GB2407914A

GB2407914A - A Cable with low radar reflections

Info

Publication number: GB2407914A
Application number: GB0325841A
Authority: GB
Inventors: Adam Wasenczuk
Original assignee: ESL Defence Ltd
Current assignee: Textron Systems Electronic Systems UK Ltd
Priority date: 2003-11-05
Filing date: 2003-11-05
Publication date: 2005-05-11
Anticipated expiration: 2023-11-05
Also published as: GB0325841D0; GB2407914B

Abstract

A cable (2) comprising an insulated conducting core (4) and isolated reflecting coaxial cylindrical conductors (6) that are spaced apart along the core (4) and that cause the cable (2) to have low radar reflections whereby the cable (2) has reduced detectability by radar.

Description

24079 1 4

A CABLE WITH LOW RADAR REFLECTIONS

This invention relates to a cable with low radar reflections. The cable may be used for towing decoys or targets. Alternatively the cable may be used as a support cable for antennas, superstructures, masts and communication links.

Metallic cables are well known for use in towing active or passive decoys or targets behind ships or aircraft. Such cables may be of a co-axial construction for facilitating the transmission of radio frequency signals or data to the towed decoy or target. The cable is often of a substantial length and such a substantial length may give rise to disadvantageous characteristic reflections of electromagnetic signals which can then be readily detected by radar detection apparatus. Such detection of the cable may interfere with the desired characteristics of the towed decoy or target, or the detection may be used by hostile radars to detect friendly activities.

US-A-5294694 discloses the coating of a cable with an absorbing polyaniline composition with the purpose of completely or partially absorbing electromagnetic energy that might otherwise be reflected by the cable. EP- A468887 discloses the use of a composite electromagnetic wave absorbing material comprising a ferro/ferro magnetic granular material. Such an electromagnetic wave absorbing material may be used to coat structures as a conformal coating, or it may be used in the form of beads distributed along the length of a cable. JP-A-110111077 discloses the use of a cable within an insulating case coated by soft magnetic powder material. The solutions proposed in each of these three patents are such that the reflection of electromagnetic radiation is affected by the absorption of energy in a resistive medium applied in some form to the outer surface of the cable.

This may adversely affect one or more of the mechanical flexibility, size and resilience of the cable.

UK Patent Application No. 9917538.2 discloses the addition of conducting cylindrical sheaths to an insulated conducting cable in order to reduce the radar cross section. The sheaths are arranged in pairs that are interconnected with conductors to apparently form a set of dipoles or interfering reflectors. The present invention describes a simpler construction in which the conducting sheaths are not connected by conductors but remain separated from one another by a dielectric medium. An understanding of the phenomenon has shown that radar cross section reduction results largely from the individual action of the sheathed sections of the cable rather than the combined response of many sheaths.

It is an aim of the present invention to obviate or reduce the above mentioned problem.

Accordingly, in one non-limiting embodiment of the present invention there is provided a cable comprising an insulated conducting core and isolated reflecting coaxial cylindrical conductors that are spaced apart along the core and that cause the cable to have low radar reflections whereby the cable has reduced detectability by radar.

The cable of the present invention is able to be produced with desired mechanical flexibility, size and resilience, whilst being able to scatter incident electromagnetic energy, thereby reducing the energy retroreflected to a radar receiver. Thus the cable of the present invention may be reduced to required dimensions which are not compromised by the ability of the cable to have the low radar reflections. Thus the cable may also be produced to have a required flexibility, thereby facilitating winding as is usually required for storage of the cable. The cable may be produced without additional coatings of non-metallic materials or powders. The cable may be produced so that its required mechanical integrity and specification are maintained.

Preferably, the cable is one in which the conductors are spaced apart by equal amounts.

The cable may be one in which the conductors are formed by periodic segmentation of an outer continuous conducting sleeve of the cable.

The cable may be one in which the co-axial geometry of the core and the conductors is such that it enables transmission of data or control signals.

The cable may exhibit significant reduction of radar cross section at chosen electromagnetic wavelengths. The reduction of the radar cross section may be extended over a wide frequency range by variation of design parameters. The appropriate choice of geometry and material properties of the cable enables the cable to perform in specific applications relating to the reduction of orthogonal radar response.

The cable may be designed and fabricated for selectively absorbing, scattering or otherwise influencing incident electromagnetic radiation. The electromagnetic energy that might otherwise be reflected by the cable is significantly reduced in directions approximately normal to the length of the cable. The cable of the present invention may be an electromagnetic co- axial cable having an outer surface comprising a longitudinal periodic structure. A characteristic of such a structure will be selective frequencies at which significantly reduced retro-reflection occurs in directions substantially normal to the length of the cable, as mentioned above. As the component elements of the cable are not significantly resistive or lossy, any reduction in energy retro-reflected in the direction normal to the length of the cable must be redirected in other directions and thereby not detectable by an illuminating radar.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 shows a cable of the present invention; Figure 2 illustrates measured cable radar cross section reduction; Figure 3 illustrates the general form of cable radar cross section over a wide bandwidth; and Figure 4 illustrates radar cross section of relatively short and long cables.

Referring to Figure 1, there is shown a cable 2 comprising an insulated conducting core 4 and isolated reflecting coaxial conductors 6.

The conductors 6 are spaced apart along the core 4 by equal amounts as shown. The conductors 6 are such that they cause the cable 2 to have low radar reflections, whereby the cable 2 has a reduced detectability by radar.

The core is a metallic core 4.

The cable 2 may be deployed such that the cable 2 is of a substantial length, but with little radar cross section in directions approximately normal to the length of the cable 2 at selected radio frequencies.

The innermost surface of the outer conductor formed by the conductors 6 and the innermost surface of the inner conductor formed by the metal core 4 form a structure that is resonant at some characteristic electromagnetic frequency. Such a resonant structure, or tuned circuit, may contain energy in the form of transverse electromagnetic waves. The fundamental resonance occurs at electromagnetic frequencies whose half- wavelengths approximate to the electrical length of the resonant structures.

At, or close to, resonance, the electrical currents flowing between adjacent elements are minimised, thereby effectively isolating each successive element from its neighbouring element. The electromagnetic fields at the extremities of each element generate an associated external field through leakage or coupling. In use of the cable 2, the cable 2 may be illuminated by an electromagnetic wave at frequencies close to the defined resonance.

Surface currents induced by the electromagnetic wave couple energy into the resonant structure, until the fields due to stored energy are equal in phase and magnitude to those of the illuminating field. At this point of equilibrium of fields, no further energy is stored in the resonator and the electromagnetic wave front proceeds as though the cable structure were not present. Thus, after the period when equilibrium of the field is established, no retro-reflection of the wave front is apparent.

The cable 2 may, by design and manufacture of its elements, provide very low radar cross section at specific frequencies. The lengths of the conductors 6 and the gaps between the conductors 6 can be designed to reduce the required retro-reflection at selected frequencies. Multiple selected narrow bands of frequencies or broadband scattering may be achieved by variation of the lengths of the conductors 6 and the gaps between them.

The cable 2 may be a tow cable or a support cable.

The conductors 6 may be regarded as an outer conductor periodically distributed along the length of the cable 2. More specifically, the conductors 6 may be formed by applying a thin dielectric coating to the core 4. Metal patterns may be deposited as indicated in order to achieve desired electromagnetic effects. Conversely, an already co-axial structure may be periodically modified to obtain the desired pattern.

Figure 2 shows the results of experimental measurements made of orthogonal radar cross section of the cable 2 of Figure 1. Measured radar cross section values are reduced by some 20dB at an electromagnetic frequency close to 8Ghz.

Figure 3 illustrates radar signal response over an extended frequency range. The general form of the cable radar cross section as a function of frequency is shown in Figure 3. The "modes" identified can be identified as 1, 2L (a family of associated nulls), 2H and 3. Modes 1 and 3 always appear with a relationship in frequency of approximately a factor of three.

The modes 2L and 2H, referred to as second mode and third mode, are not present in air-spaced systems and their particular frequencies are related to the spacing of the outer conductors. These modes 2L and 2H are associated with surface waves on the cable and are thus recognised as socalled Wood's anomalies. Multiple modes of 2L may occur on short lengths of the cable 2 and are therefore ascribed to so-called end-effects. This observation relating to the multiple modes 2L is evident in comparison of the effect of long and short cables in Figure 4.

Figure 4 illustrates the absence of the 2L multi-moding on long periodic cables, supporting the understanding that the structure is generated by end-effects, being entirely absent on electrically short lengths.

Figures 1 - 4 illustrate how the present invention is able selectively to reduce the radar cross section of a cable at a selected frequency. The radar cross section response of the described long cable exhibits a narrow-band reduction at the desired frequency. The response may be broadened to extend further than an anticipated illuminating frequency range by a spread of longitudinal dimensions of the co-axial elements of the cable.

It is to be appreciated that the embodiment of the invention described above with reference to the accompanying drawings has been given by way of example only and that modifications may be effected. Thus, for example, the cable 2 may have any suitable and appropriate number of the conductors 6. The conductors 6 may be made of any suitable and appropriate material.

Claims

1. A cable comprising an insulated conducting core and isolated reflecting coaxial cylindrical conductors that are spaced apart along the core and that cause the cable to have low radar reflections whereby the cable has reduced detectability by radar.

2. A cable according to claim 1 in which the conductors are spaced apart by equal amounts.

3. A cable according to claim 1 or claim 2 in which the conductors are formed by periodic segmentation of an outer continuous conducting sleeve of the cable.

4. A cable according to any one of the preceding claims in which the coaxial geometry of the core and the conductors is such that it enables transmission of data or control signals.

5. A cable substantially as herein described with reference to the accompanying drawings.