1. ELECTROMAGNETICS AND APPLICATIONS Lecture 20 Transmitting Antennas Luca Daniel

2. L20-2 Review of Fundamental Electromagnetic Laws Electromagnetic Waves in Media and Interfaces Digital & Analog Communications Wireless Communications oRadiation Fundamentals oTransmitting Antennas, Gain  Radiated Power from an Antenna  Antenna [Directivity] Gain  Example: Dipole Directivity Gain  Short Dipole Radiation Resistance  Arrays of Antennas  Wire Antennas oReceiving Antennas; Wireless Communicat. Systems oAperture antennas; Diffraction Acoustic waves and Acoustic antennas e.g. speakers, musical instruments, voice Outline Today

3. L20-3 I( , ,r) = Transmitted Intensity [W/m 2 ] Radiated Power For isotropic antenna: Steradian: unit of solid angle d , d  : units of radians. Spheres: span 4  steradians  d  (steradians) Isotropic: I( , ,r) area = r 2 (d  )(sin  d  ) d  steradians  r x z  main beam backlobes sidelobes Antenna pattern: I(  r) P R = Total power radiated [W] For non-isotropic antenna:

4. L20-4 Antenna [Directivity] Gain G( ,  ) Intensity at receiver: [Directivity] Gain over Isotropic, G( ,  ): Actual intensity radiated in ( ,  ) direction Intensity if P R were radiated isotropically backlobes sidelobes G( ,  ) Isotropic gain Intensity if the power were to be radiated “isotropically” [Directivity] Gain Think of it as a “shaping factor”

5. L20-5 Short Dipole Antennas – Directivity Gain Currents on Short Dipole: Radiated Intensity [W/m 2 ] (from last time):  0 I + IoIo - zz d d eff I(z) IoIo Power Radiated [W]: Gain of a short dipole:

6. L20-6 Short Dipole Antennas - Radiation Resistance Currents on Short Dipole: Radiated Intensity [W/m 2 ] (from last time): Radiation Resistance R r :  0 I + IoIo - zz d d eff I(z) IoIo IoIo + - RrRr Equivalent resistance Power Radiated [W]:

7. L20-7 How to Increase Antenna Gain G(  ) in some directions? Focus the energy Lenses Mirrors Phasing

8. L20-8 Antenna Arrays Radiation from a Pair of Short Dipoles: Example, D = /2: IoIo IoIo + - + - z I y x  I y  x D   x y φ Superposition applies Example, D = :