1、Rec. 498-2 1RECOMMENDATION 498-2IONOSPHERIC CROSS-MODULATION IN THE LF AND MF BROADCASTING BANDS(Question 44/10, Study Programme 44E/10)(1974-1978-1990)Rec. 498-2The CCIR,CONSIDERINGthat excessive radiation towards the ionosphere may result in ionospheric cross-modulation and hence harmfulinterferen
2、ce,UNANIMOUSLY RECOMMENDSthat the maximum permissible radiation at any angle of elevation should be such that annoyance due toionospheric cross-modulation does not exceed that agreed for co-channel interference (see Recommendation 560).ANNEX I*The effects of ionospheric cross-modulation in bands 5 (
3、LF) and 6 (MF) may become a problem of increasingseverity as the power of transmitters continues to increase.1. Detailed experiments on this subject have been carried out within the framework of the EBU in severalcountries, notably in the United Kingdom, in the Federal Republic of Germany Haberkant
4、and Vogt, 1966; Haberkantet al., 1971 and in the Peoples Republic of China CCIR, 1986-90. From these experiments which were carried outwith conventional amplitude-modulation double-sideband transmissions, the following results may be deduced:1.1 The percentage of cross-modulation increases practical
5、ly linearly with the power of the interfering transmitterand also increases with the depth of modulation.Note The percentage of cross-modulation is the percentage by which the carrier of the wanted transmitter is modulatedby the modulating frequencies of the interfering transmitter.1.2 Cross-modulat
6、ion depends primarily on the power radiated by the interfering transmitter in the direction of thereflection point of the wanted signal in the ionosphere.Cross-modulation of percentages less than 10% are directly proportional to the power Knight, 1973; anincrease of 3 dB in the interfering transmitt
7、er power therefore, increases the cross-modulation levels by 6 dB. Thepercentage of cross-modulation is also directly proportional to the depth of modulation of the interfering transmitterKnight, 1973.1.3 The percentage of cross-modulation decreases as the modulating frequency of the interfering tra
8、nsmitterincreases. Laboratory experiments Whythe and Reed, 1973 have shown that the subjective effect of cross-modulationcan be related to co-channel interference. To produce a given subjective grade of impairment, interference resulting fromionospheric cross-modulation requires 6 dB less input sign
9、al-to-interference ratio than does co-channel interference,providing that the cross-modulation is referred to a modulation frequency of 300 Hz.1.4 It should be noted that the studies on the problem of ionospheric cross-modulation carried out by StudyGroup 6 are summarized in Report 574.2. Figure 1 s
10、hows the percentages of cross-modulation measured in many experiments Knight, 1973. Eachmeasurement has been standardized to the value which would have been observed if the interfering transmission hadbeen radiated from a short vertical antenna with a carrier power of 100 kW and amplitude modulated
11、at 300 Hz to adepth of 80%._*This Annex is given for information.2 Rec. 498-2D01-scFIGURE 1 D01 = 22 cm = page pleineRec. 498-2 3Figure 1 includes a semi-empirical curve which shows the greatest percentage of cross-modulation, averagedover a short period, likely to be observed; the condition for thi
12、s is that the wanted signal should traverse the region of theionosphere most strongly illuminated by the interfering transmitter. Figure 1 shows that cross-modulation rises to asecond maximum when the frequency of the interfering transmitter is close to the gyromagnetic frequency. Figure 5shows a ma
13、p giving the value of the gyromagnetic frequency for different parts of the world Laitinen andHaydon, 1950.3. The effects of cross-modulation should be taken into account not only for sky-wave reception, but also forground-wave reception at the edge of the service area when at night the sky-wave is
14、no longer negligible. However, theeffect of cross-modulation is reduced approximately in the ratio of the wanted signal levels, ground-wave to sky-wave,at the receiving point.4. The percentages of ionospheric cross-modulation have been calculated for LF and MF and their dependenceon the powers of th
15、e wanted and unwanted transmitter has been determined. Results of theoretical studies and practicalexperiments have been compared. Shluyger et al., 1976.5. Preliminary conclusionsOn the basis of measurements Haberkant and Vogt, 1966; Haberkant et al., 1971 examples may be given ofthe power-flux leve
16、ls, or the transmitter power as a function of the angle of elevation, which can cause disturbance towanted transmissions.For this purpose, an assumption is first made regarding the tolerable level of the percentage of cross-modulation. According to Recommendation 560 and Report 575, a radio-frequenc
17、y protection ratio of approximately30 dB is agreed for 10% of the time in the case of a fluctuating unwanted signal. Ignoring the effect mentioned in 1.3,the same disturbing effect is produced by 3% cross-modulation for 10% of the time. It has been shown Haberkantet al., 1971 that for frequencies at
18、 the upper end of the MF broadcasting band 6 (MF) this level of cross-modulationmay be produced by a power flux within the E region of the ionosphere of about 2 W/m2(57 dB(W/m2), whichcorresponds to a maximum field strength of 27 mV/m (89 dB(V/m).Assuming a height of 100 km of the reflecting layer (
19、E region), it is possible to calculate the power radiatedfrom various types of antenna which would produce this power flux within the E region. The vertical transmittingantennas that are commonly used show a vertical radiation pattern which depends in a well-defined fashion on the height(expressed i
20、n fractions of the wavelength, ). In particular, such vertical antennas do not radiate at an angle of elevationof 90. Table I Haberkant et al., 1971 indicates, for a number of vertical transmitting antennas at different heights thetransmitter powers to be fed into these antennas to meet the above-me
21、ntioned requirements.TABLE ILength of vertical antenna 0.25 0.25 0.5 0.55 0.64 0.64 (1)Transmitter carrier power(kW)320 340 560 670 370 840(1)First side lobe compensated.It is possible to calculate the dependence of the radiated power on the angle of elevation required to producethe same power flux,
22、 covering the whole range from 0 (horizontal radiation) to 90 (vertical radiation). The results aregiven in Table II.Tables I and II give only approximate values because it is known, from theory, that ionosphericcross-modulation may be influenced by several parameters, such as the frequencies of the
23、 wanted and of the interferingtransmitter (in particular seen in their relationship to the gyro-frequency) and the polarization of emission.The powers given in Tables I and II are examples based on a small number of measurements at a frequencynear the top end of band 6 (MF); they make no allowance f
24、or the change of cross modulation with carrier frequency ofthe disturbing signal, nor do they include the effect of reduced cross-modulation at the higher audio frequencies whichpermits interfering-transmitter powers to be increased by 3 dB.4 Rec. 498-2TABLE IIAngle of elevation 0 10 20 30 40 45 50
25、60 70 80 90e.m.r.p. (dB (1 kW) or c.m.f. (1)(dB (300 V)39.5 32 27.5 24.3 22.5 22 21.5 20.2 19.3 18.7 18.5e.m.r.p. (kW) 9000 1600 570 230 190 160 140 105 85 75 70(1)e.m.r.p. : effective monopole radiated power; c.m.f. : cymomotive force.See also Recommendation 561It may be noted that services other t
26、han broadcasting have also suffered degradations due to ionosphericcross-modulation.The results of many other measurements of ionospheric cross-modulation have been compared Knight, 1973and Fig. 1 shows that 100 kW radiated from a short vertical antenna at frequencies in the lower part of the broadc
27、astband 6 (MF) produces cross-modulation which may exceed 2% for 50% of the time. It may be shown Haberkantet al., 1971 that this corresponds to a cross-modulation level of 3% exceeded for 10% of the time. The power of100 kW may therefore be directly compared with the power of 320 kW given in Table
28、I. The greater power in Table Iarises because the series of measurements on which it was based appear to give lower cross-modulation than theestimated worst case values shown by the curve in Fig. 1.Figure 1 also shows that cross-modulation levels caused by disturbing transmitters, operating either a
29、tfrequencies in band 5 (LF) or at frequencies close to the gyromagnetic frequency, may be 10 dB greater than levelsarising at frequencies in the lower part of band 6. A 5 dB reduction of disturbing-transmitter power reduces thecross-modulation level by 10 dB. Allowing for the modulation-frequency ef
30、fect we conclude that, depending on thedisturbing frequency in bands 5 (LF) and 6 (MF), transmitter powers in a range varying from the values in Tables I andII down to 7 dB lower may, at worst, give interference to a sky-wave service comparable with co-channel interferencefor 30 dB protection ratio.
31、Somewhat greater disturbing-transmitter powers may be radiated if ground-wave services, rather thansky-wave services, are to be protected from the effects of ionospheric cross-modulation, because the disturbingtransmitter influences only the sky-wave component of the received signal. If the limit of
32、 the ground-wave service areais defined as the line where the ground-wave field strength exceeds the median sky-wave field strength by 10 dB; themedian cross-modulation of the resultant signal will be 14 dB less than the median cross-modulation of the sky-wave.Disturbing-transmitter powers may there
33、fore be greater than the equivalent powers when the sky-wave is beingprotected.6. Practical application of the conclusionsThe EBU has investigated the consequences on the planning of broadcasting networks in bands 5 (LF)and 6 (MF) to be drawn from the preliminary conclusions summarized in 5 of this
34、Annex. The most urgent problem isthat of setting limits for the maximum effective monopole-radiated power as a function of the angle of elevation and typeof antenna if a certain amount of interference caused by ionospheric cross-modulation is not to be exceeded. Theconclusions drawn so far from thes
35、e studies are set out hereafter.It is recommended that the annoyance due to cross-modulation should not exceed that resulting fromco-channel interference with a protection ratio of 30 dB. However, cross-modulation, unlike co-channel interference,decreases with increasing modulation frequency, so tha
36、t subjective experiments are necessary to relate the two effects.Such experiments have been carried out, and have shown that the maximum percentage of cross-modulation could be6.3% when the interfering transmitter is 80% modulated by 300 Hz tone. It is recommended that this should be regardedas the
37、maximum acceptable limit of cross-modulation.The results of subjective assessment of the degree of annoyance by cross-modulation carried out in Chinaunder normal transmission of sound broadcasting programmes and a co-channel protection ratio of 27 dB for sky-waveservice show that the quality grade o
38、f 4 is achieved and the interference is perceptible but not annoying, when thepercentage of cross-modulation is 8.9%.Rec. 498-2 5Taking into account the dependance of cross-modulation on the carrier frequency of the unwanted emissionand the height of the reflecting layer, Fig. 2 (curve A) shows the
39、maximum effective monopole-radiated power(dB (1 kW) or cymomotive force (dB (300 V) directed vertically upwards which would produce, for 50% of the time,the percentage of cross-modulation specified above. The abscissa is the ratio of the unwanted carrier frequency fito thegyro-frequency fG(about 1.2
40、5 MHz in Europe). This curve is based on a large number of measurements in Europe andAustralia as described in 5 and Fig. 1, taking the observed values of cross-modulation as representing the worst valueslikely to occur over the most unfavourable geographical path.In practical cases, account must be
41、 taken of the vertical radiation pattern of the antenna and of the increasingdistance between the antenna and the reflecting point in directions other than vertical. Fig. 3 shows the permissibleincrease in e.m.r.p. in directions other than vertical, allowed by the increasing distance only. An additi
42、onal increase ordecrease in power resulting from the vertical diagram of the antenna has to be taken into account. For practicalapplication, the influences of increasing distance to the reflecting point and of the vertical radiation pattern of theantenna have been combined into one single correction
43、 factor P which has to be added to that read from Fig. 2. Thiscorrection factor has been calculated for vertical antennas of different electrical length l/ and horizontal dipoles0.5 long, at different heights h/ above ground, assuming a height of 85 km for the region of the ionosphere inwhich cross-
44、modulation should occur. The result of this calculation is given in Fig. 4.In a ground-wave service which is to be protected against cross-modulation at night, it may be assumed thatthe sky-wave field strength of the wanted transmitter is 10 dB below the ground-wave field strength at the service lim
45、it.Since only the sky-wave component is subject to cross-modulation, an increase of 5 dB in radiation is permissible if onlyground-wave services need be considered. This leads to curve B of Fig. 2.D02-scFIGURE 2 D02 = 6.5 cm et FIGURE 3 = 7 cm6 Rec. 498-2D03-scFIGURE 4 D03 = 18 cm = page pleineRec.
46、498-2 7D04-scFIGURE 5 D04 = 13.5 cmAs a practical example, consider a short vertical antenna in band 5 (LF) (fi/fG= 0.2). Figure 2 shows that toprotect a ground-wave service, the maximum e.m.r.p. in a vertical direction would be 20 (dB (1 kW) i.e. 100 kW.However, a short antenna produces a maximum v
47、alue of field strength in the ionosphere at an angle elevation of 45;Fig. 3 shows that an increase of 3 dB is permitted at that angle, giving an e.m.r.p. of 200 kW. However, it ismore convenient to specify the e.m.r.p. in the horizontal direction; for a short antenna this is 3 dB greater than at 45,
48、i.e. 400 kW.Accordingly, in this case, for a short vertical antenna (l/ 0.1), the value of P = + 6 dB can be read fromcurve A in Fig. 4, which results in a total power fed to the antenna of P = +26 dB (1 kW) i.e. 400 kW.Curves showing the relationship between the depth of cross-modulation at the poi
49、nt of reception and the fieldstrength at the point of reflection in the ionosphere are given in Fig. 6. They have been obtained following investigationscarried out in China. These curves may be used to evaluate the percentage of cross-modulation at a given reception pointfor different values of interfering field strength at the point of reflection in the ionosphere and to calculate approximatelythe zone of influence of cross-modulation.8 Rec. 498-2D05-s
