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.a.radiationb.magneticc.electro-magneticd.inductionContinued on next pageMCI Course 2515H 3-8 Study Unit 3, Lesson 1 Lesson 1 Exercise, ContinuedItem 5What is the purpose of a receiving antenna?____________________________________________________________________________________________________________________________MCI Course 2515H 3-9 Study Unit 3, Lesson 1 Lesson 1 ExerciseSolutionsThe table below lists the solutions to the exercise items.If you have anyquestions about these items, refer to the reference page.Item Number Answer Reference1 a 3-42 c 3-43 b 3-44 c 3-45 Converts the electro-magnetic field into 3-5energy that is delivered to a radio receiverSummaryIn this lesson, you ve learned about the roles of receiving and transmittingantennas, radiation patterns, and the different radiated fields.In the next lesson, you will learn about polarization.MCI Course 2515H 3-10 Study Unit 3, Lesson 1 LESSON 2ANTENNA POLARIZATIONOverviewIntroductionThe previous lesson described a radio wave as a moving electro-magneticfield having velocity in the direction of travel, and with components ofelectric intensity and magnetic intensity arranged at right angles to each other.This lesson expands upon that concept and shows how rotating that electro-magnetic field or polarizing it can enhance communications.ContentThis lesson discusses the concept of antenna polarization, the two primarytypes of propagation, and the advantages and applications of each type.LearningAt the end of this lesson, you should be able toObjectives" State how the polarization of a radiated wave is determined." Name the two types of antenna polarization." Identify the antenna polarization to be used when working with mediumand low frequencies." State why it is better to use horizontally polarized antennas at highfrequencies." State what type of polarization should be used at very-high and ultra-highfrequencies.In This LessonThis lesson contains the following topics:Topic See PageOverview 3-11Polarization 3-12Polarization Selection 3-14Benefits of Vertical Polarization 3-15Benefits of Horizontal Polarization 3-16Receiving Antennas 3-17Lesson 2 Exercise 3-18MCI Course 2515H 3-11 Study Unit 3, Lesson 2 PolarizationDefinitionPolarization of a radiated wave is determined by the direction of the electricfield lines of force.The two types of polarization are vertical and horizontal.VerticalIf the electric field lines of force are at right angles to the surface of the earth,Polarizationthe wave is vertically polarized.This concept is illustrated in the diagrambelow:Continued on next pageMCI Course 2515H 3-12 Study Unit 3, Lesson 2 Polarization, ContinuedHorizontalIf the electric field lines of force are parallel to the surface of the earth, thePolarizationwave is said to be horizontally polarized as shown in the diagram below:ProperA single-wire antenna is used to extract energy from a passing radio wave.OrientationTherefore, maximum reception results when the antenna is oriented so that itlies in the same direction as the transmitting antenna, which subsequentlyorients it to the electric-field component.Thus, a vertical antenna is used forefficient reception of vertically polarized waves and a horizontal antenna isused for the reception of horizontally polarized waves.In some cases, thefield rotates as the wave travels through space.Under these conditions, bothhorizontal and vertical components of the field exist and the wave is said tohave elliptical polarization.MCI Course 2515H 3-13 Study Unit 3, Lesson 2 Polarization SelectionMediumAt medium and low frequencies, ground wave transmission is usedand Lowextensively.For this reason, it is necessary to use vertical polarization.FrequenciesVertical lines of force are perpendicular to the ground, and the radio wave cantravel a considerable distance along the ground surface with a minimumamount of attenuation (loss).Because the earth acts as a fairly goodconductor at low frequencies, horizontal lines of force are shorted outlimiting the useful range of horizontally polarized waves.HighAt high frequencies with sky wave transmission, it makes little differenceFrequencieswhether horizontal or vertical polarization is used.The sky wave reflected bythe ionosphere, arrives at the receiving antenna elliptically polarized.Therefore, the transmitting and receiving antennas can be mounted eitherhorizontally or vertically.Horizontal antennas are preferred because they canbe made to radiate effectively at high angles and have inherent directionalproperties.Very- andWith frequencies in the very-high or ultra-high range, either horizontal orUltra-Highvertical polarization is satisfactory.Since the radio wave travels directly fromFrequenciesthe transmitting antenna to the receiving antenna, the original polarizationproduced at the transmitting antenna is maintained throughout the travel ofthe wave to the receiving antenna.Therefore, if a horizontal half-waveantenna is used for transmitting, a horizontal antenna must be used forreceiving.If a vertical half-wave antenna is used for transmitting, a verticalantenna must be used for receiving.MCI Course 2515H 3-14 Study Unit 3, Lesson 2 Benefits of Vertical PolarizationVehicularSimple, vertical half-wave antennas can be used to provide omni-directionalApplicationscommunication that has the ability to communicate with a moving vehicle.When antenna heights are limited to 10 feet or less, as in vehicularinstallation, vertical polarization provides a stronger received signal atfrequencies up to about 50 MHz.From approximately 50 to 100 MHz, thereis only a slight improvement over horizontal polarization with antennas of thesame height.The difference in signal strength above 1100 MHz is negligible.Over WaterFor transmission over large bodies of water, vertical polarization is decidedlybetter than horizontal when antennas are below approximately 300 feet at 30MHz.You would only need 50 feet at 85 MHz and still lower at higherfrequencies.Therefore, an ordinary antenna at mast heights, such as 40 feet,vertical polarization is advantageous for frequencies less than about 100MHz.AircraftRadiation using vertical polarization is less effected by reflections fromInterferenceaircraft flying over the transmission path.With horizontal polarization, suchreflections cause variations in the received signal strength.This factor isimportant in locations where aircraft traffic is heavy.BroadcastWith vertical polarization, less interference is produced or picked up becauseInterferenceof strong VHF and UHF broadcast transmission and reception (television andfrequency modulation), all of which use horizontal polarization.This factoris important when an antenna must be located in an urban area having severaltelevision and commercial FM broadcast stations [ Pobierz całość w 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.a.radiationb.magneticc.electro-magneticd.inductionContinued on next pageMCI Course 2515H 3-8 Study Unit 3, Lesson 1 Lesson 1 Exercise, ContinuedItem 5What is the purpose of a receiving antenna?____________________________________________________________________________________________________________________________MCI Course 2515H 3-9 Study Unit 3, Lesson 1 Lesson 1 ExerciseSolutionsThe table below lists the solutions to the exercise items.If you have anyquestions about these items, refer to the reference page.Item Number Answer Reference1 a 3-42 c 3-43 b 3-44 c 3-45 Converts the electro-magnetic field into 3-5energy that is delivered to a radio receiverSummaryIn this lesson, you ve learned about the roles of receiving and transmittingantennas, radiation patterns, and the different radiated fields.In the next lesson, you will learn about polarization.MCI Course 2515H 3-10 Study Unit 3, Lesson 1 LESSON 2ANTENNA POLARIZATIONOverviewIntroductionThe previous lesson described a radio wave as a moving electro-magneticfield having velocity in the direction of travel, and with components ofelectric intensity and magnetic intensity arranged at right angles to each other.This lesson expands upon that concept and shows how rotating that electro-magnetic field or polarizing it can enhance communications.ContentThis lesson discusses the concept of antenna polarization, the two primarytypes of propagation, and the advantages and applications of each type.LearningAt the end of this lesson, you should be able toObjectives" State how the polarization of a radiated wave is determined." Name the two types of antenna polarization." Identify the antenna polarization to be used when working with mediumand low frequencies." State why it is better to use horizontally polarized antennas at highfrequencies." State what type of polarization should be used at very-high and ultra-highfrequencies.In This LessonThis lesson contains the following topics:Topic See PageOverview 3-11Polarization 3-12Polarization Selection 3-14Benefits of Vertical Polarization 3-15Benefits of Horizontal Polarization 3-16Receiving Antennas 3-17Lesson 2 Exercise 3-18MCI Course 2515H 3-11 Study Unit 3, Lesson 2 PolarizationDefinitionPolarization of a radiated wave is determined by the direction of the electricfield lines of force.The two types of polarization are vertical and horizontal.VerticalIf the electric field lines of force are at right angles to the surface of the earth,Polarizationthe wave is vertically polarized.This concept is illustrated in the diagrambelow:Continued on next pageMCI Course 2515H 3-12 Study Unit 3, Lesson 2 Polarization, ContinuedHorizontalIf the electric field lines of force are parallel to the surface of the earth, thePolarizationwave is said to be horizontally polarized as shown in the diagram below:ProperA single-wire antenna is used to extract energy from a passing radio wave.OrientationTherefore, maximum reception results when the antenna is oriented so that itlies in the same direction as the transmitting antenna, which subsequentlyorients it to the electric-field component.Thus, a vertical antenna is used forefficient reception of vertically polarized waves and a horizontal antenna isused for the reception of horizontally polarized waves.In some cases, thefield rotates as the wave travels through space.Under these conditions, bothhorizontal and vertical components of the field exist and the wave is said tohave elliptical polarization.MCI Course 2515H 3-13 Study Unit 3, Lesson 2 Polarization SelectionMediumAt medium and low frequencies, ground wave transmission is usedand Lowextensively.For this reason, it is necessary to use vertical polarization.FrequenciesVertical lines of force are perpendicular to the ground, and the radio wave cantravel a considerable distance along the ground surface with a minimumamount of attenuation (loss).Because the earth acts as a fairly goodconductor at low frequencies, horizontal lines of force are shorted outlimiting the useful range of horizontally polarized waves.HighAt high frequencies with sky wave transmission, it makes little differenceFrequencieswhether horizontal or vertical polarization is used.The sky wave reflected bythe ionosphere, arrives at the receiving antenna elliptically polarized.Therefore, the transmitting and receiving antennas can be mounted eitherhorizontally or vertically.Horizontal antennas are preferred because they canbe made to radiate effectively at high angles and have inherent directionalproperties.Very- andWith frequencies in the very-high or ultra-high range, either horizontal orUltra-Highvertical polarization is satisfactory.Since the radio wave travels directly fromFrequenciesthe transmitting antenna to the receiving antenna, the original polarizationproduced at the transmitting antenna is maintained throughout the travel ofthe wave to the receiving antenna.Therefore, if a horizontal half-waveantenna is used for transmitting, a horizontal antenna must be used forreceiving.If a vertical half-wave antenna is used for transmitting, a verticalantenna must be used for receiving.MCI Course 2515H 3-14 Study Unit 3, Lesson 2 Benefits of Vertical PolarizationVehicularSimple, vertical half-wave antennas can be used to provide omni-directionalApplicationscommunication that has the ability to communicate with a moving vehicle.When antenna heights are limited to 10 feet or less, as in vehicularinstallation, vertical polarization provides a stronger received signal atfrequencies up to about 50 MHz.From approximately 50 to 100 MHz, thereis only a slight improvement over horizontal polarization with antennas of thesame height.The difference in signal strength above 1100 MHz is negligible.Over WaterFor transmission over large bodies of water, vertical polarization is decidedlybetter than horizontal when antennas are below approximately 300 feet at 30MHz.You would only need 50 feet at 85 MHz and still lower at higherfrequencies.Therefore, an ordinary antenna at mast heights, such as 40 feet,vertical polarization is advantageous for frequencies less than about 100MHz.AircraftRadiation using vertical polarization is less effected by reflections fromInterferenceaircraft flying over the transmission path.With horizontal polarization, suchreflections cause variations in the received signal strength.This factor isimportant in locations where aircraft traffic is heavy.BroadcastWith vertical polarization, less interference is produced or picked up becauseInterferenceof strong VHF and UHF broadcast transmission and reception (television andfrequency modulation), all of which use horizontal polarization.This factoris important when an antenna must be located in an urban area having severaltelevision and commercial FM broadcast stations [ Pobierz całość w formacie PDF ]