TERRESTRIAL MICROWAVE COMMUNICATION SYSTEM

TERRESTRIAL MICROWAVE COMMUNICATION SYSTEMK FACTORRELIABILITY OBJECTIVEFRESNEL ZONE EFFECTMICROWAVE ANTENNA SYSTEMK FACTORATMOSPHERIC REFRACTIONearths atmosphere is composed of gases, vapors and water moleculesthe atmosphere changes dynamically, it is never constant and which significantly affects radio signal propagationpropagation velocity changes with respect to the density of the medium These differences in propagation velocity result in refraction of a signal propagated through the atmosphere

K FACTORIn normal atmosphere near the earths surface, propagation velocity is approximately 99.997% of that in free space. Under normal circumstances, atmospheric density decreases linearly with altitude, resulting in a propagation velocity differential between the top and bottom of a wave front. Since the upper part of the wave front propagates through less dense atmosphere than the lower part to which it is coupled, it propagates faster than the lower part. The result is a signal path that normally tends to follow earth curvature, but to a lesser to a degree. The bending of the radio signal path caused by differences in atmospheric density is referred to as atmospheric refraction.K FACTORK FACTORdescribes the type and amount of signal refractionDefines the degree and direction of bendingAny change in the amount of beam bending caused by atmospheric condition can be expressed as a change in K.

Where: Ns is the surface refractivity

K FACTORA K factor of 1 describes a condition where there is no refraction of the signal, and it propagates in a straight line.A K factor of less than 1 describes a condition where the refracted signal path deviates from a straight line, and it arcs in the direction opposite the earth curvature.A K factor greater than 1 describes a condition where the refracted signal path deviates from a straight line, and it arcs in the same direction as the earth curvature.

K FACTORPHYSICAL EARTH BULGE- refers only to the effects of physical earth curvature.Earth bulge describes the effect of physical earth curvature along a direct path between two points on the earths surface. The earth surface appears to bulge upwards in the path, with the peak of the bulge occurring at mid-path. This assumes that the earths surface is flat, with no topological variation along the path between the two points.K FACTORThe amount of physical earth bulge along a path can be calculated from the following formula:

Where:h = Vertical distance from a horizontal reference line in feetd1 = Distance from the data point to point A in milesd2 = Distance from the data point to point B in miles

K FACTOREFFECTIVE EARTH BULGE- Effective earth bulge represents the effects of atmospheric refraction, or K, combined with physical earth bulge- Microwave signals propagated through normal atmospheric conditions do not travel in a straight line. Instead, they normally propagate in an arc with a radius approximately 1.33 (4/3) times that of true earth radius. Therefore, we refer to this condition as K=4/3, or normal earth. This refers to the amount of earth bulge that would normally result under these standard atmospheric conditions. K FACTORBecause the signal arc of a propagated signal path through normal atmosphere follows earth curvature, to a degree, this curvature effectively reduces the amount of earth bulgemaking it less than it is, when considered in strictly physical terms. When the effects of atmospheric refraction are combined with physical earth bulge, a modified profile is produced, known as effective earth bulge.Keep the following four rules in mind, since they are true under all conditions.1. When K=1, there is no refractive effect, and the signal path is a straight line. Under these conditions effective earth bulge will be equal to physical (or true) earth bulge.2. When K is less than 1, the refractive signal path arc is inverted (opposite) relative to physical earth curvature, and effective earth bulge will be greater than physical earth bulge.

K FACTOR3. When K equals a number greater than 1, the refractive signal path is an arc in the same direction as earth curvature, but may vary significantly from earth curvature, thereby reducing effective earth bulge to something less than physical earth bulge.4. When K = infinity, the refractive signal path arc follows earth curvature exactly, totally canceling any earth bulge effect, making the earth appear flat. Since the propagated signal arc follows earth curvature exactly regardless of path length, it can be stated that the relationship between the two arcs remains constant for infinity.5. When K = Negative, the refractive signal path is an arc that exceeds physical earth curvature (beyond K = infinity), and effectively reverses the curvature of the earth with respect to the signal path, making its surface appear like a bowl.K FACTORWhere:h = Vertical distance from a horizontal reference line in feetd1 = Distance from the data point to point A in milesd2 = Distance from the data point to point B in milesk = The K factor value representing atmospheric refraction

K FACTORRADIO RANGE

K FACTOR

FRESNEL ZONE EFFECT- Augustin-Jean Fresnel (1788-1827)- are a series of concentric ellipsoids that surround the path from the transmitter to the receiver- for clear line of sight Fresnel zone should be clear of obstacle- it is depends on distance and frequency

FRESNEL ZONE EFFECTFIRST FRESNEL ZONE- the first Fresnel zone is the surface containing every point for which the sum of the distances from the point to the two ends of the path is exactly one-half wavelength longer than the direct path

FRESNEL ZONE EFFECTFRESNEL ZONE CLEARANCE - May be used to analyze interference by obstacles near the path of a radio beam - Again, maximum allowable obstruction is 40%, but recommended obstruction is 20% calculating the Fresnel zone radius at any point P in between the endpoints of the link:where:Fn = The nth Fresnel Zone radius in metersd1 = The distance of P from one end in metersd2 = The distance of P from the other end in meters= The wavelength of the transmitted signal inmeters

FRESNEL ZONE EFFECT

FRESNEL ZONE EFFECT

FRESNEL ZONE EFFECTRELIABILITY OBJECTIVESUNAVAILABILITY STANDARDSAccording to International Telecommunications Union (ITU), the period of unavailable time begins when, in at least one direction of transmission, one or both of the following conditions occur for 10 consecutive seconds:Either the digital signal is interrupted (i.e. alignment or timing is lost); orBER in each second is worse than 1 x 10 -3Causes of UnavailabilityLong interruptions can usually be considered in three categories:PropagationEquipmentOtherRELIABILITY OBJECTIVESPROPAGATIONOutages related to propagation that last longer than 10 seconds are due primarily to three causes:Diffraction Loss - The dominant atmospheric fading effect, which affects availability, is due to diffraction of the radio signal- occurs when a portion of the overall wave front is obstructed by an obstacle- If this loss causes the receive signal to be attenuated to a level where the radio can no longer demodulate the signal, an outage will occurDucting - a condition that can occur if the bending of the radio beam exceeds the curvature of the Earth- Under this condition, blackout fading of the signal occurs and may last several hours- Where this condition exists, space diversity with large antenna spacing can be used to reduce its effectRELIABILITY OBJECTIVESRain - Water molecules absorb microwave energy by way of heating- The greater the size of the water droplets, the greater the amount of absorption of the microwave signal- Rain attenuation causes flat fading by attenuating the receive signal

RELIABILITY OBJECTIVESSYSTEM RELIABILITY ESTIMATESPropagation Reliabilitya.) Non-diversity Systems

Where: Undp = the path unavailability or fade probability

Where: d = path length (mi)f = frequency (GHz)FM = Fade margin (dB)

RELIABILITY OBJECTIVESb.) Diversity Systems-It suggest that there is more than one transmission path or method of transmission available between a transmitter and a receiver-Its purpose is to increase the reliability of the system by increasing its availabilityRELIABILITY OBJECTIVESEQUIPMENTEquipment Reliability

Where: U = unavailability or probability of outage Unavailability:Where: MTTR = Mean Time To Repair(traveling time, actual time to repair fault, and the availability of spares)MTBF = Mean Time Before FailureAvailability: A=(MTBF/(MTBF+MTTR))100%Even for equipment with an excellent MTBF and an MTTR of a few hours, the overall availability is unacceptable for most critical networks unless route diversity or equipment protection is employed.

RELIABILITY OBJECTIVESOTHERSThis category includes such events as planned maintenance outages, failure in the primary power supply, and catastrophic failure such as fire in an equipment room or the tower falling down.The only way to ensure that this type of failure does not lead to excessive outages is to have some form of route diversity in the network.MICROWAVE ANTENNA SYSTEMANTENNA CHARACTERISTICantenna efficiency - the effectiveness of an antenna depends upon its ability to couple or radiate energy into the air. An efficient antenna is one which wastes very little energy during the radiation process.* power gain or power ratio - a ratio of the radiated power to that of the reference antenna-- effectiveness of an entire transmitting/receiving system depends largely on impedance matching between the elements of the system--If a good impedance match is maintained between the system and the antenna throughout the operating frequency band, power transfer to and from the antenna is always maximum.MICROWAVE ANTENNA SYSTEM- The transmission line or waveguide used to transport energy to and from the antenna should have a characteristic impedance equal to that of the antenna- A proper impedance match allows all available power to be absorbed and radiated by the antenna without reflections back down the line

* standing wave ratio (SWR) is a measurement of the impedance mismatch between a transmission line and its load and is an indicator of overallsystem efficiency-- when the swr approaches a value of 1, a transmission line or waveguide approaches a perfectly matched condition

MICROWAVE ANTENNA SYSTEMantenna directivity * omnidirectional - antennas radiate and receive energy from all directions at once* directional - antennas radiate energy in lobes or beams that extend outward from the antenna in either one or two directions- directivity: If the beam is NARROW in either the horizontal or verticalplane, the antenna has a high degree of directivity in that plane- power gain: increases as the degree of directivity increases because the power is concentrated into a narrow beam and less power is required to cover the same distancereciprocity - is the ability of an antenna to both transmit and receive electromagnetic energy.

MICROWAVE ANTENNA SYSTEMREFLECTOR ANTENNAS- focus the radiated energyParabolic reflector - often used for high directivity

MICROWAVE ANTENNA SYSTEM* truncated paraboloida.) TRUNCATED (cut) so thatit is shortened vertically, the beam spreads out vertically instead of being focused- it will detect aircraft at different altitudes without changing the tilt of the antenna.

b.) cut so that it is shortenedhorizontally, the beam will spread out horizontally instead of being focused- accurately determine elevation

MICROWAVE ANTENNA SYSTEM*orange-peel paraboloid - the reflector is narrow in the horizontal plane and wide in the vertical plane-produces a beam that is wide in the horizontal plane and narrow in thevertical plane.*cylindrical paraboloid- a parabolic cross section in just onedimension which causes the reflector to be directive in one plane only.

MICROWAVE ANTENNA SYSTEMCorner reflector - consists of two flat conducting sheets that meet at an angle to form a corner- The corner reflector is normally driven by a half-wave radiator located on a line which bisects the angle formed by the sheet reflectors.

MICROWAVE ANTENNA SYSTEMHORN RADIATORS- obtain directive radiation at microwave frequencies- horns have the advantage of being useful over a wide frequency band- the larger the mouth of the horn, the more directive is the field pattern.

MICROWAVE ANTENNA SYSTEMPARABOLIC ANTENNASan antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to directthe radio wavesmain advantage of a parabolic antenna is that it has high directivitycan produce the narrowest beamwidths have some of the highest gains

MICROWAVE ANTENNA SYSTEM

MICROWAVE ANTENNA SYSTEMPARABOLIC ANTENNA FEED MECHANISMFEED- typically a low-gain type- connected to the associated radio-frequency (RF) transmitting or receiving equipment by means of a coaxial cable transmission line or waveguide

Main types of parabolic antenna feeds: Axial or front feed the feed antenna located in front of the dish at the focus, on the beam axis, pointed back toward the dish. A disadvantage of this type is that the feed and its supports block some of the beam

MICROWAVE ANTENNA SYSTEMOff-axis or offset feed - located to one side of the dish. The purpose of this design is to move the feed structure out of the beam path, so it does not block the beamwidely used in home satellite television dishes

Cassegrain- the feed is located on or behind the dish, and radiates forward,illuminating a convex hyperboloidal secondary reflector at the focus of the dish. - The radio waves from the feed reflect back off the secondary reflector to the dish, which forms the outgoing beam.

MICROWAVE ANTENNA SYSTEMGregorian similar to the Cassegrain design except that the secondary reflector is concave, (ellipsoidal) in shape.

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