Antenna system for a mobile carrier
The present invention relates to an antenna system for a mobile carrier. It applies notably to any system for transmitting and receiving wireless signals using airborne antennae and notably to those designed for spectral analysis and the location of signal transmitters. The applications may have narrow spectral band or wide spectral band.
Telecommunications applications operating on a wide frequency band, for example from 20 MHz to 3 GHz, usually require the use of several antennae or antenna networks. In addition, certain particular applications, notably the analysis of a frequency spectrum in a geographic zone, require installing the antennae or the antenna networks on a mobile carrier, for example on a carrier of the aircraft type.
In the case of airborne antenna systems, a known solution is to install the antennae directly on the fuselage of the carrier.
A first drawback of this solution is the exposure of the antennae to electromagnetic rays or interference echoes. Specifically, when the waves originating from a transmitter reach the carrier, a portion of their energy is reflected or diffracted either by the fuselage or by elements situated close to the antennae, creating interference electromagnetic waves which are mixed with the direct waves transmitted by the transmitter. In this situation, the signals captured by the antennae are difficult to use, notably in the case of receive antenna systems dedicated to the location of wireless transmissions, for which a phase of calibrating the antennae with the carrier becomes necessary. Similarly, for transmit antenna systems carrying out beam pointing or sweeping, the maintenance of the performance of such systems is also conditional upon a phase of calibration with the carrier.
A second drawback of this solution relates to the installation of an antenna network directly on the carrier. This requires as many attachment points and cable runs on the fuselage as there are antennae, which is not always acceptable or compatible with the shape of the carrier and may make the installation of the antennae difficult to carry out Moreover, once installation is complete, the latter is fixed (notably the type of antenna and the geometry of the network) and cannot change without adding additional
attachment points and cable runs in the fuselage of the carrier. This solution is neither open-ended nor modular.
A third drawback of this solution appears when an antenna network of the single-pole type must be installed on a carrier that has no conductive surface of sufficient dimensions. Specifically, in such a configuration, the antenna network can be unusable because of too many signals or interference echoes.
The aforementioned drawbacks are encountered usually on any type of mobile carrier whether it be terrestrial, naval or airborne.
One objective of the invention is to alleviate one or more of the aforementioned drawbacks by proposing an antenna system comprising at least the following elements:
o means for attachment to a mobile carrier and a support to which one or more antennae are attached,
o cables connecting the antenna(e) to a processing device,
o the support comprises at least two sides, a first side and a second side,
o the antenna(e) are attached to the second side formed of an electrically conductive material,
o the second side is masked, at least partly, from the carrier by the first side,
o the antennae are prominent.
The antennae are of the single-pole type. They may be monopole, monocone, of the blade type or of any other type of a single-pole nature. , Blade antennae are particularly well suited to the proposed invention because they are specifically designed for airborne applications.
The support must be designed so as to ensure optimal wireless operation of the antenna or antennae used. The shape and dimensions of the support are chosen mainly according to the type or types of antenna used, to the geometry of the antenna network(s), to the frequency band(s) processed, to the expected performance and to the installation requirements. Depending on the chosen application and vectorial processes used, the network geometry takes various shapes. According to one embodiment, the antennae may be placed on one or more circles, the circles being for example nested in one another, placed concentrically, or such that their centers are separated
in twos by a distance greater than the total of the radii of the two circles. Without departing from the context of the invention, any other geometry such as a linear, lacunar/ or arrow-like antenna network can be applied.
According to one embodiment, the means of attachment to a mobile carrier consist of a mast and an attachment head. The mast is connected to the attachment head which is adapted to be secured to a carrier. The antenna system according to the invention is mounted on the attachment head via the support.
A further objective of the invention is an airborne carrier carrying an antenna system according to the invention. The airborne carrier may be, for example, a helicopter, an aircraft, or a tethered balloon. One of the advantages of the signal transmissions/receptions made from airborne systems is the gain in power or in range obtained relative to ground transmissions/reception systems, the attenuation effects due to the terrain being avoided.
For certain applications, the shape of the support may be profiled and the antenna system may be enveloped by a radome, thereby shielding the antennae and improving the aerodynamic qualities of the antenna system.
Other features and advantages will appear on reading the following detailed and nonlimiting description given as an example with respect to the appended drawings which represent:
- figure 1, a bottom view of an embodiment of the antenna support according to the invention comprising two separate antenna networks,
- figure 2, a view in vertical section of an embodiment of the antenna support according to the invention comprising two separate antenna networks,
- figures 3a and 3b, two examples of a configuration of the antenna support according to the invention,
- figure 4, an illustration of an application of the invention.
Figures 1 and 2 show an embodiment of the antenna system according to the invention comprising a support provided with two separate antenna networks. Figure 1 is a bottom view of the support with a representation of the antenna attachment means, and figure 2 is a view in
vertical section in which antennae are attached to the support.
The antenna system 1 comprises a support 2 on which attachment elements 3 of antennae 4 are made on its side S2. The attachment elements 3 are, in the example, placed in two separate networks N1 and N2. The support 2 also comprises orifices 5 preferably placed at output connectors of the antennae 4. In the example of figure 2, the antennae 4 are of the "blade" type. Electric and/or microwave cables 6 are connected to the antennae 4. The antenna system 1 comprises an attachment system 7, consisting, in the example, of a mast 8 and of an attachment head 9 adapted to a carrier.
The support 2 is electrically conductive at least on its side S2. The choice of the material(s) of the support 2 depends notably on the environmental and weight constraints. It may be made of a light alloy, such as aluminum for example, or with a metalized composite material. The conductive material forming the side S2 of the support 2 forms notably an electric ground for the antennae 4 and ensures the electrical continuity between the antennae 4. The use of the support 2 makes it possible to partly or completely eliminate the interference radiations associated with the phenomena of reflection or diffraction on the carrier. Depending on the dimensions of the support 2 with respect to the carrier, the interference generated by the latter to the wireless signals is minimized to the extent of, in certain cases and for antenna systems dedicated to applications for locating wireless transmissions, making optional a phase of calibration of the antennae 4 with the carrier. In certain particular cases, the use of such a support also makes it possible to eliminate certain interference transmissions , or signals originating from the side S1 opposite to the side S2. The support 2 is rigid, most frequently with a flattened shape and preferably profiled in order to reduce the drag induced during the movement of the carrier. The size of the support 2 is mainly chosen according to the application frequency band and the expected performance, low working frequencies requiring large antenna networks. The support 2 may be hollow or solid. For example, the structure of the support 2 is a metal framework enveloped by plates of conductive material. If the support 2 is solid, it may nevertheless comprise one or more ducts suitable for running cables 6 inside. In one variant of the invention, the cables 6 run outside the support 2, thereby reducing the production constraints of the internal structure of the support 2. According to
another variant embodiment, the support 2 amounts to a simple plate of material to which the antennae 4 are attached. In this case, the cables 6 are ducted on the outside of the plate. The attachment elements 3 may consist of simple tapped holes or of any other means making it possible to attach an antenna to the support 2. Advantageously, the attachment elements 3 are adapted to the rapid removal of the antennae 4. The orifices 5 allow cables 6 to run through the support 2. If, for a given attachment element 3, no antenna is attached, it is possible to place a cover 10 closing off the orifice 5, thereby making it possible to preserve the electromagnetic properties of the support 2.
The function of the attachment system 7 is to secure the support 2 to a mobile carrier. Many forms of means of attachment to the carrier can be envisaged. In certain cases, notably when the support 2 is particularly large, and in order to gain stability, several points of attachment to the mobile carrier are provided. The cables 6 are most frequently connected to a system outside the support 2, which is why, in an embodiment illustrated in figures 2 and 3, the cables 6 run in the attachment mast 8 to emerge at the attachment head 9.
The number and arrangement of the antennae 4 on the support 1 are chosen notably according to the intended application and the expected performance. For example, for wide band receive antenna systems dedicated to wireless transmission location applications, it is possible to distribute five antennae evenly on a circle, which makes it possible to obtain 360° spatial coverage. This geometry may also be used for transmit antenna systems , carrying out beam pointing or sweeping. In order to cover a wider frequency spectrum, it is also possible to place several antenna networks 4 in circles, the circles of antennae 4 being, for example, separated from one another, or else nested, or even concentric. Without departing from the context of the invention, any other geometry such as a linear, lacunary or arrow-like antenna network can be applied.
Figures 3a and 3b show two examples of a configuration of the antenna system according to the invention. In figure 3a, a first antenna network N11 is placed on the support 2, separate from a second network N12. A distance 32 separates one end of the first antenna network N11 from
one end of the second antenna network N12. The greater this distance 32, the more the masking and coupling phenomena between these two networks are minimized. Conversely, this distance 32 is limited by the constraints of integration on the carrier. In the case of a carrier having small dimensions, a nested configuration, of which an example is shown in figure 3b, may be chosen to the detriment of a deterioration of the masking and coupling phenomena between the two antenna networks. The antennae of the first network N11 are distributed evenly on a circle with a diameter 31 corresponding to a use intended on a first frequency band B1. The second network N12 is configured in a circle with a diameter 33 greater than the diameter 31. The network N12 is therefore suitable for operating on a frequency band B2 beneath B1. The combination of the two antenna networks N11 and N12 therefore makes it possible to cover a wider frequency band than with a single network. Another similar configuration is illustrated in figure 3b, with the networks N21 and N22 placed in two concentric circles. This configuration applies notably to carriers with small dimensions.
To make the support modular, open-ended and compatible with several applications, it is possible to fashion as many attachment elements 3 as necessary for all of these applications. Therefore, when the support 2 changes use, certain antennae 4 are moved, withdrawn or added and covers 10 are applied to the attachment elements 3 that are not used. It is also possible to change antenna system 1 without having to modify or add additional attachment elements 3 in the fuselage of the mobile carrier while , changing the support 2 and while retaining the attachment system 7 on the carrier.
Figure 4 shows a possible application of the antenna system according to the invention. In this example, it involves an application of observing and locating wireless signals using a wide band airborne antenna system. A helicopter 40 carries a device 41 for processing signals and an antenna system 1 according to the invention comprising antennae 4 of the blade type. The antenna system 1 is attached so that the side S2 (figure 2) of the support 2 on which the antennae 4 are placed is shielded, at least partly, from the interference electromagnetic arrays diffused or diffracted by the carrier. In the example, the support 2 is attached beneath the fuselage, the
side S2 carrying the antennae 4 being masked, at least partly, from the mobile carrier 40 by the opposite side S1. The cables 6 are connected on the one hand to the antennae 4, and on the other hand to the processing device 41. Signals 42 originating from a ground transmitter 43 are captured by the antennae 4. Signals 42 are transmitted to the processing device 41 via the cables 6. Thanks to the presence of the support 2, the waves 44 reflected or diffracted by the helicopter 40 are mostly eliminated, thereby reducing the interference to the reception of the signals 42. The antenna system 1 may also be used for transmitting signals. In this case, the signals to be transmitted originate from the processing device 41 and are routed via the cables 6 to the antennae 4.
One advantage of the invention is to prevent considerable work for installing each of the antennae on the mobile carrier since, as only the support has to be attached thereto, only one operation is necessary for attaching an antenna system to a mobile carrier. The support provides great flexibility of use to the mobile carrier, since all the antennae are placed or removed at the same time when the support is attached to or removed from the carrier. Another advantage of the antenna support according to the invention is that it partly removes the correlation between the type of mobile carrier and the need for the antenna user. Specifically, once an appropriate arrangement of the antennae has been decided upon and put into effect on a support for the purpose of a particular application, this support may potentially be attached successively to several different carriers, provided however that said carriers are equipped with the appropriate attachment ( means.
The use of single-pole antennae makes it possible to reduce the height and therefore the space requirement of an antenna system, thereby making it easier to incorporate it onto any type of carrier. This is very important for example for the road clearance gage for land-based mobile carriers and for complying with the ground clearance of airborne mobile carriers. Using single-pole antennae requires the presence of a conductive support. The production of an antenna system according to the invention allows the use of single-pole antennae on any type of carrier whether or not it has a conductive surface by virtue of using a conductive support.

CLAIMS
1. An antenna system (1) characterized in that it comprises at least the
following elements:
o means (7) for attachment to a mobile carrier (40) and a
support (2) to which one or more antennae (4) are attached, o cables (6) connecting the antenna(e) (4) to a processing
device (41), o the support comprises at least two sides, a first side (S1) and
a second side (S2), o the antenna(e) (4) are attached to the second side (S2)
formed of an electrically conductive material, o the second side (S2) is masked, at least partly, from the
carrier (40) by the first side (S1), o the antennae (4) are prominent.
2. The antenna system as claimed in claim 1, characterized in that the shape of the support (2) is profiled.
3. The antenna system as claimed in one of the preceding claims, characterized in that the antenna(e) (4) are of the single-pole type.
4. The antenna system as claimed in claim 3, characterized in that the antenna(e) (4) are of the blade type.
5. The antenna system as claimed in one of the preceding claims, characterized in that the support (2) comprises one or more antenna networks (4).
6. The antenna system as claimed in claim 5, characterized in that the geometry of a network of antennae (4) is chosen from one of the following geometries:
o a circular geometry, o a linear geometry, o a lacunar/ geometry,
o an arrow-like geometry.
7. The antenna system as claimed in claim 6, characterized in that the
support (2) comprises at least two circular-geometry networks nested
in one another.
8. The antenna system as claimed in claim 6, characterized in that the
support (2) comprises at least two circular-geometry networks in
circles, the centers of the circles being separated in twos by a distance
greater than the total of the radii of the two circles.
9. The antenna system as claimed in one of the preceding claims,
characterized in that it is enveloped by a radome.
10. The antenna system as claimed in one of the preceding claims,
characterized in that the attachment means (7) consist of a mast (8)
and an attachment head (9).
11. The system as claimed in one of the preceding claims, characterized
in that the antenna or the antennae are transmit and/or receive
antennae.
12. Use of the system as claimed in one of the preceding claims in order
to carry out spectral and/or wireless transmission location analysis.
13. Helicopter carrying an antenna system as claimed in one of the
preceding claims.