EP2409357A1

EP2409357A1 - Antenna integrated in a vehicle structure

Info

Publication number: EP2409357A1
Application number: EP09841975A
Authority: EP
Inventors: Bengt Svensson
Original assignee: Saab AB
Current assignee: Saab AB
Priority date: 2009-03-19
Filing date: 2009-03-19
Publication date: 2012-01-25
Anticipated expiration: 2029-03-19
Also published as: WO2010107349A1; EP2409357B1; EP2409357A4; ZA201107294B

Abstract

The invention provides an antenna arranged to operate in a frequency band with a bandwidth B and a centre frequency f _c. The antenna comprises a slot integrated in a vehicle structure. The slot has a first end and a second end, The antenna also comprises a feeding arrangement with a feed line and a first connection point at a first edge of the slot and a second connection point located at a position substantially across the slot from the first connection point at a second opposite edge of the slot. The slot is arranged to be shorted at the first end and open at the second end. The impedance matching is arranged by locating the feeding arrangement to a position along the slot where antenna impedance matches the impedance of an input/output of a transceiver unit.

Description

Antenna integrated in a vehicle structure

TECHNICAL FIELD

The present invention relates to the field of antennas integrated in a vehicle structure.

BACKGROUND ART

To locate an antenna on a vehicle such as a flying structure in the form of a missile, shell or aircraft body can sometimes be difficult. In order not to disturb the aerodynamic properties, the antenna must either be incorporated into the structure or integrated in possibly protruding parts of the structure. A special complication is that the bandwidth of the antenna is related to its size and volume, which means that an antenna position allowing as big volume as possible is desired.

An attractive location can therefore be to integrate the antenna in an existing fin, or that the antenna will be a separate part designed for minima! aerodynamic influence. A well known solution is to use a monopole antenna moulded into a dielectric material.

With a monopole antenna, the resonance frequency can be chosen by varying the length of the monopole. The impedance of the monopole is however decided by several parameters such as diameter and surrounding dielectric medium. It can therefore be necessary to match the antenna to desired impedance with external matching components.

US 6097343 describes an antenna system structurally integrated into a load bearing structural member of an aircraft, such as a wing, horizontal tail section, or vertical tail fin. The antenna system includes a flared notch of non- conductive material and an antenna feed. The antenna feed can take form of a coaxial cable to opposite sides of the antenna notch. This solution however has the drawback of requiring a long notch (a notch is often also called a slot) as it is incorporated into a relatively large portion of a tail or wing section of an aircraft in order to provide an omnidirectional radiation pattern from a single antenna. The prior art antenna also requires a special shape of the notch and separate matching components. The notch extends from a lower edge of the load bearing structural member to an upper edge. A notch is a transition from a slotline to free space which requires the notch to be relatively long, at least half a wavelength. To get broad band characteristics the notch also has to be widened towards one end of the slot. This means that the notch will occupy a relatively large area of the fin which will weaken the design of the fin.

There is thus a need to achieve a more compact and robust antenna solution, easily integrated in a vehicle structure, with variable impedance without the use of external matching components.

SUMMARY

The object of the invention is to reduce at least some of the mentioned deficiencies with prior art solutions and to provide:

♦ an antenna structure and

• a method

to solve the problem to achieve a more compact and robust antenna solution, easily integrated in a vehicle structure, with variable impedance without the use of external matching components.

The object is achieved by providing an antenna arranged to operate in a frequency band with a bandwidth B and a centre frequency f_c . The antenna comprises a slot integrated in a vehicle structure. The slot has a first end and a second end. The antenna also comprises a feeding arrangement with a feed line and a first connection point at a first edge of the slot and a second connection point located at a position substantially across the slot from the first connection point at a second opposite edge of the slot, wherein the slot is arranged to be shorted at the first end and open at the second end. The impedance matching is arranged by locating the feeding arrangement to a position along the slot where antenna impedance matches the impedance of an input/output of a transceiver unit.

The object is further achieved by providing a method for arranging an antenna to operation in a frequency band with a bandwidth B and a centre frequency /_c. The antenna comprises a slot integrated in a vehicle structure. The slot has a first end and a second end. The antenna also comprises a feeding arrangement with a feed line and a first connection point at a first edge of the slot and a second connection point located at a position substantially across the slot from the first connection point at a second opposite edge of the slot, wherein the slot is shorted at the first end and open at the second end. Impedance matching is arranged by locating the feeding arrangement to a position along the slot where antenna impedance matches the impedance of an input/output of a transceiver unit.

An additional advantage is achieved by providing a vehicle comprising the antenna according to any one of claims 1-13.

Further advantages are achieved by implementing one or several of the features of the dependent claims which will be explained below.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically shows one example of an antenna according to the invention integrated in a vehicle structure.

Figure 2 schematically shows a further example of an antenna according to the invention integrated in a vehicle structure. Figure 3a-3c schematically illustrates alternative feeding arrangements.

Figure 4a-4f schematically illustrates different examples of slot configurations.

DETAILED DESCRIPTION

The invention will now be described in detail with reference to the drawings.

Henceforth in the description the invention will be exemplified with the antenna integrated in a fin of a flying structure as e.g. a missile, an aircraft or a helicopter. The invention is exemplified with a fin extending in vertical direction and with a flying structure moving in horizontal direction. The invention can however be implemented in structures of other vehicles as e.g. vessels and ground-based vehicles. The fin can also extend in any other direction and does not have to be perpendicular to the moving direction. The antenna can be integrated in any part of the vehicle structure, preferably protruding from the vehicle structure, advantageously in parts as e.g. a fin, a wing or a canard. The antenna is arranged to operate in a frequency band with a bandwidth B and a centre frequency f_c. As the antenna solution is scalable it can operate within frequency bands from VHF (Very High Frequency) to GHz bands. A typical application is in the VHF/UHF (UHF=Ultra High Frequency) band covering frequencies from 30 MHz to 3 GHz and corresponding to wavelengths from 10 m to 0,1 m. The bandwidth is typically 10-20% of the centre frequency in the frequency band.

Figure 1 shows a side view of a fin 100 made in a conductive material of the vehicle structure with an antenna 101 comprising a slot 102 integrated in the vehicle structure and a feeding arrangement 105. The antenna is connected to an input to a receiver unit and an output from a transmitter unit, or generally speaking to an input/output of a transceiver unit through a standard arrangement, well known to the skilled person and therefore not further explained here. The slot has a first end 103 and a second end 104. The slot is short circuited, or shorted, at the first end and open at the second end. By the first end being shorted is meant that the first end is surrounded by part of the conductive material. The conductive material of the vehicle structure normally has ground potential. An open end is not connected to the conductive material and has an opening towards a non-conductive material as e.g. air or a dielectric material. The slot can be filled with a dielectric material with a relative dielectric constant ε_r having a value of typically 1-4, but also higher values can be used. By filling the slot with the dielectric material the stability of the vehicle structure and the aerodynamic properties of the structure can be less affected by the introduction of the slot. The length of the slot is also reduced as the wavelength is inversely proportiona! to the square root of the relative dielectric constant ε_r. When the slot is electrically shorted at one end, the length of the slot can be shortened to % , or

substantially %\ compared to prior art notch solution, as it will work as a

Planar Inverted F Antenna (PiFA), where λ is the wavelength in the material filling the slot and corresponding to the centre frequency /_c. This PIFA has a very short extension in the lateral direction of the slot. This lateral extension can, at a maximum, be equal to the thickness of the fin. An important feature of the invention is thus to incorporate a compact PIFA solution in a vehicle structure, like a fin, by integrating a slot in the vehicle structure. In a typical application with /_c =150 MHz and ε_r =2, the length of the slot will be around 0,4 m. The antenna also comprises the feeding arrangement 105 with a feed line and with a first and a second connection point, In this example the first connection point is an RF feeding point 107 at a first edge 109 of the slot and the second connection point is a grounding point 108 located at a position substantially across the slot from the first connection point at a second opposite edge 110 of the slot. When the feed line is an unbalanced feed line, e.g. a coaxial Sine, this type of feeding arrangement is called an asymmetric feeding. Impedance matching is arranged by locating the feeding arrangement to a position along the slot where antenna impedance matches the impedance of the input/output of the transceiver unit. This has the advantage that matching components will not be required. Optionally, matching components can be added and located e.g. to the feeding arrangement or to the transceiver electronics. A common location of the RF feed point is in the vicinity of the shorted end of the slot as this in many applications corresponds to a commonly used impedance of 50-100 ohms.

Another way to realize the antenna is to etch the slot from a metal-covered substrate. The substrate can e.g. be RO4350 having a thickness of 0,76 mm which is surrounded by a dielectric materia! forming the outer shape of a part of the vehicle structure such as e.g. the fin as will be explained in association with figure 3c.

In the example of the invention shown in figure 1 the width of the slot is constant. The width is typically 10-20 mm for VHF operation, in an alternative embodiment of the invention the slot can be widened towards the first and/or the second end of the slot, thus achieving a wider bandwidth B of the antenna. This is exemplified with dotted lines 106 in figure 1.

An alternative configuration of the slot when arranged in a conductive material of the vehicle structure is shown in figure 2, with a sharp bend between a first section 201 and a second section 202 of the slot. The slot 201/202 is thus integrated in the vehicle structure. Sn this way the extension of the slot in the direction of extension of the second section can be made shorter as the electrical length of the antenna corresponds to the sum of the electrical lengths of the two sections. The slot in this configuration is then also adapted to the contours of the fin. In this example the two sections are rectilinear but they can also be slightly curved to be adjusted to the contours of the fin. The width of the second section of the slot can also be widened towards the second end 204, illustrated by dotted lines 206. The feeding arrangement 205 is in this example arranged in the vicinity of the shorted first end 203 of the slot. An angle a of the bend is in the example of figure 2 around 90 degrees but can be any angle between 0 to 180 degrees. An edge 207 of the first section of the slot coincides with an edge of the vehicle structure in this example. The edge 207 however comprises a conductive string in contact with the conductive surfaces of the vehicle structure.

The feeding arrangement to the slot and the slot can be arranged in different ways e.g. as illustrated in figures 3a-3c. The feeding arrangement in this example comprises a coaxial cable 302 as the feed line and the first and the second connection points. Figure 3a shows the fin 300 with the slot 301 integrated in the vehicle structure and arranged in a conductive material of the fin and the coaxial cable 302 with an RF conductor 303 and a shield 304. The RF conductor is connected to the first connection point, in this example an RF feeding point 305, at the first edge 307 of the slot and the shield to the second connection point, in this example a grounding point 306, located at a position substantially across the slot from the first connection point at the second opposite edge 308 of the slot. This is an example of an asymmetric feeding. The coaxial cable is at the other end connected to the electric circuits of the transceiver unit mentioned in association with figure 1.

In figure 3b the coaxial cable of the feeding arrangement is exchanged with a symmetrical feed line 310 with two conductor lines, the first conductor line 313 connected to the first connection point, in this example an RF1 feeding point 311, and the second conductor line 314 connected to the second connection point, in this example an RF2 feeding point 312, the connection points being located at opposite edges of the slot. The second connection point is located substantially across the siot from the first connection point. The symmetrical feed line can e.g. be realized on a substrate. This is an example of a symmetric feeding. The symmetrical feeding line is at the other end connected to the electric circuits of the transceiver unit mentioned in association with figure 1. Figure 3c shows an alternative feeding arrangement comprising a microstrip line as the feed line and the first and the second connection points. In this example the fin 320 is manufactured in a dielectric material. The microstrip line comprises a first 326 and a second 325 conductor separated by a substrate 321. The substrate 321 is integrated in the fin e.g. by moulding. To show details of the substrate more clearly, part of the dielectric material of the fin has been removed around the substrate in figure 3c. The substrate has a first 322 and a second 323 surface. The second surface 323 comprises the second conductor 325 which is connected to ground and also working as a ground plane. The ground plane is metallised on the second surface by any conventional methods and the slot 324 is etched from the metallised surface, i.e. the slot is formed by removing metal from an area corresponding to the shape of the slot. The slot is thus integrated in a dielectric part of the vehicle structure. The first conductor 326 of the microstrip line is provided on the first surface 322 of the substrate by any conventional means. The first conductor 326 crosses the slot 324 at a substantially right angle and continuous on the other side of the slot in an end part 327 of the first conductor, the end part having an end 328. The other end of the microstrip line is connected to the electric circuits of the transceiver unit. The microstrip line thus ends in an open end as the end 328 of the first conductor is not connected to the second conductor and has an opening towards a dielectric material. The length of the end part 327, from the end 328 to the crossing of the slot, should correspond to approximately a quarter of a wavelength in the material of the microstrip line at the centre frequency f_c. This means that the length of the end part 327 is adjusted around a length of a quarter of a wavelength such that the open end transforms to a short between the first and the second conductor just after the microstrip line has crossed the slot. The short is thus in this example accomplished non-galvanica)ly with an electromagnetic coupling. This is electrically equivalent to an alternative where the first conductor of the microstrip line has its end 328 just after it has crossed the slot and with the end 328 galvanically coupled the shortest way to the ground plane to a first point 329 located at the first edge 331 of the slot. A second point 330 is defined as located substantially across the slot from the first point 329 at the opposite second edge 332 of the slot. In this example the first connection point thus consists of the first point 329 and the second connection point consists of the second point 330. The shape of the end part 327 can be adapted to the application. When there is limited space the end part can have one bend as shown in figure 3c. The end part can also be straight or have a meander configuration with one or several bends having different bend angles.

A further alternative is to have a slot integrated in a conductive vehicle structure as e.g. a fin and a feeding arrangement comprising a microstrip line with a substrate having the first conductor 326 on the first surface 322 of the substrate and the second conductor 325 as the conductive surface of the fin. in this alternative the second surface 323 of the substrate is blank, i.e. it has no conductive area, as a metallisation. The substrate can then be located in a groove of the fin such that the first surface is facing outwards and being flush with the outer surface of the fin. The first conductor is arranged to be located relative to the slot in the same way as described for the alternative according to figure 3c. The first surface of the substrate may be protected with a thin environmental protection layer. The short between the first and the second conductor and the first and the second connection points are arranged in the same way as described for the alternative according to figure 3c.

In summary, the feeding arrangement of figure 3c can then be realized as follows:

• a microstrip line with a first conductor 326 is arranged at a first surface 322 of a substrate 321 and a second conductor 325 is arranged at a second surface 323 of the substrate 321 ,

• the second surface 323 is metallised and connected to ground, the metallised part comprising the second conductor, • the slot 324 is etched on the second surface 323,

• the first conductor 326 crosses the slot 324 at a substantially right angle and

• a short is arranged between the first 326 and the second 325 conductor, through electromagnetic or galvanic coupling, just after the first conductor of the microstrip line has crossed the slot

thus creating the first connection point consisting of a first point 329 located at the first edge 331 of the slot, being at the same side of the slot as the end 328, and the second connection point consisting of a second point 330 is defined as located at a position substantially across the slot from the first connection point at the opposite second edge 332 of the slot.

The first conductor can also cross the slot at other angles, typically the angle can be around ± 30 degrees from a right angle crossing.

Impedance matching for the alternatives described in figure 3 is accomplished in the same way as described in association with figure 1.

Some further configuration alternatives are shown in figures 4a-4e where the shorted end 401 is not located at one of the edges of the fin but is located within the area of the fin. The open end 402 always ends, by definition, at one of the edges of the fin. Figure 4c shows an example how the slot widens towards the open end 402, figure 4d shows the bend angle a being close to 180 degrees and figure 4e exemplifies a configuration with a slightly curved slot. The direction of the slot can be arbitrary as mentioned earlier. Figure 4f shows a configuration with a horizontal slot. The configuration examples of figure 2 and figures 4a to 4f are ali examples of where the first end does not end at an edge of the vehicle structure but the first end is surrounded by a conductive area. The conductive area can be either part of the vehicle structure as shown in the examples of figures 4a-4f or part of the metallisation of the substrate in the configuration according to figure 3c.

The slot can be filled with a dielectric material in all examples of the invention described above.

The invention is not limited to the embodiments above, but may vary freely within the scope of the appended claims.

Claims

1. An antenna (101) arranged to operate in a frequency band with a bandwidth B and a centre frequency f_c and comprising a slot (102, 201/202, 301, 324) integrated in a vehicle structure, the slot having a first end (103, 203, 401) and a second end (104, 204, 402), the antenna also comprising a feeding arrangement (105, 205) with a feed line and a first connection point (107, 305, 311, 329) at a first edge (109, 307, 331) of the slot and a second connection point (108, 306, 312, 330) located at a position substantially across the slot from the first connection point at a second opposite edge (110, 308, 332) of the slot, cha racte rized in that the slot is arranged to be shorted at the first end and open at the second end and that impedance matching is arranged by locating the feeding arrangement to a position along the slot where antenna impedance matches the impedance of an input/output of a transceiver unit

2. An antenna according to claim 1, characterized in that the slot is arranged in a conductive material of the vehicle structure and filled with a dielectric material.

3. An antenna according to claim 2, ch aracterized in that the feeding is asymmetric and the feeding arrangement comprises a coaxial cable (302) as the feed line and the first and the second connection points, the first connection point being an RF-feeding point (107, 305) and the second connection point being a grounding point (108, 306).

4. An antenna according to claim 2, ch aracterized in that the feeding is symmetric and the feeding arrangement comprises a symmetric feeding line (310) as the feed line and the first and the second connection points, the first connection point being an RF1 feeding point (311) and the second connection point being an RF2 feeding point (312).

5. An antenna according to claim 1 , c h a r a c t e r i z e d in that the slot is arranged to be etched from a metal covered substrate, the substrate being surrounded by a dielectric material forming the outer shape of a part of the vehicle structure.

6. An antenna according to claim 5, c h a r a c t e r i z e d in that the feeding arrangement comprises a microstrip line as the feed fine and the first and the second connection point, the feeding being realized with:

• the microstrip line with a first conductor (326) arranged at a first surface (322) of a substrate (321) and a second conductor (325) arranged at a second surface (323) of the substrate (321),

• the second surface (323) being metallised and connected to ground, the metallised part comprising the second conductor,

• the slot (324) being etched on the second surface (323) • the first conductor (326) crossing the slot (324) at a substantially right angle

• a short arranged between the first (326) and the second (325) conductor, through electromagnetic or galvanic coupling, just after the first conductor of the microstrip line has crossed the slot

thus creating the first connection point consisting of a first point (329) located at the first edge (331) of the slot, being at the same side of the slot as the end (328), and the second connection point consisting of a second point (330) defined as located at a position substantially across the slot from the first connection point at the second opposite edge (332) of the slot.

7. An antenna according to claim 2, c h a r a c t e r i z e d in that the antenna comprises a feeding arrangement with a microstrip line comprising a substrate (321) having a first conductor (326) on a first surface (322) of the substrate and the conductive material of the vehicle structure as a second conductor (325) of the microstrip line.

8. An antenna according to any one of claims 1 - 7, ch a ra cte ri ze d in that the slot is widened towards at least one end of the slot.

9. An antenna according to any one of the claims 1 - 8, ch a ra cte ri zed in that the first end of the s!ot is surrounded by a conductive area.

10. An antenna according to any one of the claims 1 - 9, c h a ra cte ri zed in that the feeding arrangement is located in the vicinity of the first end.

11. An antenna according to any one of the claims 1 - 10, characte rized in that the electrical length of the slot from the first end to the second end is substantially %\ λ being the wavelength in the material filling the slot corresponding to the centre frequency f_c.

12. An antenna according to any one of the claims 1 - 11, chara cterized in that the slot has a bend and/or is curved.

13. An antenna according to any one of the claims 1 - 12, cha racterized in that the antenna is arranged to operate within a

VHF/UHF frequency band.

14. A vehicle structure equipped with antenna according to any one of the claims 1-13,

15. A method for arranging an antenna to operation in a frequency band with a bandwidth B and a centre frequency f_c and comprising a slot (102, 201/202, 301, 324) integrated in a vehicle structure, the slot having a first end (103, 203, 401) and a second end (104, 204, 402) the antenna also comprising a feeding arrangement (105, 205) with a feed line and a first connection point (107, 305, 311, 329) at a first edge (109, 307, 331) of the slot and a second connection point (108, 306, 312, 330) located at a position substantially across the slot from the first connection point at a second opposite edge (110, 308, 332) of the slot, characterized in that the slot is shorted at the first end and open at the second end and that impedance matching is arranged by locating the feeding arrangement to a position along the slot where antenna impedance matches the impedance of an input/output of a transceiver unit.