IEMS 5701 Wireless Communica7ons Part II: Radio Wave Propaga7on Basics Antenna Basics Electromagne7c Waves • Wireless signals propagate in the air in form of electromagne7c waves Recalling Some High School Physics • Sta7c electric field • Sta7c magne7c field . Recalling Some High School Physics • Ampere’s Law • Faraday’s law of induc7on . then both fields must be considered together as a coupled electromagne7c field using Maxwell’s equa7ons . t) • Magne7c field B(x. t) • If either E or B are 7me dependent. y. y.Electromagne7c Field • Electric field E(x. z. z. Star7ng at the Beginning Maxwell’s Equa7ons published in 1865 • A set of par7al differen7al equa7ons that form the founda7on of classical electrodynamics . EM Wave Propaga7on in Free Space • Maxwell’s equa7ons in a simple media with no current and free charge: ∇iE = 0 ∇iB = 0 ∇×E= − ∂B ∂t ∇ × B = µ0 ε 0 ∂E ∂t • These equa7ons predicts EM waves . Radio Spectrum . Radio Spectrum . What Happens When an EM Wave Encounters a New Media • • • • Reflec7on Diffrac7on ScaVering Absorp7on . .Reflec7on • Wave hits an object with dimension much larger than the radio wavelength will be par7ally reflected off the surface of the object • Amount of reflec7on depends on the incident angle. materials. etc. Diffrac7on • Apparent bending of waves around small obstacles and the spreading out of waves past small openings • The effect of diffrac7on is most pronounced when the wavelength is roughly similar to the dimensions of the diffrac7ng objects . ScaVering • Waves are forced to deviate from a straight trajectory by one or more localized non-‐ uniformi7es in the propaga7on medium • Rayleigh ScaVering: – EM wave scaVered by par7cles that are much smaller in diameter than the wavelength . g. heat . e..Absorp7on • The absorp7on of EM wave is o[en called aVenua7on • The energy of EM wave is taken up by some maVer and transformed to other forms of energy. Update • We have learned how radio waves propagate in the open air • We now discuss – Antennas: to convert EM waves in a wired circuit to EM waves radiated in the open air and vice versa . A Simplified Block Diagram of Wireless Communica7on System • Example: speech or music transmission via microphone . Antenna . Radia7on PaVerns Omni-‐direc7onal Direc7onal Direc7onal Direc7onal . Antennas and Antenna Arrays . Antenna Parameters • Efficiency: the ra7o of power actually radiated by an antenna to the electrical power it receives • Bandwidth: the range of frequency within which the performance of the antenna conforms to a specified standard • Direc7vity: the ra7o between the maximum radia7on intensity radiated by the antenna in the maximum direc7on and the intensity radiated by a hypothe7cal isotropic antenna radia7ng the same total power . Antenna Parameters (Cont’d) • Aperture of an antenna: the area that captures energy from a passing radio wave • Gain: the ra7o between the intensity radiated by the antenna in a given direc7on at an arbitrary distance divided by the intensity radiated at the same distance by an hypothe7cal isotropic antenna For example: G = 4πη A λ2 . e.. i.Reciprocity Between Transmit and Receive Antennas • A transmit antenna setup can be used to serve as a receive antenna • The antenna gains are the same. Gr=Gt .