Microwave Waveguides by Engr. LP Patunob

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Information about Microwave Waveguides by Engr. LP Patunob
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Published on December 30, 2008

Author: dhudz04

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Slide 1: 1 Microwave Communication & Waveguides by Lotis P. Patunob, M.Eng., ECE Slide 2: 2 Microwave Communication & Waveguides Microwaves An electromagnetic waves with frequencies that ranges from approximately 500 MHz to 300 GHz or more. And its wavelengths fall between 1cm and 60 cm. Wavelength The distance between repeating units of a propagating wave of a given frequency. Designated by lambda (?). Slide 3: 3 Microwave Communication & Waveguides Slide 4: 4 Microwave Communication & Waveguides Categories of Microwave Systems: A. Short haul – used to carry information for relatively short distances, e.i. between cities within the same state. Slide 5: 5 Microwave Communication & Waveguides Categories of Microwave Systems: A. Long haul – used to carry information for relatively long distances, such as interstate. Slide 6: 6 Microwave Communication & Waveguides Advantages of Microwave Radio: 1. Radio systems do not require a right-of-way acquisition between stations. 2. Each station requires the purchase or lease of only a small area of land. 3. Because of their high operating frequencies, microwave radio systems can carry large quantities of information. 4. Short wavelengths, require relatively small antennas. 5. Radio signals are more easily propagated around physical obstacles, such as high mountains Slide 7: 7 Microwave Communication & Waveguides Disadvantages of Microwave Radio: 1. It is more difficult to analyze and design circuits at microwave frequencies. 2. Measuring techniques are more difficult to perfect and implement at microwave frequencies. 3. It is difficult to implement conventional circuit components at microwave frequencies. 4. Transient time is more critical at microwave frequencies. 5. Microwave frequencies propagate in a straight line, which limits their use to line-of-sight applications. Slide 8: 8 Microwave Communication & Waveguides Applications of Microwave: 1. Telephone communications. 2. Radar 3. Space Communications 4. Heating Slide 9: 9 Microwave Communication & Waveguides Microwave Parameters: It is the loss that would be obtained between two isotropic antennas in free space, where there are no ground influences or obstructions. A. Free Space Path Loss, LFS It is defined as a loss incurred by an electromagnetic wave as it propagates in a straight line through a vacuum with no absorption or reflection of energy from nearby objects. Slide 10: 10 Microwave Communication & Waveguides Note: signal strength is 1/8 distance; & antenna gain 8 aperture. Slide 11: 11 Microwave Communication & Waveguides Microwave Parameters: General Equation: B. Parabolic Antenna Gain, G where: D = antenna diameter in m ? = signal wavelength in m ? = efficiency Slide 12: 12 Microwave Communication & Waveguides Microwave Parameters: Antenna Gain for Typical Values of ? (0.55 to 0.75): Parabolic Antenna Gain for Typical Values of ? (0.55 to 0.75) in Metric system: Slide 13: 13 Microwave Communication & Waveguides Microwave Parameters: Parabolic Antenna Gain for Typical Values of ? (0.55 to 0.75) in English system: Slide 14: 14 Microwave Communication & Waveguides Microwave Parameters: C. Fade Margin, FM It is an attenuation allowance so that anticipated fading will still keep the signal above specified minimum RF input. It considers the nonideal and less predictable characteristics of a radio wave propagation such as multipath propagation and terrain sensitivity. Slide 15: 15 Microwave Communication & Waveguides Microwave Parameters: Fade Margin in Metric system: Fade Margin in English system: Slide 16: 16 Microwave Communication & Waveguides Microwave Parameters: where: R = propagation reliability Slide 17: 17 Microwave Communication & Waveguides Microwave Parameters: Slide 18: 18 Microwave Communication & Waveguides Microwave Parameters: D. System Reliability Estimates D.1. Propagation Reliability for Non-diversity Systems: where: Undp = the path unavailability or fade probability where: d = path length in mi f = frequency in GHz FM = fade margin in dB Slide 19: 19 Microwave Communication & Waveguides Diversity It suggests that there is more than one transmission path or method of transmission available bet. a transmitter and a receiver. Its purpose is to increase the reliability of the system by increasing its availability Frequency diversity It simply modulates two different RF carrier frequencies with the same information. At the destination, both are demodulated but the one yields the better quality is selected. Slide 20: 20 Microwave Communication & Waveguides Space diversity The output of a transmitter is fed to two or more antennas that are physically separated by an appreciable number of wavelengths. Diversity Receiver diversity It is using more than one receiver for a single RF channel. Slide 21: 21 Microwave Communication & Waveguides Microwave Parameters: D. System Reliability Estimates D.2. Propagation Reliability for Diversity Systems: where: Udiv = the path unavailability or fade probability where: Idiv = the diversity improvement factor Slide 22: 22 Microwave Communication & Waveguides Microwave Parameters: D. System Reliability Estimates D.3. Equipment Reliability: where: U = unavailability or probability of outage where: MTTR = mean time to repair MTBF = mean time before failure Slide 23: 23 Microwave Communication & Waveguides Microwave Parameters: E. Received Signal Level, RSL It is the difference from the nominal transmitter output, antenna transmit and receive gain, from that of the fixed losses of transmit and receive side and its path loss. where: LTX and LRX = transmitter and receiver total insertion losses in dB GT and GR = transmit and receive antenna gains in dB Slide 24: 24 Microwave Communication & Waveguides Microwave Parameters: RSL = FM + Threshold (receiver) where: FM = Fade Margin in dB Threshold (receiver) = receiver minimum RF input in dBm; Cmin where: LFS = Free Space Loss in dB Po(dBm) = Transmitter Output Power in dBm Slide 25: 25 Microwave Communication & Waveguides Microwave Parameters: F. System Gain, Gs (dB) It is the difference between the nominal output power of a transmitter and the minimum rf input power to a receiver. Slide 26: 26 Microwave Communication & Waveguides Microwave Parameters: where: Lf(dB) = transmission line loss bet. the distribution network and its respective antenna (dB) Lb(dB) = total coupling or branching loss in the distribution network bet. the output of a transmitter or receiver and the transmission line (dB) Slide 27: 27 G. Fresnel Zone and Fresnel Radius Fresnel zone – the area where the interference is constructive. Microwave Communication & Waveguides Slide 28: 28 Microwave Communication & Waveguides If a reflected signal is bounced within an odd-numbered Fresnel zone, it would arrive at the receiver in “phase addition” with the direct signal. G. Fresnel Zone and Fresnel Radius Slide 29: 29 Microwave Communication & Waveguides Fresnel zones – are a series of concentric ellipsoids that surround the path from the transmitter to the receiver. Fresnel zone radius, (F1) in Metric System: Slide 30: 30 Microwave Communication & Waveguides Fresnel zone radius, (F1) in English System: nth Fresnel zone radius (Fn): Fresnel zone clearance (Fc) - it takes into account the unusual conditions that occur in the atmosphere. Slide 31: 31 Microwave Communication & Waveguides H. Passive Repeater Gain of a Passive Repeater where: A = the actual area of the passive repeater in (ft2 ) ? = wavelength = c/f in (ft) ? = alpha, the angle bet. the incident wave and the reflected wave in (°) Slide 32: 32 Microwave Communication & Waveguides I. Net Path Loss, NPL: It is the total loss of the system. Example: A plane passive reflector 10x16 ft is erected 21 miles from one active site and only 1 mile from the other and ? = 50°. The operating frequency is 2000 MHz. Determine the net path loss of the system. Slide 33: 33 Microwave Communication & Waveguides Example: In a microwave communication system with a normal temperate and average terrain has the following parameters: a. Operating frequency = 4 GHz b. Path length = 25 mi c. Tx/Rx antenna diameter = 3 ft. d. Transmitter Output Power = 1 W e. Threshold(receiver) = - 80 dBm f. Tx total insertion loss = 5 dB g. Rx total insertion loss = 4 dB Deermine: LFS(dB) , FM(dB) & % Reliability Slide 34: 34 Microwave Communication & Waveguides Waveguides It is a conducting tube through which the energy is transmitted, in the form of electromagnetic waves. It is an alternative to cable for frequency of 1 Ghz and above. Slide 35: 35 Microwave Communication & Waveguides Electromagnetic Wave It is made up of magnetic and electric fields that are at right angles to each other and at right angles to the direction of propagation. It travels in a straight line at approximately the speed of light. Slide 36: 36 Microwave Communication & Waveguides Modes of Propagation - the possible direction of distribution of energy 1. Transverse Electric (TE) – has the electric field transverse the direction of propagation, while the magnetic field is along the propagation direction 2. Transverse Magnetic (TM0) – has the magnetic field at right angles to the direction of propagation along the guide, and the electric field in the direction of propagation. Classification of Modes of Propagation: Slide 37: 37 Microwave Communication & Waveguides Format: TEm,n where: n = indicates the no. of half wave variation of the electric field along the y or b (height) dimension. m = indicates the no. of half wave variation of the electric field along the x or a (width) dimension. Slide 38: 38 Microwave Communication & Waveguides where: arrows = represent the E field perpendicular to the sides of the guide. x’s = represent the H field that is going into the waveguide. dots = represent the H field that is coming out of the waveguide. Slide 39: 39 Microwave Communication & Waveguides Types of Waveguides: A. Rectangular – used when energy must be coupled from the source to a load and both are fixed in place since they are smaller than circular waveguides for a given wavelength. General formula for cut off wavelength, ?c: Cut off wavelength for TEm,0: Slide 40: 40 Microwave Communication & Waveguides Cut off wavelength for TE1,0: where: TE1,0 = called the dominant mode, the mode for the lowest frequency that can be propagated in a waveguide x = the width of the waveguide y = the height of the waveguide Note: x = ?/2 for dominant mode means no propagation Slide 41: 41 Microwave Communication & Waveguides B. Circular – used for rotating systems such as radar antenna where: K = 1.84 for dominant mode Example: What is the cut off wavelength that a 2.5 cm wide waveguide will support the dominant mode (m = 1)? How about for the next mode (m = 2)? Slide 42: 42 Microwave Communication & Waveguides Key wavelength formula for rectangular/circular waveguide: Slide 43: 43 Microwave Communication & Waveguides Group Velocity, Vg The actual speed at which a signal travels down the guide. Phase Velocity, Vp The rate at which the wave appears to move along the wall of the guide. Note: Vg·Vp = Vc2 Slide 44: 44 Microwave Communication & Waveguides Waveguide Characteristic Impedance: TE mode: TM mode: Example: A 6 GHz signal is to be propagated in a waveguide whose width is 7.5 cm. Calculate the characteristic impedance for TE1,0 mode and TM1,1 mode if the thickness is 3.75 cm. Slide 45: 45 Microwave Communication & Waveguides Example: A 6 GHz signal is to be propagated in the dominant mode in a rectangular waveguide if its group velocity is to be 90% of the speed of light, what must be width of the guide? Slide 46: 46 THE END

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