propagat.ppt

Propagation modelsWhat are they for?Regulatory vs. scientific issues.Modes of propagation.The models.

ITU Recommendations on Radiowave Propagation

Modes of propagation & propagation lossFree spaceGround wave. Diffraction around a smooth earth. Ground reflections. Effect of terrain.Ionospheric, including sporadic ETropospheric: refraction, super-refraction and ducting, forward scatteringDiffraction over knife edge & rounded edgeAtmospheric attenuationVariability & Statistics

Free space propagationEIRP (watts) to pfd (w/m^2) = P/(4.pi.D^2) equivalent to (dBW 11 -20.log(D))EIRP (watts) to E (V/m) = sqrt(30.P)/DEIRP (kW) to E (V/m) = 173*sqrt(P)/DkmAlso: pfd (W/m^2)=E^2/Z0=E^2/(120.pi)

Free space lossNote that EIRP(W) to pfd(W/m^2) is frequency independentEIRP(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.

Ionospheric propagationMost relevant up to about 30 MHzMany modes of propagation: a complicated topic.Sporadic E can be important up to about 70 MHz. (ITU-R P.534)Highly variable

TroposphericVariations of radio refractive indexNormal change with height causes greater than line-of-sight range. Often taken into account by assuming increased radius for the earth e.g. (4/3)Temperature inversions can cause ducting, with relatively low attenuation over large distances beyond the horizonSmall scale irregularities are responsible for forward scatter propagation.Rain scatter can sometimes be a dominant mode.

ObstaclesTerrain features, and buildings, usually attenuate signals. (NB in some circumstances knife edge diffraction can enhance propagation beyond the horizon)The OKUMURA-HATA model calculates attenuation taking account of the percentage of buildings in the path, as well as natural terrain features.

Is an Obstruction Obstructing?

Fresnel ellipsoids and Fresnel zones

In 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:

(1)

where n is a whole number characterizing the ellipsoid and n 1 corresponds

to 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 is

given by the following formula:

(2)

or, in practical units:

(3)

where f is 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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(Radio Horizon)

An approximation to the 0.6 Fresnel clearance path length

The 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

km(30)

where:

Df:frequency-dependent term

km(30a)

Dh:asymptotic term defined by horizon distances

km(30b)

f:frequency (MHz)

h1, h2 :antenna heights above smooth earth (m).

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Knife Edge diffraction

see 4.1 and 4.3)

R,

and

d

,

d

,

d

(For definitions of

2

1

2

1

Geometrical elements

FIGURE 6

2

d

b)

2

< 0

h

1

d

1

c)

d

2

1

R

h

1

d

2

d

1

d

1

a)

2

d

2

0

h >

0526-06

_1084883044.doc

0526-06

h >

0

2

d

2

a)

1

d

1

d

2

d

1

h

R

1

2

d

c)

1

d

1

h

< 0

2

b)

d

2

FIGURE 6

Geometrical elements

1

2

1

2

(For definitions of

d

,

d

,

d

and

R,

see 4.1 and 4.3)

Atmospheric attenuationStarts becoming relevant above about 5 GHzDepends primarily, but not exclusively on water vapour content of the atmosphereVaries according to location, altitude, path elevation angle etc.Can add to system noise as well as attenuating desired signalPrecipatation has a significant effect

Propagation modelsThe ITU recommendations give many approved methods and modelsTwo popular methods are are theOkumura-Hata and the Longley Rice

Okumura-Hata method

E 69.82(6.16 log f13.82 log H1+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 200m

H2:mobile station antenna height above ground (m) in the range 1 to 10 m

d:distance (km)

a(H2) =(1.1 log f(0.7) H2((1.56 log f(0.8)

b =1 for d(20 km

b =1(0.140.000187 f0.00107

) (log [0.05d])0.8

for d>20 km

where:

H1/

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Longley-Rice model TRANSMISSION LOSS PREDICTIONS FOR TROPOSPHERIC COMMUNICATION CIRCUITS

Longley Rice has been adopted as a standard by the FCCMany software implementations are available commerciallyIncludes 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. ]

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...

Problems with modelsAll 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 different answers.May need a statistical interpretationNeed 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?

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.