamaterial-filled cylindrical waveguide is performed. The dispersion relations for different configurations of metamaterials [double positive (DPS)–graphene–DPS, DPS–graphene–double negative (DNG), DNG–graphene–DPS, and DNG–graphene–DNG] are simulated by solving Maxwell’s equations for cylindrical symmetry and implementing impedance boundary conditions at the interface. The electromagnetic response of graphene is modeled using Kubo’s formalism. The influence of the geometrical parameters of the waveguide structure, the chemical potential of graphene and the parameters of partnering materials on the dispersion curve, the effective mode index, and the phase velocity is presented. It is observed that the existence of graphene along with metamaterials provides better control and tuning of the propagation of the surface waves. The backward surface waves, forward surface waves, and slow surface waves for the fundamental mode are studied for different waveguide configurations. The results are found to be in accordance with the published literature. These results may have potential applications in tuning surface waves, waveguide technology, modulators, backward-wave amplifiers, traveling-wave masers, frequency selectors, circular polarizers, switching and phase compensation, and graphene-based slow-light devices.

Investigations into Helix Slow-Wave Structure Assisted by Double-Negative Metamaterial

A helical slow-wave structure consisting of a metallic helix supported by double-negative metamaterial (DNG-MTM) rods, symmetrically arranged around the helix, in a metal envelope was analyzed, considering the frequency-dependent permeability and the permittivity of the DNG-MTM following Lorentz and Drude models. The analysis was validated against the 3-D electromagnetic simulation using CST Studio and was extended for studying the effects of the DNG-MTM parameters on the dispersion and interaction impedance of the structure. Tunability of the structure over multiple-frequency bands was also demonstrated. The possible means of metal MTM layering of the radial faces of the discrete support rods were suggested in a more realistic structure configuration. The structure exhibited a high value of interaction impedance as compared to its traditional counterpart, promising enhanced device efficiency and miniaturization. The structure has potential applications in backward-wave amplifiers and backward-wave oscillators.

Noise-figure expressions for crossed-field amplifiers

Noise-figure expressions for crossed-field amplifiers are presented in a compact and unified form by making use of normal-mode amplitudes. Expressions are given for space-charge-wave amplifiers and cyclotron-wave amplifiers, and are applicable for both forward- and backward-wave amplifiers when the power gain appropriate to each amplifier is used.

Microwave tubes and semiconductor devices, G. D. Sims and I. M. Stephenson

The object of the book, as stated by the authors, is to present an up-to-date and readable survey of the principles of operation and uses of microwave valves. On this score, it is the reviewer's feeling that the authors have succeeded. The scope of the book covers practically all microwave tube types, including traveling-wave tubes, klystrons, backward-wave amplifiers, parametric amplifiers, magnetrons, etc. There is a chapter on masers and on semiconductor devices.

Theory and potential applications of backward waves in nonperiodic inhomogeneous waveguides

Backward waves can propagate in an inhomogeneous waveguide of circular cross-section when the geometry and the properties of the materials are suitable. The phenomenon is related to a degeneracy of the H11 and E11 modes at cutoff, and this suggests where to look for the most pronounced effect. The behaviour of the group velocity is studied in detail, in the neighbourhood of the degeneracy, for a number of values of the guide radius. A discussion is given of the possibilities of applying the phenomenon to backward-wave amplifiers, travelling-wave masers, a frequency selector, a circular polariser, switching and phase compensation.

Low-noise backward-wave amplifiers

The characteristics of backward-wave amplifiers are briefly reviewed and related to general receiver problems which exist in broad-band high-power systems. It is emphasized that a backward-wave amplifier of sufficiently low noise figure offers potential solution to several of these problems and would be more useful in some cases than a conventional low-noise traveling-wave tube. The problems encountered in noise reduction in backward-wave amplifiers are discussed and compared with those for forward-wave amplifiers. Gain, bandwidth, and noise-figure characteristics of experimental S-band tubes are presented. Very low noise figures have been achieved for tuning ranges up to half an octave with minimum programming of the gun potentials. A noise figure of 3.7 db at the center of the tuning range has been attained and was reproducible after several hundred hours of operation. This node figure is well below the usual theoretical minimum and is made possible by the unique geometry of the annular cathode and gun. The results to date indicate that the backward-wave type of amplifier is feasible for receiver applications and may satisfy the increasingly apparent need for rapidly tunable selective preamplification.

Conditions for minimum noise generation in backward-wave amplifiers

The noise figure of a backward-wave amplifier can be minimized by optimizing the standing-wave ratio and position of the standing wave of noise power on the beam in the precircuit region. These optimum conditions are presented as detailed functions of tube parameters for the common types of backward-wave amplifiers and are compared with the characteristics of conventional traveling-wave tubes. The problem of achieving maximum tuning range while maintaining a noise figure close to the theoretical minimum value is discussed and general design criteria and methods are evolved. Although the voltage-tuned characteristics of backward-wave amplitiers impose rigorous requirements on the noise space-charge-wave transducer, it is shown that wide low-noise tuning ranges are possible even with no programming of the noise reducing electrodes.

Propagation in a crossed field periodic structure

A field theory of a crossed field periodic structure is given. Taking into account the effect of space charge, an integral equation method is employed to obtain a general dispersion equation for a structure supporting any space harmonic. The case of pure scalloping is treated in detail and the results are applied to forward and backward-wave amplifiers and backward-wave oscillators. Criteria for gain and oscillations are derived and corresponding expressions for the gain and start oscillation current are obtained. These results include the effect of all space harmonics.

Low Noise Tunable Preamplifiers for Microwave Receivers

An investigation of noise reduction in backwardwave amplifiers has yielded two significant results. 1) This type of amplifier has been demonstrated to be capable of very-low noise figures and therefore constitutes an entirely new class of microwave receiver tubes. Tube noise figures less than 6 db for a 25 per cent tuning range and less than 4.5 db for a 10 per cent tuning range of the amplifier's narrow pass band have been attained. 2) Tube noise figures of well under 4 db have been measured, which are the lowest recorded to date on any type of microwave tube. This performance results from a special low-noise gun which was developed for use with hollow electron beams and which features a new type of beam launching mechanism. The experimental S-band tubes and "Christmas-tree gun" are described. Detailed noise performance of the tubes is presented, as well as other data relating to the operation of the tube as a receiver component. On the basis of these experiments, it is concluded that still lower noise figures are possible using the basic concepts of this new gun, not only for backward-wave amplifiers, but also for other types of microwave tubes.

The gain and bandwidth characteristics of backward-wave amplifiers

The problem of designing backward-wave amplifiers to yield specified pass band characteristics is discussed. Detailed universal curves of gain and bandwidth for both single-circuit and cascade amplifiers are presented. Simple charts are developed which facilitate the design procedure. Experimental results are correlated with the theory and indicate its usefulness. The skirt-selectivity of cascade amplifiers is discussed. A specific design example is given to demonstrate the design method and to indicate the range of bandwidths obtainable from helix-type amplifiers.

The Cascade Backward-Wave Amplifier: A High-Gain Voltage-Tuned Filter for Microwaves

The characteristics of a backward-wave circuit as a beam modulator and as a beam demodulator are investigated theoretically and experimentally. A basic modulator-demodulator configuration, consisting of two periodic circuits (e.g., helices) separated by an arbitrary transducer section, constitutes the basis for a new class of backward-wave tubes. These "cascade backward-wave amplifiers" behave as narrow-band amplifiers whose pass bands can be tuned electronically over a wide range of frequencies. They overcome most of the inherent disadvantages of the simple single-circuit backward-wave amplifier and feature high gain well removed from the oscillation region, high off-signal rejection, internal circuit terminations which smooth out variations in operating characteristics with frequency, and provision for adjusting bandwidth electronically. The characteristics of an experimental model at S-band are presented and compared with theory. Analysis indicates that a minimum noise figure in the same range as that of a conventional traveling-wave tube, i.e., about 6 db, should be attainable.

The Minimum Noise Figure of Microwave Beam Amplifiers

A matrix description of microwave amplifiers such as klystrons, traveling-wave tubes, and backward-wave amplifiers, in which an electron beam interacts with longitudinal RF fields, is developed. Certain relations between the matrix elements are derived as a consequence of the conservation of energy and these relations set a lower limit to the noise figure attainable with amplifiers of this class. It is shown that the minimum noise figure of any amplifier of this type with lossless RF structures is identical with that already found by several authors for the traveling-wave tube and is entirely determined by the noise parameters of the beam. These in turn depend only on conditions in the immediate neighborhood of the cathode. Special cases involving lossy structures are investigated and in each case the presence of loss is shown to increase the noise figure. The method is also applied to calculate the minimum noise figure of a double-stream amplifier.