1、10 CCIR VOLUME*V 90 4855232 05003LL1 b = Rec. 341-2 RECOMMENDATION 341-2* THE CONCEPT OF TRANSMISSION LOSS FOR RADIO LINKS* The CCIR, (1959-1 982- 1986) CONS1 DERING (a) that in a radio link between a transmitter and a receiver, the ratio between the power supplied by the transmitter and the power a
2、vailable 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 polarization, etc.; (6) that it is desirable to standardize the
3、terminology and notations employed to characterize transmission loss and its components; (c) that Recommendation 525 provides the free-space reference conditions for propagation, UNANIMOUSLY RECOMMENDS that, to describe the characteristics of a radio link involving a transmitter, a receiver, their a
4、ntennas, the associated circuits and the propagation medium, the following terms, definitions and notations should be employed: 1. Total loss (of a radio link)* (symbols: L/ or AI) The ratio, usually expressed in decibels, between the power supplied by the transmitter of a radio link and the power s
5、upplied to the corresponding receiver in real installation, propagation and operational conditions. Note. - 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: - before. or after the rad
6、io 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 decibels, for a radio link, of the radio frequency p
7、ower input to the terminals of the transmitting 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, Le. the power which would be delivered to the load
8、 if the impedances were conjugately matched. Note 2. - The system loss may be expressed by: L, = 10 log (p,/pa) = Pi - Pa where pf : radio frequency power input to the terminals of the transmitting antenna and pa: resultant radio frequency signal power available at the terminals of the receiving ant
9、enna. Note 3. - The system loss excludes losses in feeder lines but includes all losses in radio-frequency circuits associated with the antenna, such as ground losses, dielectric losses, antenna loading coil losses, and terminating resistor losses. , * * This Recommendation should be brought to the
10、attention of the CMV. Throughout this Recommendation, capital letters are used to denote the ratios, expressed in decibels, of the corresponding quantities designated with lower-case type, e.g. Pr = 10 log pI. Pr is the input power to the transmitting antenna, expressed in decibels relative to 1 W w
11、hen pI is the input power in watts. A graphical depiction of this and subsequent definitions is shown in .Fig. i. * , * , 11 CCIR VOLUMEaV 90 W 4855212 0500317 L - - Rec. 341-2 13 2. Reference standard antennas In the study of propagation over radio links in different frequency bands, a number of re
12、ference antennas are used and referred to in CCIR texts. According to Recommendation 311 this reference should be cleat;ly defined in each case with respect to an isotropic antenna. The power gain of an antenna is defined as the ratio, usually expressed in decibels, of the power required at the inpu
13、t 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 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
14、be considered for a specified polarization. (a) absolute or isotropic gain (Gi), when the reference antenna is an isotropic anteona isolated in space; (b) gain relative to a half-wave dipole (Gd), when the reference antenna is a half-wave dipole isolated in space, whose equatorial plane contains the
15、 given direction; (c) gain relative to a short vertical antenna (Gy), when the reference antenna is a linear conductor much shorter than one quarter of the wavelength, normal to the surface af a perfectly conducting plane which contains the given direction. Depending on the choice of the reference a
16、ntenna a distinction is made between: (The power gain corresponds to the maximum directivity for lossless antennas.) Table I gives the directivity G, for some typical reference antennas. The corresponding values of the cymomotive force are also shown for a radiated power of 1 kW. TABLE I - Directivi
17、ty for typical reference antertnas 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 vertical monopole on a perfecty conducting ground (3 Quarter wave monopole on a perfectly condu
18、cting ground 1 1.5 i .65 3 3.3 O 1.75 2.15 4.8 5.2 Cymomotive force for a radiated power of i kW- (v) 173 212 222 300 314 (9 Gf =. 10 1% gf The values of Gr.( g,) equal the values of GI ( gr) for antennas in free space. See Annex II for values of G, for antennas oh a perfectly conducting ground. ()
19、In the case of the hertzian dipole, it is assumed that the antenna is near a perfectly conducting ground: ANNEX II ,. Influence ofjhe environment on the antennas When antennas are installed on or near the ground and the ground-wave propagation mode is used (Le. h A, At+ O; - due to the vector additi
20、on of the direct and reflected rays the free-space value of the power flux is multiplied by : 2 cos (kh, sin y) (1 + Al) This is equivalent to the change in directive gain due t the presence of the reflecting surface. The multiplying factor has the value of 2 when h, = h, = O. The effective capture
21、area of the receiving antenna is given by: I 1.5.h2 cos2 y a,e = 471 (1 + Ar) The following should be noted: - - - the capture area in the direction of the transmitting antenna is decreased by cos2 y; the change in radiation resistance is based on equation (7), where A, and h, are replaced by A, and
22、 h,; the free-space value of the capture area is multiplied by -; thus the presence of the reflecting surface 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 definition) when h, = h, = O it is important to note that this is no
23、t the appropriate value to use for g,; the correct value for g, is 1.512 = g,/4. (1 + 4) - * This relation is derived by S. A. Schelkunoff in Chapters VI and IX f the book Electromagnetic Waves, D. Van Nostrand Co., 1943. CCIR VOLUME*V 90 4855212 0500339 5 = Rec. 341-2 15 Since p; = Su, expressions (6) and (8) may be combined to give an expression for the transmission loss between two short vertical loss-free electric dipoles above a plane perfectly conducting surface. . Consider two cases. h, = hr = O; A, = Ar = 1; y=o L = Lbf - 3.5- dB Thus L is. equal to the free-space value.
