1、 Recommendation ITU-R P.525-3 (09/2016) Calculation of free-space attenuation P Series Radiowave propagation ii Rec. ITU-R P.525-3 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the radio-frequency spectrum by all radiocommuni
2、cation services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted. The regulatory and policy functions of the Radiocommunication Sector are performed by World and Regional Radiocommunication Conferences and Radioco
3、mmunication Assemblies supported by Study Groups. Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of Resolution ITU-R 1. Forms to be used for the submission of patent statements and licensing decla
4、rations by patent holders are available from http:/www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found. Series of ITU-R Recommendations (Also available online at http:/
5、www.itu.int/publ/R-REC/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F Fixed service M Mobile, radiodetermination, amateur and related satellite services P Radiowave
6、propagation RA Radio astronomy RS Remote sensing systems S Fixed-satellite service SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service systems SM Spectrum management SNG Satellite news gathering TF Time signals and frequency standards
7、 emissions V Vocabulary and related subjects Note: This ITU-R Recommendation was approved in English under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2016 ITU 2016 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without w
8、ritten permission of ITU. Rec. ITU-R P.525-3 1 RECOMMENDATION ITU-R P.525-3 Calculation of free-space attenuation (1978-1982-1994-2016) Scope Recommendation ITU-R P.525 provides methods to calculate the attenuation in free space. Keywords Free space, attenuation, telecommunication links The ITU Radi
9、ocommunication Assembly, considering a) that free-space propagation is a fundamental reference for radio-engineering, recommends 1 that the methods in Annex 1 be used for the calculation of attenuation in free space. Annex 1 1 Introduction As free-space propagation is often used as a reference in ot
10、her texts, this Annex presents relevant formulae. 2 Basic formulae for telecommunication links Free-space propagation may be calculated in two different ways, each of which is adapted to a particular type of service. 2.1 Point-to-area links If there is a transmitter serving several randomly-distribu
11、ted receivers (broadcasting, mobile service), the electric field is calculated at a point located at some appropriate distance from the transmitter by the expression: d pe 30 (1) where: e : r.m.s. field strength (V/m) (see Note 1) p : equivalent isotropically radiated power (e.i.r.p.) of the transmi
12、tter in the direction of the point in question (W) (see Note 2) d : distance from the transmitter to the point in question (m). 2 Rec. ITU-R P.525-3 Equation (1) is often replaced by equation (2) which uses practical units: kmkWm V /m 173 dpe (2) where: emV/m: r.m.s field strength (mV/m) pkW: equiva
13、lent isotropically radiated power (e.i.r.p.) of the transmitter in the direction of the point in question (kW) dkm: distance from the transmitter to the point in question (km). For antennas operating in free-space conditions the cymomotive force may be obtained by multiplying together e and d in equ
14、ation (1). Its dimension is volts. Note 1 If the wave is elliptically polarized and not linear, and if the electric field components along two orthogonal axes are expressed by ex and ey, the left-hand term of equation (1) should be replaced by e ex y2 2 . ex and ey can be deduced only if the axial r
15、atio is known. e should be replaced by e 2 in the case of circular polarization. Note 2 In the case of antennas located at ground level (typically at relatively low frequencies) with vertical polarization, radiation is generally considered only in the upper half-space. When the ground is assumed to
16、be plane and perfectly conducting, the power flux-density for a given radiated power is doubled, as compared with an antenna in free space. (Alternatively, when considering field strengths, the field strength is similarly increased by 3 dB.) This should be taken into account in determining the radia
17、ted power (and is already included in Recommendations ITU-R P.368 and ITU-R P.341, Annex 2). 2.2 Point-to-point links With a point-to-point link it is preferable to calculate the free-space attenuation between isotropic antennas, also known as the free-space basic transmission loss (symbols: Lbf or
18、A0), as follows (see Recommendation ITU-R P.341): dLbf 4lo g20mmmmmmdB (3) where: Lbf : free-space basic transmission loss (dB) d : distance : wavelength, and d and are expressed in the same unit. Equation (3) can also be written using the frequency instead of the wavelength. Lbf = 32.4 20 log 20 lo
19、g dmmmmmmdB (4) where: f : frequency (MHz) d : distance (km). 2.3 Relations between the characteristics of a plane wave There are also relations between the characteristics of a plane wave (or a wave which can be treated as a plane wave) at a point: Rec. ITU-R P.525-3 3 22 4120 rpes(5) where: s : po
20、wer flux-density (W/m2) e : r.m.s. field strength (V/m) pr : power (W) available from an isotropic antenna located at this point : wavelength (m). 3 The free-space basic transmission loss for a radar system (symbols: Lbr or A0r) Radar systems represent a special case because the signal is subjected
21、to a loss while propagating both from the transmitter to the target and from the target to the receiver. For radars using a common antenna for both transmitter and receiver, a radar free-space basic transmission loss, Lbr, can be written as follows: Lbr 103.4 20 log 40 log d 10 log mmmmmmdB (6) wher
22、e: : radar target cross-section (m2) d : distance from the radar to the target (km) f : frequency of the system (MHz). The radar target cross-section of an object is the ratio of the total isotropically equivalent scattered power to the incident power density. 4 Conversion formulae On the basis of f
23、ree-space propagation, the following conversion formulae may be used. Field strength for a given isotropically transmitted power: E Pt 20 log d 74.8 (7) Isotropically received power for a given field strength: Pr E 20 log f 167.2 (8) Free-space basic transmission loss for a given isotropically trans
24、mitted power and field strength: Lbf Pt E 20 log f 167.2 (9) Power flux-density for a given field strength: S E 145.8 (10) where: Pt : isotropically transmitted power (dB(W) Pr : isotropically received power (dB(W) E : electric field strength (dB(V/m) f : frequency (GHz) d : radio path length (km) Lbf : free-space basic transmission loss (dB) 4 Rec. ITU-R P.525-3 S : power flux-density (dB(W/m2). Note that equations (7) and (9) can be used to derive equation (4).
