1、 Recommendation ITU-R P.684-6(02/2012)Prediction of field strength at frequencies below about 150 kHzP SeriesRadiowave propagationii Rec. ITU-R P.684-6 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the radio-frequency spectru
2、m 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 Radiocommunication Co
3、nferences 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 statement
4、s 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 avail
5、able 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 satellite
6、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 signals an
7、d 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, 2009 ITU 2009 All rights reserved. No part of this publication may be reproduced, by any means
8、whatsoever, without written permission of ITU. Rec. ITU-R P.684-6 1 RECOMMENDATION ITU-R P.684-6 Prediction of field strength at frequencies below about 150 kHz (Question ITU-R 225/3) (1990-1994-2001-2003-2005-2009-2012) The ITU Radiocommunication Assembly, considering a) that there is a need to giv
9、e guidance to engineers for the planning of radio services in the frequency band below 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
10、 1 000 kHz; a waveguide mode method for frequencies below about 60 kHz, based on a theoretical 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, reco
11、mmends 1 that the following methods be used, taking particular note of the cautions 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
12、the information in this Recommendation includes values of f cos i exceeding 150 kHz. 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
13、 energy paths between a given transmitter and receiver are represented geometrically 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 ionospher
14、ic reflections, depending on whether the propagation in question involves one or more 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
15、taken into account, which is not the case for HF. The wave-hop method may be justified by the fact that, with oblique incidence, the dimensions of the section of altitude in which propagation takes place are equal to or greater than several wavelengths. 2 Rec. ITU-R P.684-6 With this method it is ne
16、cessary to know the values of the reflection coefficients of the incident wave on the 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
17、characteristics (conductivity and permittivity) of the ground at the transmitting and 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.
18、 In this method, the propagation is analysed as the sum of the waves corresponding to 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 depe
19、nds on practical consideration of numerical calculations. 1.3 In the case of VLF at distances 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 t
20、he other hand, requires only a limited number of paths, including the ground wave, and 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 addi
21、tion of the field due to a large number of paths whereas, since the series of modes converge 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 dist
22、ances for frequencies down to 10 kHz, and it is possible to limit the number of paths 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 theo
23、ry, the sky-wave field (strength and phase) at a point is treated as the resultant of 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 t
24、he field due to the sky wave. The sky-wave field is calculated by applying the theory 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 geometr
25、y of a path comprising a single hop is shown in Fig. 1. The surface of the Earth is defined 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 Rand Rbeyond the critical points such that
26、the first sky-wave hop propagates into the diffraction or shadow zone. Rec. ITU-R P.684-6 3 2.2 Calculating the ray-path field strength The cymomotive force corresponding to the electric field radiated from a short vertical dipole is given by: tupV 300= V (1) where ptis radiated power (kW). The fiel
27、d strength of the downcoming sky wave, before reflection at the ground in the vicinity of the receiving antenna, is given by: mV/m cos| tutFDRLVE = (2) where: L : sky-wave path length (km) |R|: the ionospheric reflection coefficient which gives the ratio of the electric field components parallel to
28、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.684-6 If reception is by a small in-plane loop antenna located on the surface of the Earth, the effect
29、ive field strength of the sky wave is: mV/m cos2| rtusFFDRLVE = (3) For reception by a short vertical antenna equation (3) becomes: mV/m ) (cos2|2rtusFFDRLVE = (4) where Fris the appropriate receiving antenna factor. For propagation over great distances, the wave-hop method can be extended to includ
30、e 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: mV/m cos2|2|2|1| rtgGusFFRDDRRLVE = (5) where: DG : divergence factor caused by the spherical Earth, approximately equal
31、 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 ionospheric reflection coefficients will not be equal, because the
32、 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 Angles of elevation and ionospheric incidence The ray path geometry fo
33、r 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 corresponds to typical daytime conditions and in Fig. 3 for an effe
34、ctive 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 frequencies below about 50 kHz. 2.2.2 Path length and differenti
35、al 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 reflection heights of 70 and 90 km, corresponding to daytime and ni
36、ght-time conditions. A propagation velocity of 3 105km/s is assumed. Rec. ITU-R P.684-6 5 FIGURE 2 Departure and arrival angles, , and ionospheric angles of incidence, i, for typical daytime conditions (h = 70 km). The dashed curve includes the effects of atomospheric refraction 6 Rec. ITU-R P.684-6
37、 FIGURE 3 Departure and arrival angles, , and ionospheric angles of incidence, i, for typical nightime conditions (h = 90 km). The dashed curve includes the effects of atomospheric refraction Rec. ITU-R P.684-6 7 0684-04h = 90 kmFIGURE 4Differential time delay between surface wave and one, two and t
38、hree hop sky wavesGreat-circle hop length, (km)dA: 3 hopB: 2 hopC: 1 hopD: limiting range2.2.3 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
39、 factors, Ftand 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 characteristi
40、cs (conductivity and permittivity), as shown in Table 1. 8 Rec. ITU-R P.684-6 0684-05FIGURE 5Ionospheric focusing factor daytimeFocusing factor,DGreat-circle hop length, (km)d0684-06FIGURE 6Ionospheric focusing factor night-timeFocusing factor,DGreat-circle hop length, (km)dRec. ITU-R P.684-6 9 10 R
41、ec. ITU-R P.684-6 Rec. ITU-R P.684-6 11 12 Rec. ITU-R P.684-6 TABLE 1 Conductivity, (S/m) Permittivity, Sea water 5 80 0Land 2 10315 0Polar ice 2.5 1053 00: permittivity of free space The curves were calculated assuming an effective Earths radius, 8 480 km, which is 4/3 of its actual value to take a
42、ccount of atmospheric refraction effects. The factors F are the ratio of the actual field strength 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 (s
43、ee Figs. 1 to 3). 2.2.5 Ionospheric reflection coefficients |R| Values of the ionospheric reflection 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 freque
44、ncy 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 vertical and oblique incidence are indicated based upon the results given in numerous reports. In all cases the ionospheric reflection c
45、oefficient data given in the various references mentioned have been modified, if necessary, to account for ionospheric focusing, antenna factors, etc., so that the results of the measurements are consistent with the analysis technique given here. The concept of an effective frequency f cos i for whi
46、ch reflection coefficients are constant cannot, however, always be relied on. The curves in Fig. 10 are derived from data obtained at steep incidence (d 500 km) and the f cos i concept is likely to be approximately correct for such distances. At intermediate distances, however, the concept of an equ
47、ivalent frequency is likely to lead to substantial errors in reflection coefficient, because in such circumstances the reflection coefficient and polarization of the wave change rapidly with distance. While many data have been incorporated into the curves in Fig. 10, which show that the ionospheric
48、reflection coefficient varies with time of day (midnight and noon) and season, much more work is needed to establish clearly how it varies over the epoch of the solar cycle. There is clearly a solar cycle variation (see Fig. 11) in that reflection coefficients are larger in sunspot maximum years at
49、the very low frequencies, whereas at medium frequencies they are smaller. The physical interpretation of this fact is as follows. During sunspot maximum years, the base of the ionosphere is lower and the electron density gradient is steeper than during sunspot minimum years. Thus, VLF waves which are reflected from this lower layer are more strongly reflected in sunspot maximum years, whereas MF waves, that are reflected above this lower layer, are more strongly absorbed. Clearly, the transition bet
