antennas

antennas - CS263: Wireless Communications and Sensor...

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© 2004 Matt Welsh – Harvard University 1 CS263: Wireless Communications and Sensor Networks Matt Welsh Lecture 3: Antennas, Propagation, and Spread Spectrum September 30, 2004
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© 2004 Matt Welsh – Harvard University 2 Today's Lecture Antennas and gain Propagation, fading and loss models Spread Spectrum techniques Frequency Hopping Direct Sequence Spread Spectrum (DSSS) Use in 802.11 and 802.11b
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© 2004 Matt Welsh – Harvard University 3 Antennas Antennas are conductors that radiate and collect EM energy Lots of types. .. Antennas generally designed for a certain range of frequencies
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© 2004 Matt Welsh – Harvard University 4 Radiation Patterns Most antennas do not operate equally well in all directions. An antenna's radiation pattern represents the energy it transmits/collects in each direction in space Simplest antenna is a free-space isotropic radiator An idealized point in space that radiates energy in all directions equally. Antenna gains are measured in dBi Power output in a direction compared to that produced by a perfect isotropic antenna Example: an 8 dBi Yagi antenna improves on an isotropic antenna by 8 dB in its “preferred” direction 8 dB = 10 log R, so R = 6.3 But . .. the total power radiated by the antenna is the same!!
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© 2004 Matt Welsh – Harvard University 5 Radiation Patterns Most antennas do not operate equally well in all directions. An antenna's radiation pattern represents the energy it transmits/collects in each direction in space Horizontal Vertical Scale shown in dB, normalized to 0 dB as the maximum gain.
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© 2004 Matt Welsh – Harvard University 6 Radiation Patterns Most antennas do not operate equally well in all directions. An antenna's radiation pattern represents the energy it transmits/collects in each direction in space
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© 2004 Matt Welsh – Harvard University 7 Propagation in Maxwell Dworkin Propagation in real environments is very complex!
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© 2004 Matt Welsh – Harvard University 8 What causes fading? Free Space Loss Radio signal spreading out over space Interference with other transmitters Or other RF sources, e.g., microwave ovens Fast fading As receiver moves over distances of ½ the wavelength, very large variations in received signal strength occur! RF absorption by obstacles, the atmosphere, etc.
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antennas - CS263: Wireless Communications and Sensor...

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