1、 Rec. ITU-R P.1147-4 1 RECOMMENDATION ITU-R P.1147-4 Prediction of sky-wave field strength at frequencies between about 150 and 1 700 kHz (Question ITU-R 225/3) (1995-1999-2003-2005-2007) Scope This Recommendation provides a prediction procedure for the frequency range between about 150 and 1700 kHz
2、, for path length between 50 to 12 000 km. The ITU Radiocommunication Assembly, considering a) that there is a need to give guidance to engineers in the planning of broadcast services in the LF and MF bands; b) that it is important, for stations working in the same or adjacent frequency channels, to
3、 determine the minimum geographical separation required to avoid interference resulting from long-distance ionospheric propagation; c) that portions of this frequency range are being shared by broadcasting and other services in different Regions, an accurate method for predicting interference levels
4、 is needed to maintain efficient and orderly utilization of those portions of the spectrum, recommends that the following method should be adopted for use, taking particular note of the discussion on accuracy given in Annex 1. 1 Introduction This method predicts values of the night-time sky-wave fie
5、ld strength for a given power radiated from one or more vertical antennas, when measured by a loop antenna at ground level aligned in a vertical plane along the great-circle path to the transmitter. The method has been based on measurements made in the frequency bands allocated to broadcasting and a
6、pplies for paths of length 50 to 12 000 km for those LF and MF bands in particular. For a discussion on daytime propagation, see Annex 2. Figures 1, 2 and 3 are an essential part of the prediction method. Geomagnetic maps are included for convenience in Figs. 5, 6 and 10. The remaining Figures and A
7、ppendix 1 provide additional information to simplify the use of the method. 2 Rec. ITU-R P.1147-4 2 Annual median night-time field strength The predicted sky-wave field strength is given by: E = V + E0 Lt= V + GS Lp+ A 20 log p La Lt Lr(1) where: E : annual median of half-hourly median field strengt
8、hs (dB(V/m) for a given transmitter cymomotive force, V, and at a given time, t, relative to sunset or sunrise as appropriate E0: annual median of half-hourly median field strengths (dB(V/m) for a transmitter cymomotive force of 300 V at the reference time defined in 2.1 V : transmitter cymomotive f
9、orce (dB above a reference cymomotive force of 300 V) (see 2.2) GS: sea-gain correction (dB) (see 2.3) Lp: excess polarization-coupling loss (dB) (see 2.4) A : a constant. At LF, A = 110.2. At MF, A = 107 except for propagation paths whose midpoints are situated in the part of Region 3 south of para
10、llel 11 S. In those cases, A = 110 La: loss factor incorporating effects of ionospheric absorption and related factors (see 2.6) Lt: hourly loss factor (dB) (see 2.7) Lr:loss factor incorporating effect of solar activity ( 2.8). Figure 4 shows E0as a function of ground distance, d, for various geoma
11、gnetic latitudes when GS, Lpand R are all zero; where R is the twelve-month smoothed international relative sunspot number. 2.1 Reference time The reference time is taken as six hours after the time at which the Sun sets at a point S on the surface of the Earth. For paths shorter than 2 000 km, S is
12、 the mid-point of the path. On longer paths, S is 750 km from the terminal where the Sun sets last, measured along the great-circle path. 2.2 Cymomotive force The transmitter cymomotive force V (dB(300 V) is given as: V = P + GV+ GH(2) where: P : radiated power (dB(1 kW) GV: transmitting antenna gai
13、n factor (dB) due to vertical directivity, given in Fig. 1 H: transmitting antenna gain factor (dB) due to horizontal directivity. For directional antennas, GHis a function of azimuth. For omnidirectional antennas, GH= 0. Rec. ITU-R P.1147-4 3 2.3 Sea gain The sea gain GSis the additional signal gai
14、n when one or both terminals is situated near the sea, but it does not apply to propagation over fresh water. GSfor a single terminal is given by: GS= G0 c1 c2for (c1+ c2) 6 500 km; and at LF G0= 4.1 dB when d 5 000 km, where d is the ground distance between the two terminals. The correction c1is gi
15、ven by: 0111Grsc = (5) where: s1: distance of terminal from sea, measured along great-circle path (km) r1= 103G20/Q1 f km f : frequency (kHz) Q1= 0.30 at LF and 1.4 at MF. The correction c2is given by: =22021rsGc for s245I where I is the magnetic dip, N or S (degrees) at the terminal and is the path
16、 azimuth measured in degrees from the magnetic E-W direction, such that | | 90. Lpshould be evaluated separately for the two terminals, because of the different values of and I that may apply, and the two Lpvalues added. The most accurate available values of magnetic dip and declination (e.g. see Fi
17、gs. 5 and 6) should be used in determining and I. Figure 7 shows values of Lpcalculated from equation (8). Rec. ITU-R P.1147-4 5 1147-01 12 11 10 9 8 7 6 5 4 3 2 101234102103104255 25d (km)GV(dB)h = 0.6 0.5 0.4 0.25 h: antenna heightNote 1 d d For 10 000 km 45, Lr= b(R/100) (p/1 000) dB (12) 8 Rec.
18、ITU-R P.1147-4 where: b = (| | 45)/3 except in Europe where b = 1 is to be used (13) regardless of latitude. Paths longer than 3 000 km are divided into two equal sections as described in 2.6. The value of Lrfor each section is derived and added together. FIGURE 4a Curves showing E0for LF when GS, L
19、pand R are all zero, for constant geomagnetic latitudes Rec. ITU-R P.1147-4 9 3 Day-to-day and short-period variations of night-time field strengths The difference, (w), where w is typically 10 or 1, at a specific time relative to sunset or sunrise, between the field strength exceeded for w % of the
20、 time and the annual median value is given by: at LF: (10) = 6.5 dB (14) and (1) = 11.5 dB (15) at MF: (10) = 0.2 | | 2 dB (16) and (1) = 0.2 | | + 3 dB (17) In equation (16), (10) is greater than or equal to 6 dB but less than or equal to 10 dB. In equation (17), (1) is greater than or equal to 11
21、dB but less than or equal to 15 dB. FIGURE 4b Curves showing E0for MF when GS, Lpand R are all zero, for constant geomagnetic latitudes 10 Rec. ITU-R P.1147-4 Rec. ITU-R P.1147-4 11 1147-06180 160 140 120 100 80 60 40 20 0 20 40 60 80 100 120 140 160 1809080706050403020101020N304050607080900180 160
22、140 120 100 80 60 40 20 0 20 40 60 80 100 120 140 160 1809080706050403020101020N304050607080900FIGURE 6Map of magnetic declination (epoch 1975.0)(Source: Magnetic variation (epoch 1975.0) Chart No. 42 World U.S. Defence Mapping Agency Hydrographic Center)LatitudeLongitude12 Rec. ITU-R P.1147-4 1147-
23、07Rec. ITU-R P.1147-4 13 14 Rec. ITU-R P.1147-4 Rec. ITU-R P.1147-4 15 16 Rec. ITU-R P.1147-4 Rec. ITU-R P.1147-4 17 1147-11ONDAppendix 1 This Appendix contains equations that may be used in lieu of Figs. 3 and 11 for hourly loss factor, and sunset and sunrise times respectively. For the purpose of
24、this Appendix, the following additional symbols are used. List of symbols : geographic latitude of a point on the path (degrees) : geographic longitude of a point on the path (degrees) S : local mean time of sunset or sunrise at a point (h). North and East coordinates are considered positive, and So
25、uth and West coordinates negative. 18 Rec. ITU-R P.1147-4 1 Hourly loss factor: LtThese equations may be used instead of the curves in Fig. 3, within the stated limits of t. For hours between these times (i.e. near midnight) set Lt= 0. Lt(sunset) = 12.40 9.248 t + 2.892 t2 0.3343 t3for 1 1, there is
26、 no sunset or sunrise. From cos H, obtain H in degrees; for sunrise 180 (foE) sec i, where foE is the critical frequency of the E layer and i is the angle of incidence at the E layer, then the wave will penetrate the E layer and be reflected from the F layer. This is most likely to occur at the high
27、est frequencies in the MF band at ground distances less than 500 km, especially late at night and during the sunspot minimum period. The method can still be used provided p is calculated for an F-layer reflection height of 220 km and the cymomotive force V is calculated for the corresponding angle o
28、f elevation. Measurements made in the United States of America suggest that Fig. 3 (hourly loss factor) is likely to be accurate for frequencies near 1 000 kHz in a year of low solar activity. As the frequency deviates in either direction from about 1 000 kHz, particularly during transition hours, a
29、ppreciable errors may result. These measurements also suggest that the magnitude of the effect of solar activity at two hours after sunset is considerably greater than that at six hours after sunset. Thus, in a year of high solar activity, the difference between field strengths at six hours after su
30、nset and two hours after sunset can be considerably greater than that shown in Fig. 3. 20 Rec. ITU-R P.1147-4 At night, MF sky-waves propagating in temperate latitudes are strongest in spring and autumn and are weakest in summer and winter, the summer minimum being the more pronounced. The overall v
31、ariation may be as much as 15 dB at the lowest frequencies in the MF band, decreasing to about 3 dB at the upper end of the band. At LF the seasonal variation at night has the opposite trend, with a pronounced summer maximum. The seasonal variation is much smaller in tropical latitudes. Annex 2 A di
32、scussion on daytime sky-wave propagation 1 LF cases Midday field strengths at LF are 7 to 45 dB lower than the values at midnight. The difference is dependent on frequency, distance and season (see also Recommendation ITU-R P.684). 2 MF cases Available data show that midday sky-wave field strengths
33、display a consistent seasonal variation pattern with maximum occurring in winter months. The average winter-month field strength is about 10 dB stronger than the annual median value and the winter-to-summer ratio can exceed 30 dB. The annual median value of midday field strength is about 43 dB lower than its counterpart at six hours after sunset. Field strength exceeded for 10% of the days of the year is about 13 dB stronger than the annual median value. See also the ITU-R Handbook The ionosphere and its effects on radiowave propagation.
