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Lecture 20 Transmitting Antennas

Lecture 20 Transmitting Antennas. 6.013. ELECTROMAGNETICS AND APPLICATIONS. Luca Daniel. Outline. Review of Fundamental Electromagnetic Laws Electromagnetic Waves in Media and Interfaces Digital & Analog Communications Wireless Communications Radiation Fundamentals

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Lecture 20 Transmitting Antennas

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  1. Lecture 20Transmitting Antennas 6.013 ELECTROMAGNETICS AND APPLICATIONS Luca Daniel

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

  3. Isotropic: main beam sidelobes backlobes Radiated Power PR = Total power radiated [W] I(,,r) = Transmitted Intensity [W/m2] For isotropic antenna: For non-isotropic antenna: Steradian: unit of solid angle d, d: units of radians. Spheres: span 4 steradians dW (steradians) Antenna pattern: z area = r2(d)(sin  d) dW steradians I(,,r)  I(q,f,r) r  x

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

  5. Short Dipole Antennas – Directivity Gain z z Currents on Short Dipole:  deff I I(z) Radiated Intensity [W/m2] (from last time): + d Io - 0 Io Power Radiated [W]: Gain of a short dipole:

  6. Equivalent resistance + Rr Io - Short Dipole Antennas - Radiation Resistance z z Currents on Short Dipole:  deff I I(z) Radiated Intensity [W/m2] (from last time): + d Io - 0 Io Power Radiated [W]: Radiation Resistance Rr:

  7. How to Increase Antenna Gain G(q,f) in some directions? Focus the energy Lenses Mirrors Phasing

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

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