1、RECOMMENDATION ITU-R P.341-5* THE CONCEPT OF TRANSMISSION LOSS FOR RADIO LINKS* (1959- 1982- 1986- 1994- 1995- 1999) The Radiocommunication Assembly, considering a) that in a radio link between a transmitter and a receiver, the ratio between the power supplied by the transmitter and the power availa
2、ble at the receiver input depends on several factors such as the losses in theantennas or in the transmission feed lines, the attenuation due to the propagation mechanisms, the losses due to faulty adjustment of the impedances or polarization, etc.; b) that it is desirable to standardize the termino
3、logy and notations employed to characterize transmission loss and its components; c) that Recommendation ITU-R P.525 provides the free-space reference conditions for propagation, recommends that, to describe the characteristics of a radio link involving a transmitter, a receiver, their antennas, the
4、 associated circuits and the propagation medium, the following terms, definitions and notations should be employed: 1 The ratio, usually expressed in decibels, between the power supplied by the transmitter of a radio link and the power supplied to the corresponding receiver in real installation, pro
5、pagation and operational conditions. NOTE 1 - It is necessary to specify in each case the points at which the power supplied by the transmitter and the power supplied to the receiver are determined, for example: Total loss (of a radio link)* (symbols: LI or Al) - - before or after the radio-frequenc
6、y filters or multiplexers that may be employed at the sending or the receiving end; at the input or at the output of the transmitting and receiving antenna feed lines. 2 The ratio, usually expressed in decibels, for a radio link, of the radio-frequency power input to the terminals of the transmittin
7、g antenna and the resultant radio-frequency signal power available at the terminals of the receiving antenna. NOTE 1 - The available power is the maximum real power which a source can deliver to a load, i.e., the power which would be delivered to the load if the impedances were conjugately matched.
8、NOTE 2 - The system loss may be expressed by: System loss (symbols: L, or A,) L, = lOlog(p,/p,) = Pf - P, dB where: pt: radio-frequency power input to the terminals of the transmitting antenna pa : resultant radio-frequency signal power available at the terminals of the receiving antenna. * This Rec
9、ommendation should be brought to the attention of the Coordination Committee for Vocabulary (CCV). Throughout this Recommendation, capital letters are used to denote the ratios (dB) of the corresponding quantities designated with lower-case type; e.g. P, = 10 log pp P, is the input power to the tran
10、smitting antenna (dB) relative to 1 W when pt is the input power (W). A graphical depiction of this and subsequent definitions is shown in Fig. 1. * * STDsITU-R RECMN P.341-5-ENGL 1999 W 4855212 053b878 258 M 19 Rec. IT-R P.341-5 NOTE 3 - The system loss excludes losses in feeder lines but includes
11、ali losses in radio-frequency circuits associated with the antenna, such as ground losses, dielectric losses, antenna loading coil losses, and terminating resistor losses, FIGURE 1 Graphical depiction of terms used in the transmission loss concept 3 Transmission loss (of a radio link) (symbols: L or
12、 A) The ratio, usually expressed in decibels, for a radio link between the power radiated by the transmitting antenna and the power that would be available at the receiving antenna output if there were no loss in the radio-frequency circuits, assuming that the antenna radiation diagrams are retained
13、. NOTE 1 - The transmission loss may be expressed by: L = L, - LI, - L, dB STDmITU-R RECMN P.34P-S-ENGL P999 Y855212 053b879 diffraction loss as for ground waves; STDmITU-R RECMN P.3Y5-5-ENGL 1999 W Y855252 053bAA0 90b Rec. ITU-R P.341-5 21 - effective reflection or scattering loss as in the ionosph
14、eric case including the results of any focusing or defocusing due to curvature of a reflecting layer; - polarization coupling loss; this can arise from any polarization mismatch between the antennas for the particular ray path considered; aperture-to-medium coupling loss or antenna gain degradation,
15、 which may be due to the presence of substantial scatter phenomena on the path; effect of wave interference between the direct ray and rays reflected from the ground, other obstacles or atmospheric layers. - - ANNEX 1 1 Antenna directivity Directivity in a given direction is defined as the ratio of
16、the intensity of radiation (the power per unit solid angle (steradian), in that direction, to the radiation intensity averaged over all directions. When converting transmission loss, or, in specific cases, ray path transmission loss to basic transmission loss the plane wave directivities for the tra
17、nsmitting and receiving antennas at the particular direction and polarization must be taken into account. In cases where the performance of the antenna is influenced by the presence of local ground or other obstacles (which do not affect the path) the directivity is the value obtained with the anten
18、na in situ. In the particular case of ground wave propagation withantennas located on or near the ground, although the directivity of the receiving antenna, Gr, is determined by the above definition, the aperture for signai capture, and hence the available power, is reduced below its free-space valu
19、e. Thus the value to be used for Gr must be reduced (see Annex 2). 2 Antenna gain The power gain of an antenna is defined as the ratio, usually expressed in decibels, of the power required at the input of a loss-free reference antenna to the power supplied to the input of the given antenna to produc
20、e, in a given direction, the same field strength or the same power flux-density at the same distance. When not specified otherwise, the gain refers to the direction of maximum radiation. The gain may be considered for a specified polarization. 3 Reference standard antennas In the study of propagatio
21、n over radio links in different frequency bands, a number of reference antennas are used and referred to in IT-R texts. Depending on the choice of the reference antenna a distinction is made between: - - absolute ur isotropic gain (Gi), when the reference antenna is an isotropic antenna isolated in
22、space; gain relative to a halfwave dipole (Gd), when the reference antenna is a half-wave dipole isolated in space, whose equatorial plane contains the given direction; gain relative to a short vertical antenna (G,), when the reference antenna is a linear conductor much shorter than one quarter of t
23、he wavelength, normal to the surface of a perfectly conducting plane which contains the given direction. - (The power gain corresponds to the maximum directivity for lossless antennas.) Table 1 gives the directivity, G, for some typical reference antennas. The corresponding values of the cymomotive
24、force are also shown for a radiated power of 1 kW. - STD-ITU-R RECMN P.343-5-ENGL 3999 = 4855232 0536883 842 22 Rec. ITU-R P.341-5 TABLE 1 Directivity for typicai ieference antennas and its relation to cymomotive force Reference antenna Isotropic in free space Hertzian dipole in free space Haif-wave
25、 dipole in free space Hertzian dipole, or a short vertical monopole on a perfectly conducting ground(2) Quarter wave monopole on a perfectly conducting ground gt 1 1.5 1.65 3 3.3 Cymomotive force for a radiated power of 1 kW (VI O 1.75 2.15 4.8 5.2 173 212 222 300 3 14 (I) G, = 10 log g, The values
26、of G,(gr) equal the values of G, (g,) for antennas in free space. See Annex 2 for values of G, for antennas on a perfectly conducting ground. In the case of the hertzian dipole, it is assumed that the antenna is near a perfectly conducting ground. (2) ANNEX 2 Influence of the environment on the ante
27、nnas When antennas are installed on or near the ground and the ground-wave propagation mode is used (i.e. h h; At = A,.+ O; + O L = Lbf - 3.5 - 6.0 dB NOTE 1 - It should be noted that the formulae taking into account the presence of an infinite reflective plane cannot tend to the free-space formulae even when the antenna heights tend to infinity. I
