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)