propagat (2)
TRANSCRIPT
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Propagation models
What are they for?Regulatory vs. scientific issues.
Modes of propagation.
The models.
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ITU Recommendations on Radiowave Propagation
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Modes of propagation &
propagation loss Free space
Ground wave. Diffraction around a smooth earth.
Ground reflections. Effect of terrain. Ionospheric, including sporadic E
Tropospheric: refraction, super-refraction andducting, forward scattering
Diffraction over knife edge & rounded edge Atmospheric attenuation
Variability & Statistics
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Free space propagation
EIRP (watts) to pfd (w/m^2) = P/(4.pi.D^2)
equivalent to (dBW11 -20.log(D))
EIRP (watts) to E (V/m) = sqrt(30.P)/D
EIRP (kW) to E (V/m) = 173*sqrt(P)/Dkm
Also: pfd (W/m^2)=E^2/Z0=E^2/(120.pi)
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Free space loss
Note that EIRP(W) to pfd(W/m^2) is
frequency independent
EIRP(W) to Prx(W) in isotropic antenna is:
Prx={Peirp/(4.pi.D^2)}*{lambda^2/(4.pi)}
I.e. isotropic to isotropic antenna free-space
loss increases as frequency squared.
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Ground wave propagation
Most relevant for low frequencies (
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Ionospheric propagation
Most relevant up to about 30 MHz
Many modes of propagation: a complicated
topic.
Sporadic E can be important up to about 70
MHz. (ITU-R P.534)
Highly variable
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Tropospheric
Variations of radio refractive index
Normal change with height causes greater than
line-of-sight range. Often taken into account byassuming increased radius for the earthe.g. (4/3)
Temperature inversions can cause ducting, withrelatively low attenuation over large distances
beyond the horizon Small scale irregularities are responsible for
forward scatter propagation.
Rain scatter can sometimes be a dominant mode.
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Obstacles
Terrain features, and buildings, usually
attenuate signals. (NB in some
circumstances knife edge diffraction canenhance propagation beyond the horizon)
The OKUMURA-HATA model calculates
attenuation taking account of the percentageof buildings in the path, as well as natural
terrain features.
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Is an Obstruction Obstructing?
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Fresnel ellipsoids and Fresnel zonesIn studying radiowave propagation between two points A and B, the
intervening space can be subdivided by a family of ellipsoids, known
as Fresnel ellipsoids, all having their focal points at A and B such that
any point M on one ellipsoid satisfies the relation:
2ABMBAM n (1)
where n is a whole number characterizing the ellipsoid and n 1 correspondsto the first Fresnel ellipsoid, etc., and is the wavelength.As a practical rule, propagation is assumed to occur in line-of-sight, i.e. with
negligible diffraction phenomena if there is no obstacle within the first Fresnel ellipsoid.
The radius of an ellipsoid at a point between the transmitter and the receiver isgiven by the following formula:
2/1
21
21
dd
ddnRn (2)
or, in practical units:
2/1
21
21
)(550
fddddnRn (3)
wherefis the frequency (MHz) and d1 and d2 are the distances (km) between transmitter
and receiver at the point where the ellipsoid radius (m) is calculated.
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An approximation to the 0.6 Fresnel clearance path lengthThe path length which just achieves a clearance of 0.6 of the first Fresnel zone
over a smooth curved earth, for a given frequency and antenna heights h1 and h2,
is given approximately by:
D06 hf
hf
DD
DD
km (30)
where:
Df: frequency-dependent term
210000389.0 hhf km (30a)
Dh: asymptotic term defined by horizon distances
)(1.4 21 hh km (30b)
f: frequency (MHz)h1, h2: antenna heights above smooth earth (m).
(Radio Horizon)
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h > 0
2
d2
a)
1
d1
1
d1h< 0
2
b)
d2
FIGURE 6
Geometrical elements
1 2 1 2(For definitions ofd, d , d andR,see 4.1 and 4.3)
h > 0
2
d2
a)
1
d1
1
d1h< 0
2
b)
d2
FIGURE 6
Geometrical elements
1 2 1 2(For definitions ofd, d , d andR,see 4.1 and 4.3)
Knife Edge diffraction
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2
0
2
4
6
8
10
12
14
16
18
20
J()
(dB)
FI GURE 7
K ni f e-edgediffraction loss
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Atmospheric attenuation
Starts becoming relevant above about 5 GHz
Depends primarily, but not exclusively on water
vapour content of the atmosphere Varies according to location, altitude, path
elevation angle etc.
Can add to system noise as well as attenuating
desired signal
Precipatation has a significant effect
Specific attenuation due to atmospheric gases
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0676-0
H O2
H O2
102
10
10 1
10 2
1
10 3
2
5
5
2
5
2
5
2
5
2
Specificattenuation(dB/km)
3.52 52 2
102101
Dry airDry airTotal
Frequency,f(GHz)
Pressure: 1 013 hPaTemperature: 15 CWater vapour: 7.5 g/m3
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Propagation models
The ITU recommendations give many
approved methods and models
Two popular methods are are the
Okumura-Hata
and the
Longley Rice
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1546-18
1 200 m
600 m
300 m
150 m
75 m
20 m
10 m
120
110
100
90
80
70
60
50
40
30
20
10
0
10
20
30
40
50
60
70
80
10 100 1 000
h1 = 1 200 m
h1 = 10 m
1
Distance (km)
Fieldstrength
(dB(V/m))for1kWe.r.p.
50% of locations
h2: representative clutter height
FIGURE 18
2 000 MHz, land path, 10% time
Maximum (free space)
Transmitting/base
antenna heights, h1
37.5 m
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Okumura-Hata methodE 69.82 6.16 logf 13.82 logH1 + a(H2) (44.9 6.55 log(H1)(log d)b
where:
E: field strength (dB(V/m)) for 1 kW e.r.p.f: frequency (MHz)
H1: base station effective antenna height above ground (m) in the range 30 to 200 m
H2: mobile station antenna height above ground (m) in the range 1 to 10 m
d: distance (km)a(H2) = (1.1 logf 0.7)H2 (1.56 logf 0.8)
b = 1 for d 20 km
b = 1 (0.14 0.000187f 0.00107 1 ) (log [0.05d])0.8
for d > 20 kmwhere:
1H H1/210,0000071 H
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Longley-Rice model
TRANSMISSION LOSS PREDICTIONS FOR
TROPOSPHERIC COMMUNICATION
CIRCUITS
Longley Rice has been adopted as a standard by the FCC
Many software implementations are available
commercially
Includes most of the relevant propagation modes [multiple
knife & rounded edge diffraction, atmospheric attenuation,
tropospheric propagation modes (forward scatter etc.),
precipitation, diffraction over irregular terrain,
polarization, specific terrain data, atmospheric
stratification, different climatic regions, etc. etc. ]
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NRAO: TAP model(SoftWright implementation with the Terrain Analysis
Package
Notes on The Prediction of Tropospheric Radio Transmission Loss Over Irregular Terrain
(the Longley-Rice Model) propagation in the Terrain Analysis Package (TAP).
The Longley-Rice model predicts long-term median transmission loss over irregular
terrain relative to free-space transmission loss. The model was designed for frequencies
between 20 MHz and 40 GHz and for path lengths between 1 km and 2000 km.
...
This implementation is based on Version 1.2.2 of the model, dated September 1984. Note
also that the version 1.2.2 implemented by SoftWright does not utilize several other
corrections to the model proposed since the method was first published (see A. G. Longley,
"Radio propagation in urban areas," OT Rep. 78-144, Apr. 1978; and A. G. Longley,"Local variability of transmission loss- land mobile and broadcast systems," OT Rep., May
1976).
Technical Foundation
...
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Problems with models
All models have limitations: e.g. Longley Rice doesnt
include ionosphere, so limited applicability at lower
frequencies. Some skill is needed in choosing the right
model for the right circumstances.
Accuracy is limited. Different models can give differentanswers.
May need a statistical interpretation
Need good input data (e.g. terrain models)
Any model needs fairly universal acceptance, to avoid
legal arguments. Acceptance may be more important than
accuracy.
What is the height of a radio telescope?
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Where does this leave us?
In spite of the difficulties, propagation models
have come a long way.
We cant live without them. The best guide we have to whether a given
terrestrial transmission will cause interference to a
radio telescope.
The best guide we have as to whether a given size
of coordination zone will be adequate.