This document discusses antennas and propagation. It begins by defining antennas and their use for transmission and reception of electromagnetic energy. It then describes common antenna types including dipole antennas and parabolic reflective antennas. The document discusses concepts like antenna gain, effective area, and the relationship between them. It also covers propagation modes including ground-wave, sky-wave, and line-of-sight propagation. Key challenges like multipath propagation, fading, and different types of noise are summarized. Finally, it discusses techniques to compensate for errors during transmission.

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2. 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
 Antenna characteristics essentially the same when
sending or receiving electromagnetic energy
2

3. Example: Quarter-Wave Antenna
Source: A VISUAL TOUR OF CLASSICAL ELECTROMAGNETISM,
MIT Physics 8.02: Electromagnetism I
3

4. Types of Antennas
 Isotropic antenna (idealized)
 A point of space that 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
4

5. Antenna: Gain and Effective Area
 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
"l = carrier wavelength
2
G Ae f Ae p
2
= p =
4 4
2
c
l
5

6. More on Duality
 Electromagnetic signals have time domain
as well as frequency domain
representations.
 Waveforms in time domain and frequency
domain has duality.
 Convolution and multiplication in t.d. and
f.d. have duality.
 Light/electromagnetic wave has duality.
 Wave
 Particle
6

7. Propagation Modes
 Ground-wave propagation
 Sky-wave propagation
 Line-of-sight (LOS) propagation
7

8. Ground Wave Propagation
 Follows contour of the earth
 Can propagate considerable distances
 Frequencies up to 2 MHz
 Example
 AM radio
Signal
propagation
transmit
antenna receive
Earth
antenna
8

9. Ionosphere Refraction
 Reflection effect caused by refraction
Source: http://www.tpub.com/neets/book10/40e.htm
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10. 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
 Picks up thousands of kilometers
 Examples
 Amateur radio
 CB radio
 Long-distance broadcast
Ionosphere
transmit
antenna receive
Earth
antenna 10

11. 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
Earth
transmit
antenna
receive
antenna
11

12. Line-of-Sight Equations
 Optical line of sight
d = 3.57 h
 Effective, or radio, line of sight
d = 3.57 Kh
• 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:
( ) 1 2 d = 3.57 Kh + Kh
• h1, h2: heights of the two antennas
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13. LOS Transmission Impairments
 Attenuation and attenuation distortion
 Free space loss
 Noise
 Atmospheric absorption
 Multipath
 Refraction
 Thermal noise
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14. 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
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15. Free Space Loss
 Free space loss, ideal isotropic antenna
4 2 4
d fd
P
r
t p
= p =
• Pt = signal power at transmitting antenna
• Pr = signal power at receiving antenna
"l = carrier wavelength
• d = propagation distance between antennas
• c = speed of light
• where d and l are in the same units (e.g., meters)
 Recast
( ) ( )
2
2
2
c
P
l
L = -20log( ) + 20log(d ) + 21.98 dB dB l
= 20log( f ) + 20log(d ) -147.56 dB
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16. Free Space Loss with Antenna Gains
 Free space loss accounting for gain of other
antennas
P
t
4 2 2 = = l =
2 2
cd
2
f A A
d
A A
p
G G
d
P
2
l
• Gt = gain of transmitting antenna
• Gr = gain of receiving antenna
• At = effective area of transmitting antenna
• Ar = effective area of receiving antenna
 Recast as
( ) ( ) ( ) ( )
r r t r t r t
( ) ( ) ( ) dB t r A A d L log 10 log 20 log 20 - + = l(
= -20log ) + 20log( ) -10log( ) +169.54dB t r f d A A
16

18. So Many Noises
 Thermal Noise
 Intermodulation noise
 Crosstalk
 Impulse Noise
How much is the noise?
 Ratio of signal energy per bit to noise power
density per Hertz
E=
S /
R
b  Related to SNR
N
N
0 0
 Suppose the bit rate R increases, the
transmission signal power S should increase
to maintain the same E/N.
b018

19. Thermal Noise
 Thermal noise due to agitation of electrons
 Present in all electronic devices and transmission
media; cannot be eliminated
 Function of temperature
 Particularly significant for satellite communication
 Thermal noise present in a bandwidth of B Hz (in
watts):
N = kTB
• k = Boltzmann's constant
• T = temperature, in kelvins (absolute temperature)
19

20. Other Noises
 Intermodulation noise – occurs if signals with
different frequencies share the same medium
 A signal produced at a frequency that is the sum or
difference of original frequencies
 Nonlinearity in transmitter, receiver systems.
 Crosstalk
 Unwanted coupling between signal paths
 Dominates in unlicensed ISM bands
 Impulse noise – irregular pulses or noise spikes
 Short duration and of relatively high amplitude
 Caused by external electromagnetic disturbances, or
faults and flaws in the communications system
 Primary source of error in digital data transmission
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21. Other Impairments
 Atmospheric absorption – water vapor
and oxygen contribute to attenuation
 Rain and fog are significant damaging factors
 Multipath – obstacles reflect signals so
that multiple copies with varying delays are
received
 Refraction – bending of radio waves as
they propagate through the atmosphere
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22. Multipath Propagation
Figure 5.10 Sketch of three important propagation
mechanisms: Reflection (R), Scattering (S), Diffraction (D)
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23. Multipath Propagation
 Reflection - occurs when signal
encounters a surface that is large relative
to the wavelength of the signal
 Diffraction - occurs at the edge of an
impenetrable body that is large compared
to wavelength of radio wave
 Scattering – occurs when incoming signal
hits an object whose size in the order of the
wavelength of the signal or less
23

24. The Effects of Multipath Propagation
 Multiple copies of a signal may arrive at different
phases
 If phases add destructively, the signal level relative to
noise declines, making detection more difficult
 Intersymbol interference (ISI)
 One or more delayed copies of a pulse may arrive at
the same time as the primary pulse for a subsequent
bit
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25. Fading
 Time variation of received signal power caused
by changes in the medium or path, e.g., rainfall,
mobility etc.
 Fast fading: over a distance of a half of
wavelength
 Slow fading: change in average received power
over larger distances
 Fading pattern on frequencies: flat fading or
selective fading
 Rayleigh fading: e.g. urban settings
 Rician fading: LOS, e.g. indoor
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26. Error Compensation Mechanisms
 Forward error correction
 Adaptive equalization
 Diversity techniques
 Space diversity: use of collocated directional
antennas
 Frequency diversity: eg. using spread
spectrum
 Time diversity: using TDMA
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27. Recap
 Antennas
 Propagation modes
 LOS loss model
 Noise: types and causes
 Other impairments esp. multipath
 Fading
 Try Problems 5.2, 13 ~ 17 in William
Stallings Text Book.
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