Advantages in addition to Limitations Magnetron oscillator
Johnston, Kathy, Features and News Reporter has reference to this Academic Journal, PHwiki organized this Journal PH0101 Unit 2 Lecture 5 Microwaves Properties Advantages Limitations Applications Magnetron oscillator Electro Magnetic Spectrum Microwaves Microwaves are electromagnetic waves whose frequencies range from about 300 MHz 300 GHz (1 MHz = 10 6 Hz in addition to 1 GHz = 10 9 Hz) or wavelengths in air ranging from 100 cm 1 mm. The word Microwave means very short wave, which is the shortest wavelength region of the radio spectrum in addition to a part of the electromagnetic spectrum.
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Properties of Microwaves Microwave is an electromagnetic radiation of short wavelength. They can reflect by conducting surfaces just like optical waves since they travel in straight line. Microwave currents flow through a thin outer layer of an ordinary cable. Microwaves are easily attenuated within short distances. They are not reflected by ionosphere Advantages in addition to Limitations 1. Increased b in addition to width availability: Microwaves have large b in addition to widths compared to the common b in addition to s like short waves (SW), ultrahigh frequency (UHF) waves, etc. For example, the microwaves extending from = 1 cm – = 10 cm (i.e) from 30,000 MHz 3000 MHz, this region has a b in addition to width of 27,000 MHz. 2. Improved directive properties: The second advantage of microwaves is their ability to use high gain directive antennas, any EM wave can be focused in a specified direction (Just as the focusing of light rays with lenses or reflectors) Advantages in addition to Limitations 3. Fading effect in addition to reliability: Fading effect due to the variation in the transmission medium is more effective at low frequency. Due to the Line of Sight (LOS) propagation in addition to high frequencies, there is less fading effect in addition to hence microwave communication is more reliable. 4. Power requirements: Transmitter / receiver power requirements are pretty low at microwave frequencies compared to that at short wave b in addition to .
Advantages in addition to Limitations 5.Transparency property of microwaves: Microwave frequency b in addition to ranging from 300 MHz 10 GHz are capable of freely propagating through the atmosphere. The presence of such a transparent window in a microwave b in addition to facilitates the study of microwave radiation from the sun in addition to stars in radio astronomical research of space. Applications Microwaves have a wide range of applications in modern technology, which are listed below Telecommunication: Intercontinental Telephone in addition to TV, space communication (Earth to space in addition to space to Earth), telemetry communication link as long as railways etc. Radars: detect aircraft, track / guide supersonic missiles, observe in addition to track weather patterns, air traffic control (ATC), burglar alarms, garage door openers, police speed detectors etc. 3.Commercial in addition to industrial applications Microwave oven Drying machines textile, food in addition to paper industry as long as drying clothes, potato chips, printed matters etc. Food process industry Precooling / cooking, pasteurization / sterility, hat frozen / refrigerated precooled meats, roasting of food grains / beans. Rubber industry / plastics / chemical / as long as est product industries Mining / public works, breaking rocks, tunnel boring, drying / breaking up concrete, breaking up coal seams, curing of cement. Drying inks / drying textiles, drying / sterilizing grains, drying / sterilizing pharmaceuticals, leather, tobacco, power transmission. Biomedical Applications ( diagnostic / therapeutic ) diathermy as long as localized superficial heating, deep electromagnetic heating as long as treatment of cancer, hyperthermia ( local, regional or whole body as long as cancer therapy).
Other Applications Identifying objects or personnel by non contact method. 5. Light generated charge carriers in a microwave semiconductor make it possible to create a whole new world of microwave devices, fast jitter free switches, phase shifters, HF generators, etc. Magnetron oscillator Magnetrons provide microwave oscillations of very high frequency. Types of magnetrons Negative resistance type Cyclotron frequency type Cavity type Description of types of magnetron Negative resistance Magnetrons Make use of negative resistance between two anode segments but have low efficiency in addition to are useful only at low frequencies (< 500 MHz). Cyclotron frequency Magnetrons Depend upon synchronization between an alternating component of electric in addition to periodic oscillation of electrons in a direction parallel to this field. Useful only as long as frequencies greater than 100 MHz. Cavity Magnetrons Depend upon the interaction of electrons with a rotating electromagnetic field of constant angular velocity. Provide oscillations of very high peak power in addition to hence are useful in radar applications Cavity Magnetrons Fig (i) Major elements in the Magnetron oscillator Cavity Magnetron Anode Assembly Construction Each cavity in the anode acts as an inductor having only one turn in addition to the slot connecting the cavity in addition to the interaction space acts as a capacitor. These two as long as m a parallel resonant circuit in addition to its resonant frequency depends on the value of L of the cavity in addition to the C of the slot. The frequency of the microwaves generated by the magnetron oscillator depends on the frequency of the RF oscillations existing in the resonant cavities. Description Magnetron is a cross field device as the electric field between the anode in addition to the cathode is radial whereas the magnetic field produced by a permanent magnet is axial. A high DC potential can be applied between the cathode in addition to anode which produces the radial electric field. Depending on the relative strengths of the electric in addition to magnetic fields, the electrons emitted from the cathode in addition to moving towards the anode will traverse through the interaction space as shown in Fig. (iii). In the absence of magnetic field (B = 0), the electron travel straight from the cathode to the anode due to the radial electric field as long as ce acting on it, Fig (iii) a. Cavity Magnetrons Fig (ii) Cross sectional view of the anode assembly Description If the magnetic field strength is increased slightly, the lateral as long as ce bending the path of the electron as given by the path b in Fig. (iii). The radius of the path is given by, If the strength of the magnetic field is made sufficiently high then the electrons can be prevented from reaching the anode as indicated path c in Fig. (iii)), The magnetic field required to return electrons back to the cathode just grazing the surface of the anode is called the critical magnetic field (Bc) or the cut off magnetic field. If the magnetic field is larger than the critical field (B > Bc), the electron experiences a greater rotational as long as ce in addition to may return back to the cathode quite faster. Fig (iii) Electron trajectories in the presence of crossed electric in addition to magnetic fields (a) no magnetic field (b) small magnetic field (c) Magnetic field = Bc (d) Excessive magnetic field Working Fig (iv) Possible trajectory of electrons from cathode to anode in an eight cavity magnetron operating in mode
Working The RF Oscillations of transient nature produced when the HT is switched on, are sufficient to produce the oscillations in the cavities, these oscillations are maintained in the cavities reentrant feedback which results in the production of microwaves. Reentrant feedback takes place as a result of interaction of the electrons with the electric field of the RF oscillations existing in the cavities. The cavity oscillations produce electric fields which fringe out into the interaction space from the slots in the anode structure, as shown in Fig (iv). Energy is transferred from the radial dc field to the RF field by the interaction of the electrons with the fringing RF field. Working Due to the oscillations in the cavities, the either sides of the slots (which acts as a capacitor) becomes alternatively positive in addition to negative in addition to hence the directions of the electric field across the slot also reverse its sign alternatively. At any instant the anode close to the spiraling electron goes positive, the electrons gets retarded in addition to this is because; the electron has to move in the RF field, existing close to the slot, from positive side to the negative side of the slot. In this process, the electron loses energy in addition to transfer an equal amount of energy to the RF field which retard the spiraling electron. On return to the previous orbit the electron may reach the adjacent section or a section farther away in addition to transfer energy to the RF field if that part of the anode goes positive at that instant.
Working This electron travels in a longest path from cathode to the anode as indicated by a in Fig (iv), transferring the energy to the RF field are called as favoured electrons in addition to are responsible as long as bunching effect in addition to give up most of its energy be as long as e it finally terminates on the anode surface. An electron b is accelerated by the RF field in addition to instead of imparting energy to the oscillations, takes energy from oscillations resulting in increased velocity, such electrons are called unfavoured electrons which do not participate in the bunching process in addition to cause back heating. Every time an electron approaches the anode in phase with the RF signal, it completes a cycle. This corresponds to a phase shift 2. For a dominant mode, the adjacent poles have a phase difference of radians, this called the – mode. Fig (v) Bunching of electrons in multicavity magnetron Working At any particular instant, one set of alternate poles goes positive in addition to the remaining set of alternate poles goes negative due to the RF oscillations in the cavities. AS the electron approaches the anode, one set of alternate poles accelerates the electrons in addition to turns back the electrons quickly to the cathode in addition to the other set alternate poles retard the electrons, thereby transferring the energy from electrons to the RF signal. This process results in the bunching of electrons, the mechanism by which electron bunches are as long as med in addition to by which electrons are kept in synchronism with the RF field is called phase focussing effect. electrons with the fringing RF field.
Working The number of bunches depends on the number of cavities in the magnetron in addition to the mode of oscillations, in an eight cavity magnetron oscillating with – mode, the electrons are bunched in four groups as shown in Fig (v). Two identical resonant cavities will resonate at two frequencies when they are coupled together; this is due to the effect of mutual coupling. Commonly separating the pi mode from adjacent modes is by a method called strapping. The straps consist of either circular or rectangular cross section connected to alternate segments of the anode block. Per as long as mance Characteristics Power output: In excess of 250 kW ( Pulsed Mode), 10 mW (UHF b in addition to ), 2 mW (X b in addition to ), 8 kW (at 95 GHz) Frequency: 500 MHz 12 GHz Duty cycle: 0.1 % Efficiency: 40 % – 70 % Applications of Magnetron Pulsed radar is the single most important application with large pulse powers. Voltage tunable magnetrons are used in sweep oscillators in telemetry in addition to in missile applications. Fixed frequency, CW magnetrons are used as long as industrial heating in addition to microwave ovens.
Johnston, Kathy Features and News Reporter
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