1、4 ITU-R RECMN*PN- 343-3 94 4855232 0,522365 766 Rec. ITU-R PN.341-3 RECOMMENDATION ITU-R PN.341-3* THE CONCEPT OF TRANSMISSION LOSS FOR RADIO LINKS* (1959- 1982- 1986-1994) The ITU Radiocommunication Assembly, considering a) that in a radio link between a transmitter and a receiver, the ratio betwee
2、n the power supplied by the transmitter and the power available at the receiver input depends on several factors such as the losses in the antennas or in the transmission feed lines, the attenuation due to the propagation mechanisms, the losses due to faulty adjustment of the impedances or polarizat
3、ion, etc.; b) that it is desirable to standardize the terminology and notations employed to characterize transmission loss and its components; c) that Recommendation ITU-R PN.525 provides the free-space reference conditions for propagation, recommends that, to describe the Characteristics of a radio
4、 link involving a transmitter, a receiver, their antennas, the associated circuits and the propagation medium, the following terms, definitions and notations should be employed: 1. Total loss (of a radio link)* (symbols: LI or Al) The ratio, usually expressed in decibels, between the power supplied
5、by the transmitter of a radio link and the power supplied to the corresponding receiver in real installation, propagation 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
6、determined, for example: before or after the radio-frequency 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. System loss (symbols: L, or A,) The ratio, usually expressed in de
7、cibels, for a radio link, of the radio-frequency power input to the terminals of the transmitting antenna and the resultant radio-frequency signal power available at the terminals of the receiving antenna. Nore 1 - The available power is the maximum real power which a source can deliver to a load, i
8、.e., the power which would be delivered to the load if the impedances were conjugately matched. * This Recommendation 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 corre
9、sponding quantities designated with lower-case type, e.g. P, = 10 log p,. P, is the input power to the transmitting antenna (dB) relative to 1 W when p, is the input power (W). A graphical depiction of this and subsequent definitions is shown in Fig. I. * * ITU-R RECMNJPN- 341-3 74 = 4855212 0522366
10、 bT2 Rec. ITU-R PN.341-3 5 Note 2 -The system loss may be expressed by: Ls = 10 log (p diffraction loss as for ground waves; effective reflection or scattering loss as in the ionospheric case including the results of any focusing or defocusing due to curvature of a reflecting layer; - - - ITU-R RECM
11、NPN. 343-3 w m 4855212 0522168 475 m Rec. ITU-R PN.341-3 7 - 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, which may be due to the presence of subs
12、tantial 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 the intensity of radiation (the power per
13、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 transmitting and receiving antennas at the pa
14、rticular 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 antenna in situ. In the particular case of grou
15、nd wave propagation with antennas located on or near the ground, although the directivity of the receiving antenna G, is determined by the above definition, the aperture for signal capture, and hence the available power, is reduced below its free-space value. Thus the value to be used for G, must be
16、 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 produce, in a given direction, the same field st
17、rength 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 propagation over radio links in different frequency
18、 bands, a number of reference antennas are used and referred to in ITU-R texts. Depending on the choice of the reference antenna a distinction is made between: - - absolute or isotropic gain (Gi), when the reference antenna is an isotropic antenna isolated in space; gain relative to a haywave dipole
19、 (GJ, 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 (Gv), when the reference antenna is a linear conductor much shorter than one quarter of the wavelength, normal to the surface of
20、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 force are also shown for a radiated power o
21、f 1 kW. 8 - ITU-R RECMN*PN- 341-3 94 H 4855232 0522369 301 = Rec. ITU-R PN.341-3 TABLE 1 Directivity for typical reference antennas and its relation to cymomotive force Reference antenna Isotropic in free space Hertzian dipole in free space Half-wave dipole in free space Hertzian dipole, or a short
22、vertical monopole on a perfectly conducting ground(*) 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 I73 212 222 300 314 (1) Gt = 10 log g, The values of G,(g,) equal the values of G, (gt) for antennas
23、in free space. See Annex 2 for values of G, for antennas on a perfectly conducting ground. C2) In the case of the hertzian dipole, it is assumed that the antenna is near a perfectly conducting ground. ANNEX 2 Influence of the environment on the antennas When antennas are installed on or near the gro
24、und and the ground-wave propagation mode is used (Le. h h, particularly when using frequencies less than 30 MHz) the free-space value of the antenna radiation resistance is modified by the presence of the ground. Consequently the power flux density at the receiving antenna (resulting from the vector
25、 sum of direct and reflected rays) is dependent on the height of the transmitting antenna, and the effective capture area of the receiving antenna is dependent on the height of the antenna above the ground. The influence of the environment on the operation of a pair of antennas (forming an elementar
26、y circuit) is illustrated by considering the transmission loss between two vertical loss-free short electric dipoles at heights h, and h, above a plane perfectly conducting surface. The separation, d, along the surface is very large compared to the wavelength h. 1. The power flux-density s (W/m2) at
27、 height h, is given by: p; cos4 Iq 2 x 1.5 2 cos (k ht sin v) = 4nd2(1 + At) where: P; : d, h, h, h power radiated by the transmitting antenna (W) are expressed in metres and: At = (2 k 3 hJ2 si;“,“,: - cos 2 k ht with A, = 1 when h, = O. ITU-R RECMN*PN. 343-3 94 485.5212 0522370 023 W Rec. ITU-R PN
28、.3413 9 Equation (6) assumes that h, h, and h are all much less than d. The following should be noted: - - - - the distance between the antennas is increased to d sec w, the electric field due to the dipole varies as cos y, the free-space radiation resistance is multiplied by (1 + A,), due to the ve
29、ctor addition of the direct and reflected rays the free-space value of the power flux is multiplied by: 2 2 cos (k hf sin y) (1 + Al) This is equivalent to the change in directivity due to the presence of the reflecting surface. The multiplying factor has the value of 2 when h, = h, = O. 2. The effe
30、ctive capture area of the receiving antenna is given by: 1.5 h2 cos2 Ir 4(1 + Ar) a, = The following should be noted: - the capture area in the direction of the transmitting antenna is multiplied by cos2 effects; the change in radiation resistance is based on equation (7), where A, and h, are replac
31、ed by A, and h,; the free-space value of the capture area is multiplied by l/(l + A,) by the presence of the reflecting plane; thus the presence of the reflecting plane reduces the capture area below its free-space value by a factor of 2 when h, = h, = O; since g, has the value 2 x 1.5 (by definitio
32、n) when h, = h, = O it is important to note that this is not the appropriate value to use for g,; the correct value for g, is 1.5/2 = gt/4. due to directional - - - 3. Since the total power collected by the receiving antenna is given by pa = sa, expressions (6) and (8) may be combined to give an exp
33、ression for the transmission loss between two short vertical loss-free electric dipoles above a plane perfectly conducting surface. 1 cos2 (k hf sin u/) L = Lbf - 6.0 - 10 log (1.5 COS2 + Ar) + At) dB (9) Consider two limiting cases: a) Antennas on the su$ace b) Very large antenna heights L = Lbf - 3.5 - 6.0 dB Note J - 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.
