1、CCIR RECMN*444 92 4855212 0520260 280 Rec. 844 RECOMMENDATION 844 101 IONOSPHERIC FACTORS AFFECTING FREQUENCY SHARING IN THE VHF (30-300 MHz) BAND (Question 3816) 1997) The CCIR, considering a) that the ionosphere, while primarily responsible for reflection of radio waves below approximately 30 MHz,
2、 is capable of supporting propagation in the VHF (30-300 MHz) band under some conditions, during relatively short periods of time, and in certain areas of the world; b) consideration when planning radio systems which share frequencies; that propagation mechanisms which exist for relatively short per
3、iods of time should be taken into 4 that frequency sharing is important in the efficient use of the radio spectrum, recommends that the following information be taken into account when planning radio systems which make use of frequency sharing in the VHF (30-300 MHz) band. 1. Introduction Radio prop
4、agation at VHF is mainly controlled by physical objects, such as terrain and ground cwer (clutter), and tropospheric factors, principally refraction. However, ionospheric propagation over long distances at VHF can take place with relatively small losses. Such propagation events may be significant in
5、 causing interference at VHF, particularly for systems requiring high reliability. 2. Summary of potential ionospheric interference mechanisms Table 1 provides a summary of ionospheric propagation mechanisms which may be significant causes of interference at VHF. Further information on the more impo
6、rtant mechanisms is given in the following sections. 3. Normal F-region propagation Near the peak of the solar cycle long-distance propagation via the F2 layer can occur for a significant fraction of the time at frequencies above 30 MHz. This effect extends to 70 MHz at low latitudes. Figures i. 7 a
7、nd 3 show values of the MUF for a path length of 4000 km exceeded for 1% of hours for three seasons at sunspot maximum. 4. Trans-equatorial propagation (TEP) Strong transmission can occur, particularly during high sunspot years, over long North-South paths spanniny the geomagnetic equator. There app
8、ear to be two types of trans-equatorial propagation characterized by the times of peak OccurTence, fading characteristics and modes of propagation. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesCCIR RECMN*BLiY IT! YBSCiT!itc! OSi?Oi?
9、bit 557 D 102 8 a2 18 Rec. 844 I O in I COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec. 844 I a“ 2s 1: o I l I I I a .r( E 3 W U sa 9 O COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Informatio
10、n Handling Services CCIR RECflN*KBLiY 72 Lid55212 O5202b3 T9T 104 Rec. 844 FIGURE 1 MUF for a path length of 4 O00 km exceeded durlng 1% of hours - December soistlce: sunspot maxlmum The first type of TEP, which is called flic afternoon type, has the characteristics: - a per normally strong steady s
11、ignals with a low fading ratc and a small Doppler spread (about f2-4 Hz); path lengths of about (i-90 km and sometimes longer. - - The second type of TEP, which is called the evening typc, generally supports higher frequencies than the afternoon type and has very different characteristics: - - a per
12、ik occurrence around 2XKl-2300 li LMT; high signal strengths but with deep and rapid fading at rates up to about 15 Hz and a large Doppler spread which sometimes excccds 40 Hz; path lengths usuaily shorter tiinn for thc aftcnioon-typc mode, being about 3 001)-6000 km. - COPYRIGHT International Telec
13、ommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesCCIR RECMN*844 92 4855232 0520264 926 I Rec. 844 FIGURE 2 MUF for a path length of 4 O00 km exceeded during 1 % of hours - June solstice; sunspot maximum 1 o5 5. Sporadic-E propagation Sporadic-E ionization appears a
14、s an intensification in ionization in the form of a horizontal sheet of about 1 km average thickness and a horizontal dimension of the order of 100 km. The height is commonly 100 to 120 km. Such sporadic-E layers can cause abnormal VHF propagation for periods lasting for several hours. The occurrenc
15、e of sporadic-E propagation decreases with increasing frequency, but can be a sipificatit cause of interference at frequencies up to about 135 MHz. Recommendation 534 provides a method for calculating sporadic-E field strengths and probability of occurrence. 6. Meteor-trail ionization Scattering fro
16、m ionization due to meteor-trails can produce VHF interference over ranyes up to approximately 2000 km. Although individual meteor trails remain effective for periods measured in seconds. meteor bursts can support continuous or near-continuous propagation for much longer periods of time. The effect
17、of meteor trails on VHF propagation shows spatial, diurnal and seasonal variations. Further information may be found in Recommendation 843. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesCCZR RECMN*KB44 92 U 4KB552i12 05202b5 Abi2 106
18、 Rec. 844 FIGURE 3 MUF for a path length OP 4 O00 km exceeded during 1% of hours - equinox; sunspot maximum 7. Auroral lonlzatlon Field-aligned irregularities in thc auroral zones may appear during magnetically disturbed periods. Such ionization can produce significant reflections causing propagation which is normally off the great-circle path. This may cause interference at VHF frequencies, mainly in mid to high geomagnetic latitudes. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services
