Mobile Comms. Systems Antennas for Bases and Mobiles
Mobile Comms. Systems
Antennas for Bases and Mobiles
Mobile Comms. Systems
• 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
4 HV
Dααπ
≈
Mobile Comms. Systems
• In reception, antennas are characterised by the effective area
and by the effective length
where• Rr: radiation resistance
General AspectsGeAs (2/2)
GAe πλ4
2
=
0
ZRD r
e πλ=l
Mobile Comms. Systems
• 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)
Mobile Comms. Systems
• Simple models consider a thin dipole, centrally fed.
Simple DipolesSiDi (2/7)
x
y
zz = l
z = -l
θ
ϕ
r
2a
IAVA
z
x
y
r
2a
IA
VA
z = l
z = -lφ
θ(ZA= VA/IA)
Mobile Comms. Systems
• 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
Simple DipolesSiDi (3/7)
[ ] ( )θθ
θπ
θ uE)sin(
cos)cos(cos2
),( 0 ll kkr
eIZjrjkr
m −=
−
Mobile Comms. Systems
• Radiation patterns, in the vertical plane:
Simple DipolesSiDi (4/7)
[Source: Balanis, 1997]
2 l <<λ
2 l =λ/4
2 l =λ/2
2 l =3λ/42 l =λ
Mobile Comms. Systems
• Characteristics of some dipoles:
Simple DipolesSiDi (5/7)
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
Mobile Comms. Systems
• Monopole parameters can be obtained from the dipole ones.
• Emon = Edip , 0 ≤ θ ≤ π/2• ZA
mon = ZAdip/2
• Dmon = 2 Ddip
Simple DipolesSiDi (6/7)
l
Mobile Comms. Systems
• 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.
Simple DipolesSiDi (7/7)
Mobile Comms. Systems
• Folded dipoles have a bandwidth larger than simple ones.
Other DipolesOtDi (1/5)
2l
2a
d
2a
dd
Mobile Comms. Systems
• Asymmetric dipoles enable to change the radiation pattern and the input impedance.
Other DipolesOtDi (2/5)
z
l
z = 0
12l
l2
Mobile Comms. Systems
• Non-collinear dipoles can be used to increase directivity.
Other DipolesOtDi (3/5)
x
y
z
l
θ
ϕ
r
φφ l
φdφd
Mobile Comms. Systems
• Dipoles can be covered by a lossy dielectric, in order to increase the bandwidth, although it decreases efficiency.
Other DipolesOtDi (4/5)
Mobile Comms. Systems
• Loaded dipoles are also used, for matching impedance or increasing directivity.
Other DipolesOtDi (5/5)
2l2d
L /20
L /20
L /20
L /20
2dL
Mobile Comms. Systems
• In uniform collinear arrays, the geometry is:
Collinear Transversal ArraysCoTA (1/7)
z
θ
I e0j2δ
I e0jδ
I 0
d
d
da
da
I0ej2δ
I0ejδ
I0
θ
Mobile Comms. Systems
• 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
Collinear Transversal ArraysCoTA (2/7)
)2/sin()2/sin(
2)1(
1
)1(
γγγ
γ
aNj
N
n
nja
Ne
eF
a
a
−
=
−
=
=∑
Mobile Comms. Systems
• In broadside arrays, with isotropic antennas, one hasδ = 0hence,
• directions of nulls
• direction of side lobe adjacent to the main one
Collinear Transversal ArraysCoTA (3/7)
⎪⎩
⎪⎨
⎧
∈≠≤∈
⎟⎠
⎞⎜⎝
⎛±=
IN,/
IN,
/arccos 0
ppNndNn
n
dNn
a
aaaa
n λλ
θ
1,/2
3arccos >>⎟⎠
⎞⎜⎝
⎛±≅ a
aas N
dN λθ
Mobile Comms. Systems
• null beam width
• half power beam width
Collinear Transversal ArraysCoTA (4/7)
1/,/
2 1010]rad[0 >>≅= −− λλ
θθα aaaa
dNdN
1/,/
886.0]rad[dB 3 >>≅ λ
λα aa
aadN
dN
Mobile Comms. Systems
• The array factor is characterised by: • maximum
|Fa|max = |Fa(γ = 0)| = Na
• largest side lobe
• side lobes level
Collinear Transversal ArraysCoTA (5/7)
1,3
2|)(| >>≅ aa
sa NNFπ
θ
dB5.13|)(|
||log20 max ≅⎟⎠
⎞⎜⎝
⎛=
sa
a
FFSLLθ
Mobile Comms. Systems
• The array directivity can be expressed as D = Dele Drel
where• Dele: directivity of the array element• Drel: relative directivity of the array
and
Collinear Transversal ArraysCoTA (6/7)
1/,2 >>≅ λλ aa
aarel dNdND
[rad]dB 3
77.1α
≅relD
Mobile Comms. Systems
• The directivity for an array of dipoles is
Collinear Transversal ArraysCoTA (7/7)
[Source: Rudge et al., 1983]
D[dBi]
da/λ
Mobile Comms. Systems
• 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)
Mobile Comms. Systems
• Example of dipole
Base Station AntennasBaSA (2/10)
[Source: Kathrein., 1999]Vertical plane
Mobile Comms. Systems
• Example of monopole
Base Station AntennasBaSA (3/10)
[Source: Allen Telecom, 1994]Vertical plane
Mobile Comms. Systems
• Example of collinear omnidirectional array
Base Station AntennasBaSA (4/10)
[Source: Jaybeam, 1995]Horizontal plane
Mobile Comms. Systems
• Example of sector array
Base Station AntennasBaSA (5/10)
[Source: Jaybeam, 1995]Horizontal plane
Mobile Comms. Systems
• Example of sector array
Base Station AntennasBaSA (6/10)
[Source: Jaybeam, 1995]Vertical plane
Mobile Comms. Systems
• Example of sector array with tilting
Base Station AntennasBaSA (7/10)
Vertical plane [Source: Kathrein., 1999]
Mobile Comms. Systems
• The supporting structure can have a big influence on the radiation pattern.
Base Station AntennasBaSA (8/10)
[Source: Allen Telecom, 1994]
Mobile Comms. Systems
• 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
Base Station AntennasBaSA (9/10)
Mobile Comms. Systems
• bandwidth• connectors• maximum input power• weight• wind resistance• tower mounting• antennas material
Base Station AntennasBaSA (10/10)
Mobile Comms. Systems
• MT antennas can be:• monopoles;• asymmetric dipoles;• helices; • micro-strips;• patches.
Mobile Terminal AntennasMoTA (1/6)
Mobile Comms. Systems
Mobile Terminal AntennasMoTA (2/6)
[Source: Seibersdorf, 2000]
Mobile Comms. Systems
Mobile Terminal AntennasMoTA (3/6)
[Source: Seibersdorf, 2000]
Mobile Comms. Systems
• The user influences antenna performance
Mobile Terminal AntennasMoTA (4/6)
[Source: Jensen & Rahma-Samii, 1995]
Mobile Comms. Systems
• Monopoles are typical for vehicles
• In non-metallic vehicles, half wave length dipoles are used.
Mobile Terminal AntennasMoTA (5/6)
λ/4
5λ/83λ/4
λ/4
Mobile Comms. Systems
• The location of the antenna on a vehicle influences the radiation pattern.
Mobile Terminal AntennasMoTA (6/6)
[Source: Rudge et al., 1983]
Mobile Comms. Systems
• 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)
Mobile Comms. Systems
• The noise figure can be estimated from
NoiseNois (2/4)
[Source: Hall, 1979]
GalacticAtmospheric
made-Man Rural
lResidentiaBusiness
⎪⎭
⎪⎬
⎫
Mobile Comms. Systems
• 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
MT
NoiseNois (3/4)
⎟⎟⎠
⎞⎜⎜⎝
⎛= 2
2
[dB] log10b
m
m
bbm d
dnnNΔ
Mobile Comms. Systems
• Noise power depends on vehicular traffic density.
NoiseNois (4/4)
[Source: Lee, 1993]
Mobile Comms. Systems
• 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)
Mobile Comms. Systems
• 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)
Mobile Comms. Systems
• 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• …
Exposure to RadiationExRa (3/24)
Mobile Comms. Systems
• 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”.
Exposure to RadiationExRa (4/24)
Mobile Comms. Systems
• 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.
Exposure to RadiationExRa (5/24)
Mobile Comms. Systems
• Peak reference levels:
Exposure to RadiationExRa (6/24)
[Source: CENELEC, 1998]
Mobile Comms. Systems
• Average reference levels:
Exposure to RadiationExRa (7/24)
[Source: CENELEC, 1998]
Mobile Comms. Systems
• Average reference levels:
Exposure to RadiationExRa (8/24)
[Source: CENELEC, 1998]
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]
Mobile Comms. Systems
• Average reference levels:
Exposure to RadiationExRa (9/24)
[Source: CENELEC, 1998]
10.000.16561.52 000UMTS
GSM
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]
Mobile Comms. Systems
• 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.
Exposure to RadiationExRa (10/24)
Mobile Comms. Systems
• 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.
Exposure to RadiationExRa (11/24)
Mobile Comms. Systems
• 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.
Exposure to RadiationExRa (12/24)
Mobile Comms. Systems
• 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.
Exposure to RadiationExRa (13/24)
Mobile Comms. Systems
• 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.
Exposure to RadiationExRa (14/24)
Mobile Comms. Systems
• SAR can be calculated by
where• σ[S/m]: conductivity;• E[V/m]: electric field magnitude;• ρ[g/m3]: specific density.
Exposure to RadiationExRa (15/24)
ρσ 2
ESAR =
Mobile Comms. Systems
• 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.
Exposure to RadiationExRa (16/24)
Mobile Comms. Systems
• 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.
Exposure to RadiationExRa (17/24)
λ
22 antff
Ld =
Mobile Comms. Systems
• 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.
Exposure to RadiationExRa (18/24)
Mobile Comms. Systems
• 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.
Exposure to RadiationExRa (19/24)
Mobile Comms. Systems
• Example of a very bad installation:
Exposure to RadiationExRa (20/24)
Mobile Comms. Systems
• Model for the numerical evaluation of SAR in a head.
Exposure to RadiationExRa (21/24)
[Source: Dimbylow & Mann, 1994]
Mobile Comms. Systems
• Simulation of SAR inside a head:
Exposure to RadiationExRa (22/24)
[Source: Dimbylow & Mann, 1994]
[W/kg]
Mobile Comms. Systems
• Measurement of SAR inside a model of a head:
Exposure to RadiationExRa (23/24)
[Source: Seibersdorf, 2000]
Mobile Comms. Systems
• Radiation into the head can be reduced via the use of headsets:
Exposure to RadiationExRa (24/24)
[Source: Ericsson, 1997]
Mobile Comms. Systems
• 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)