ITU-R M 1075-1994 Leaky Feeder Systems in the Land Mobile Services《陆地移动业务中的漏泄馈源系统》.pdf
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1、118 Rec. ITU-R M.l75 RECOMMENDATION ITU-R M. 1075 LEAKY FEEDER SYSTEMS IN THE LAND MOBILE SERVICES (Question IT-R 3618) (1994) The ITU Radiocommunication Assembly, considering a) that conventional methods of propagation are inadequate in an underground or confined environment; b) that the use of lea
2、ky feeder systems is established as a solution to those propagation problems; c that all types of mobile radio service may require communications in these environments; d) mobile service over a wide range of frequencies in the HF, VHF and UHF frequency bands; that systems using the leaky feeders and
3、 related techniques have now been in use for many years in the land e) required. For example, open stretches of railways or roads; that leaky feeders are also an effective means of communication where linear rather than area coverage is f) that such use of leaky feeders above ground can usefully con
4、tribute to spectrum conservation; s feeder system as required; that a wide variety of active system techniques is now available for extending the range of cover of a leaky h) techniques for wider use, that it is desirable to summarize the general characteristics and applications of leaky feeders and
5、 related recommends 1. environment where conventional methods of propagation are known to be inadequate; that leaky feeders should be used wherever mobile communication is required in any underground or confined 2. required along the length of a road or a railway or similar linear feature in the ope
6、n: that the use of leaky feeders should be considered where dedicated or augmented mobile communication is 3. that Annex 1 should be referred to for the basic characteristics of leaky feeder and associated techniques; 4. applications. that Annex 2 should be referred to for broad guidance on selectin
7、g types of leaky feeders for specific COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec. ITU-R M.1075 ANNEX 1 119 General aspects of leaky feeder systems in the land mobile service 1. Introduction A leaky feeder is a form of transmis
8、sion line that enables radiocommunication to take place with or between mobile sets in its vicinity through its leakage fields, while substantially providing the linear range of the system through its internai propagation properties. Such systems are used in confined spaces, such as tunnels, mines o
9、r large buildings, where natural propagation is inadequate. They may be completely self-contained or extensions of conventional surface systems otherwise using free radiation: in the latter case the feeds for the underground coverage may either be taken “off air” or share a base station with the sur
10、face antenna directly. Conversely, a primarily underground system may interface with a surface antenna to provide local cover, for example, of the precincts of a mine. Leaky feeders may also be used on the surface, to confine radio coverage and thus improve spectrum efficiency. The term “leaky feede
11、r” can also be taken to include a non-leaky cable with periodic discrete radiators, mode converters or spaced aerials. 2. Classification of leaky feeders The following types of leaky feeder are commonly used: a) bifilar lines; b) continuously leaky coaxial cables; c) coaxial cables with periodic ape
12、rtures; d) cables with mode converters. Types a) and b) are intrinsically non-radiating in the sense that a cable of infinite length extending in free space can only carry waves guided by the structure. However, any discontinuity along the cable causes mode conversion and radiation. In type c), the
13、periodic apertures are radiating discontinuities and act like elements of an antenna array. Maximum radiation is obtained in oblique directions determined basically by the ratio of the spatial interval to the wavelength. In type d), the mode converters or radiating elements are separate discontinuit
14、ies acting in isolation. 2.1 Basic performance parameters The performance of a leaky feeder system may be characterized by two parameters: - longitudinal attenuation; - coupling loss. The longitudinal attenuation is governed primarily by the factors which apply to normal transmission lines, such as
15、construction, conductor size and dielectric. Additionally there is a small loss component attributable to the leakage (or mode converters). COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMN*M= 1075 99 I 4855232 0523553 792 I
16、120 Rec. ITU-R M.1075 The coupling loss is, in general terms, the power loss between the feeder and a mobile antenna in its vicinity. For the commonly used coaxial types of leaky cable it is dependent on the degree of shielding in the feeder construction, the configuration of the shield or conductor
17、s and the permittivity of the dielectric. For a given cable construction it should also be noted that the coupling loss is also dependent upon: - the environment in which the cable is mounted; - the cable mounting position; - the characteristics, position and orientation of the mobile antenna; - the
18、 operating frequency The longitudinal attenuation of cables designed for a low coupling loss can increase substantially with close mounting of the cable to a wall structure or other cables or with surface contamination by grime and moisture, except for cables specifically designed to minimize this e
19、ffect such as the triaxial (tri-coaxial) cables and inherently non-leaky cables using discrete mode converters. In the same process as the increase in attenuation the coupling loss is usually decreased (i.e. the received signal is increased locally). For a given coupling loss, various types of cable
20、 are not necessarily subject to the same increase in attenuation by these effects of proximity and contamination. Bifilar lines are by far the most sensitive, followed by longitudinally slotted cables and lastly by the various types of cable with numerous small holes. Some braided types of cable are
21、 so insensitive to proximity effects that they may be run in normal hangers along with power cables, telephone lines and other such conductors without undue degradation of their performance in the VHF range. 2.2 Bifilar lines Bifilar lines (nominally balanced) in general have a low coupling loss and
22、 a lower longitudinal attenuation for a given conductor size in comparison with coaxial cables. For these reasons they have generally the lowest cost of all leaky cables. However, they can be sensitive to mounting position and surface contamination at VHF and more especially at UHF. If a bifilar cab
23、le is given a tight twist (e.g. several turns per wavelength) the balance is normally improved against proximity effects, thereby improving the longitudinal propagation but in the same process the coupling loss may be increased by up to 15 dB. 2.3 Coaxial cables 2.3.1 Continuously leaky coaxial cabl
24、es This type (with either two or three coaxial conductors) includes the loosely braided cables, cables with continuous slots and cables with discrete apertures or slots separated by distances much smaller than the wavelength. Since these coaxial cables have an imperfect outer conductor, part of the
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