Wireless Networks Spring 2005: Antennas, Propagation, and Signal Encoding, Thesis of Neuroscience

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Antennas & Propagation

Signal Encoding

CSG 250

Spring 2005

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

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

Propagation Modes

Ground-wave propagation

Sky-wave propagation

Line-of-sight propagation

Ground Wave Propagation

Sky Wave Propagation

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

o (^) Amateur radio o (^) CB radio

Line-of-Sight Propagation

 Transmitting and receiving antennas must be

within line of sight

o (^) Satellite communication – signal above 30 MHz not reflected by ionosphere o (^) Ground communication – antennas within effective line of site due to refraction

 Refraction – bending of microwaves by the

atmosphere

o (^) Velocity of electromagnetic wave is a function of the density of the medium o When wave changes medium, speed changes o (^) 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/ d = 3. 57 h d = 3. 57 Κ h

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:

o (^) Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal o (^) Signal must maintain a level sufficiently higher than noise to be received without error o (^) Attenuation is greater at higher frequencies, causing distortion

Free Space Loss

Free space loss equation can be recast:

⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = =  π d P P L r t dB 4 10 log 20 log = − 20 log ( λ )+ 20 log( d )+ 21. 98 dB 20 log ( ) 20 log( ) 147. 56 dB 4 20 log ⎟= + − ⎠ ⎞ ⎜ ⎝ ⎛ = f d c π fd

Free Space Loss

 Free space loss accounting for gain of antennas

• G t = gain of transmitting antenna
• (^) G r = gain of receiving antenna
• (^) A t = effective area of transmitting antenna
• (^) A r = effective area of receiving antenna ( ) ( ) ( ) ( ) r r t r t r t t f A A cd A A d G G d P P 2

4 = = = λ λ π