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Some Important Pointers on Antenna Theory



  • To create radiation, there must be an acceleration/deceleration of charge (i.e. changing current or AC current/voltage). This also implies that DC won't work on straight conductors as radiators unless the conductor is bent or has discontinuities (presence of centripetal acceleration in charge)
  • If pulses of shorter and more compact duration are fed into a radiating element, stronger radiation with more frequency components will occur (I don't know by what magnitude radiation is amplified but it could serve as a possible driver of Microwave-generated hurricanes, though I doubt that the amplification factor could reach that high).
  • Radiated fields do not need the continuous presence of electric charge to sustain their existence.
  • An omnidirectional radiator is non-directional in one dimension while it is directional in other dimensions. An isotropic radiator is non-directional.
  • The regions of antenna radiation can be divided into 3 parts:

-Reactive Near Field Region

Exists at R<0.62*sqrt((D^3)/lambda)

-Radiating Near Field (Fresnel) Region

Exists at R<2*(D^2)/lambda

-Far Field (Fraunhofer) Region

Exists at greater distances than above

(D-antenna diameter; lambda-wavelength)

  • The power density of radiated EM waves in the far field are mostly real values of the poynting vector. (The poynting vector is a way of expressing the amount of power associated with an EM wave and is expressed as the cross-product of the electric and magnetic field intensities) Also, the imaginary part is usually related to the inductive and capacitive characteristics of the radiated EM wave.
  • There is no such thing as an isotropic radiator. It only exists in our imagination.
  • The term directive gain has been "deprecated" by the 1983 version of the IEEE Standard Definitions of Terms for Antennas.
  • The directivity of an isotropic source is unity.
  • The 2 most popular methods in analysis of modern antenna problems are the Integral Equation method and Geometrical Theory of Diffraction.
The Integral Equation method is best used with analyzing electrically small/wire-type antennas. 2 integral equations are formulated in the analysis, the electric field integral equation and the magnetic field integral equation.
(sometimes the 2 methods are combined)
  • The total efficiency of an antenna depends on the product of its reflection, conduction, and dielectric efficiencies.
  • Right Hand Polarization- clockwise electric field rotation
  • Left Hand Polarization- counter-clockwise electric field rotation
  • In circular polarization, the magnitudes of the electric and magnetic fields are the same and their phase differences are odd multiples of 90 degrees.
  • In elliptical polarization, the magnitudes of the electric and magnetic fields are NOT the same OR their phase differences are not multiples of 90 degrees (whatever the magnitude).
  • If the phase differences are multiples of 180 degrees, then the radiated EM wave is linearly polarized.
  • The figure of merit for polarization is the polarization efficiency/polarization mismatch/loss factor.
  • Right hand polarized antennas cannot receive left hand polarized EM waves and vice versa.
  • The free-space loss factor is the loss due to the spherical spreading of the EM energy radiated.
  • Friis Transmission Equation relates the transmit and receive powers through the transmitter and receiver efficiencies, reflection coefficients, directivities, polarizations (Note: the dot product becomes zero when the transmitter polarization is vertical and the receiver polarization is horizontal - the dot product of i and j is zero thus the entire equation becomes equated to zero!!) and the free space loss.
  • The Radar Range Equation is just like Friis Transmission Equation only with the added echo area factor divided by 4*pi.
  • Every object with a temperature above absolute zero radiates energy (Stefan-Boltzmann) and this relationship can be used for antennas (antenna temperature/brightness temperature).
  • Analysis - specify the source, require the fields
  • Synthesis - specify the fields, find the source
  • Infinitesimal wire dipoles are used to represent top-hat loaded (capacitor plate) antennas.
  • Slightly rough surface- rms height much smaller than wavelength
  • Very rough surface- rms height is much greater than wavelength
  • Electrically small loop antennas are poor radiators.
  • Superdirective antennas- antennas whose directivities are much larger than the directivity of a reference antenna of the same size

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