Antenna Propagation

Antennas & Propagation
CS 6710
Spring 2010
Rajmohan Rajaraman

Introduction
An antenna is an electrical conductor or system of conductors o Transmission - radiates electromagnetic energy into space o Reception - collects electromagnetic energy from space

In two-way communication, the same antenna can be used for transmission and reception Radiation Patterns
 Radiation pattern o Graphical representation of radiation properties of an antenna o Depicted as two-dimensional cross section

 Beam width (or half-power beam width) o Measure of directivity of antenna o Angle within which power radiated is at least half of that in most preferred direction

 Reception pattern o Receiving antenna’s equivalent to radiation pattern

 Omnidirectional vs. directional antenna

Types of Antennas
 Isotropic antenna (idealized) o Radiates power equally in all directions

 Dipole antennas o Half-wave dipole antenna (or Hertz antenna) o Quarter-wave vertical antenna (or Marconi antenna)

 Parabolic Reflective Antenna o Used for terrestrial microwave and satellite applications o Larger the diameter, the more tightly directional is the beam Antenna Gain
Antenna gain o Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) Expressed in terms of effective area o Related to physical size and shape of antenna

Antenna Gain
 Relationship between antenna gain and effective area 4!Ae 4!f 2 Ae
G= 2 =
2
" c •
•
•
•
•

G = antenna gain
Ae = effective area f = carrier frequency c = speed of light (≈ 3 x 108 m/s) λ = carrier wavelength

Propagation Modes
Ground-wave propagation
Sky-wave propagation
Line-of-sight propagation

Ground Wave Propagation

Ground Wave Propagation
Follows contour of the earth
Can Propagate considerable distances
Frequencies up to 2 MHz
Example
o AM radio

Sky Wave