1、 Recommendation ITU-R P.684-7 (09/2016) Prediction of field strength at frequencies below about 150 kHz P Series Radiowave propagation ii Rec. ITU-R P.684-7 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the radio-frequency sp
2、ectrum by all radiocommunication 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 Radiocommunicati
3、on Conferences and Radiocommunication 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 stat
4、ements and licensing declarations 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
5、available online at http:/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 satel
6、lite services P Radiowave 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 signa
7、ls and frequency standards 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 m
8、eans whatsoever, without written permission of ITU. Rec. ITU-R P.684-7 1 RECOMMENDATION ITU-R P.684-7 Prediction of field strength at frequencies below about 150 kHz (Question ITU-R 225/3) (1990-1994-2001-2003-2005-2009-2012-2016) Scope This Recommendation provides waveguide and wave-hop methods for
9、 the prediction of field strength at frequencies below about 150 kHz. Keywords Ionospheric propagation, ELF, VLF, LF, ray-path, sky-wave The ITU Radiocommunication Assembly, considering a) that there is a need to give guidance to engineers for the planning of radio services in the frequency band bel
10、ow about 150 kHz; b) that the following methods have been developed: a wave-hop treatment for frequencies above about 60 kHz, based on a statistical analysis of field strength measurements in the band 16 kHz to about 1 000 kHz; a waveguide mode method for frequencies below about 60 kHz, based on a t
11、heoretical model of the Earth and the ionosphere, employing ionospheric model parameters determined from propagation data; a method for the frequency band 150-1 700 kHz, described in Recommendation ITU-R P.1147, recommends 1 that the following methods be used, taking particular note of the cautions
12、on accuracy in their application to certain regions as discussed in Annex 2. 1 Introduction Two methods are available for theoretically calculating the field strength of ELF, VLF and LF signals. It may be noted that the information in this Recommendation includes values of f cos i exceeding 150 Hz.
13、The use of this information for frequencies exceeding 150 kHz is not recommended. Recommendation ITU-R P.1147 gives information for frequencies above 150 kHz. 1.1 The wave-hop method is that in which electromagnetic energy paths between a given transmitter and receiver are represented geometrically
14、as is done in the case of HF. This method should be used at LF and, for distances less than 1 000 km, at VLF. The method treats radio transmission as taking place along certain paths defined by one or more ionospheric reflections, depending on whether the propagation in question involves one or more
15、 hops, as well as a ground wave. The total field is then the vectorial resultant of the fields due to each path. In view of the long wavelengths concerned, the diffraction of the waves by the Earths surface must be taken into account, which is not the case for HF. The wave-hop method may be justifie
16、d by the fact that, with oblique 2 Rec. ITU-R P.684-7 incidence, the dimensions of the section of altitude in which propagation takes place are equal to or greater than several wavelengths. With this method it is necessary to know the values of the reflection coefficients of the incident wave on the
17、 ionosphere. These vary greatly with frequency, length and geographic and geomagnetic coordinates of transmission path, time of day, season, and epoch of the solar cycle. It is also necessary to know the electrical characteristics (conductivity and permittivity) of the ground at the transmitting and
18、 receiving sites, since the finite conductivity of the Earth affects the vertical radiation patterns of the terminal antennas. 1.2 The waveguide mode method should be used at VLF for distances greater than 1 000 km. In this method, the propagation is analysed as the sum of the waves corresponding to
19、 each of the different types of propagation in the Earth-ionosphere waveguide, analogous to a mode as defined for waveguides in the microwave region. The selection of the method to be used for field calculation depends on practical consideration of numerical calculations. 1.3 In the case of VLF at d
20、istances of less than 1 000 km and for LF in general, the series of modes are slightly convergent and the calculations then require adding together vectorially a large number of components. The wave-hop theory, on the other hand, requires only a limited number of paths, including the ground wave, an
21、d it is particularly convenient for the long-distance propagation of LF, taking into account, if possible, the diffraction. For VLF at distances of more than 1 000 km, the wave-hop theory requires the vectorial addition of the field due to a large number of paths whereas, since the series of modes c
22、onverge rapidly, sufficient accuracy can be obtained by adding together only a small number of modes. But in many cases for calculation with sufficient accuracy it is possible to use the wave-hop model at large distances for frequencies down to 10 kHz, and it is possible to limit the number of paths
23、 to be taken into account to three or in rare cases to four. Propagation at ELF also may be described in terms of a single waveguide mode. 2 Wave-hop propagation theory 2.1 General description According to this theory, the sky-wave field (strength and phase) at a point is treated as the resultant of
24、 the fields created by different waves propagated directly from the transmitter in one or more hops. The total field at this point is then the resultant of the field due to the wave diffracted by the ground and of the field due to the sky wave. The sky-wave field is calculated by applying the theory
25、 of rays in the regions where the methods of geometric optics are applicable and by integrating the effects of diffraction or by applying the full wave theory in regions where optics are no longer valid. The geometry of a path comprising a single hop is shown in Fig. 1. The surface of the Earth is d
26、efined by r a and a smooth reflecting ionospheric layer located at r a h. It is convenient to distinguish three cases. In the first, the receiving antenna located at R and R beyond the critical points such that the first sky-wave hop propagates into the diffraction or shadow zone. Rec. ITU-R P.684-7
27、 3 FIGURE 1 Ray path geometry for the wave-hop radio propagation theory (first hop sky wave) P. 0 6 8 4 - 0 1r = a + hr = aTTcT RRcRig2.2 Calculating the ray-path field strength The cymomotive force corresponding to the electric field radiated from a short vertical dipole is given by: tu pV 300 V (1
28、) where pt is radiated power (kW). The field strength of the downcoming sky wave, before reflection at the ground in the vicinity of the receiving antenna, is given by: m V / m c o s| tut FDRLVE (2) where: L : sky-wave path length (km) |R| : the ionospheric reflection coefficient which gives the rat
29、io of the electric field components parallel to the plane of incidence D : ionospheric focusing factor Ft : transmitting antenna factor : the angle of departure and arrival of the sky wave at the ground, relative to the horizontal. 4 Rec. ITU-R P.684-7 If reception is by a small in-plane loop antenn
30、a located on the surface of the Earth, the effective field strength of the sky wave is: m V / m c o s2| rtus FFDRLVE (3) For reception by a short vertical antenna equation (3) becomes: m V / m ) ( c o s2|2 rtus FFDRLVE (4) where Fr is the appropriate receiving antenna factor. For propagation over gr
31、eat distances, the wave-hop method can be extended to include sky waves reflected more than once from the ionosphere. For example for a two-hop sky wave, the field strength received by a receiving loop antenna can be represented simply as: m V / m c o s2 |2|2|1| rtgGus FFRDDRRLVE (5) where: DG : div
32、ergence factor caused by the spherical Earth, approximately equal to D1 |Rg| : effective reflection coefficient of the finitely conducting Earth L : total propagation path of the two-hop ray-path |R1| and |R2| : ionospheric reflection coefficients for the first and second reflection. In general, the
33、 ionospheric reflection coefficients will not be equal, because the polarizations of the incident waves are not the same. However, in the simple method for calculating field strengths given here, for propagation at very oblique angles of incidence, |R1| |R2| as a first order approximation. 2.2.1 Ang
34、les of elevation and ionospheric incidence The ray path geometry for determining the angles of departure and arrival of the sky wave at the ground, and ionospheric angles of incidence, i, is shown on Figs. 2 and 3. These angles are given in Fig. 2 for an effective reflection height of 70 km which co
35、rresponds to typical daytime conditions and in Fig. 3 for an effective reflection of 90 km which corresponds to typical night-time conditions. The effects of atmospheric refraction on the departure and arrival angles are included and shown by the dashed curve although they are probably not valid for
36、 frequencies below about 50 kHz. 2.2.2 Path length and differential time delay To calculate L the sky-wave path length and estimates of the diurnal phase changes, Fig. 4 is used. This shows the differential time delay between the surface wave and the one, two or three hop sky wave for ionospheric re
37、flection heights of 70 and 90 km, corresponding to daytime and night-time conditions. A propagation velocity of 3 105 km/s is assumed. Rec. ITU-R P.684-7 5 FIGURE 2 Departure and arrival angles, , and ionospheric angles of incidence, i, for typical daytime conditions (h = 70 km). The dashed curve in
38、cludes the effects of atomospheric refraction P. 0 6 8 4 - 0 20 5.0 2.0 1.G reat -ci rcl e l en g t h , d (k m)A : n eg at i v e iorangles()degreesh = 7 0 kmdhi10 20 50 100 200 500 1 0 0 0 2 0 0 0 5 0 0 0 1 0 0 0 0100502010521iA6 Rec. ITU-R P.684-7 FIGURE 3 Departure and arrival angles, , and ionosp
39、heric angles of incidence, i, for typical nightime conditions (h = 90 km). The dashed curve includes the effects of atomospheric refraction P. 0 6 8 4 - 0 30 , 50 , 20 , 1G reat -ci rcl e h o p l en g t h , (k m) dA: n eg at i v e ioranglesdegrees()Ai10 20 50 100 200 500 1 0 0 0 2 0 0 0 5 0 0 0 1 0
40、0 0 0100502010521h = 9 0 kmdhiRec. ITU-R P.684-7 7 FIGURE 4 Differential time delay between surface wave and one, two and three hop sky waves P. 0 6 8 4 - 0 4Timedelays()G reat -ci rcl e h o p l en g t h , (k m) dA : 3 hopB: 2 hopC: 1 hopD : l i mi t i n g ran g eDDD7090709070 ABC40501002005001 0 0
41、02 0 0 010 20 50 100 200 500 1 0 0 0 2 0 0 0 5 0 0 0 1 0 0 0 0h = 9 0 km2.2.3 Focusing factor The ionospheric focusing factor, D, for a spherical Earth and ionosphere is shown in Fig. 5 for daytime conditions and in Fig. 6 for night-time conditions. 2.2.4 Antenna factors The antenna factors, Ft and
42、Fr, which account for the effect of the finitely conducting curved Earth on the vertical radiation pattern of the transmitting and receiving antennas are shown in Figs. 7 to 9. Factors are calculated for land, sea and ice conditions which are defined by their electrical characteristics (conductivity
43、 and permittivity), as shown in Table 1. 8 Rec. ITU-R P.684-7 FIGURE 5 Ionospheric focusing factor daytime P. 0 6 8 4 - 0 5Focusingfactor, DG re at -c i rc l e h o p l en g t h , (k m) d321500 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 02050100150= 2 0 0 k H zFIGURE 6 Ionospheric focusing factor night-time 0 6 8
44、 4 - 0 6Focusingfactor,DG reat -ci rcl e h o p l en g t h , (k m) d321500 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 02050100150 = 2 0 0 k H zRec. ITU-R P.684-7 9 FIGURE 7 Antenna factor Sea water P. 0 6 8 4 - 0 7A n g l e ab o v e h o ri zo n d eg rees, ( )Antennafactoror,FFtr = 8 0 = 5 /S m= 4 / 3 6 3 6 0 km01
45、0 5 0 5 1 0 1 5 2 0252525251 410 310 21011020010050500 = 20 kHz10 Rec. ITU-R P.684-7 FIGURE 8 Antenna factor Land P. 0 6 8 4 - 0 8A n g l e ab o v e h o ri zo n , (d eg rees )Antennafactor,orFFtr = 1 5 = 2 1 0 /S m= 4 / 3 6 3 6 0 km03252525251 410 310 21011010 5 0 5 1 0 1 5 2 01510050200500 20 kHzRe
46、c. ITU-R P.684-7 11 FIGURE 9 Antenna factor Ice at 4 C P. 0 6 8 4 - 0 9A n g l e ab o v e h o ri zo n , (d eg rees )Antennafactor,orFFtr = 3 0 = 0 , 0 2 5 1 0 /3S m = 4 / 3 6 3 6 0 km252525251 410 310 21011010 5 0 5 1 0 1 5 2 01510050 = 20 kHz12 Rec. ITU-R P.684-7 TABLE 1 Conductivity, (S/m) Permitt
47、ivity, Sea water 5 80 0 Land 2 103 15 0 Polar ice 2.5 105 3 0 0: permittivity of free space The curves were calculated assuming an effective Earths radius, 8480 km, which is 4/3 of its actual value to take account of atmospheric refraction effects. The factors F are the ratio of the actual field str
48、ength to the field strength that would have been measured if the Earth were perfectly conducting. Negative values of refer to propagation beyond the geometric optical limiting range for a one hop sky wave (see Figs. 1 to 3). 2.2.5 Ionospheric reflection coefficients |R| Values of the ionospheric ref
49、lection coefficient |R| are shown in Fig. 10 for solar cycle minimum. To take account of frequency and distance changes, the values of |R| are given as a function of f cos i, where f is the transmitted frequency and i is the ionospheric angle of incidence. Curves are shown for night during all seasons, and for day conditions during winter and summer. Measured values at v
