26-05-2012, 03:58 PM
Antennas and Propagation
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
Reception pattern
Receiving antenna’s equivalent to radiation pattern
Types of Antennas
Isotropic antenna (idealized)
Radiates power equally in all directions
Dipole antennas
Half-wave dipole antenna (or Hertz antenna)
Quarter-wave vertical antenna (or Marconi antenna)
Parabolic Reflective 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)
Effective area
Related to physical size and shape of antenna
Relationship between antenna gain and effective area
G = antenna gain
Ae = effective area
f = carrier frequency
c = speed of light (» 3 ´ 108 m/s)
= carrier wavelength
Propagation Modes
Ground-wave propagation
Sky-wave propagation
Line-of-sight propagation
Ground Wave Propagation
Follows contour of the earth
Can Propagate considerable distances
Frequencies up to 2 MHz
Example
AM radio
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
Line-of-Sight Propagation
Transmitting and receiving antennas must be within line of sight
Satellite communication – signal above 30 MHz not reflected by ionosphere
Ground communication – antennas within effective line of site due to refraction
Refraction – bending of microwaves by the atmosphere
Velocity of electromagnetic wave is a function of the density of the medium
When wave changes medium, speed changes
Wave bends at the boundary between mediums
Line-of-Sight Equations
Optical line of sight
Effective, or radio, line of sight
d = distance between antenna and horizon (km)
h = antenna height (m)
K = adjustment factor to account for refraction, rule of thumb K = 4/3
Maximum distance between two antennas for LOS propagation:
h1 = height of antenna one
h2 = height of antenna two
LOS Wireless Transmission Impairments
Attenuation and attenuation distortion
Free space loss
Noise
Atmospheric absorption
Multipath
Refraction
Thermal noise
Attenuation
Strength of signal falls off with distance over transmission medium
Attenuation factors for unguided media:
Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal
Signal must maintain a level sufficiently higher than noise to be received without error
Attenuation is greater at higher frequencies, causing distortion
Free Space Loss
Free space loss, ideal isotropic antenna
Pt = signal power at transmitting antenna
Pr = signal power at receiving antenna
= carrier wavelength
d = propagation distance between antennas
c = speed of light (» 3 ´ 10 8 m/s)
where d and are in the same units (e.g., meters)
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
Reception pattern
Receiving antenna’s equivalent to radiation pattern
Types of Antennas
Isotropic antenna (idealized)
Radiates power equally in all directions
Dipole antennas
Half-wave dipole antenna (or Hertz antenna)
Quarter-wave vertical antenna (or Marconi antenna)
Parabolic Reflective 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)
Effective area
Related to physical size and shape of antenna
Relationship between antenna gain and effective area
G = antenna gain
Ae = effective area
f = carrier frequency
c = speed of light (» 3 ´ 108 m/s)
= carrier wavelength
Propagation Modes
Ground-wave propagation
Sky-wave propagation
Line-of-sight propagation
Ground Wave Propagation
Follows contour of the earth
Can Propagate considerable distances
Frequencies up to 2 MHz
Example
AM radio
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
Line-of-Sight Propagation
Transmitting and receiving antennas must be within line of sight
Satellite communication – signal above 30 MHz not reflected by ionosphere
Ground communication – antennas within effective line of site due to refraction
Refraction – bending of microwaves by the atmosphere
Velocity of electromagnetic wave is a function of the density of the medium
When wave changes medium, speed changes
Wave bends at the boundary between mediums
Line-of-Sight Equations
Optical line of sight
Effective, or radio, line of sight
d = distance between antenna and horizon (km)
h = antenna height (m)
K = adjustment factor to account for refraction, rule of thumb K = 4/3
Maximum distance between two antennas for LOS propagation:
h1 = height of antenna one
h2 = height of antenna two
LOS Wireless Transmission Impairments
Attenuation and attenuation distortion
Free space loss
Noise
Atmospheric absorption
Multipath
Refraction
Thermal noise
Attenuation
Strength of signal falls off with distance over transmission medium
Attenuation factors for unguided media:
Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal
Signal must maintain a level sufficiently higher than noise to be received without error
Attenuation is greater at higher frequencies, causing distortion
Free Space Loss
Free space loss, ideal isotropic antenna
Pt = signal power at transmitting antenna
Pr = signal power at receiving antenna
= carrier wavelength
d = propagation distance between antennas
c = speed of light (» 3 ´ 10 8 m/s)
where d and are in the same units (e.g., meters)