Research paper in Microwaves

I. Microwave Tubes Basic Definition A high-vacuum tube designed for operation in the frequency region from approximately 3000 to 300,000 MHz. Two considerations distinguish a microwave tube from vacuum tubes used at lower frequencies: the dimensions of the tube structure in relation to the wavelength of the signal that it generates or amplifies, and the time during which the electrons interact with the microwave field. Wavelength Characteristic In the microwave region, wavelengths are in the order of centimeters; resonant circuits are in the forms of transmission lines that extend a quarter of a wavelength from the active region of the microwave tube. With such short circuit dimensions the internal tube structure constitutes an appreciable portion of the circuit. For these reasons a microwave tube is made to form part of the resonant circuit. Leads from electrodes to external connections are short, and electrodes are parts of surfaces extending through the envelope directly to the external circuit that is often a coaxial transmission line or cavity. Periodicity At microwaves, the period of signal is in the range of 0.001-1 nanosecond. Only if transit time is less than a quarter of the signal period do significant numbers of electrons exchange appreciable energy with the signal field. Transit time is reduced in several ways. Electrodes are closely spaced and made planar in configuration, and high interelectrode voltages are used. Basic Operation Tubes designed by the foregoing principles are effective for wavelengths from a few meters to a few centimeters. At shorter wavelengths different principles are necessary Instead of collecting the electron beam at a plate formed by the opposite side of the resonant circuit, the beam is allowed to pass into a field-free region before reacting further with an external circuit. The electron cloud can be deflected by a strong static magnetic field so as to revolve and thereby react several times with the signal field before reaching the plate. Instead of producing the field in one or several resonant circuits, the field can be supported by a distributed structure along which it moves at a velocity comparable to the velocity of electrons in the beam. The electron beam is then directed close to this structure so that beam and field interact over an extended interval of time. II. Klystrons Basic Definition

A type of vacuum tube used as an amplifier and/or oscillator for UHF and microwave signals. It is typically used as a high-power frequency source in such applications as particle accelerators, UHF TV transmission and satellite earth stations. Moreover, it is an evacuated electron-beam tube in which an initial velocity modulation imparted to electrons in the beam results

subsequently in density modulation of the beam. A klystron is used either as an amplifier in the microwave region or as an oscillator. As an Amplifier For use as an amplifier, a klystron receives microwave energy at an input cavity through which the electron beam passes. The microwave energy modulates the velocities of electrons in the beam, which then enters a drift space. Here the faster electrons overtake the slower to form bunches. In this manner, the uniform current density of the initial beam is converted to an alternating current. The bunched beam with its significant component of alternating current then passes through an output cavity to which the beam transfers its ac energy. As an Oscillator Klystrons may be operated as oscillators by feeding some of the output back into the input circuit. More widely used is the reflex oscillator in which the electron beam itself provides the feedback. The beam is focused through a cavity and is velocity-modulated there, as in the amplifier. The cavity usually has grids to concentrate the electric field in a short space so that the field can interact with a slow, low-voltage electron beam. Leaving the cavity, the beam enters a region of dc electric field opposing its motion, produced by a reflector electrode operating at a potential negative with respect to the cathode. The electrons do not have enough energy to reach the electrode, but are reflected in space and return to pass through the cavity again. The points of reflection are determined by electron velocities, the faster electrons going farther against the field and hence taking longer to get back than the slower ones. Reflex oscillators are used as signal sources from 3 to 200 GHz. They are also used as the transmitter tubes in line-of-sight radio relay systems and in low-power radars. Basic Operation

A klystron tube makes use of speed-controlled streams of electrons that pass through a resonating cavity. Electrons in a klystron are accelerated to a controlled speed by the application of several hundred volts. As the electrons leave the heated cathode of the tube, they are directed through a narrow gap into a resonating chamber, where they are acted upon by an

RF signal. The electrons bunch together and are directed into one or more additional chambers that are tuned at or near the tube's operating frequency. Strong RF fields are induced in the chambers as the electron bunches give up energy. These fields are ultimately collected at the output resonating chamber. Most commonly used Klystron tubes Two Cavity Amplifier In the two-chamber klystron, the electron beam is injected into a resonant cavity. The electron beam, accelerated by a positive potential, is constrained to travel through a cylindrical drift tube in a straight path by an axial magnetic field. While passing through the first cavity, the electron beam is velocity modulated by the weak RF signal. In the moving frame of the electron beam, the velocity modulation is equivalent to a plasma oscillation. Plasma oscillations are rapid oscillations of the electron density in conducting media such as plasmas or metals.(The frequency only depends weakly on the wavelength). So in a quarter of one period of the plasma frequency, the velocity modulation is converted to density modulation, i.e. bunches of electrons. As the bunched electrons enter the second chamber they induce standing waves at the same frequency as the input signal. The signal induced in the second chamber is much stronger than that in the first.

Oscillator The two-cavity amplifier klystron is readily turned into an oscillator klystron by providing a feedback loop between the input and output cavities. Two-cavity oscillator klystrons have the advantage of being among the lowest-noise microwave sources available, and for that reason have often been used in the illuminator systems of missile targeting radars. The two-cavity oscillator klystron normally generates more power than the reflex klystrontypically watts of output rather than milliwatts. Since there is no reflector, only one high-voltage supply is necessary to cause the tube to oscillate, the voltage must be adjusted to a particular value. This is because the electron beam must produce the bunched electrons in the second cavity in order to generate output power. Voltage must be adjusted to vary the velocity of the electron beam (and thus the frequency) to a suitable level due to the fixed physical separation between the two cavities. Often several "modes" of oscillation can be observed in a given klystron.

Muticavity A klystron in which there is at least one cavity between the input and output cavities, each of which remodulates the beam so that electrons are more closely bunched. In all modern klystrons, the number of cavities exceeds two. A larger number of cavities may be used to increase the gain of the klystron, or to increase the bandwidth. Reflex Kylstron A single-cavity klystron in which the electron beam is reflected back through the cavity resonator by a repelling electrode having a negative voltage; used as a microwave oscillator. The electrons are fired into one end of the tube by an electron gun. After passing through the resonant cavity they are reflected by a negatively charged reflector electrode for another pass through the cavity, where they are then collected. The electron beam is velocity modulated when it first passes through the cavity. The formation of electron bunches takes place in the drift space between the reflector and the cavity .

This effect is used to good advantage for automatic frequency control in receivers, and in frequency modulation for transmitters. The level of modulation applied for transmission is small enough that the power output essentially remains constant. At regions far from the optimum voltage, no oscillations are obtained at all. This tube is called a reflex klystron because it repels the input supply or performs the opposite function of a klystron. II.TWT Basic Definition Traveling wave tubes (TWT) are wideband amplifiers. They take therefore a special position under the velocity-modulated tubes. On reason of the special low-noise characteristic often they are in use as an active RF amplifier element in receivers additional. The bandwidth of a broadband TWT can be as high as one octave, although tuned (narrowband) versions exist, and operating frequencies range from 300 MHz to 50 GHz. The voltage gain of the tube can be of the order of 70 decibels.

Basic Grouping Low-power TWT for receivers Occurs as a highly sensitive, low-noise and wideband amplifier in radar equipments High-power TWT for transmitters These are in used as a pre-amplifiers for high-power transmitters. Physical Construction and Functional Describing

Physical Construction of a TWT

Amplified Helix Signal The TWT contains an electron gun which produces and then accelerates an electron beam along the axis of the tube. The surrounding magnet provides a magnetic field along the axis of the tube to focus the electrons into a tight beam. The helix, at the center of the tube, is a coiled wire that provides a low-impedance transmission line for the RF energy within the tube. The RF input and output are coupled onto and removed from the helix by waveguide directional couplers that have no physical connection to the helix. The attenuator prevents any reflected waves from traveling back down the helix.

Basic Operation The electron- beam bouncing already starts at the beginning of the helix and reaches its highest expression on the end of the helix. If the electrons of the beam were accelerated to travel faster than the waves traveling on the wire, bunching would occur through the effect of velocity modulation. Velocity modulation would be caused by the interaction between the traveling-wave fields and the electron beam. Bunching would cause the electrons to give up energy to the traveling wave if the fields were of the correct polarity to slow down the bunches. The energy from the bunches would increase the amplitude of the traveling wave in a progressive action that would take place all along the length of the TWT. Characteristics of a TWT The attainable power-amplification are essentially dependent on the following factors: constructive details (e.g. length of the helix) electron beam diameter (adjustable by the density of the focusing magnetic field) power input voltage on the helix

Different types of TWT Ring-Loop TWT

Ring-Loop slow wave Structure

A Ring Loop TWT uses loops as slow wave structure to tie the rings together. These devices are capable of higher power levels than conventional helix TWTs, but have significantly less bandwidth of 515 percent and lower cut-off frequency of 18 GHz. The feature of the ringloop slow wave structure is high coupling impedance and low harmonic wave components. Therefore ring-loop traveling wave tube has advantages of high gain (4060 Decibels), small dimension, higher operating voltage and less danger of the backward wave oscillation.

Ring-Bar TWT

Ring-Bar slow wave Structure The Ring-Bar TWT has got characteristics likely the Ring-Loop TWT. The slow wave structure can be made easier by cut-out the structure of a copper tube.

Coupled-cavity TWT

Coupled-cavity slow wave Structure The Coupled-cavity TWT uses a slow wave structure of a series of cavities coupled to one another. The resonant cavities are coupled together with a transmission line. The electron beam (shown in figure 9 as red beam) is velocity modulated by an RF input signal at the first resonant cavity. This RF energy (displayed as blue arrow) travels along the cavities and induces RF voltages in each subsequent cavity. If the spacing of the cavities is correctly adjusted, the voltages at each cavity induced by the modulated beam are in phase and travel along the transmission line to the output, with an additive effect, so that the output power is much greater than the power input. III. BWO Basic Definition The Backward-Wave Oscillator (BWO) or also known as carcinotron is a microwavefrequency, velocity-modulated modulated tube that operates on the same principle as the TWT. However, a traveling wave that moves from the electron gun end of the tube toward the collector is not used in the BWO. Instead, the BWO extracts energy from the electron beam by using a backward wave that travels from the collector toward the electron gun (cathode). Otherwise, the electron bunching action and energy extraction from the electron beam is very similar to the actions in a TWT.

Construction

The typical BWO is constructed from a folded transmission line or waveguide that winds back and forth across the path of the electron beam, as shown in figure 2-16. The folded waveguide in the illustration serves the same purpose as the helix in a TWT. The fixed spacing of the folded waveguide limits the bandwidth of the BWO. Since the frequency of a given waveguide is constant, the frequency of the BWO is controlled by the transit time of the electron beam. The transit time is controlled by the collector potential. Thus, the output frequency can be changed by varying the collector voltage, which is a definite advantage. As in the TWT, the electron beam in the BWO is focused by a magnet placed around the body of the tube Types of BWO M-type BWO The M-type carcinotron, or M-type backward wave oscillator, uses crossed static electric field E and magnetic field B, similar to the magnetron, for focusing an electron sheet beam drifting perpendicularly to E and B, along a slow-wave circuit, with a velocity E/B. Strong interaction occurs when the phase velocity of one space harmonic of the wave is equal to the electron velocity. Both Ez and Ey components of the RF field are involved in the interaction (E y parallel to the static E field). Electrons which are in a decelerating E z electric field of the slowwave, lose the potential energy they have in the static electric field E and reach the circuit. The sole electrode is more negative than the cathode, in order to avoid collecting those electrons having gained energy while interacting with the slow-wave space harmonic. O-type BWO The O-type carcinotron, or O-type backward wave oscillator, uses an electron beam longitudinally focused by a magnetic field, and a slow-wave circuit interacting with the beam. A collector collects the beam at the end of the tube.

References Microwave Tubes. Retrieved August 14, 2010 from http://www.answers.com/topic/microwavetube Klystron Tubes Retrieved August 14, 2010 from .http://www.answers.com/topic/klystron Klystron Tubes. Retrieved August 14, 2010 from http://en.wikipedia.org/wiki/Klystron Traveling Wave Tube. Retrieved August 14, 2010 from http://www.radartutorial.eu/08. transmitters/tx13.en.html Backward Wave Oscillator .Retrieved August 14, 2010 from http://www.tpub.com/content/neets /14183/css/14183_102.htm Backward Wave Oscillator .Retrieved August 14, 2010 from http://www.tpub.com/content/neets /14183/css/14183_103.htm Backward Wave Oscillator .Retrieved August 14, 2010 from http://en.wikipedia.org/wiki /Backward_wave_oscillator#The_Slow-wave_structure Backward Wave Oscillator. Retrieved August 14, 2010 from http://www.tpub.com/content /armycomsystems/SS03444/SS034440032.htm