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36 Cards in this Set

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