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
25、transmission-line energy travels outside the cable as a leakage field. Basic cable properties that (together with environmental factors) determine the coupling loss are a) the internal propagation velocity and b) the surface transfer impedance of the outer conductor. It is preferable to characterize
26、 cables by quoting these quantities, which can be measured in the laboratory, rather than an arbitrarily and artificially defined “coupling loss”. Propagation velocity is determined by the internal dielectric constant, while surface transfer impedance is determined by the structure of the outer cond
27、uctor and is increased, for example, by increasing the size or number of apertures. Increase of surface transfer impedance results in a proportional increase in the coupled signal, and is normally accompanied also by an increase in longitudinal attenuation depending on the precise cable structure. T
28、he sensitivity of the longitudinal attenuation to mounting position also becomes more marked but there is evidence that this is more a function of “hole size” than of surface transfer impedance in itself, and so a large number of smaller holes is better than fewer large holes in this respect. COPYRI
29、GHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMN*N* LO75 94 I 4855232 0523552 629 W Rec. ITU-R M.1075 121 Increase of internal cable velocity (e.g. through the internal dielectric constant) increases the total energy in the leakag
30、e field, though this relationship is not linear and involves other parameters. The field close to the cable is actually reduced, but the spread of the field increases. The congestion radius, within which most of the power of the coaxial-mode leakage field is contained, is of the order of 60 cm at 10
31、0 MHz and varies in inverse ratio to the dielectric constant of the internal cable insulation. It is also directly proportional to wavelength. The mechanism by which a mobile antenna outside the congestion radius is coupled to the coaxial-mode is the diffraction of the coaxial-mode leakage fields by
32、 the inhomogeneities within the congestion radius; these are the inhomogeneities and irregularities in the environment, cable construction and suspension brackets and obstacles of all kinds. This diffraction is a random process: furthermore, in tunnels the resulting coupling loss does not significan
33、tly depend on the distance of the antenna from the cable; this is because the diffracted fields are carried by numerous waveguide modes. 2.3.2 Coaxial cables with periodic apertures Coaxial cables having discrete apertures with dimensions and/or periodicity comparable with the wavelength are a sourc
34、e of radiation. A coaxial cable with zig-zag slot arrays can be treated as an antenna array and the transmitted energy in the cable is little affected by the radiation from the slots. The radiating field propagates as a cylindrical wave. Properly located zig-zag slots in coaxial cables improve radia
35、tion in the transverse direction and it is possible to manufacture cables with different coupling losses suitable for grading (see 5 3.2.1). 2.4 Cables with mode converters, radiating devices or antennas These are ordinary bifilar or coaxial lines fitted with devices to convert part of the energy ca
36、rried by the line into guided modes propagating outside the line or into radiated spherical waves; the devices are installed with either regular or irregular spacing at points determined by the propagation conditions prevailing in the surrounding environment. A radiating device for coaxial cables ca
37、n take the form of a slot cut around the circumference, forming a complete interruption of the outer conductor to which the circuit elements are added so that only part of the power transported by the cable is radiated. The spacing of the mode converters or radiators may vary from 100 m to 1 km depe
38、nding on the system input power, the position of the cable in the tunnel and the single-wire mode or tunnel-mode attenuation (whichever is predominant in the conditions). The conversion rate may be adjusted by a suitable choice of circuit components. A common rate is approximately lo%, which gives r
39、ise to an insertion loss of about 0.5 dB per converter. Mode converter systems have been constructed using a short section of continuously leaky cable inserted in the conventional “non-leaky” cable. Such a section acts as a continuous network of circumferential slots. 3. Systems aspects 3.1 Basic sy
40、stem A basic leaky feeder system for two-way communication comprises a base-station transmitter and receiver connected to a leaky cable which provides communication to conventional mobile stations as shown in Fig. 1. By placing the base station at the centre of the cable the longitudinal range can b
41、e effectively doubled relative to an end-fed system. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMN*N* 1075 9Y = 4855232 0523553 565 = 122 Rec. ITU-R M.1075 FIGURE 1 Basic system B *I Li L2 A: base station LI : coupling lo
42、ss B: leakycable b: cable insertion loss C: mobile I Wt 3.1.1 Communication range The basic equation relating the parameters (in dB) of a leaky feeder system is: SYSTEM LOSS = LINE LOSS + COUPLING LOSS where - - SYSTEM LOSS is the ratio of the transmitter output power to the power at the receiver in
43、put terminal; LINE LOSS is the longitudinal attenuation of the line between the base station and the point on the line nearest the mobile set; - COUPLING LOSS is the ratio of the power in the line at the point nearest the mobile set to the power at the antenna terminal of the mobile set. In the case
44、 of centre-fed systems the splitting loss of 3 dB has to be first subtracted from the system loss. This relationship enables the approximate range of a simple leaky feeder system to be predicted provided the basic parameters of the cable, radio sets and the particular environment are known. Typicall
45、y, ranges of between 1 and 2 km are achievable. Coupling losses as low as 30 dB have been recorded in favourable conditions (e.g. with bifilar lines in a poor state of balance), but more typical figures range from 60 to 90 dB. Short-distance variations of 20 dB, to a Rayleigh-type distribution, are
46、common in tunnels and mines and result from multipath effects. To take these into account it is normally wise to work to a coupling loss figure of at least 90 dB for systems using a cable that is reasonably insensitive to mounting arrangements. In the case of widely spaced discrete radiators or ante
47、nnas rather than a continuously leaky cable the natural propagation of the tunnel has to be considered between radiators. Below cut-off frequency of the tunnel a different mechanism applies and the single-wire propagation of the feeder has to be considered instead. 3.2 More complex systems 3.2.1 Gra
48、ded cable The basic range of a single base station may be extended by suitably grading the cable along its length, so that coupling loss reduces (at the expense of longitudinal attenuation or proximity effects) with distance from the base station, this technique has been used extensively in Japan. C
49、OPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services ITU-R RECMN*M* 1075 94 4855212 0523554 4T1 C A Rec. ITU-R M.1075 123 E 3.2.2 Multiple base stations The range of the basic system may be extended as required by deploying further base stations, which may be separately controlled or under common control with the first. Lack of synchronism between successive base transmitters gives rise to objectionable “overlap” effects, but these may be minimized by leaving a gap between feeder extremities in the regions affected. The transmit
