Radiating cable and method

Abstract

A radiating cable 100 comprises an inner conductor 110, an outer conductor 130 comprising a plurality of apertures 135, and an insulator 120 between the inner and outer conductors. The outer conductor

Description

TECHNOLOGICAL FIELD Examples of the disclosure relate to a radiating cable. Some relate to a radiating cable for use in multiple-input multiple-output (MIMO) systems. BACKGROUND A radiating cable, which can also be known as a leaky feeder, is a type of antenna that is used in indoor environments such underground mines, tunnels, metal hulled ships. These locations may be difficult to cover using conventional antennae. In use, a radiating cable is run along the length of tunnels and the radiating cable can emit and receive radio waves, functioning as an extended antenna. The radiating cable has apertures in its outer conductor to allow the radio signals to leak into or out of the cable along its length. The orientation of the apertures determines the polarity of the signals emitted and received by the radiating cable. BRIEF SUMMARY According to various, but not necessarily all, examples there is provided a radiating cable comprising: an inner conductor; an outer conductor and an insulator. The outer conductor comprises a plurality of apertures. The insulator is between the inner conductor and the outer conductor. The outer conductor has a cross-sectional shape comprising at least one substantially straight side. The plurality of apertures are positioned at least partially in the at least one substantially straight side. In some but not necessarily all examples, an axis is defined from a start to an end of the at least one side and the at least one side has a straightness variation ratio defined as the ratio of: the maximum distance between the at least one side and the axis in a direction perpendicular to the axis, and a minimum wavelength of a signal emitted or received by the radiating cable. The straightness variation ratio may be less than 1:100. The at least one side may have a change in angle of orientation along a length of the at least one side of less than 20°. The outer conductor may have a cross-sectional shape comprising at least two substantially straight sides. The at least two substantially straight sides may be substantially parallel. The outer conductor may have a symmetrical cross-sectional shape. The outer conductor may have a rotationally symmetrical cross-sectional shape. The outer conductor may have a cross-sectional shape comprising a rectangular portion with partial ellipse portions connected with the rectangular portion. The outer conductor may have a stadium cross-sectional shape. The outer conductor may have a substantially rectangular, triangular or other polygonal cross-sectional shape. The cross-sectional shape of the inner conductor may be non-circular. The cross-sectional shape of the inner conductor may be substantially similar to the cross-sectional shape of the outer conductor. The plurality of apertures may be located periodically along a length of the radiating cable. The apertures may be slots. The apertures may be substantially aligned and may be parallel to each other. According to various, but not necessarily all, examples there is provided a system comprising: a first radiating cable; and a second radiating cable. The second radiating cable comprises an inner conductor; an outer conductor comprising a plurality of apertures; and an insulator between the inner conductor and the outer conductor. The first radiating cable may be configured to receive and emit signals of a first polarity. The second radiating cable may be configured to receive and emit signals of a second polarity different to the first polarity. The signals of a first polarity may be vertically polarized signals, and the signals of a second polarity may be horizontally polarized signals. The apertures of the outer conductor of the first radiating cable may be oblique to an axial direction of the first radiating cable. The apertures of the outer conductor of the second radiating cable may be perpendicular to an axial direction of the second radiating cable. According to various, but not necessarily all, examples there is provided a method of manufacturing a radiating cable. The method comprises providing an inner conductor, providing an outer conductor comprising a plurality of apertures; and providing an insulator between the inner conductor and the outer conductor. The outer conductor has a cross-sectional shape comprising at least one substantially straight side. The plurality of apertures are positioned at least partially in the at least one substantially straight side. According to various, but not necessarily all, embodiments there is provided an apparatus comprising means for performing at least part of one or more methods described herein. The description of a function and / or action should additionally be considered to also disclose any means suitable for performing that function and / or action. Functions and / or actions described herein can be performed in any suitable way using any suitable method. According to various, but not necessarily all, embodiments there is provided examples as claimed in the appended claims. While the above examples of the disclosure and optional features are described separately, it is to be understood that their provision in all possible combinations and permutations is contained within the disclosure. It is to be understood that various examples of the disclosure can comprise any or all the features described in respect of other examples of the disclosure, and vice versa. Also, it is to be appreciated that any one or more or all the features, in any combination, may be implemented by / comprised in / performable by an apparatus, a method, and / or computer program instructions as desired, and as appropriate. The description of a function should additionally be considered to also disclose any means suitable for performing that function BRIEF DESCRIPTION Some examples will now be described with reference to the accompanying drawings in which: FIG. 1 shows an example of the subject matter described herein; FIG. 2 shows another example of the subject matter described herein; FIG. 3 to 3c shows further examples of the subject matter described herein; FIG. 4a and 4b shows further examples of the subject matter described herein; FIG. 5 shows another example of the subject matter described herein; FIG. 6 shows another example of the subject matter described herein; and FIG. 7 shows another example of the subject matter described herein. The figures are not necessarily to scale. Certain features and views of the figures can be shown schematically or exaggerated in scale in the interest of clarity and conciseness. For example, the dimensions of some elements in the figures can be exaggerated relative to other elements to aid explication. Similar reference numerals are used in the figures to designate similar features. For clarity, all reference numerals are not necessarily displayed in all figures. DETAILED DESCRIPTION The Figures illustrate examples of a radiating cable 100. The radiating cable 100 comprises an inner conductor 110; an outer conductor 130, and an insulator 120 positioned between the inner conductor 110 and the outer conductor 130. The outer conductor 130 comprises a plurality of apertures 135. The outer conductor 130 has a cross-sectional shape comprising at least one substantially straight side 131. The plurality of apertures 135 are positioned at least partially in the at least one substantially straight side 131. If the side of a radiating cable 100 is not straight then the apertures 135 positioned within the side will also be non-straight. This means that signals emitted or received through the apertures 135 are less strongly polarized and there are more mutually coupled polarized radiation components than if the apertures 135 were straight. An outer conductor 130 with a cross-sectional shape comprising at least one substantially straight side 131 therefore produces the technical effect of increasing the cross-polar discrimination (XPD) of the radiating cable 100. This is useful for a number of applications, including for multiple-input and multiple-output (MIMO) systems. FIG. 1 shows a first example of a radiating cable 100 according to examples of the disclosure. The radiating cable 100 is for radiating electromagnetic energy and functions as an antenna. The radiating cable 100 is configured to emit and receive radio frequency signals. In the illustrated example, the radiating cable 100 is a coaxial cable. The inner conductor 110, insulator 120 and outer conductor 130 are concentric and the radiating cable 100 is a transmission line. A radiating cable 100 may also be called a leaky feeder 100, or a leaky coaxial cable 100. The inner conductor 110 forms the core of the cable 100. It may be formed of a metal such as copper or steel. The insulator 120 surrounds the inner conductor 110 and may be a dielectric insulator. The insulator 120 may be arranged radially outside of the inner conductor 110. The insulator 120 provides for electric isolation between the inner conductor 110 and the outer conductor 130. The outer conductor 130 surrounds the insulator 120, and may be formed of a metal such as copper or steel. The outer conductor 130 may be arranged radially outside of the insulator 120. In some examples the outer conductor 130 is formed of a woven metallic braid. In some examples the outer conductor 130 is corrugated to improve flexibility. The corrugations may occur at different positions along the outer conductor 130 than the substantially flat / straight regions 131 where the apertures 135 are positioned. In some examples the radiating cable 100 further comprises an insulating jacket (not shown) which surrounds the outer conductor 130. The insulating jacket may be arranged radially outside of the outer conductor 130. The outer conductor 130 forms a shield for signals. In most coaxial cables, the outer conductor 130 does not have apertures 135 and so the outer conductor 130 would restrict a signal's electric and magnetic fields to the insulator 120 dielectric, with little leakage of signal occurring. However, in radiating cables 100 the outer conductor 130 comprises a plurality of apertures 135 and so signals can be emitted and received by the radiating cable 100. In the illustrated example, the apertures 135 are through holes which pass through the outer conductor 130 but not the insulator 120. In this example the apertures 135 are slots, are elongate, are substantially aligned and are parallel to each other, and are located periodically along a length of the radiating cable 100. In some examples the apertures 135 are rectangular, in other examples they may have a different shape such as elliptical. The apertures 135 may serve as antenna apertures 135 which enable an efficient leakage or transmission of radiation from the inside of the radiating cable 100 to a surrounding volume and / or vice versa. The illustrated apertures 135 are orientated vertically, perpendicular to the length of the radiating cable 100 and perpendicular to the axial direction of the radiating cable 100. As such, the illustrated apertures 135 are for horizontally polarized signals. The outer conductor 130 has a cross-sectional shape comprising at least one substantially straight side 131. In the example of FIG. 1, the outer conductor 130 has a cross-sectional shape which is a portion / segment of a circle, and may be considered to be semi-circular. In other examples, the outer conductor 130 may have a different cross-sectional, as can be seen in FIGs 2, 3 and 4. The at least one substantially straight side 131 is substantially non-curved and substantially non-kinked. The at least substantially straight one side 131 forms a substantially planar side of the outer conductor 130. In this document the at least substantially straight one side 131 may be referred to as the at least one side 131 for brevity. The at least one side 131 is substantially parallel to an axis from the start to the end of the at least one side 131. The start and end of the at least one side 131 being where the at least one side 131 forms a vertex with another side of the outer conductor 130. In some examples, an axis is defined from a start to an end of the at least one side 131, and the at least one side 131 has a straightness deviation ratio defined as the ratio of: the maximum distance between the at least one side 131 and the axis in a direction perpendicular to the axis, and the length of the axis. In some examples, the straightness deviation ratio is less than 1:10, such as less than 1:20, less than 1:50, less than 1:100, or less than 1:200. In some examples, the at least one side 131 has a straightness variation ratio defined as the ratio of: the maximum distance between the at least one side 131 and the axis in a direction perpendicular to the axis, and a minimum wavelength of a signal emitted or received by the radiating cable 100. In some examples, the straightness variation ratio is less than 1:10, such as less than: 1:30, less than 1:100, or less than 1:300. This ensures that the at least one side 131 is sufficiently straight compared to the length of a wavelength of the signal, which increases XPD. In some examples the minimum wavelength is a minimum desired wavelength for signals emitted and / or received. The minimum wavelength may be the minimum wavelength used for signals. The minimum wavelength may be the minimum wavelength which produces less than a predetermined loss value for the signal. The minimum wavelength may be related to the operational bandwidths of the radiating cable 100. A straightness value can be defined as the inverse of the straightness deviation ratio or the inverse of the straightness variation ratio. In some examples, the at least one side 131 has a change in angle of orientation along a length of the at least one side 131 of less than 20°, such as less than 10°, less than 5°, less than 2°, or less than 1°. In some examples, the change in angle is a change in orientation from a start of the at least one side 131 to an end of the at least one side 131. In some examples, the change in angle is between an orientation of the at least one side 131 at a first end of the at least one side 131, and an orientation of the at least one side 131 at a second end of the at least one side 131. The change in angle may be between a tangent to the at least one side 131 at a first end of the at least one side 131, and a tangent to the at least one side 131 at a second end of the at least one side 131. The illustrated inner conductor 110 has a cross-sectional shape which is non-circular. Specifically, the illustrated inner conductor 110 has a non-circular elliptical cross-sectional shape. In other examples the inner conductor 110 has a circular cross-sectional shape. In some examples the inner conductor 110 may have a different cross-sectional shape. Due to the cross-sectional shape of the outer conductor 130 not being circular, a non-circular cross-sectional shape can reduce the signal loss of the radiating cable 100. In some examples the cross-sectional shape of the inner conductor 110 is substantially similar to the cross-sectional shape of the outer conductor 130, whilst is others, such as the illustrated example, they are dissimilar. FIG. 2 shows a second example of a radiating cable 100 according to examples of the disclosure. The second example of a radiating cable 100 may comprise some or all of the features of the first example of a radiating cable 100. The second example of a radiating cable 100 is similar to the first example of a radiating cable 100 with some differences. In the example of FIG. 2, the outer conductor 130 has a cross-sectional shape comprising two substantially straight sides 131a, 131b. In the illustrated example, the at least two substantially straight sides 131,131b are substantially parallel and are substantially aligned. In the illustrated example, apertures 135 are only positioned on one of the straight sides 131a. However in other examples apertures 135 are positioned on both of the straight sides 131a, 131b. The apertures 135 are slanted and thus are oblique to the length of the radiating cable 100 and oblique to an axial direction of the radiating cable 100. As such, the illustrated apertures 135 are for vertically polarized signals. The illustrated apertures 135 alternate the direction of slanting. The outer conductor 130 of the radiating cable 100 may a number of different shapes. FIGs 3a to 3c show examples of cross-sectional shapes for radiating cables 100. The examples of radiating cables 100 of FIGs 3a to 3c may comprise some or all of the features of the examples of a radiating cable 100 of FIGs 1 and 2. FIG. 3a shows an example of a radiating cable 100 comprising an outer conductor 130 with a substantially rectangular cross-sectional shape. FIG. 3b shows an example of a radiating cable 100 comprising an outer conductor 130 with a substantially stadium cross-sectional shape. FIG. 3c shows an example of a radiating cable 100 comprising an outer conductor 130 with a cross-sectional shape comprising a rectangular portion with partial ellipse portions connected with the rectangular portion. The partial ellipse portions are located at opposite ends of the rectangular portion. In the example of FIG. 3c the portions are portions / segments of a non-circular ellipse, whilst in the example of FIG. 3b the portions are portions / segments of a circular ellipse. In the example of FIG. 2 the outer conductor 130 also has a cross-sectional shape comprising a rectangular portion with partial ellipse portions connected with the rectangular portion. In FIG. 2 the partial ellipse portions comprisea much smaller proportion of an ellipse than in FIGS 3b and 3c. In the illustrated examples the ellipse portions on opposite sides of the rectangular portion are substantially the same size and shape. In other examples they may different sizes and / or shapes. In the examples of FIGs 1, 2 and 3 the outer conductor 130 has a symmetrical cross-sectional shape. This leads to reduced signal loss, such as reduced longitudinal loss, when compared to other cross-sectional shapes with one substantially straight side 131 with the same cut-off frequency. This, therefore, leads to better performance. In some examples, the outer conductor 130 has a rotationally symmetrical cross-sectional shape. In some examples, the outer conductor 130 has a reflectionally symmetrical cross-sectional shape. In some examples, such as in FIGs 2 and 3, the outer conductor 130 has cross-sectional shape which is a reflectionally symmetrical across two perpendicular lines of symmetry. In some examples, the outer conductor 130 has a substantially triangular or other polygonal cross-sectional shape. In some examples, the outer conductor 130 has a substantially rounded other polygonal cross-sectional shape, such as a rounded rectangle. FIG 4a shows a sixth example of a radiating cable 100-1 according to examples of the disclosure. The sixth example of a radiating cable 100-1 may comprise some or all of the features of the first, second, third, fourth and fifth examples of a radiating cable 100 as shown in FIGs 1, 2 and 3. FIG 4b shows a seventh example of a radiating cable 100-2 according to examples of the disclosure. The seventh example of a radiating cable 100-2 may comprise some or all of the features of the first, second, third, fourth, fifth and sixth examples of a radiating cable 100. In each of FIGs 4a and 4b, on the left is shown a cross-sectional view of the respective radiating cable 100, and on the right is shown a side view of the respective radiating cable 100. In the side views a portion of the outer conductor 130 and a portion of the insulator 120 have been removed so that the insulator 120 and inner conductor 110 can be seen. In the example of FIG. 4a, the illustrated apertures 135 are orientated vertically, and are perpendicular to the length of the radiating cable 100-1 and perpendicular to the axial direction of the radiating cable 100-1. As such, the illustrated apertures 135 are for horizontally polarized signals. The illustrated apertures 135 are arranged in groups of two. In the example of FIG. 4b, the apertures 135 are slanted, and are oblique to the length of the radiating cable 100-2 and oblique to an axial direction of the radiating cable 100-2. As such, the illustrated apertures 135 are for vertically polarized signals. The illustrated apertures 135 are arranged in groups of four. In some but not necessarily all examples, a system comprises a first radiating cable 100-1; and a second radiating cable 100-2. One example of such as system would comprise the radiating cables 100 of each of FIGs 4a and 4b. Both of the radiating cables 100 can be run along the length of a tunnel, and may be substantially parallel to each other. In the example of FIG. 4 both of the radiating cables 100 have an outer conductor 130 of with a cross-sectional shape comprising at least one substantially straight side 131. In other examples, the outer conductor 130 of one of the radiating cables 100 has a cross-sectional shape comprising no substantially straight sides 131. In some examples, the first radiating cable 100-1 is configured to receive and emit signals of a first polarity, and the second radiating cable 100-2 is configured to receive and emit signals of a second polarity different to the first polarity. The second polarity may be orthogonal I perpendicular to the first polarity. For example, signals of a first polarity may be vertically polarized signals and signals of a second polarity may be horizontally polarized signals. A system comprising radiating cables 100 with different polarities is useful for multiple-input and multiple-output (MIMO) systems. A high cross-polar discrimination (XPD) of the radiating cables 100 is useful for MIMO in order to distinguish between the different signals. In the illustrated example, the apertures 135 of the outer conductor 130 of the first radiating cable 100-1 are oblique to an axial direction of the first radiating cable 100-1, and the apertures 135 of the outer conductor 130 of the second radiating cable 100-2 are perpendicular to an axial direction of the second radiating cable 100-2. In the example of FIG. 4, the first and second radiating cables 100 have substantially the same cross-sectional shape, whilst in other examples they may have different cross-sectional shapes. In the example of FIG. 4, the first and second radiating cables 100 have different cross-sectional sizes, whilst in other examples they may have substantially the same cross-sectional size. Radiating cables 100 are a wireless solution for communication systems in underground mines, rail and transit tunnels, subways, metal-hulled ships, high traffic large sport-stadiums, buildings with metal supporting structures and structures, which are otherwise difficult to cover by other antennas. Depending on the radiating elements I apertures 135 on the outer conductor 130 of a radiating cable 100, the radiating cable 100 can radiate vertical or horizontal polarized waves relative to the ground plane. Examples of the disclosure improve the decoupling between these two polarized propagations by radiating cables 100, and so such a radiating cable 100 is suitable for use in multiple antenna systems like multiple-input multiple-output (MIMO) systems. With this, a suitable MIMO system can be implemented, for example in a tunnel. One solution for MIMO is using decoupled cross-polarized propagation simultaneously. Therefore, a radiating cable 100 with high cross-polar discrimination (XPD) is useful. Some, but not necessarily all example radiating cables 100 include a stadium shaped cross-section, which provides a good decoupling between the two cross-polarizations (high XPD). Such radiating cables 100 provides a flatter area for the slots / apertures 135 compared to a bent I curved area which is provided by a circular coaxial cable. In some examples, the XPD is increased, while the characteristic impedance of the radiating cable 100 is at or close to 50 fl. The cross-sectional shape may be selected such that the longitudinal loss of the radiating cable 100 is slightly increased whilst its XPD is significantly increased compared to a circular coaxial cable. The cross-sectional shape may be selected such that the longitudinal loss of the radiating cable 100 is reduced, and potentially minimized, compared to other cross-sectional shapes which have at least one substantially straight side 131. For a radiating cable 100 with a circular cross-sectional shape coaxial, the bent / curved slots I apertures 135 on the outer conductor 130 provide more mutually coupled polarized radiation components than the straight slots I apertures 135 produced by radiating cables 100 with at least one substantially straight side 131. Thus, the use of physically straight apertures 135 instead of curved apertures 135 increases the XPD of a radiating cable 100. Physically straight apertures 135 can be formed if the outer conductor 130 has a cross-sectional shape with a substantially straight side 131. An example of such a shape would be rectangular as seen in Fig 3a. However, radiating cables 100 having cross-sectional shapes with elliptical portions, as seen in Figs 1,2, 3b, 3c, 4a and 4b, have more mechanical flexibility than for rectangular cross sectional shapes, and can also be easier to manufacture. A slot I aperture 135 on the outer conductor 130 of a cable 100 can disturb the electric field and let power leak from the cable 100. Periodically using this phenomenon (such as with periodic slots 135 etched on the outer conductor 130), leads to a radiating cable 100, which radiates based on the concept of periodic leaky wave antennas. In some examples, such as in FIG. 2, alternating mirrored slots 135 may be used. Depending on the aperture 135 shape the radiation could be vertically or horizontally polarized. Radiating cables 100 with a circular cross-section have slots 135 on the outer conductor 130 will be bent and therefore both polarizations will be emitted simultaneously. The decoupling of these two polarizations is a relevant factor for the use of a radiating cable 100 in a MIMO system. In examples of the disclosure, a radiating cable 100 provides a flat area for etching the slots 135, which increases the decoupling between the two polarizations since the bending effect is eliminated. In some examples, a cross-sectional shape comprising a rectangular portion with partial ellipse portions connected with the rectangular portion could be parameterized by six parameters: ‘a’, ’b’, and ‘d’ for the inner conductor 110 and outer conductor 130, where d is the length of the straight sides 131 and a and b are the length of the two semi-axis of the ellipse. Transmission line properties (characteristic impedance, longitudinal loss, and the cut-off frequency) of radiating cables 100 with such a shape were calculated using a 2D-finite difference method and the six parameters were sweep to produce data for a variety of shapes. After that, the cases with 50 ohms characteristic impedance + / - 3 ohms were chosen, and the case with a similar cut-off frequency (compared to a circular coaxial cable) and with the highest ‘d’ was chosen as the optimal case. For example, FIG. 5 shows a proposed cross-section for a cut-off frequency equal to 4800 MHz. FIG. 6 shows a graph in which the top curve depicts the XPD of the Stadium-shaped radiating cable 100 of FIG. 5 and the bottom curve depicts the XPD of the circular coaxial radiating cable of FIG. 5. The radiating cables 100 for both curves comprise the vertically polarized slot configuration depicted in Fig. 4b. FIG. 6 shows that the XPD increases 5.7 dB by using this proposed cross-sectional shape instead of a circular coaxial radiating cable. This occurs even though the apertures 135 in both models are identical other than being bent or straight. This shows a benefit of radiating cables 100 with at least one substantially straight side. A radiating cable 100 with the selected shape can be fabricated. This may involve a stadium-shaped tool being fabricated. For example, 3D-printing may be used. After which the radiating cable 100 can be metalized. FIG. 7 illustrates a method 700 of manufacturing a radiating cable 100. At block 702, the method comprises providing an inner conductor 110. At block 704, the method 700 comprises providing an outer conductor 130 comprising a plurality of apertures 135. At block 706, the method 700 comprises providing an insulator 120 between the inner conductor 110 and the outer conductor 130. The outer conductor 130 has a cross-sectional shape comprising at least one substantially straight side 131. The plurality of apertures 135 are positioned at least partially in the at least one substantially straight side 131. The blocks illustrated in the accompanying Figs may represent steps in a method and / or sections of code in a computer program. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some blocks to be omitted. Where a structural feature has been described, it may be replaced by means for performing one or more of the functions of the structural feature whether that function or those functions are explicitly or implicitly described. An operational radiating mode (operational bandwidth) is a frequency range over which an antenna (such as a radiating cable 100) can efficiently operate. An operational radiating mode (operational bandwidth) may be defined as where the return loss |S111 (S parameter) of the antenna is less than an operational threshold T such as, for example, 3 or 4 dB and where the a radiated efficiency (er) is greater than an operational threshold such as for example -3dB in an efficiency plot. A radiating cable 100 is a non-resonant antenna, and may be called a travelling wave antenna. A radiating cable 100 functions as a periodic leaky wave antenna. Radiation efficiency is the ratio of the power delivered to the radiation resistance of the antenna (Rrad) to the total power delivered to the antenna: er = (Rrad) / (RL + Rrad), where RL = loss resistance (which covers dissipative losses in the antenna itself). It should be understood that “radiation efficiency” does not include power lost due to poor VSWR (mismatch losses in the matching network which is not part of the antenna as such, but an additional circuit). The “total radiation efficiency” comprises the “radiation efficiency” and power lost due to poor VSWR in dB. The efficiency operational threshold could alternatively be expressed in relation to “total radiation efficiency” rather than “radiation efficiency”. The radiating cable 100 and any radio frequency circuitry may be configured to operate in a plurality of operational frequency bands. The radiating cable 100 is configured to transmit and / or receive electromagnetic signals in a plurality of operational frequency bands. For example, the operational frequency bands may include (but are not limited to) Long Term Evolution (LTE) (US) (734 to 746 MHz and 869 to 894 MHz), Long Term Evolution (LTE) (rest of the world) (791 to 821 MHz and 925 to 960 MHz), amplitude modulation (AM) radio (0.535-1.705 MHz); frequency modulation (FM) radio (76-108 MHz); Bluetooth (2400-2483.5 MHz); wireless local area network (WLAN) (2400-2483.5 MHz); hiper local area network (HiperLAN) (5150-5850 MHz); global positioning system (GPS) (1570.42-1580.42 MHz); US - Global system for mobile communications (US-GSM) 850 (824-894 MHz) and 1900 (1850 - 1990 MHz); European global system for mobile communications (EGSM) 900 (880-960 MHz) and 1800 (1710 - 1880 MHz); European wideband code division multiple access (EU-WCDMA) 900 (880-960 MHz); personal communications network (PCN / DCS) 1800 (1710-1880 MHz); US wideband code division multiple access (US-WCDMA) 1700 (transmit: 1710 to 1755 MHz , receive: 2110 to 2155 MHz) and 1900 (1850-1990 MHz); wideband code division multiple access (WCDMA) 2100 (transmit: 1920-1980 MHz, receive: 2110-2180 MHz); personal communications service (PCS) 1900 (1850-1990 MHz); time division synchronous code division multiple access (TD-SCDMA) (1900 MHz to 1920 MHz, 2010 MHz to 2025 MHz), ultra wideband (UWB) Lower (3100-4900 MHz); UWB Upper (6000-10600 MHz); digital video broadcasting - handheld (DVB-H) (470-702 MHz); DVB-H US (1670-1675 MHz); digital radio mondiale (DRM) (0.15-30 MHz); worldwide interoperability for microwave access (WiMax) (2300-2400 MHz, 2305-2360 MHz, 2496-2690 MHz, 3300-3400 MHz, 3400-3800 MHz, 5250-5875 MHz); digital audio broadcasting (DAB) (174.928-239.2 MHz, 1452.96- 1490.62 MHz); radio frequency identification low frequency (RFID LF) (0.125-0.134 MHz); radio frequency identification high frequency (RFID HF) (13.56-13.56 MHz); radio frequency identification ultra high frequency (RFID UHF) (433 MHz, 865-956 MHz, 2450 MHz), frequency allocations for 5G may include, for example, 700MHz, 410 MHz - 7125 MHz (FR1), 24250 MHz - 52600 MHz (FR2), 3.6-3.8GHz, 24.25-27.5GHz, 31,8-33.4GHz, 37.45-43.5, 66-71GHz, mmWave, and >24GHz). The above-described examples find application as enabling components of: automotive systems; telecommunication systems; electronic systems including consumer electronic products; distributed computing systems; media systems for generating or rendering media content including audio, visual and audio visual content and mixed, mediated, virtual and / or augmented reality; personal systems including personal health systems or personal fitness systems; navigation systems; user interfaces also known as human machine interfaces; networks including cellular, non-cellular, and optical networks; ad-hoc networks; the internet; the internet of things; virtualized networks; and related software and services. The term ‘comprise’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use ‘comprise’ with an exclusive meaning then it will be made clear in the context by referring to ‘comprising only one...’ or by using ‘consisting.’ In this description, the wording ‘connect’, ‘couple’ and ‘communication’ and their derivatives mean operationally connected / coupled / in communication. It should be appreciated that any number or combination of intervening components can exist (including no intervening components), i.e., to provide direct or indirect connection / coupling / communication. Any such intervening components can include hardware and / or software components. As used herein, the term "determine / determining" (and grammatical variants thereof) can include, not least: calculating, computing, processing, deriving, measuring, investigating, identifying, looking up (for example, looking up in a table, a database, or another data structure), ascertaining and the like. Also, "determining" can include receiving (for example, receiving information), accessing (for example, accessing data in a memory), obtaining and the like. Also, "determine / determining" can include resolving, selecting, choosing, establishing, and the like. In this description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term ‘example’ or ‘for example’ or ‘can’ or ‘may’ in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus ‘example’, ‘for example’, ‘can’, or ‘may’ refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example as part of a working combination but does not necessarily have to be used in that other example. As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two or more elements are joined by “and” or “or” mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements. Although examples have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the claims. Features described in the preceding description may be used in combinations other than the combinations explicitly described above. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. The description of a feature, such as an apparatus or a component of an apparatus, configured to perform a function, or for performing a function, should additionally be considered to also disclose a method of performing that function. For example, description of an apparatus configured to perform one or more actions, or for performing one or more actions, should additionally be considered to disclose a method of performing those one or more actions with or without the apparatus. Although features have been described with reference to certain examples, those features may also be present in other examples whether described or not. The term ‘a’, 'an’ or ‘the’ is used in this document with an inclusive notan exclusive meaning. That is any reference to X comprising a / an / the Y indicates that X may comprise only one Y or may comprise more than one Y unless the context clearly indicates the contrary. If it is intended to use ‘a’, ‘an’ or ‘the’ with an exclusive meaning then it will be made clear in the context. In some circumstances the use of ‘at least one’ or ‘one or more’ may be used to emphasis an inclusive meaning but the absence of these terms should not be taken to infer any exclusive meaning. The presence of a feature (or combination of features) in a claim is a reference to that feature or (combination of features) itself and to features that achieve substantially the same technical effect (equivalent features). The equivalent features include, for example, features that are variants and achieve substantially the same result in substantially the same way. The equivalent features include, for example, features that perform substantially the same function, in substantially the same way to achieve substantially the same result. In this description, reference has been made to various examples using adjectives or adjectival phrases to describe characteristics of the examples. Such a description of a characteristic in relation to an example indicates that the characteristic is present in some examples exactly as described and is present in other examples substantially as described. The above description describes some examples of the present disclosure however those of ordinary skill in the art will be aware of possible alternative structures and method features which offer equivalent functionality to the specific examples of such structures and features described herein above and which for the sake of brevity and clarity have been omitted from the above description. Nonetheless, the above description should be read as implicitly including reference to such alternative structures and method features which provide equivalent functionality unless such alternative structures or method features are explicitly excluded in the above description of the examples of the present disclosure. Whilst endeavoring in the foregoing specification to draw attention to those features believed to be of importance the Applicant may seek protection via the claims in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not emphasis has been placed thereon. l / we claim:

Claims

1. A radiating cable comprising:an inner conductor;an outer conductor comprising a plurality of apertures; andan insulator between the inner conductor and the outer conductor;wherein the outer conductor has a cross-sectional shape comprising at least one substantially straight side, and wherein the plurality of apertures are positioned at least partially in the at least one substantially straight side.

2. The radiating cable of claim 1, wherein an axis is defined from a start to an end of the at least one side;wherein the at least one side has a straightness variation ratio defined as the ratio of: the maximum distance between the at least one side and the axis in a direction perpendicular to the axis, and a minimum wavelength of a signal emitted or received by the radiating cable; andwherein the straightness variation ratio is less than 1:100.

3. The radiating cable of claim 1 or 2, wherein the at least one side has a change in angle of orientation along a length of the at least one side of less than 20°.

4. The radiating cable of claim 1, 2 or 3, wherein the outer conductor has a cross-sectional shape comprising at least two substantially straight sides.

5. The radiating cable of claim 4, wherein the at least two substantially straight sides are substantially parallel.

6. The radiating cable of any of the preceding claims, wherein the outer conductor has a symmetrical cross-sectional shape.

7. The radiating cable of claim 6, wherein the outer conductor has a rotationally symmetrical cross-sectional shape.

8. The radiating cable of claim 6 or 7, wherein the outer conductor has a cross-sectional shape comprising a rectangular portion with partial ellipse portions connected with the rectangular portion.

9. The radiating cable of claim 8, wherein the outer conductor has a stadium cross-sectional shape.

10. The radiating cable of any of claims 1 to 7, wherein the outer conductor has a substantially rectangular, triangular or other polygonal cross-sectional shape.

11. The radiating cable of any of the preceding claims, wherein the cross-sectional shape of the inner conductor is non-circular.

12. The radiating cable of claim 11, wherein the cross-sectional shape of the inner conductor is substantially similar to the cross-sectional shape of the outer conductor.

13. The radiating cable of any of the preceding claims, wherein the plurality of apertures are located periodically along a length of the radiating cable.

14. The radiating cable of any of the preceding claims, wherein the apertures are slots.

15. The radiating cable of any of the preceding claims, wherein the apertures are substantially aligned and are parallel to each other.

16. A system com pri si ng:a first radiating cable according to any of claims 1 to 15; anda second radiating cable comprising:an inner conductor;an outer conductor comprising a plurality of apertures; andan insulator between the inner conductor and the outer conductor.

17. The system of claim 16, wherein the second radiating cable is a radiating cable according to any of claims 1 to 15.

18. The system of claim 16 or 17, wherein the first radiating cable is configured to receive and emit signals of a first polarity, and the second radiating cable is configured to receive and emit signals of a second polarity different to the first polarity.

19. The system of claim 18, wherein the signals of a first polarity are vertically polarized signals, and the signals of a second polarity are horizontally polarized signals.

20. The system of any of claims 16 to 19, wherein the apertures of the outer conductor of the first radiating cable are oblique to an axial direction of the first radiating cable, and wherein the apertures of the outer conductor of the second radiating cable are perpendicular to an axial direction of the second radiating cable.

21. A method of manufacturing a radiating cable comprising:providing an inner conductor;providing an outer conductor comprising a plurality of apertures; andproviding an insulator between the inner conductor and the outer conductor;wherein the outer conductor has a cross-sectional shape comprising at least one substantially straight side, and wherein the plurality of apertures are positioned at least partially in the at least one substantially straight side.

22. The method of claim 21, wherein an axis is defined from a start to an end of the at least one side;wherein the at least one side has a straightness variation ratio defined as the ratio of: the maximum distance between the at least one side and the axis in a direction perpendicular to the axis, and a minimum wavelength of a signal emitted or received by the radiating cable; andwherein the straightness variation ratio is less than 1 / 100.

23. The method of claim 21 or 22, wherein the at least one side has a change in angle of orientation along a length of the at least one side of less than 20°.

24. The method of claim 21, 22 or 23, wherein the outer conductor has a symmetrical cross-sectional shape.

25. The method of any of claims 21 to 24, wherein the cross-sectional shape of the inner conductor is non-circular.

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