Introduction

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

oyb/acs 1

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

Hall, 2005, Chapter 5)

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

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Types of Antennas Isotropic antenna (idealized)

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

Radiation Patterns Radiation pattern

Graphical representation of radiation properties of an antenna

Depicted as two-dimensional cross section

Beam width (or half-power beam width) Measure of directivity of antenna

5

Radiation patterns

isotropic

ldirectiona

P

PG

6

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

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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

8

9

10

outdoor enclosure featuring a wide band 2.5GHz panel antenna

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)

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Helical Antenna

Other antennas

Patch (microstrip) antenna

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

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Antenna Gain 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

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Antenna Gain Relationship between antenna gain and effective

area

G = antenna gain Ae = effective area f = carrier frequency c = speed of light ( 3 108 m/s) = carrier wavelength

2

2

2

44

c

AfAG ee

14

15

Propagation Models

Ground Wave (GW) Propagation: < 3MHz Sky Wave (SW) Propagation: 3MHz to 30MHz Effective Line-of-Sight (LOS) Propagation: >

30MHz

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Ground Wave Propagation

– Follows contour of the earth.– Can propagate considerable distances.– Frequency bands: ELF, VF, VLF, LF, MF.– Spectrum range: 30Hz ~ 3MHz, e.g. AM radio.

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Sky Wave Propagation

– 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.

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Line-of-Sight Propagation

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.

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LOS calculations

hearth

optical horizon

radio horizon

dr

do

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

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Line-of-Sight EquationsEffective, 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 = 4/3

hd 57.3

Maximum distance between two antennas for LOS propagation:

2157.3 hhd

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LOS Wireless Transmission Impairments

Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise

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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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Free Space Loss Free space loss, ideal isotropic antenna

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)

2

2

2

2 44

c

fdd

P

P

r

t

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Free Space Loss Free space loss equation can be recast:

d

P

PL

r

tdB

4log20log10

dB 98.21log20log20 d

dB 56.147log20log204

log20

df

c

fd

25

dBdf

df

df

cdf

c

fd

c

fdd

P

PL

r

tdB

56.147)log(20)log(20

82054.994.904.12)log(20)log(20

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

log20log204log20)log(20)log(20

4log20

4log10

4log10

8

2

2

2

2

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Free Space Loss Free space loss accounting for gain of other

antennas can be recast as

rtdB AAdL log10log20log20

dB54.169log10log20log20 rt AAdf

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Categories of Noise Thermal Noise Intermodulation noise Crosstalk Impulse Noise

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Noise (1)

Thermal noise due to thermal agitation of electrons. Present in all electronic devices and transmission media. As a function of temperature. 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.

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Noise (2): Noise on digital data

Error in bits

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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 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

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Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz

(in watts):

or, in decibel-watts (dBW),

BTN log10 log 10k log10 BT log10 log 10dBW 6.228

TBN k

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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

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

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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.

In decibel unit,

where PS = Signal Power, PN = Noise Power

Higher SNR means better quality of signal.

N

SdB P

P1010logSNR

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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:

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.

SNR1log2 BC

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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 related

to the transmission bit rate, R, as Tb = 1/ R.

Thus,

In decibels:

bSb TPE

TR

P

N

RP

N

E SSb

k

/

00

dB

b

N

E

0

TRP dBS 101010)( 10logk10log10log

– 228.6 dBW

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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 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

BER versus Eb/N0 plot

where B = Bandwidth, R = Bit rate

Higher Eb/N0, lower BER

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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

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

reflection

diffraction scattering