Antennas and Propagation
Dec 18, 2015
Antennas and Propagation
Lecture Learning Outcomes
Understand the radiation pattern of an antenna and calculate parameters for different antenna types.
Understand the basis of signal propagation.
Lecture Learning Outcomes
Understand the concepts associated with LoS transmissions.
Been able to calculate noise parameters, antenna gain and transmission losses for different types of antennas in LoS transmissions.
Class ContentsAntennas• Radiation Patterns• Antenna Types & GainsPropagation Modes• Ground Wave• Sky Wave• Line of SightLine of Sight Transmission• Attenuation• Free Space Loss• Noise• Atmospheric Absorption• Multipath• RefractionFading in the Mobile Environment• Multipath Propagation
Antennas
An antenna is an electrical conductor or system of conductors used either for radiating electromagnetic energy into space or for collecting electromagnetic energy from space.
is a graphical representation of the radiation properties of an antenna as a function of space coordinates.
Radiation Patterns
Radiation patterns are almost always depicted as 2-dimensional cross section of the three-dimensional pattern
The Isotropic Antenna
An Isotropic Antenna radiates power in all directions equally. (Omnidirectional Antenna)
Beam Width (Half-Power Width)
Is the angle within which the power radiated by the antennais at least half of what is in the most preferred radiationposition.
Directional Antenna: Power radiated in the direction of B is greater than that radiated in the direction of A
• Half-Wave Dipole (Hertz Antenna)
• Quarter Wave Dipole (Marconi Antenna)
Antenna Types & GainsDipoles
Half-WaveDipole
MarconiAntenna
Dipole Radiation Pattern
Parabolic Reflective Antenna
(a) Parabola Properties
(b) Parabolic Antenna: principle of operation
(c) Radiation Pattern
Typical beam width for parabolic antennas at 12 GHz
Antenna Diameter (m) Beam Width (degrees)
0.5 3.50.75 2.331.0 1.751.5 1.1662.0 0.8752.5 0.75.0 0.35
Antenna GainIs a measure of directionality of an antenna
It is defined the power output in a particular direction compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna).
2
2e
2e A4A4
Gc
f
velengthcarrier wa
m/s)(3x10ligth of speed
frequencycarrier
area effectiveA
gain antenna G
8
e
c
f
Effective Area of typical antennas
Type of Antenna Effective Area Ae (m2)
Power Gain(Relative to Isotropic)
Isotropic 1
Infinitesimal Dipole or loop
1.5
Half-Wave Dipole 1.64
Parabolic (face area A)
4/2
4/5.1 2
4/64.1 2
A56.0 2/A7
Propagation Modes
Ground Wave Propagation Sky Wave Propagation Line of Sight
Ground Wave
• Frequency Below 2 MHz
• Slowed down wave front due to EM current induced into the earth. (downwards tilt)
• Suffer from difraction and scattering from the atmosphere
• Classical Example: AM radio
Sky Wave
• Frequency between 2 and 30 MHz
• Transmitted signal is refracted by the ionosphere and reflectedBy the earth.
• Bouncing allows signal to be picked up thousands of kilometresfrom the transmitter.
• Classical Example: Amateur radio, CB radio and internationalbroadcast (BBC & Voice of America)
Line of Sight
• Above 30 MHz, ground wave and sky wave do not operate
• There is no reflection from the ionosphere (allowing satellite communications not beyond the horizon and back).
• For Ground Based communications, the antennas need to bein LOS with each other.
hd 57.3
Optical LOS with no intervening obstacles
hd K57.3
Radio LOSK is and adjustment factor used to compensate for therefraction
Optical and radio LOS
Optical and radio LOSMaximum distance between two antennas (radio LOS) with K=4/3
2121 12.4KK57.3 hhhhd
• h is measured in metres• d is measured in kilometres• K depends on weather conditions
PerfectStandard Atmosphere
Ideal Without mist
Average Sub-standard
Light Mist
HardSurface Ducts,
ground mist
BadWet Mist
over water
Typical Mild Climate (Non tropical), air mix day and night
Dry, Mountainous without mist
Plains, some mist
Tropical Coast Coast
K 1,33 1,33 1 1 0,66 0,66 0,5 0,5 0,4
Line of Sight Transmission
Sources of Impairment
Attenuation & Attenuation Distortion Noise Atmospheric Absorption Multipath Refraction
Attenuation & Attenuation Distortion
Defined as the loss of strength of the signal over the communicationschannel. It is a complex function of the distance and the make of theatmosphere.
Attenuation Distortion
Occurs when the frequency components of the received signalhave different relative strengths than the frequency componentsof the transmitted signal.
Attenuation
Strength on the received signal (solved using amplifiers or repeaters
in the communications path).
SNR considerations (must be high enough to avoid errors in the transmission) – solved using amplifiers of repeaters.
Attenuation increase with frequency (known as attenuation distortion) – solved using equalizing techniques across a band of frequencies.
Factors encountered when dealing with attenuation
Free Space Loss
Is the ratio of power radiated by the transmitter antennato the power received by the receiver antenna.
It is usually expressed in dB
2
2
R
T 4
P
PL
d
PT=transmitted power (W)PR=received power (W)d = distance = wavelength (same units as distance
:
dB56.147)log(20)log(20L
LPP
dB
dBR(dB)T(dB)
fdf is expressed in Hzd is expressed in m
dB56.147)log(20)log(20PP R(dB)T(dB) fd
IsotropicAntenna
Free Space Loss – Other Antennas
For non-isotropic antennas, the gain of the antenna, with respectto isotropic, should be taken into consideration:
RT GG
d
2
2
R
T 4
P
PL
dB56.147)Glog(G10)log(20)log(20PP RTR(dB)T(dB) fd
Expressed in dB:
PT(dB) and PR(dB) must be expressed in the same dB unit: dBW or dBmThe gains inside the logarithm should be expressed in adimensionalQuantities. If expressed in dB, they should be in dBi
dB56.147GG)log(20)log(20PP R(dBi))T(dBR(dB)T(dB) ifd
Free Space Loss – Other Antennas
Free space loss can also be expressed in terms of effective area:
dB54.169)AAlog(20)log(20)log(20L eReTdB fd
Noise
Thermal Noise
Intermodulation Noise
Crosstalk
Impulsive Noise
Noise are unwanted signals that combine and distortthe signal intended for transmission and reception in a communications system.
Thermal Noise
Due to thermal agitation of electrons
It is present in all electronic devices and transmissionmedia.
It is a function of the temperature
The amount of thermal noise is defined as noise power densityin watts per 1 Hz of bandwidth.
W/Hz)(0 TkN
K is the Boltzmann’s constant: 1.3803x10-23 J/KT is the absolute temperature in Kelvins
Thermal Noise
At room temperature (250 C), the noise power density is:
dBW/Hz203K))15.298()J/K(1038.1log(10 23dB0 N
B)log(10)log(10dBW6.2280 TN
For any given bandwidth B, the noise present in the band is:
B0 TkN
in dBW
Intermodulation Noise
Produced when there is nonlinearities in the transmitter, receiver or transmission system, when 2 or more different frequencies share the medium.
The effect is the production of new signals at frequencies that are the sum or difference of the original frequency and multiples of those frequencies.
Cross Talk
Defined as unwanted coupling between signal paths.
Can occur when unwanted signals are picked up by microwave antennas or by electrical coupling between twisted pair (in guided media transmissions)
Can be identified when in the telephone line, another conversation can be heard.
Typically is in the same order of magnitude or less than the Thermal Noise
Impulsive Noise
Non-continuous noise consisting of irregular pulse or noise spikes of short duration and relatively high amplitude.
Causes include external electromagnetic disturbances (lightning) and faults and flaws in the communication system.
It is a minor concern in analogue signals, but is a major concern when dealing with digital data transmissions
Impulsive Noise
In a voice communication, impulsive noise will generate clicks and crackles of short duration, however, the conversation will still be intelligible.
Example
In a digital transmission, a small spark of energy (10 ms in duration) would wash out 560 bits of data being transmitted at 56 kbps.
Ratio of Signal Energy per bit to Noise Power Density
The short name for this equivalent is the Eb/N0
expression
The advantage of Eb/N0 over SNR is that the latter depends on the bandwidth
Ratio of Signal Energy per bit to Noise Power Density
A signal containing a binary data transmitted at a data rate of R, is subjected to thermal noise N0
The Energy per bit in such a signal is:
bb TSE S = signal powerTb = time needed to transmitt 1 bit:
Tb = 1/Rk = Boltzman Constant (1.3803x1023 J/K)T = Temp in Kelvin
The expression Eb/No can be written:
TR
S
R
SE
00b
kN
N
log(T)10-dBW6.228)Rlog(10SE
dBW
dB0
b
N
Ratio of Signal Energy per bit to Noise Power Density
Example:
Suppose a signal encoding technique requires that Eb/N0 = 8.4 dB for a bit error rate of 10-4. If the effective noise temperature is 290K (room temperature) and the data rate is 2.4 Kbps, what received signal level is required to overcome thermal noise
Solution:
dBW8.161S
)462.(10-6.228)38.3(10SdB4.8
log(T)10-dBW6.228)Rlog(10SE
dBW
dBW
dBW
dB0
b
N
Achievable Spectral Density
The parameter N0 is the noise power density in watts/hertz. The noise in a signal with a bandwidth B is:
BN 0N
Substituting in the Eb/N0 expression
RN
BS
R
SE
00b
NN
Considering that the Shannon’s capacity formula (in bps)
12N
S
)NS1(logBC
BC
2
Achievable Spectral Density
Equating the channel capacity C with the data rate R, and using the Eb/N0 expression:
.12C
BE B
C
0b
N
This expression is a formula that relates the achievable spectral efficiency C/B to Eb/No
Atmospheric Absorption
Additional loss between the transmitting and receiving antenna.
The main contributors are the water vapour and oxygen present in the atmosphere.
Water Vapour generates attenuation peaks at frequencies close to 22 GHz
Absorption due to oxygen has a peak in the vicinity of 60 GHz
Rain and Fog cause scattering of radio waves that results in attenuation
Multipath
Occurs in environments where there is no direct LOS between the transmitting and receiving antenna due to the presence of intervening obstacles.
Obstacles can reflect the signal creating multiple copies that arrive at delayed times to the receiver. This copies acts as noise to the received signal.
Refraction
Is the bend that suffer radio waves when propagating through the atmosphere
It is caused by changes of speed of the signal with altitude or by other spatial changes in atmospheric conditions.
Normally the speed of the signal increases with altitude, causing the radio waves to bend downwards.
Fading in the Mobile Environment
Fading refers to the time variation of the received signal power caused by changes in the transmission medium or path(s).
The most important fading mechanism is multipath propagation.
Multipath propagation
Reflection (surface > wavelength)
Diffraction(edge of body >
wavelength)
Scattering(obstacle = wavelength)
Effects of multipath propagation
Copies of the signal arriving at different phases.
If copies add destructively, SNR declines
Signal interpretation then becomes difficult.
Intersymbol interference (ISI)