antennas for bases and mobiles - técnico lisboa ... · pdf filenamely concerning the...

Mobile Comms. Systems

Antennas for Bases and Mobiles

• In transmission, antennas are characterised by the gainG = η Dwhere

• η: efficiency• D: directivity

• In general, directivity can be approximated as a function of the half power beam width in the vertical and horizontal planes

General AspectsGeAs (1/2)

dB3dB3

Dααπ

• In reception, antennas are characterised by the effective area

and by the effective length

where• Rr: radiation resistance

General AspectsGeAs (2/2)

GAe πλ4

e πλ=l

• Antennas used in BSs and MTs should have, in principle, an omnidirectional radiation pattern in the azimuthal plane.

• In the VHF/UHF bands, dipoles (and monopoles) are usually used, either isolated or in arrays (besides other types of antennas).

• The analysis of dipoles can be done:• analytically, assuming a sinusoidal current

distribution;• numerically, using the Method of Moments.

Simple DipolesSiDi (1/7)

• Simple models consider a thin dipole, centrally fed.

zz = l

z = -l

z = -lφ

θ(ZA= VA/IA)

• The current distribution is approximated byI(z) = Im sin[k (l - |z|)] , |z| ≤ lthe electric field being

• In order to have a unique lobe in the vertical radiation pattern, the dipole should have 2 l ≤ λ

[ ] ( )θθ

θ uE)sin(

cos)cos(cos2

),( 0 ll kkr

eIZjrjkr

m −=

• Radiation patterns, in the vertical plane:

[Source: Balanis, 1997]

2 l <<λ

2 l =λ/4

2 l =λ/2

2 l =3λ/42 l =λ

• Characteristics of some dipoles:

5.193.822.151.76D [dBi]

150 - j110

(>>) – j(>>)

73 + j42

(<<)– j(>>)ZA [Ω]

32.48.78.90.αV 3dB [o]5/411/2<<12l/ λ

• Monopole parameters can be obtained from the dipole ones.

• Emon = Edip , 0 ≤ θ ≤ π/2• ZA

mon = ZAdip/2

• Dmon = 2 Ddip

• In general, one should analyse the influence of (metallic) structures in the vicinity of antennas, namely concerning the radiation pattern.

• Other types of antennas are used as well:• patches;• helices;• geometrically complex dipoles;• micro-strips.

• Folded dipoles have a bandwidth larger than simple ones.

Other DipolesOtDi (1/5)

• Asymmetric dipoles enable to change the radiation pattern and the input impedance.

• Non-collinear dipoles can be used to increase directivity.

φφ l

φdφd

• Dipoles can be covered by a lossy dielectric, in order to increase the bandwidth, although it decreases efficiency.

• Loaded dipoles are also used, for matching impedance or increasing directivity.

• In uniform collinear arrays, the geometry is:

Collinear Transversal ArraysCoTA (1/7)

I e0j2δ

I e0jδ

I0ej2δ

I0ejδ

• The total field radiated by an array with Naelements isE = Ea Fa

where• Ea: field radiated by a single element• Fa: array factor

γ = kda cos(θ) + δ

)2/sin()2/sin(

γγγ

• In broadside arrays, with isotropic antennas, one hasδ = 0hence,

• directions of nulls

• direction of side lobe adjacent to the main one

⎪⎩

⎪⎨

∈≠≤∈

⎟⎠

⎞⎜⎝

⎛±=

/arccos 0

ppNndNn

n λλ

3arccos >>⎟⎠

⎞⎜⎝

⎛±≅ a

dN λθ

• null beam width

• half power beam width

2 1010]rad[0 >>≅= −− λλ

θθα aaaa

886.0]rad[dB 3 >>≅ λ

λα aa

• The array factor is characterised by: • maximum

|Fa|max = |Fa(γ = 0)| = Na

• largest side lobe

• side lobes level

2|)(| >>≅ aa

sa NNFπ

dB5.13|)(|

||log20 max ≅⎟⎠

⎞⎜⎝

FFSLLθ

• The array directivity can be expressed as D = Dele Drel

where• Dele: directivity of the array element• Drel: relative directivity of the array

1/,2 >>≅ λλ aa

aarel dNdND

[rad]dB 3

77.1α

≅relD

• The directivity for an array of dipoles is

[Source: Rudge et al., 1983]

D[dBi]

• On the horizontal plane, radiation patterns are classified into:

• omnidirectional;• sector.

• On the vertical plane, radiation patterns are classified into:

• horizontal;• tilted.

• Diversity in reception is usually used in order to improve system performance.

Base Station AntennasBaSA (1/10)

• Example of dipole

[Source: Kathrein., 1999]Vertical plane

• Example of monopole

[Source: Allen Telecom, 1994]Vertical plane

• Example of collinear omnidirectional array

[Source: Jaybeam, 1995]Horizontal plane

• Example of sector array

[Source: Jaybeam, 1995]Horizontal plane

• Example of sector array

[Source: Jaybeam, 1995]Vertical plane

• Example of sector array with tilting

Vertical plane [Source: Kathrein., 1999]

• The supporting structure can have a big influence on the radiation pattern.

[Source: Allen Telecom, 1994]

• The choice of antennas needs to consider:• frequency • gain• radiation pattern• half power beam width• side lobe level • front to back ratio• polarisation • input impedance• maximum voltage standing wave ratio

• bandwidth• connectors• maximum input power• weight• wind resistance• tower mounting• antennas material

• MT antennas can be:• monopoles;• asymmetric dipoles;• helices; • micro-strips;• patches.

Mobile Terminal AntennasMoTA (1/6)

[Source: Seibersdorf, 2000]

• The user influences antenna performance

[Source: Jensen & Rahma-Samii, 1995]

• Monopoles are typical for vehicles

• In non-metallic vehicles, half wave length dipoles are used.

5λ/83λ/4

• The location of the antenna on a vehicle influences the radiation pattern.

[Source: Rudge et al., 1983]

• Human originated noise predominates in mobile communications.

• In cellular systems, noise is less important than interference.

• Noise average power can be estimated fromN[dBm] = -174 + 10 log(Δf[Hz]) + F [dB]

where• Δf: signal bandwidth• F: noise figure

NoiseNois (1/4)

• The noise figure can be estimated from

NoiseNois (2/4)

[Source: Hall, 1979]

GalacticAtmospheric

made-Man Rural

lResidentiaBusiness

⎪⎭

⎪⎬

• Noise power is not equal by the BS and by the MT.• Considering that noise comes mainly from

vehicular motors, the difference in noise power between the MT and the BS can be estimated from

where• nb,nm: number of sources at the BS and the MT• db,dm: average distance from sources to BS and

NoiseNois (3/4)

⎟⎟⎠

⎞⎜⎜⎝

⎛= 2

[dB] log10b

dnnNΔ

• Noise power depends on vehicular traffic density.

NoiseNois (4/4)

[Source: Lee, 1993]

• The concern of the population on possible health hazards from radiation is natural and appropriate.

• The question is not whether radiation is harmful, but rather knowing the thresholds beyond which there can be health hazards.

• A basic principle of science is that the results of a study are taken as valid only after being reproduced and checked by the scientific community.

Exposure to RadiationExRa (1/24)

• The effects of radiation depend on frequency.

Exposure to Radiation

power line

AM radio

FM radio TV

microwave oven

heat lamp

tanning booth

medical x-rays

static field

X-rayELF(electric power)

Frequency (Hz)Wavelength (meters)

Microwave (MW)

Ultraviolet (UV)

102 104 106 108 102012101010 1014 1016 1018

1106 104 10-2 10-4 10-6 10-8 10-10 10-12102

Infrared (IR)

Radio (RF)

cell phones

VISIBLE

Thermal

High induced currents

Heating

Non-IonizingOptical

Electronic excitation

Photochemical effects

Non-thermal

Low induced currents

Broken bonds

DNA Damage

Ionizing

[Source: Foster, 2001]

ExRa (2/24)

• Several international bodies have established recommendations

• WHO – World Health Organisation (EHD –Environmental Health Division).

• IRPA – International Radiation Protection Association (INIRC – International Non-Ionising Radiation Committee)

• CENELEC – Comité Européen de Normalisation Electrotechnique.

• ANSI – American National Standards Institute• …

• The European Council recommendation, dated July 1999, states that:

• “has as its objective the protection of the health of the public and it therefore applies, in particular, to relevant areas where members of the public spend significant time”.

• “only established effects were used as a basis for the proposed exposure restrictions”.

• “Such basic restrictions and reference levels should apply to all radiations emitted by electromagnetic fields”.

• Several parameters can be used to quantify exposure to radiation:

• Specific Absorption Rate (SAR) (power dissipated by mass unit – W/g): not possible to measure under normal operating conditions.

• Electric (E) and magnetic (H) fields magnitude, and power density (S) of radiated waves: easy to measure.

• In the frequency bands of mobile communications, it is enough to measure only one of these parameters.

• Peak reference levels:

[Source: CENELEC, 1998]

• Average reference levels:

20.07328.10 < < 400f/2000.0037f 1/21.375f 1/2400 < < 2 00010.000.16561.52 000 < < 300 000

---0.73/f87./f 1/21 < < 10---0.73/f87.0.15 < < 1---5. 87.0.003 < < 0.15

S[W/m2]

H[A/m]

E[V/m]

f[MHz]

10.000.16561.52 000UMTS

System

9.400.16059.61 8809.500.16159.91 900

10.000.16561.52 170

8.550.15356.91 7104.800.11542.69604.450.11041.0890

S[W/m2]

H[A/m]

E[V/m]

f[MHz]

• Radiation can have an effect in human beings:• thermal physical effects – well established;• non-thermal physical effects – research still

being conducted;• psycho-somatic effects – clearly associated to

risk perception.

• Thermal effects:• Temperature higher than 45o C, for a short time:

pain by thermal effects.• Temperature higher than 45o C, for a long time:

burns, tissue damaged by thermal effects.• Radiation of several W/kg, for a long time:

excessive thermal load over the body, leading to serious burns.

• Some non-thermal effects have been raised:• headache, dizziness, etc.;• possibility of electromagnetic interference with

the nervous system.• There are still no definitive conclusions on this

matter.

• Psycho-somatic effects cannot be neglected:• they are a consequence of a perception of

danger, if even it does not exist;• the well known example is the one of people

feeling symptoms even when antennas are not connected.

• It is clear a consequence of the alarm raised by the media.

• Besides health hazard effects, there are others that, indirectly, can constitute a risk, associated to electromagnetic compatibility:

• interference with pace-makers, hearing aids, etc.;

• interference with hospital equipments of high sensitivity (e.g., in intensive care units);

• interference with navigation and communication devices in airplanes.

• SAR can be calculated by

where• σ[S/m]: conductivity;• E[V/m]: electric field magnitude;• ρ[g/m3]: specific density.

ρσ 2

ESAR =

• Electric, or magnetic, field magnitudes can be measured directly, via wideband field probes, with standard antennas.

• Power density is not measured directly, but rather via the receiver.

• The problem of exposure to electromagnetic radiation must be seen from two different perspectives:

• exposure to BSs;• exposure to MTs.

• The far field border from an antenna can be estimated from

where• Lant: largest linear dimension of the antenna

• The exposure to MTs is in the near field of the antenna, while for BSs it can be in either the near or the far fields, depending on the conditions.

22 antff

• Comparison between MTs and BSs (GSM and UMTS):

• MTs radiate in a single frequency, while BSs radiate in several (power sum being needed).

• MTs radiate 1/8 of time (in GSM), while BSs do it continuously.

• Power radiated by BS is higher than that of MTs, but not much.

• MTs radiate discontinuously, as a function of voice.

• There is power control in both MTs and BSs.

• For the largest majority of buildings with BS antennas on top, power levels inside buildings are considerably below the recommended values, because:

• antennas radiate mainly in the horizontal plane;• concrete attenuates between 10 and 20 dB.

• In many cases, power is higher in buildings in front of the reference one, because (besides the previous ones):

• distances are small;• glass attenuates between 1 and 2 dB.

• Example of a very bad installation:

• Model for the numerical evaluation of SAR in a head.

[Source: Dimbylow & Mann, 1994]

• Simulation of SAR inside a head:

[Source: Dimbylow & Mann, 1994]

[W/kg]

• Measurement of SAR inside a model of a head:

• Radiation into the head can be reduced via the use of headsets:

[Source: Ericsson, 1997]

• GeAs - General Aspects• SiDi - Simple Dipoles• OtDi - Other Dipoles• CoTA - Collinear Transversal Arrays• BaSA - Base Station Antennas• MoTA - Mobile Terminal Antennas• Nois - Noise• ExRa - Exposure to Radiation

Table of ContentsToC (1/1)

antennas for bases and mobiles - técnico lisboa ... · pdf filenamely concerning the...

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