Radiating coaxial cable

Abstract

 The invention relates to radiating coaxial cables designed for radiating electromagnetic energy in radiated mode over a broad frequency band, comprising an outer conductor provided with a periodic pattern (M) of aperture groups repeated along the length of said outer conductor with a constant spacing (s) between successive groups of apertures, wherein the periodic pattern of apertures and/or the constant spacing (s) between successive groups of apertures are provided in such way that the radiating coaxial cable operates, for the highest frequency band the cable is designed for, at a radiating angle Θ₁ between 150° and 180° with respect to the axis of the radiating cable.
In a preferred embodiment the pattern of apertures in each group involves a number of apertures (n) of at least 10 and the distance between successive apertures is s / 2n ± 20%.

Description

[0001] The present invention relates generally to a radiating coaxial cable, and more particularly, to a radiating coaxial cable having equally-spaced groups of apertures for generating electromagnetic waves.

[0002] Radiating coaxial cables are particularly appropriate to allow radio communication links with mobile equipment in indoor environments such as tunnels, mines, underground railways and buildings.

[0003] The use of radiating coaxial cables in these environments is particularly important as a result of the development of mobile communication systems (radio links, cellular phone, cordless telephone, wireless computer network, etc.).

[0004] Nowadays, these mobile communications systems operate in a very large spectrum the frequencies of which are allocated at an international level. Starting from the low frequencies, the bands are allocated as follows (these figures are only indicative and may vary with countries):

• 74 to 87 MHz : Private mobile radio;
• 88 to 108 MHz : FM radio broadcast;
• 145 to 175 MHz : Private mobile radio;
• around 225 MHz : Digital Audio Broadcast (DAB);
• 380 to 470 MHz : Private mobile radio and TETRA networks;
• 824 to 894 MHz : TDMA IS-54 and CDMA IS 95 mobile communication networks;
• 870 to 960 MHz : GSM 900, GSM R and TETRA mobile communication networks;
• 1710 to 1880 MHz : GSM 1800 networks;
• 1885 to 2200 MHz : UMTS networks.

[0005] Moreover, such radiating coaxial cables can also be used in outdoor or indoor environments to restrict the radio coverage in a narrow lateral corridor along an axis, e.g. a transport route, a railway, a defined path in a workshop, etc. Restricting the radio coverage in a certain width may be required to avoid interference with neighbour transmitters operating at the same radio frequency.

[0006] Various types of radiating cables are known; they consist of a coaxial cable comprising an inner conductor surrounded by a dielectric and by an outer conductor of tubular form. The outer conductor includes apertures which generate an electromagnetic radiation. The outer conductor is covered by an insulating outer sheath.

[0007] The apertures in the outer conductor may be of various types, for example a longitudinal slot over the entire length of the cable, or numerous small holes very close to each other. There also exist cables in which the outer conductor consists of a loose braiding, or sometimes of a layer of wires wound in a spiral around the dielectric. The common characteristic of these cables is that the total length of the outer conductor includes apertures separated by a distance considerably shorter than the wavelength of the radiated signal. All these cables operate in a mode known as "coupled mode" and the radiated energy propagates in a direction parallel to the cable. With these cables, the signal received by a receiving antenna falls off rapidly when the distance between the antenna and the cable increases. Moreover, the received signal fluctuates greatly when the receiving antenna is moved along a path parallel to the cable.

[0008] A more recent technique has proposed cables known as "radiated mode cables" in which the outer conductor includes an aperture (or a group of apertures) which is reproduced with a constant spacing s, this spacing being of the same order of magnitude as the wavelength of the signal to be radiated. The radiation produced by the radiated mode cables propagates in a radial direction (fig. 1), forming an angle θ₁ with the cable axis lying between 0° and 180°.

[0009] It is known by those skilled in the art that a radiated mode cable produces a main mode which propagates in a direction forming an angle θ₁ with the axis of the cable; this angle is given by the formula


where :

s :

aperture group spacing (in metres);

λ :

signal wavelength in the air (in metres);

ε_r :

relative dielectric constant of the cable (coefficient).

[0010] In the above expression, the direction of reference for measuring θ₁ is the direction of the cable end fed by the radio frequency generator, as illustrated by the enclosed Figure 1.

[0011] A radiated mode cable operates in this way in a band from λ_start to λ_end where λ_start and λ_end correspond to θ₁ = 0° and 180° respectively. These wavelengths (in the air) λ_start and λ_end are linked respectively to the frequencies f_start and f_end (in MHz) by

[0012] It is known by those skilled in the art that the ratio fend / f_start is given by

[0013] With the dielectric usually used between the inner and outer conductors, √ε_r is generally lying between ≅ 1.1. and ≅ 1.15. Consequently, fend / f_start varies between ≅ 14 and ≅ 21.

[0014] Hereinafter most calculations are carried out with √ε_r = 1.136 which is the most frequent value with dielectrics presently used. It should be stressed, however, that the conclusions which will be drawn will generally also be valid if √ε_r is not equal to this particular value.

[0015] The enclosed Figure 1 shows the graph of θ₁ versus f/f_start calculated for √ ε_r = 1.136. This figure shows that θ₁ begins at 0° when f is equal to f_start. Then, θ₁ increases with f up to 180° when f = f_end which is equal to 15.71 f_start. Below f_start and above fend, the cable operates in coupled mode.

[0016] Compared to coupled mode cables, the main advantages of the radiated mode cables are:

• a lower coupling loss;
• a coupling loss which increases less rapidly in the radial direction;
• a field which fluctuates less when moving parallel to the axis of the cable.

[0017] However, it is also known by those skilled in the art that the third advantage above disappears when the frequency reaches 2 f_start if some precautions are not adopted since there appears a second order mode which propagates in a direction θ₂ different from θ₁ and which interferes with the main mode. According to the relation (1), θ₁ ≅ 94° (for √ε_r = 1.136) when f = 2 f_start. If f continues to increase, a third mode appears when f = 3 f_start and so on for all the f_start multiples. As a consequence, the higher the frequency, the more numerous are the secondary modes which all propagate in different directions θ_i. These interferences between the main and secondary modes result in rather large field strength fluctuations along the cable.

[0018] If we consider first the case of narrow band radiating cables i.e. the cables used at only one or several frequencies very close to each other (this is the case if the cable is only used for one radio communication application listed above), prior art cables were generally designed to have the θ₁ angle very close to 90° in the frequency band for which the cable is intended. The main reasons are avoiding the secondary mode which appears for θ₁ higher than about 94° and also because, with most aperture types, the radiation decreases in the directions nearly parallel to the cable axis (i.e. with θ₁ close to 0° or 180°).

[0019] Formula (1) indicates that choosing a spacing s ≅ λ gives rise to θ₁ ≅ 90° as √ε_r ≅ 1. This is the reason why prior art narrow band radiating cables are designed with the aperture group spacing approximately equal to the wavelength (in the air) for which the cable is intended.

[0020] The enclosed Figure 2 illustrates a specific embodiment of such prior art narrow band radiating cables; in this embodiment, each aperture group includes two slots slanted in opposite directions and the group spacing is approximately equal to the wavelength.

[0021] If we consider now the case of wide band radiating cables, i.e. cables which must exhibit satisfactory performances in the frequency band allocated to several mobile communication applications, the main problem to solve is the field fluctuations due to the interference produced by the secondary modes described earlier. Several solutions have been proposed to cancel or to reduce to an acceptable level the intensity of the secondary modes, at least on a frequency band from 2 f_start to k x f_start where k depends on the efficiency of the solution. Generally, k varies from 3 to 5 or even 7 with the best solutions. If we refer to figure 1, this means that the performances deteriorate (there are large field strength fluctuations along the cable) if θ₁ exceeds 115°, 135° or 145°, with k equal to 3, 5 and 7 respectively, and with √ε_r = 1.136.

[0022] It must be mentioned that if √ε_r ≅ 1.1, the θ₁ values which correspond to k equal to 3, 5 and 7 are respectively 114°, 133° and 143°; these values are close to those obtained for √ε_r = 1.136. Similar conclusions apply if √ε_r ≅ 1.2.

[0023] Figure 1 also shows that θ₁ raises very rapidly from 0° to 35° when f increases from f_start to 1.1 f_start. This band is too narrow to be of any interest in practice and it results that prior art wide band radiating cables are generally designed to have θ₁ lying between ≅ 35° and an angle θ_max comprised between 115° and 145° (θ_max depends on the efficiency of the solution used to cancel or attenuate the secondary modes) in the frequency bands for which they are is intended. This also means that (in the best case) the direction θ₁ into which the wave generated by the radiating cable propagates, lies within an angle of about 110° centred on the direction perpendicular to the cable axis.

[0024] As a consequence, prior art wide band radiating cables are designed by choosing the aperture spacing s in order to have θ₁ lying between ≅ 35° and θ_max in the frequency bands for which the cable is intended. Such cables can be used at frequencies where θ₁ > θ_max, but the performances deteriorate due to the interference between the main mode and secondary modes insufficiently attenuated.

[0025] The following specific documents illustrate the state of the art referred to here above

[0026] DE-A-2, 812, 512 describes a pattern which, with the aim of producing a periodic profile in the direction of the radiating cable axis consists of apertures of the same size and of the same shape, the density of which varies periodically along the cable. As the holder of this patent indicates, the purpose of such a pattern is to produce a periodic profile of the radiation intensity in the direction of the axis of the cable. Moreover, this document does not give the extent of the frequency band in which the secondary modes are attenuated.

[0027] GB-A-1, 481, 485 describes a periodic pattern consisting of two main slots and four auxiliary slots. The auxiliary slots are arranged on either side of each of the main slots. In this device, the secondary modes appearing at the frequencies lying between f_start and 5 f_start are negligible or almost zero. Moreover, a pattern of greater size would include ten slots and, consequently, would be difficult to produce in practice, since the total length of the apertures would be such that it would weaken the mechanical strength of the outer conductor.

[0028] FR-A-2 685 549 describes a pattern including N apertures, the useful frequency band of which lies between f_start and N x f_start.

[0029] The patterns described in these last two documents have the drawback that the apertures are present over almost the whole length of the cable, which has the effect of reducing the mechanical strength. It is well known, in fact, that deformations of the cable or of the apertures in the outer conductor may greatly affect the performances obtained. Another drawback of these known solutions is the difficulty of producing long slanted slots with different inclinations on certain types of cable constructions.

[0030] DE-G-9, 318, 420 describes a solution which uses a corrugated outer conductor. No mention is made of the elimination of secondary modes.

[0031] EP 0 765 002 A2 describes a solution for a narrow band cable which uses a periodic pattern consisting of two opposed slots elongated in the axial direction. The pattern spacing is approximately equal to one wave length in order to radiate in a direction θ₁ close to 90°.

[0032] US 6,292,071 B1 describes a solution for a wide band coupled mode cable which uses groups of apertures separated by a spacing varying between 8 and 10 m. Such an embodiment has the drawbacks of the coupled mode cables.

[0033] WO 99/17401 describes a solution for a radiated mode cable which is based on a principle similar to the one shown in figure 2 but in which each slanted slot is replaced by a group of circular or elongated holes.

[0034] BE 1010528 describes a radiating cable operating in radial direction for a specific frequency band, which comprises an outer conductor provided with a periodic pattern of aperture groups with defined spacing, corresponding to A / (√ε_r - 1), where A is the wavelength of the lowest frequency at which the cable operates and ε_r is the dielectric constant of the cable. The number of apertures in each pattern group ranges from 1 up to 10.

[0035] It is an object of the present invention is to provide an improved narrow band radiating cable exhibiting a low coupling loss over a frequency band of about one octave.

[0036] Another object of the present invention is to provide an improved narrow band radiating cable exhibiting small field strength fluctuations over a frequency band of about one octave and hence will allow the attainment of low bit error rates when used for digital communications and minimises distortions when used for analogue communications.

[0037] A further object of the present invention is to provide a wide band radiating cable which provides a large band in which the performances are comparable to prior art wide band cables and a band the length of which is about one octave in which the cable features a lower coupling loss and smaller field strength variations.

[0038] It has indeed been found surprisingly, in accordance with the present invention, that the foregoing objectives can be reached by providing a radiating coaxial cable which includes groups of apertures separated by a constant spacing s chosen in such a way that θ₁ varies in the interval between about 150° and 180° in the highest frequency band the cable is intended for.

[0039] This invention thus provides for a radiating coaxial cable designed for radiating electromagnetic energy in radiated mode over a broad frequency band, comprising an outer conductor provided with a periodic pattern (M) of aperture groups repeated along the length of said outer conductor with a constant spacing (s) between successive groups of apertures, wherein said periodic pattern of apertures and/or said constant spacing (s) between successive groups of apertures are provided in such way that the radiating coaxial cable operates, for the highest frequency band the cable is designed for, at a radiating angle ⊖₁ , in accordance with the formula


wherein ε_r is the relative dielectric constant of the radiating cable
and A is the wavelength corresponding to said frequency in the air, essentially between 150° and 180° with respect to the axis of the radiating cable (where the direction of reference for measuring ⊖₁ is the direction of the cable end fed by the radio frequency generator).

[0040] According to preferred embodiments of the invention, the periodic pattern of apertures in each group may in particular involves a number of apertures (n) of at least 10 , more particularly of at least 14, and whereas the distance between successive apertures preferably corresponds to

[0041] According to still another preferred embodiment of the invention the distance between the left end of the first aperture and the left end of the last aperture in each group may very suitably correspond to (n - 1) s / 2n, whereas the length of the section without apertures, between the left end of the last aperture of one group and the left end of the first aperture of a next group may in particular correspond to (n + 1) s / 2n.

[0042] According to a preferred feature of the invention the radiating cables more specifically involve a dielectric constant corresponding to a √ε_R value between 1.1 and 1.3.

[0043] According to a further preferred feature of the invention, radiating cables having optimal performance for wavelengths between λ_opt.1 and λ_opt.2 can be obtained by specifically selecting the spacing (s) between successive groups of apertures so that

[0044] Further embodiments and other details of the invention will become apparent from the following detailed description, having reference to the attached drawings, in which

Figure 1

represents a graph of the angle θ₁ versus f/f_start calculated for √ε_r = 1.136 ;

Figure 2

illustrates an aperture group spacing according to the state of the art ;

Figure 3

illustrates one preferred embodiment of the spacing between successive groups of apertures according to the invention

Figure 4

illustrates the distance between apertures in accordance with a preferred embodiment of the invention ;

Figures 5-8

illustrate several preferred embodiments of aperture groups in accordance with the invention.

[0045] Figure 1 illustrates the fact that the frequency band for θ₁ varying from 150° to 180° corresponds to approximately one octave (i.e. from about 7.9 f_start to 15.71 f_start if √ε_r = 1.136).

[0046] The width of the band where θ₁ varies from 150° to 180° depends on √ε_r. For the lowest √ε_r value, i.e. ≅ 1.1, the band is slightly larger than one octave; the ratio of limits of this band ≅ 2.3. In the description of the invention, we shall assume that this band corresponds to one octave, even if it is actually slightly larger when √ε_r ≅ 1.1.

[0047] It has been discovered that, at the low end of the above mentioned octave, (i.e. for θ₁ ≅ 150°), the coupling loss is 6 dB lower than for a prior art coaxial radiating cable designed to have θ₁ ≅ 90° and exhibiting the same longitudinal attenuation. The coupling loss continues to decrease when θ₁ increases and the gain corresponds to 10 dB with θ₁ ≅ 161°; the lowest coupling loss is obtained when θ₁ is between 170° and 180°.

[0048] Furthermore, it has been found that, for θ₁ = 150°, the field strength variations are typically less than 3 dB peak to peak when the receiving antenna is orientated for maximum response.

[0049] Designing a radiating cable which works with θ₁ in the interval between about 150° and 180° requires an excellent secondary mode cancellation or attenuation up to the frequency ≅ 15.71 f_start with √ε_r = 1.136 and up to ≅ 21 f_start with √ε_r = 1.1.

[0050] A radiating cable according to the present invention can also be used at lower frequencies (which corresponds to θ₁ < 150°) but the performances are slightly impaired (higher coupling loss and larger field strength variations compared to what is obtained with θ₁ = 150°). Consequently, a wide band coaxial radiating cable according to the present invention provides a larger frequency band than wide band prior art cables.

[0051] Figure 3 shows one of the preferred embodiments of the present invention. It includes groups of n slots (with n is larger than 10 and preferably equal to or larger than 14) reproduced at a constant spacing s measured between the left end of two successive slot groups. The distance between the axis of two successive slots within a group is equal to s/2n ± Δ (where Δ represents about 20% of s/2n) as shown in figure 4. It results that the distance between the left end of the first slot and the left end of the last slot within a group is equal to (n-1)s/2n. The group of slots is followed by a section without any slot, the length of which is equal to (n+1)s/2n if measured between the left end of the last slot of a group and the left end of the first slot of the next group.

[0052] The spacing s must be chosen in order that θ₁ ≅ 150° at the bottom of the octave in which the performances must be optimised; this octave is delimited by the frequencies (in MHz) f_opt and 2 f_opt which correspond respectively to the wavelengths (in the air) λ_opt and λ_opt/2.
λ _opt is linked to f_opt by the expression

[0053] The condition θ₁ ≅ 150° at frequency f_opt can be written, if we consider expression (1)

[0054] As cos 150° = - 0,866 and for √ε_r =1.136, we obtain the following condition :

[0055] In principle, if √ε_r is different from 1.136, the condition (9) should be recalculated. In practice however, such a difference has only a small impact; indeed, choosing s ≅ 3.7 λ_opt with √ε_r ≅ 1.1 gives rise to θ₁ ≅ 146° which is at less than 3% of the target value.

[0056] There is a second condition which imposes that θ₁ = 180° at the top of the frequency band in which performances optimisation is required, i.e. for λ = λ_opt/2. From figure 1, it is obvious that this condition is always satisfied if s is chosen according to the expression (9).

[0057] A coaxial radiating cable, according to the present invention, with a spacing s given by the expression (9) provides a low coupling loss and small field strength variations in the octave between λ_opt and λ_opt/2.

[0058] If the optimisation is required on a frequency band which is less than one octave, for example between the wavelengths λ_opt1 and λ_opt2 (with λ_opt2 > λ_opt1/2), the condition (9) becomes

[0059] The second condition which imposes that θ₁ =180° will be satisfied if

[0060] As cos 180° = - 1, we obtain for √ε_r =1.136

[0061] If √ε_r ≅ 1.1, this condition is :

[0062] For √ε_r = 1.136, the spacing s is chosen within the interval

for √ε_r ≅ 1.1, the spacing s is chosen within the interval

[0063] As these intervals are large, s is chosen to avoid having resonant frequencies in the frequency bands of interest.

[0064] As a first example, we consider a radiating cable optimised for the frequency band allocated to the TETRA communication standards and to Private Mobile Radio (PMR) systems. This frequency band extends from 380 to 470 MHz. The wavelengths in the air λ_opt1 and λ_opt2 are respectively equal to 79 and 64 cm. We shall assume that √ε_r = 1.136. To satisfy the conditions (10) and (12), the length of the pitch s is chosen within the interval [292 cm ; 467 cm] and to avoid having any resonant frequencies in the bands of interest.

[0065] For example, a spacing s = 350 cm involves that θ₁ varies from 155.6° to 162.6° in the frequency band from 380 to 470 MHz.

[0066] A radiating cable according to the present invention and with a spacing s chosen within the interval [292 cm ; 467 cm] works also, with lower performances, at frequencies outside the 380 to 470 MHz band and can be used as wide band cable. For example with s = 350 cm, the cable operates in radiated mode, with satisfactory performances, between about 40 and 600 MHz.

[0067] As a second example, we consider a radiating cable optimised for the transmission of the TDMA IS-54, CDMA IS 95 and GSM 900 mobile communication standards the frequency band of which extends from 824 to 960 MHz. The wavelengths in the air λ _opt1 and λ _opt2 are respectively equal to 36 and 31 cm. We shall assume that √ε_r = 1.136. To satisfy the conditions (10) and (12), the spacing s is chosen within the interval [135 cm ; 226 cm] and to avoid having resonant frequencies in the bands of interest.

[0068] A radiating cable according to the present invention and with a spacing s chosen within the interval [135 cm ; 226 cm] works also, with lower performances, at frequencies outside the 870 to 960 MHz band and can be used as a wide band cable. For example with s = 200 cm, the cable operates in radiated mode, with satisfactory performances, between about 70 and 1050 MHz.

[0069] As a third example, we shall consider a radiating cable optimised for the frequency band allocated to Wireless Local Area Network (WLAN) working above 5 GHz. The precise frequency band extends from 5150 to 5850 MHz. The wavelengths in the air λ _opt1 and λ _opt2 are respectively equal to about 6 and 5 cm. We shall assume that √ε_r = 1.136. To satisfy the conditions (10) and (12), the spacing s is chosen within the interval [22 cm ; 36 cm] and to avoid having resonant frequencies in the bands of interest.

[0070] For example, a spacing s equal to 32 cm involves that θ₁ varies from 162.6° to 167.5° in the frequency band from 5150 to 5850 MHz.

[0071] A radiating cable according to the present invention and with a spacing s chosen within the interval [22 cm ; 36 cm] works also, with lower performances, at frequencies outside the 5150 to 5850 MHz band and can be used as a wide band cable. For example with s = 32 cm, the cable would operate in radiated mode, with satisfactory performances, between about 440 and 6500 MHz.

[0072] The rectangular slots perpendicular to the cable axis as shown in figure 4 is one of the preferred embodiments. The distance between the axis of two successive slots must be equal to s/2n ± Δ. The length and the width of the slot are chosen to control the coupling loss.

[0073] Other embodiments allow to achieve the same effect. For example, the slot may be slanted with respect to the cable axis. The slot may also have rounded corners. The aperture may also have an elliptical or oval shape with the main axis either perpendicular, parallel or slanted with respect to the cable axis. The aperture may also be circular.

[0074] The single aperture may also be replaced by a plurality of smaller identical apertures located along the same circumference as illustrated in figure 5. In this particular embodiment, the distance between two successive circumferences must be equal to p/2n ± Δ.

[0075] The single aperture may also be replaced by a plurality of smaller identical apertures not necessarily located along the same circumference as illustrated in figure 6. In this particular embodiment, the distance between two successive pluralities of small apertures must be equal to s/2n ± Δ.

[0076] The single aperture may also be replaced by a plurality of different smaller apertures located along the same circumference as illustrated in figure 7. In this particular embodiment, the small apertures are not necessarily identical and the distance between two successive circumferences must be equal to s/2n ± Δ.

[0077] The single aperture may also be replaced by a plurality of smaller different apertures not necessarily located along the same circumference as illustrated in figure 8. In this particular embodiment, the distance between two successive pluralities of small apertures must be equal to s/2n ± Δ.

[0078] The single aperture may also be replaced by different pluralities of smaller apertures not necessarily located along the same circumference. In this particular embodiment, the different pluralities of smaller apertures must have approximately equivalent radiation properties and the distance between two successive pluralities of small apertures must be equal to

Claims

1. Radiating coaxial cable designed for radiating electromagnetic energy in radiated mode over a broad frequency band, comprising an outer conductor provided with a periodic pattern (M) of aperture groups repeated along the length of said outer conductor with a constant spacing (s) between successive groups of apertures, characterised in that said periodic pattern of apertures and/or said constant spacing (s) between successive groups of apertures are provided in such way that the radiating coaxial cable operates, for the highest frequency band the cable is designed for, at a radiating angle ⊖₁ , in accordance with the formula


wherein ε_r is the relative dielectric constant of the radiating cable and A is the wavelength corresponding to said frequency in the air,
essentially between 150° and 180° with respect to the axis of the radiating cable.

2. Radiating cable according to claim 1, characterised in that the periodic pattern of apertures in each group involves a number of apertures (n) of at least 10 and the distance between successive apertures is s / 2n ± 20%.

3. Radiating cable according to any one of claims 1 and 2, characterised in that the periodic pattern of apertures in each group involves a number of apertures (n) of at least 14 and the distance between successive apertures is s / 2n ± 20%.

4. Radiating cable according to any one of claims 2 and 3, characterised in that the distance between the left end of the first aperture and the left end of the last aperture in each group is (n - 1) s / 2n, whereas the length of the section without apertures, between the left end of the last aperture of one group and the left end of the first aperture of a next group is (n + 1) s / 2n.

5. Radiating cable according to any one of the preceding claims, characterised in that the dielectric constant of the radiating cable corresponds to a √ε_R value between 1.1 and 1.3.

6. Radiating cable according to any one of the preceding claims, having optimal performance for wavelengths between λ_opt.1 and λ_opt.2, characterised in that the spacing (s) between successive groups of apertures is selected so that

Drawing