Operation Of TWT And Magnetrons

Operation Of TWT And Magnetrons A traveling-wave tube (TWT) is an electronic device used to amplify radio frequency signals to high power, usually in an electronic assembly known as a traveling-wave tube amplifier (TWTA). The bandwidth of a broadband TWT can be as high as three octaves, although tuned (narrowband) versions exist, and operating frequencies range from 300Â  MHz to 50Â  GHz. The voltage gain of the tube can be of the order of 70 decibels. Traveling-Wave Tubes Traveling-wave tubes (TWTs) are high-gain, low- noise, wide and width microwave amplifiers, capable of gains of 40 dB or more, with bandwidths of over an octave. (A bandwidth of 1 octave is one in which the upper frequency is twice the lower frequency.) TWTs have been designed for frequencies as low as 300 MHz and as high as 50 GHz. The primary use for TWTs is voltage amplification (although high-power TWTs, with characteristics similar to those of a power klystron, have been developed). Their wide bandwidth and low-noise characteristics make them ideal for use as RF amplifiers. CONSTRUCTION: The device is an elongated vacuum tube with an electron gun (a heated cathode that emits electrons) at one end. A magnetic containment field around the tube focuses the electrons into a beam, which then passes down the middle of a wire helix that stretches from the RF input to the RF output, the electron beam finally striking a collector at the other end. A directional coupler, which can be either a waveguide or an electromagnetic coil, fed with the low-powered radio signal that is to be amplified, is positioned near the emitter, and induces a current into the helix. The helix acts as a delay line, in which the RF signal travels at near the same speed along the tube as the electron beam. The electromagnetic field due to the RF signal in the helix interacts with the electron beam, causing bunching of the electrons (an effect called velocity modulation), and the electromagnetic field due to the beam current then induces more current back into the helix (i.e. the current builds up and thus is amplified as it passes down). A second directional coupler, positioned near the collector, receives an amplified version of the input signal from the far end of the helix. An attenuator placed on the helix, usually between the input and output helicies, prevents reflected wave from travelling back to the cathode. Higher powered TWTs usually contain beryllium oxide ceramic as both a helix support rod and in some cases, as an electron collector for the TWT because of its special electrical, mechanical, and thermal properties. OPERATION AND WORKING While the electron beam in a klystron travels primarily in regions free of RF electric fields, the beam in a TWT is continually inter- acting with an RF electric field propagating along an external circuit surrounding the beam. To obtain amplification, the TWT must propagate a wave whose phase velocity is nearly synchronous with the dc velocity of the electron beam. It is difficult to accelerate the beam to greater than approximately one- fifth the velocity of light. Therefore, the forward velocity of the RF field propagating along the helix must be reduced to nearly that of the beam. The phase velocity in a waveguide, which is uniform in the direction of propagation, is always greater than the velocity of light. However, this velocity can be reduced below the velocity of light by introducing a periodic variation of the circuit in the direction of propagation. The simplest form of variation is obtained by wrapping the circuit in the form of a helix, whose pitch is equal to the desire d slowing factor. TWT MIXER.- A TWT is also used as a micro- wave mixer. By virtue of its wide bandwidth, the TWT can accommodate the frequencies generated by the heterodyning process (provided that the frequencies have been chosen to be within the range of the tube). The desired frequency is selected by the use of a filter on the output of the helix. A TWT mixer has the added advantage of providing gain as well as simply acting as a mixer. TWT MODULATION.- A TWT can be modulated by applying the modulating signal to a modulator grid. The modulator grid can be used to turn the electron beam on and off, as in pulsed microwave applications, or to control the density of the beam and its ability to transfer energy to the traveling wave. Thus, the grid can be used to amplitude modulate the output. TWT OSCILLATOR.- A forward-wave TWT can be constructed to serve as a microwave oscillator. Physically, a TWT amplifier and an oscillator differ in two major ways. The helix of the oscillator is longer than that of the amplifier, and there is no input connection to the oscillator. TWT oscillators are often called backward-wave oscillators (BWOs) or carcintrons. The Traveling-Wave Tube The TRAVELING-WAVE TUBE (twt) is a high-gain, low-noise, wide-bandwidth microwave amplifier. It is capable of gains greater than 40 dB with bandwidths exceeding an octave. (A bandwidth of 1 octave is one in which the upper frequency is twice the lower frequency.) Traveling-wave tubes have been designed for frequencies as low as 300 megahertz and as high as 50 gigahertz. The twt is primarily a voltage amplifier. The wide-bandwidth and low-noise characteristics make the twt ideal for use as an RF amplifier in microwave equipment. The physical construction of a typical twt is shown in figure 2-13. Fig-2 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 directional couplers that have no physical connection to the helix. If the RF energy is transported on coaxial cables, the coaxial couplers are wound in a helical manner similar to that shown in figure 2. If the RF energy is transported in waveguides, waveguide directional couplers are used. The attenuator prevents any reflected waves from traveling back down the helix. Physical construction of a twt. A simplified version of twt operation is shown in fig below. In the figure, an electron beam is passing along a nonresonant transmission line represente d by a straight wire. The input to the transmission line is an RF wave which travels on the line from input to output. The line will transport a wide range of RF frequencies if it is terminated in the characteristic impedance of the line. The electromagnetic waves traveling down the line produce electric fields that interact with the electrons of the beam. Fig:-3 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, as shown in figure . However, because the waves travel along the wire at the speed of light, the simple twt shown in figure 3 will not work. At present no way is known to accelerate an electron beam to the speed of light. Since the electron beam cannot travel faster than the wave on the wire, bunching will not take place and the tube will not work. The twt is therefore designed with a delay structure to slow the tra veling wave down to or below the speed of the electrons in the beam. A common twt delay structure is a wire, wound in the form of a long coil or helix, as shown in figure , view (A). The shape of the helix slows the effective velocity of the wave along the common axis of the helix and the tube to about one-tenth the speed of light. The wave still travels down the helix wire at the speed of light, but the coiled shape causes the wave to travel a much greater total distance than the electron beam. The speed at which the wave travels down the tube can be varied by changing the number of turns or the diameter of the turns in the helix wire. The helical delay structure works well because it has the added advantage of causing a large proportion of electric fields that are parallel to the electron beam. The parallel fields provide maximum interaction between the fields and the electron beam. In a typical twt, the electron beam is directed down the center of the helix while, at the same time, an RF signal is coupled onto the helix. The electrons of the beam are velocity-modulated by the electric fields produced by the RF signal. Amplification begins as the electron bunches form and release energy to the signal on the helix. The slightly amplified signal causes a denser electron bunch which, in turn, amplifies the signal even more. The amplification process is continuous as the RF wave and the electron beam travel down the length of the tube. Any portion of the twt output signal that reflects back to the input will cause oscillations within the tube which results in a decrease in amplification. Attenuators are placed along the length of the helix to prevent reflections from reaching the input. The attenuator causes a loss in amplitude, as can be seen in figure , view (B), but it can be placed so as to minimize losses while still isolating the input from the output. The rel atively low efficiency of the twt partially offsets the advantages of high gain and wide bandwidth. The internal attenuator reduces the gain of the tube, and the power required to energize the focusing magnet is an operational loss that cannot be recovered. The twt also produces heat which must be dissipated by either air-conditioning or liquid-cooling systems. All of these factors reduce the overall efficiency of the twt, but the advantages of high gain and wide bandwidth are usually enough to overcome the disadvantages. THE MAGNETRON The MAGNETRON, shown in figure 4-A, is a self-contained microwave oscillator that operates differently from the linear-beam tubes, such as the twt and the klystron. Figure 4-B is a simplified drawing of the magnetron. CROSSED-ELECTRON and MAGNETIC fields are used in the magnetron to produce the high-power output required in radar and communications equipment. Figure 4.A.-Magnetron Figure4 b.-Magnetron The magnetron is classed as a diode because it has no grid. A magnetic field located in the space between the plate (anode) and the cathode serves as a grid. The plate of a magnetron does not have the same physical appearance as the plate of an ordinary electron tube. Since conventional inductive- capacitive (LC) networks become impractical at microwave frequencies, the plate is fabricated into a cylindrical copper block containing resonant cavities which serve as tuned circuits. The magnetron base differs considerably from the conventional tube base. The magnetron base is short in length and has large diameter leads that are carefully sealed into the tube and shielded. The cathode and filament are at the center of the tube and are supported by the filament leads. The filament leads are large and rigid enough to keep the cathode and filament structure fixed in position. The output lead is usually a probe or loop extending into one of the tuned cavities and coupled into a waveguide or coaxial line. The plate structure, shown in figure 5, is a solid block of copper. The cylindrical holes around its circumference are resonant cavities. A narrow slot runs from each cavity into the central portion of the tube dividing the inner structure into as many segments as there are cavities. Alternate segments are strapped together to put the cavities in parallel with regard to the output. The cavities control the output frequency. The straps are circular, metal bands that are placed across the top of the block at the entrance slots to the cavities. Since the cathode must operate at high power, it must be fairly large and must also be able to withstand high operating temperatures. It must also have good emission characteristics, particularly under return bombardment by the electrons. This is because most of the output power is provided by the large number of electrons that are emitted when high-velocity electrons return to strike the cathode. The cathode is indirectly heated and is constructed of a high- emission material. The open space between the plate and the cathode is called the INTERACTION SPACE. In this space the electric and magnetic fields interact to exert force upon the electrons. Figure 5.-Cutaway view of a magnetron The magnetic field is usually provided by a strong, permanent magnet mounted around the magnetron so that the magnetic field is parallel with the axis of the cathode. The cathode is mounted in the center of the interaction space. BASIC MAGNETRON OPERATION.-Magnetron theory of operation is based on the motion of electrons under the influence of combined electric and magnetic fields. The following information presents the laws governing this motion. The direction of an electric field is from the positive electrode to the negative electrode. The law governing the motion of an electron in an electric field (E field) states: The force exerted by an electric field on an electron is proportional to the strength of the field. Electrons tend to move from a point of negative potential toward a positive potential. This is shown in figure 6. In other words, electrons tend to move against the E field. When an electron is being accelerated by an E field, as shown in figure 6, energy is taken from the field by the electron. Figure 6.-Electron motion in an electric field The law of motion of an electron in a magnetic field (H field) states: The force exerted on an electron in a magnetic field is at right angles to both the field and the path of the electron. The direction of the force is such that the electron trajectories are clockwise when viewed in the direction of the magnetic field. This is shown in figure 7. Figure 7.-Electron motion in a magnetic field In figure 7, assume that a south pole is below the figure and a north pole is above the figure so that the magnetic field is going into the paper. When an electron is moving through space, a magnetic field builds around the electron just as it would around a wire when electrons are flowing through a wire. In figure 7 the magnetic field around the moving electron adds to the permanent magnetic field on the left side of the electrons path and subtracts from the permanent magnetic field on the right side. This action weakens the field on the right side; therefore, the electron path bends to the right (clockwise). If the strength of the magnetic field is increased, the path of the electron will have a sharper bend. Likewise, if the velocity of the electron increases, the field around it increases and the path will bend more sharply. A schematic diagram of a basic magnetron is shown in figure 8A. The tube consists of a cylindrical plate with a cathode placed along the center axis of the p late. The tuned circuit is made up of cavities in which oscillations take place and are physically located in the plate. When no magnetic field exists, heating the cathode results in a uniform and direct movement of the field from the cathode to the plate, as illustrated in figure 8B. However, as the magnetic field surrounding the tube is increased, a single electron is affected, as shown in figure 9. In figure 9, view (A), the magnetic field has been increased to a point where the electron proceeds to the plate in a curve rather than a direct path. Figure 8A.-Basic magnetron. SIDE VIEW Figure 9.-Effect of a magnetic field on a single electron In view (B) of figure 9, the magnetic field has reached a value great enough to cause the electron to just miss the plate and return to the filament in a circular orbit. This value is the CRITICAL VALUE of field strength. In view (C), the value of the field strength has been increased to a point beyond the critical value; the electron is made to travel to the cathode in a circular path of smaller diameter. View (D) of figure 9. shows how the magnetron plate current varies under the influence of the varying magnetic field. In view (A), the electron flow reaches the plate, so a large amount of plate current is flowing. However, when the critical field value is reached, as shown in view (B), the electrons are deflected away from the plate and the plate current then drops quickly to a very small value. When the field strength is made still greater, as shown in view (C), the plate current drops to zero. When the magnetron is adjusted to the cutoff, or critical value of the plate current, and the electrons just fail to reach the plate in their circular motion, it can produce oscillations at microwave frequencies. These oscillations are caused by the currents induced electrostatically by the moving electrons. The frequency is determined by the time it takes the electrons to travel from the cathode toward the plate and back again. A transfer of microwave energy to a load is made possible by connecting an external circuit between the cathode and the plate of the magnetron. Magnetron oscillators are divided into two classes: NEGATIVE-RESISTANCE and ELECTRON-RESONANCE MAGNETRON OSCILLATORS. A negative-resistance magnetron oscillator is operated by a static negative resistance between its electrodes. This oscillator has a frequency equal to the frequency of the tuned circuit connected to the tube. An electron-resonance magnetron oscillator is operated by the electron transit time required for electrons to travel from cathode to plate. This oscillator is capable of generati ng very large peak power outputs at frequencies in the thousands of megahertz. Although its average power output over a period of time is low, it can provide very high-powered oscillations in short bursts of pulses.

Public Policing Versus Private Security Essay

When comparing private security to public law enforcement there are both many similarities and differences (COPS, 2012). Private security is paid by a private company or agency, whereas public policing is paid by government salaries and taxpayers (COPS, 2012). Public police officers have the authority to enforce laws and protect and serve society (COPS, 2012). Private security officers are paid to protect private property and personnel (COPS, 2012). Most of the duties that public policing and private security officers perform are similar (COPS, 2012). â€Å"Private security and public law enforcement share many of the same goals: preventing crime and disorder, identifying criminals, and ensuring the security of people and property† (COPS, 2012, P. 1). As there are two private security practitioners for every one sworn law enforcement officer, effective partnerships can act as a much needed force multiplier (COPS, 2012). The services that both officers perform are to achieve si milar goals, to prevent, and deter crimes (COPS, 2012). Public policing and private security officers serve as leaders while performing their duties (COPS, 2012). Public policing and private security officers have a positive impact on the criminal justice system (COPS, 2012). The impact that both of these companies have on the criminal justice system could be beneficial if both would team up, work together, and look toward the future (COPS, 2012). There are also several differences between public policing and private security (COPS, 2012).Even though both public policing and private security perform the same type of work, private security protects organizations and personnel (COPS, 2012). Public policing has the authority to enforce the laws and maintain order in society (COPS, 2012). Private security officers perform duties, where they protect, and deter crimes on private property (COPS, 2012). There are several distinct differences between public policing and private security (COPS, 2012). Public policing performs the following duties: maintains order, protects and serves the community, controls traffic, prevents crimes, and arrests offenders (COPS, 2012). Private security officers perform the following duties: serves as an escort, patrols business grounds and surrounding areas, such as parking lots, security guards, and transports valuables (COPS, 2012). Another difference between public policing and private security is  private security has the main concern of protecting corporate and personnel, although public policing has the main concern of public safety and seeking to enforce the laws of the criminal justice system (COPS, 2012). If a private security guard witnesses a crime outside their grounds or area of protection, it is their choice whether or not to engage in the situation (COPS, 2012). Another difference between public policing and private security is that public policing earns respect from the community, whereas private security does not earn that same r espect because they work for a company or an agency for profit (COPS, 2012). The community will have different views regarding private security because they do it for money (COPS, 2012). Public police officers do get paid but by the government, not a private organization (COPS, 2012). This brings questions about whether or not private security organizations are running their business with the right intentions in mind (COPS, 2012). Before the community will respect them they want to know money is not the main motivation (COPS, 2012). Both private security and public policing have several similarities and their duties overlap in several ways (COPS, 2012). Though still a minority, both of these agencies do employ women along with men (COPS, 2012). Both of these jobs use uniforms to show people who they are, deter crime, and show their authority (COPS, 2012). Both private security and public policing perform duties that uphold the law and keep the community and organizations crime-free (COPS, 2012). The leadership roles of both of the private and public sectors are fairly similar (COPS, 2012). They both have a paramilitary ranking system (COPS, 2012). Within the public policing ranking system there are officers, detectives, and sergeants (COPS, 2012). Each rank reports to the highest position with their rank (COPS, 2012). The officers report to the highest ranking officer within the government agency (COPS, 2012). Within the private sector, the positions are similar to public with a sergeant, corporal, and senior patrol officer (COPS, 2012). The company owner would be the highest rank within the private sector (COPS, 2012). Public policing and private security both play important roles in the criminal justice system (COPS, 2012). Both roles deter crime with their presence (COPS, 2012). Both roles can make an arrest, investigate crimes, and prevent crimes (COPS, 2012). Public policing and private security help keep members of the community safe (COPS, 2012). Another similarity of  public policing and private security is that both sectors need to have training (COPS, 2012). Depending on the duty, private security may not need as much training as public policing but both need training to perform their duties effectively (COPS, 2012). The different roles that public policing and private security play in the criminal justice system are both important (COPS, 2012).Public policing is bound by enforcing the laws and policies (COPS, 2012). Private security focuses more on keeping the company and personnel safe (COPS, 2012).Most private security is not bound by the same regulations that public police have to follow, such as reading an offender their Miranda Rights (COPS, 2012). There are three important elements that make up a comprehensive security plan, which are physical, personnel, and information security (COPS, 2012). The physical aspect of the security plan is building design, fences, locks, lighting, and alarm systems (COPS, 2012). Another aspect of physical security is security personnel (COPS, 2012). The physical aspect of the security plan may be the main focus on protecting and deterring crime (COPS, 2012). The personnel aspect of a security plan is protecting people within a company or organization and this comes from the presence of the security guard (COPS, 2012). Another aspect of personnel security is identification badges (COPS, 2012). Identification badges allow security officers to check the identity and the security clearance of individuals who come into the company (COPS, 2012). The information aspect of a comprehensive security plan has background checks (COPS, 2012). Another aspect of information security would be to put certain papers through a shredder and dispose important documents properly (COPS, 2012). One more aspect of information security would be to encrypt messages and codes (COPS, 2012). Encrypting files would provide security, so that no one would be able to access information (COPS, 2012). Public policing and private security of different similarities and differences; however, both have common goals in mind to protect and serve (COPS, 2012). Both of the goals of these agencies intertwine within each other (COPS, 2012). Both roles are important within the criminal justice system (COPS, 2012). Public policing has to abide by the laws and regulations that affect society that private security do not have to follow, such as reading a suspect their Miranda rights (COPS, 2012). The presence of private security helps provide services to the public police by handling small crimes, such as shoplifting,  security issues, business security, a nd surveillance (COPS, 2012). The service that private security provides frees up public police because in the past police officers had to respond to an abundance of calls (COPS, 2012). Private security does not protect society; their main focus is protecting companies and personnel (COPS, 2012).Public policing and private security need to work together as they head into the future to make society a safer place (COPS, 2012). However, the integrating of the public and private law enforcement needs to be a smooth transition. There are reservations about the integrations because of the main difference that comes to mind about the private sector is money. Regardless of anything, private security agencies are businesses making money. Usually, businesses are run in a manner so that they will make money. Law enforcement is a human services field, not a place to run as a business. Therefore, people will wonder if integration between the two sectors will change the focal point to a money-making business rather than a human services field. When money becomes the focal point of a human services field, things will be run differently and that may not be the best interest at heart for public policing. Privatizing everything would change the priorities, goals, and conditions of the entire justice system. References Private Security and Public Law enforcement. (2012). Retrieved from http://www.cops.usdoj.gov/Default.asp?Item=2034

Howard Gardner Biography

Howard Gardner was born on July 11, 1943 in Scranton, Pennsylvania. His parents were refugees from the period of the Nazis, in Germany. As a child he loved music, he later became a great pianist. As a young man he enrolled at Harvard University. Gardner started to study other careers but ended getting inspired by the works of Jean Piaget to study developmental psychology.He is married to Ellen Winner, a developmental psychologist who teaches at Boston College, and they have four children together He spent some time working with two different types of groups, normal and gifted children and brain-damaged adults, Gardner began developing a theory designed to synthesize his research and observations. In 1983, he wrote Frames of Mind  which outlined his theory of multiple intelligences. Gardner believed that people had multiple different ways of thinking and learning.He has since identified and described eight different kinds of intelligence which are: Visual-spatial intelligence Gardne r also identifies spatial ability as one which lasts longest into old age, Linguistic-verbal intelligence which Gardner takes account of the importance of language in thought, and also in terms of music , Mathematical intelligence, kinesthetic intelligence, Musical intelligence in which Gardner investigates neurological basis for the musical ability, Interpersonal intelligence is related to the ways in which we understand and respond to other people, Intrapersonal intelligence is mostly about our cognitive ability to understand ourselves as human beings and Naturalistic intelligence has to do with an individual's ability to perceive patterns in nature and to classify them. He has also believed that there might have been another possible addition of a ninth type which he refers to as â€Å"existential intelligence. Gardner’s theory has had one of the greatest impact in education In 1986 he started to teach at Harvard Graduate School of Education and began his role at Project Zero, which is a research group that focuses in human cognition with a special focus on the arts and was created by by the philosopher Nelson Goodman with the aim of improving learning in the Arts through research But Over this period Project Zero expanded from its original arts learning base to include research into learning across all types of things. Howard Gardner's theory of multiple intelligences has not been accepted within academic psychology. However, it has met with a strongly positive response from many educators. After all, Gardner has been a great psychologist and his theory has inspired many educators, whether it hasn’t or has been accepted in psychology education.

PSY 301 Week 3 Assignment Persuasion Who What To Whom Essay

This paperwork comprises PSY 301 Week 3 Assignment Persuasion Who What To Whom Persuasion: Who, What, To Whom â€Å"As we explore persuasion, we can divide the persuasive communication into three parts: the communicator, the message, and the audience. First, we will deal with what characteristics of persuaders make people more likely to be persuaded. Next, we will think about characteristics of the message that lead people to change. Finally, we will explore what characteristics of the audience can lead them to be persuaded.† (Feenstra, 2011, p. 88) For your assignment this week, provide an in-depth analysis of the three parts of persuasion. Please reference the bullet points below to complete your assignment. Who –†¦show more content†¦This article is jam packed with tips to help you have a fantastic, worthwhile college experience. Psychology - General Psychology Persuasion: Who, What, To Whom In your textbook, Feenstra (2011) states, â€Å"As we explore persuasion, we can divide the persuasive communication into three parts: the communicator, the message, and the audience.† (p. 88). For your assignment this week, construct a paper that provides an in-depth analysis of the three parts of persuasion. Address the following points in your paper: 1. Who – Describe the Characteristics of the Persuader: What influences our ability to become persuaded by someone? What specific characteristics must this person possess? Be sure to address the impact of credibility, physical attractiveness, and likeability in your response. Why do we respond well to those who possess such characteristics? Would we respond the same to an unattractive, angry, or non-credible person? Why not? 2. What – Discuss the Characteristics of the Message: What attributes are inherent in persuasive messages? How are we influenced by the emotion, framing, narratives, and rational appeals in the messaging we receive? What is the significance of the sleeper effect? 3. To Whom – Examine the Characteristics of the Audience: Why do different audiences perceive messages in different ways? What is the role of culture, gender, and self-esteem inShow MoreRelatedHuman Resources Management150900 Words   |  604 Pagesmanagement as a strategic business contributor. Explain why HR professionals and operating managers must view HR management as an interface. Discuss why ethical issues and professionalism affect HR management as a career field. ââ€"  ââ€"  ââ€"  ââ€"  ââ€"  3 HR TRANSITIONS HR Management Contributes to Organizational Success More effective management of human resources (HR) increasingly is being seen as positively affecting performance in organizations, both large and small. A joint venture between