05-02-2013, 02:07 PM
Antennas and Propagation
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Introduction
An antenna is an electrical conductor or
system of conductors
Transmission - radiates electromagnetic energy
into space
Reception - collects electromagnetic energy
from space
In two-way communication, the same
antenna can be used for transmission and
reception
Radiation Patterns
Radiation pattern
Graphical representation of radiation properties of an
antenna
Depicted as two-dimensional cross section
Beam width (or half-power beam width)
Measure of directivity of antenna
Angle within which power radiated is at least half of that
in most preferred direction
Reception pattern
Receiving antenna’s equivalent to radiation pattern
Omnidirectional vs. directional antenna
Antenna Gain
Antenna gain
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
Related to physical size and shape of antenna
Sky Wave Propagation
Signal reflected from ionized layer of atmosphere
back down to earth
Signal can travel a number of hops, back and
forth between ionosphere and earth’s surface
Reflection effect caused by refraction
Examples
Amateur radio
CB radio
International broadcasts