Definitions of terms relating to propagation in non-ionized media




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Rec. ITU-R P.310-9

RECOMMENDATION ITU-R P.310-9*

DEFINITIONS OF TERMS RELATING TO PROPAGATION
IN NON-IONIZED MEDIA

(1951-1959-1966-1970-1974-1978-1982-1986-1990-1992-1994)

Rec. ITU-R PN.310-9

The ITU Radiocommunication Assembly,



considering

a) that it is important to have agreed definitions of propagation terms used in the texts of ITU-R Study Groups,



recommends

1. that the list of definitions annexed hereto be adopted for incorporation in the vocabulary.

VOCABULARY OF TERMS
USED IN RADIO PROPAGATION IN NON-IONIZED MEDIA





Term

Definition

A.

Terms related to radio waves

A1.

Cross-polarization

The appearance, in the course of propagation, of a polarization component which is orthogonal to the expected polarization.

A2.

Cross-polarization discrimination

For a radio wave transmitted with a given polarization, the ratio at the reception of the power received with the expected polarization to the power received with the orthogonal polarization.







Note 1 – The cross-polarization discrimination depends both on the characteristics of the antenna and on the propagation medium.

A3.

Cross-polarization isolation

For two radio waves transmitted with the same frequency with the same power and orthogonal polarization, the ratio of the co-polarized power in a given receiver to the cross-polarized power in that receiver.

A4.

Depolarization

A phenomenon by virtue of which all or part of the power of a radio wave transmitted with a defined polarization may no longer have a defined polarization after propagation.

B.

Terms related to ground effects on radio-wave propagation

B1.

Free-space propagation

Propagation of an electromagnetic wave in a homogeneous ideal dielectric medium which may be considered of infinite extent in all directions.

B2.

Line of sight propagation

Propagation between two points for which the direct ray is sufficiently clear of obstacles for diffraction to be of negligible effect.

B3.

Radio horizon

The locus of points at which direct rays from a point source of radio waves are tangential to the surface of the Earth.







Note 1As a general rule, the radio and geometric horizons are different because of atmospheric refraction.

B4.

Penetration depth

The depth within the Earth at which the amplitude of a radio wave incident at the surface falls to a value 1/e (0.368) of its value at the surface.




B5.

Smooth surface; specular surface

A surface separating two media which is sufficiently large and the irregularities of which are sufficiently small to cause specular reflection.







Note 1 – In practice, the minimum size of the surface corresponds to the first Fresnel zone and the importance of irregularities is estimated using the Rayleigh criterion.

B6.

Rough surface

A surface separating two media which does not fulfil the smooth surface conditions and the irregularities of which are randomly located and cause diffuse reflection.

B7.

Diffuse reflection coefficient

The ratio of the amplitude of the incoherent wave reflected from a rough surface to the amplitude of the incident wave.

B8.

Measure of terrain irregularity;h

A statistical parameter which characterizes the variations in ground height along part or all of a propagation path.







Note 1 – For example, h is often defined as the difference between the heights exceeded by 10% and 90% respectively of the terrain heights measured at regular intervals (the interdecile height range) along a specified section of a path.

B9.

Obstacle gain

The enhancement in field strength, which can occur at one end of a transmission path including an isolated obstacle, with respect to the field strength which would occur at the same point if this obstacle were removed.

B10.

Site shielding

The reduction in the level of interference radio signals reaching an antenna situated near the ground, due to natural or artificial obstacles in the vicinity of the antenna.

B11.

Site shielding factor

The ratio, generally expressed in dB, of the level of an interfering radio signal which would occur without any site shielding, to the actual level of the interfering radio signal with site shielding.

C.

Terms related to tropospheric effects on radio-wave propagation

C1.

Troposphere

The lower part of the Earth’s atmosphere extending upwards from the Earth’s surface, in which temperature decreases with height except in local layers of temperature inversion. This part of the atmosphere extends to an altitude of about 9 km at the Earth’s poles and 17 km at the equator.

C2

Temperature inversion (in the troposphere)

An increase in temperature with height in the troposphere.

C3.

Mixing ratio

The ratio of the mass of water vapour to the mass of dry air in a given volume of air (generally expressed in g/kg).

C4.

Refractive index; n

Ratio of the speed of radio waves in vacuo to the speed in the medium under consideration.

C5.

Refractivity; N

One million times the amount by which the refractive index n in the atmosphere exceeds unity:

N (n 1) 106

C6.

N-unit

A dimensionless unit in terms of which refractivity is expressed.

C7.

Modified refractive index

The sum of the refractive index n of the air at height h and the ratio of this height to the radius of the Earth, a:

n


C8.

Refractive modulus; M

One million times the amount by which the modified refractive index exceeds unity:

M

C9.

M-unit

A dimensionless unit in terms of which refractive modulus M is expressed.

C10.

Standard refractivity gradient

A standard value of vertical gradient of refractivity used in refraction studies; namely –40 N/km. This corresponds approximately to the median value of the gradient in the first kilometre of altitude in temperate regions.

C11.

Standard radio atmosphere

An atmosphere having the standard refractivity gradient.

C12.

Reference atmosphere for refraction

An atmosphere in which n(h) decreases with height as given in Recommendation ITU-R P.453.

C13.

Sub-refraction

Refraction for which the refractivity gradient is greater (i.e. positive or less negative) than the standard refractivity gradient.

C14.

Super-refraction

Refraction for which the refractivity gradient is less (i.e. more negative) than the standard refractivity gradient.

C15.

Effective radius of the Earth

Radius of a hypothetical spherical Earth, without atmosphere, for which propagation paths are along straight lines, the heights and ground distances being the same as for the actual Earth in an atmosphere with a constant vertical gradient of refractivity.







Note 1 – The concept of effective radius of the Earth implies that the angles with the horizontal planes made at all points by the transmission paths are not too large.







Note 2 – For an atmosphere having a standard refractivity gradient, the effective radius of the Earth is about 4/3 that of the actual radius, which corresponds to approximately 8 500 km.

C16.

Effective Earth-radius factor, k

Ratio of the effective radius of the Earth to the actual Earth radius.







Note 1 – This factor k is related to the vertical gradient dn/dh of the refractive index n and to the actual Earth radius a by the equation:



C17.

Ducting layer

A tropospheric layer characterized by a negative M gradient, which consequently may generate a tropospheric radio-duct if the layer is sufficiently thick compared with the wavelength.

C18.

Tropospheric radio duct

A quasi-horizontal stratification in the troposphere within which radio energy of a sufficiently high frequency is substantially confined and propagates with much lower attenuation than would be obtained in a homogeneous atmosphere.







Note 1 – The tropospheric radio-duct consists of a ducting layer and, in the case of an elevated duct, the portion of the underlying atmosphere in which the refractive modulus exceeds the minimum value attained in the ducting layer.

C19.

Ground-based duct (Surface duct)

A tropospheric radio-duct in which the lower boundary is the surface of the Earth.




C20.

Elevated duct

A tropospheric radio-duct in which the lower boundary is above the surface of the Earth.

C21.

Duct thickness

The difference in height between the upper and lower boundaries of a tropospheric radio-duct.

C22.

Duct height

The height above the surface of the Earth of the lower boundary of an elevated duct.

C23.

Duct intensity

The difference between the maximum and minimum values of the refractive modulus in a tropospheric radio-duct.







Note 1 – The intensity of a duct is the same as that of its ducting layer.

C24.

Ducting

Guided propagation of radio waves inside a tropospheric radio-duct.







Note 1 – At sufficiently high frequencies, a number of electromagnetic modes of guided propagation can coexist in the same tropospheric radio duct.

C25.

Trans-horizon propagation

Tropospheric propagation between points close to the ground, the reception point being beyond the radio horizon of the transmission point.







Note 1 – Trans-horizon propagation may be due to a variety of tropospheric mechanisms such as diffraction, scattering, reflection from tropospheric layers. However ducting is not included because in a duct there is no radio horizon.

C26.

Tropospheric-scatter propagation

Tropospheric propagation due to scattering from many inhomogeneities and discontinuities in the refractive index of the atmosphere.

C27.

Hydrometeors

Concentrations of water or ice particles which may exist in the atmosphere or be deposited on the surface of the Earth.







Note 1 – Rain, fog, clouds, snow and hail are the main hydrometeors.

C28.

Aerosols

Small particles in the atmosphere (other than fog or cloud droplets) which do not fall rapidly under gravity.

C29.

Precipitation-scatter propagation

Tropospheric propagation due to scattering caused by hydrometeors, mainly rain.

C30.

Multipath propagation

Propagation of the same radio signal between a transmission point and a reception point over a number of separate propagation paths.

C31.

Scintillation

Rapid and random fluctuation in one or more of the characteristics (amplitude, phase, polarization, direction of arrival) of a received signal, caused by refractive index fluctuations of the transmission medium.

C32.

Gain degradation; antenna to medium coupling loss

The apparent decrease in the sum of the gains (expressed in decibels) of the transmitting and receiving antennas when significant scattering effects occur on the propagation path.

C33.

Precipitation rate; rainfall rate; rain rate

A measure of the intensity of precipitation expressed by the increase in the height of water reaching the ground per unit time.







Note 1 – Rain rate is generally expressed in millimetres per hour.



* Radiocommunication Study Group 3 made editorial amendments to this Recommendation in 2000 in accordance with Resolution ITU-R 44.


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