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36 Cards in this Set
- Front
- Back
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What are the 4 elements of a communications system
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-transmitter
-communications medium -receiver -noise |
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what is the equasion for wavelength
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3x10^8/f
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what is the frequency of AM radio stations
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300-3000kHz
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what do you have to do to transmit audio signals
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they need to be formatted to travel through the air
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what is the common frequency of an AC power line
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30-300Hz
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what is antenna reciprocity
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when the characteristics and performance of an antenna are the same
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how do you figure the height of an antenna
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984/f=height in feet
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what is ERP
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effective radiated power=ApPt
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what are the requirements for the reflection of a radio signal
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any conducting surface will reflect radio waves
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what is refraction
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the bending of a wave due to the physical makeup of the medium it passes through
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what causes refraction
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the different densities of the air
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what is diffraction
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the bending of radio waves around objects into shadow zones
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describe ground waves
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waves that remain close to the earth and follow the curvature of the earth, it has vertical polarization, strongest at low and medium frequencies
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what is the standard fequency range of ground waves
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30kHz-3MHz
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describe sky waves
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radiated into the upper atmosphere and bent back to the earth, refraction rarely occurs above 50MHz
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what is the standard frequency range of sky waves
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3-30MHz
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describe space waves
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direct waves, line of sight communication
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what is the standard frequency range of space waves
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above 30MHz
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when does the worst fading occur
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when transmitted signals take multiple paths to a receiver
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what is fading
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the variation in signal amplitude at the receives caused by the characteristics of the signal path and changes in it
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what are some causes of fading
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-distance variation
-enviornment variation -multiple pathes going into the same antenna at different angles |
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what are some ways to allievate fading
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-use widebands rather than narrow
-freq division-use 2 transmitters and 2 receivers to send signals -spacial diversity-two receivers spaced apart |
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how do you determine the distance between the transmitting antenna and the horizon in a space wave
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d=square root(2*Ht of antenna)
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is the carrier frequency lower than the modulating frequency
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no
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what mathmatical operation does an amplitude modulator perform
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it computes the product of the carrier and modulator signals
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what is the formula for the modulation index
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m=Vm/Vc
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what does the frequency of the envelope equal
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the baseband freqency
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what are two ways to change a signal so that it can carry a signal
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-change the amplitude
-change the frequency modulation |
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what causes distortion/overmodualtion
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when Vm is greater than Vc
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what is the ideal condition for a modulation index
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when it =1
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what creates sidebans in AM
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modulation
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what is Bandwidth
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the range of frequencies
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what is the range of AM radio stations
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540-1600kHz
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what information does a carrier send
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none
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What are the advantages to SSB
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-less power used
-less BW -noise reduction b/c of narrow band -less fading |
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what are the disadvantages to SSB
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-hard to demodulate
-can guess at the carrier frequency -sends low level carrier frequencies and amplifies |
