Transmission Media -Cont

™   Unguided Media o    Unguided   Media   or   Wireless   Communication   transport   Electromagnetic waves without a physical conductor o    Instead signals are broadcast through Air and are available to anyone who has a receiver capable of receiving them    Radio Frequency Allocation o    The  section  of  EM  spectrum  defined  as  Radio  Communication  is divided into 8 ranges called BANDS o    BANDS  are  rated  from  very  low  frequency  (VLF)  to  extremely  high frequency (EHF) Types of Propagation: -Surface -Tropospheric -Ionospheric -Line-of-Sight -Space Radio Technology considers the earth as surrounded by two layers of atmosphere: TROPOSHERE o    It  is  the  portion  of  the  atmosphere  extending  outwards  approx.  30  miles from the earth's surface o    It contains what we call as AIR o    Clouds,   wind,   Temp.   variation   and   weather   in   general   occur   in   the Troposhere as does jet plane travel IONOSPHERE o    It is the layer of atmosphere above the troposhere but below space o    It is beyond what we think of as atmosphere Surface Propagation o    In surface propagation, radio waves travel through the lowest layer of the atmosphere, hugging the earth o    At  the  lowest  frequencies  signal  emanate  in  all  direction  from  the  tx antenna and follow the curvature of the planet o    Distance depends on the amount of power in the signal Troposhpheric Propagation This can work in two ways:    Line-of-Sight: -A signal can be directed in a straight from Antenna to antenna    Broadcast: -Signal   is   broadcasted   at   an   angle   into   the   upper   layers   of   the troposphere from where it is reflected back to earth's surface o    The  first  method  demands  that  both  transmitter  and  receiver  be  placed within line-of-sight distances and is limited by the curvature of the earth o    The second method allows greater distances to be covered Ionoshpheric Propagation o    Higher  frequency  radio  waves  are  radiated  towards  the  ionosphere  where they are reflected back to the earth o    The  difference  in  density  between  troposhphere  and  ionosphere  causes each radio wave to speed up and change direction bending back to earth o    Allows greater distances to be covered by lower power output Line of Sight Propagation o    Very high frequencies signals are transmitted in straight line from antenna to antenna o    Antennas  must  be  directional  facing  each  other  or  either  tall  enough  or close enough to each other to avoid earth's curvature o    Radio waves can reflect of the objects in the middle and can reach late to the receiver. These late signals distort signal Space Propagation o    A broadcast signal is received by the orbiting satellites which rebroadcasts the signal to the intended receiver on the earth