Antennas and Propagation(William Stallings, “Wireless Communications and Networks” 2nd Ed, Prentice-

Hall, 2005, Chapter 5)

by Ya Bao http://eent3.sbu.ac.uk/staff/baoy

b/acs 1

Introduction

An antenna is an electrical conductor or system of conductors Transmission - radiates electromagnetic energy

into space

Reception - collects electromagnetic energy

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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)

Radiates power equally in all directions

Dipole antennas

Half-wave dipole antenna (or Hertz

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antenna)

Quarter-wave vertical antenna (or Marconi antenna)

Parabolic Reflective Antenna

Radiation Patterns

Radiation pattern

Graphical representation of radiation properties of an antenna

Depicted as two-dimensional

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Depicted as two-dimensional cross section

Beam width (or half-power beam width)

Measure of directivity of antenna

Radiation patterns

5isotropic

ldirectiona

P

PG

Three-dimensional antenna radiation patterns. The top shows the directive pattern of a horn antenna, the bottom

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horn antenna, the bottom shows the omnidirectionalpattern of a dipole antenna.

or as separate graphs in the vertical plane (E or V plane) and horizontal plane (H plane). This is often known as a polar diagram

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8

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outdoor enclosure featuring a wide band 2.5GHz panel antenna

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Gain (max) 16 dBi (+-0.5 dB)

Frequency 2300 - 2700 MHz

3 dB beamwidth 30° (± 5°)

Front to back (F/B ratio) 20 dB (± 3 dB)

Helical Antenna

Other antennas

Patch (microstrip) antenna

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Multiband antenna: for GSM 900+GSM 1800+GSM 1900+Bluetooth; or GSM and 3G

Antenna Gain

Antenna gain

Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic

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perfect omnidirectional antenna (isotropic antenna)

Effective area

Related to physical size and shape of antenna

Antenna Gain

Relationship between antenna gain and effective area

2

2

2

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c

AfAG ee

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G = antenna gain

Ae = effective area

f = carrier frequency

c = speed of light ( 3 108 m/s)

= carrier wavelength

c

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Propagation Models

Ground Wave (GW) Propagation: < 3MHz

Sky Wave (SW) Propagation: 3MHz to 30MHz

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Effective Line-of-Sight (LOS) Propagation: > 30MHz

Ground Wave Propagation

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– Follows contour of the earth.

– Can propagate considerable distances.

– Frequency bands: ELF, VF, VLF, LF, MF.

– Spectrum range: 30Hz ~ 3MHz, e.g. AM radio.

Sky Wave Propagation

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– Signal reflected from ionized layer of upper atmosphere back down to earth, which can travel a number of hops, back and forth between ionosphere and earth’s surface.

– HF band with intermediate frequency range: 3MHz ~ 30MHz.

– e.g: International broadcast.

Line-of-Sight Propagation

Tx. and Rx. antennas are in the effective ‘line of sight’ range.

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Tx. and Rx. antennas are in the effective ‘line of sight’ range. Includes both LOS and non-LOS (NLOS) caseFor satellite communication, signal above 30 MHz not reflected by ionosphere.For ground communication, antennas within effective LOS due to refraction.

Frequency bands: VHF, UHF, SHF, EHF, Infrared, optical lightSpectrum range : 30MHz ~ 900THz.

LOS calculations

earth

optical horizon

radio horizon

dr

do

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What is the relationship between h and d ?

– For optical LOS:

hdo 3.57

– For effective or radio LOS:

hdr K3.57

where h = antenna height (m)d = distance between

antenna and horizon (km)K = adjustment factor for

refraction, K = 4/3

Line-of-Sight Equations

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

hd 57.3

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K = adjustment factor to account for refraction, rule of thumb K = 4/3

Maximum distance between two antennas for LOS propagation:

2157.3 hhd

LOS Wireless Transmission Impairments

Attenuation and attenuation distortion

Free space loss

Noise

Atmospheric absorption

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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

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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

2

2

2

244

c

fdd

P

P

r

t

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Pt = signal power at transmitting antenna

Pr = signal power at receiving antenna

= carrier wavelength

d = propagation distance between antennas

c = speed of light ( 3 108 m/s)

where d and are in the same units (e.g., meters)

cPr

Free Space Loss

Free space loss equation can be recast:

d

P

PL

r

tdB

4log20log10

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Pr

dB 98.21log20log20 d

dB 56.147log20log204

log20

df

c

fd

cdf

c

fd

c

fdd

P

PL

r

tdB

log20log204log20)log(20)log(20

4log20

4log10

4log10

2

2

2

2

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dBdf

df

df

56.147)log(20)log(20

82054.994.904.12)log(20)log(20

)103log(2094.904.12)log(20)log(20 8

Free Space Loss

Free space loss accounting for gain of other antennas can be recast as

rtdB AAdL log10log20log20

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rtdB AAdL log10log20log20

dB54.169log10log20log20 rt AAdf

Categories of Noise

Thermal Noise

Intermodulation noise

Crosstalk

Impulse Noise

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Impulse Noise

Noise (1)

Thermal noise due to thermal agitation of electrons.

Present in all electronic devices and transmission media.

As a function of temperature.

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Uniformly distributed across the frequency spectrum, hence often referred as white noise.

Cannot be eliminated – places an upper bound on the communication system performance.

Can cause erroneous to the transmitted digital data bits.

Noise (2): Noise on digital data

29Error in bits

Thermal Noise

The noise power density (amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor) is:

W/Hz k0 TN

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W/Hz k0 TN

N0 = noise power density in watts per 1 Hz of bandwidthk = Boltzmann's constant = 1.3803 10-23 J/KT = temperature, in kelvins (absolute temperature)

0oC = 273 Kelvin

Thermal Noise

Noise is assumed to be independent of frequency

Thermal noise present in a bandwidth of B Hertz (in watts):

TBN k

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or, in decibel-watts (dBW),

BTN log10 log 10k log10

BT log10 log 10dBW 6.228

TBN k

Noise Terminology Intermodulation noise – occurs if signals with

different frequencies share the same medium Interference caused by a signal produced at a frequency

that is the sum or difference of original frequencies

Crosstalk – unwanted coupling between signal

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Crosstalk – unwanted coupling between signal paths

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

Signal to Noise Ratio – SNR (1)

Ratio of the power in a signal to the power contained in the noise present at a particular point in the transmission.

Normally measured at the receiver with the attempt to eliminate/suppressed the unwanted noise.

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eliminate/suppressed the unwanted noise.

In decibel unit,

where PS = Signal Power, PN = Noise Power

Higher SNR means better quality of signal.

N

SdB

P

P1010logSNR

Signal to Noise Ratio – SNR (2)

SNR is vital in digital transmission because it can be used to sets the upper bound on the achievable data rate.

Shannon’s formula states the maximum channel capacity (error-free capacity) as:

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(error-free capacity) as:

Given the knowledge of the receiver’s SNR and the signal bandwidth, B. C is expressed in bits/sec.

In practice, however, lower data rate are achieved.

For a fixed level of noise, data rate can be increased by increasing the signal strength or bandwidth.

SNR1log 2 BC

Expression of Eb/N0 (1)

Another parameter that related to SNR for determine data rates and error rates is the ratio of signal energy per bit, Eb to noise power density per Hertz, N0; → Eb/N0.

The energy per bit in a signal is given by: PS = signal power & Tb = time required to send one bit which can be

bSb TPE

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PS = signal power & Tb = time required to send one bit which can be related to the transmission bit rate, R, as Tb = 1/ R.

Thus,

In decibels:

TR

P

N

RP

N

E SSb

k

/

00

dB

b

N

E

0

TRP dBS 101010)( 10logk10log10log

– 228.6 dBW

Expression of Eb/N0 (2)

As the bit rate R increases, the signal power PS relative to the noise must also be increased to

maintain the required Eb/N0.

The bit error rate (BER) for the

BER versus Eb/N0 plot

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The bit error rate (BER) for the data sent is a function of Eb/N0

(see the BER versus Eb/N0 plot).

Eb/N0 is related to SNR as:

R

BSNR

R

B

P

P

N

E

N

Sb

0

where B = Bandwidth, R = Bit rate

Higher Eb/N0, lower BER

Wireless Propagation Mechanisms

Basic types of propagation mechanisms Free space propagation

LOS wave travels large distance with obstacle-free

Reflection

Wave impinges on an object

reflection

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Wave impinges on an object which is large compared to the wave-length

Diffraction

Occurs when wave hits the sharp edge of the obstacles and bent around to propagate further in the ‘shadowed’ regions – Fresnel zones.

Scattering

Wave hits the objects smaller than itself. e.g. street signs and lamp posts.

Lamp post

diffraction scattering