Vehicle integrated antenna TECHNICAL FIELD The present invention relates to the field of low signature antennas integrated in a vehicle structure according to the preamble of claim 1. BACKGROUND ART There is a need today for creating a low radar signature for different objects such as e.g. aircrafts, i.e. to design aircrafts having a low radar visibility. Significant progress has been achieved in a number of problem areas as e.g.: • Intake/exhaust • Cockpit/canopy • Hull or fuselage shape • Absorbers • Armament but there is often a problem with reducing the passive signature of the aircraft sensors such as antennas. A number of solutions have been proposed for antennas with a low radar signature or a low Radar Cross Section, RCS. There are two main problems with existing solutions for creating low RCS with low frequency antenna arrays integrated in a vehicle structure such as a wing edge. Henceforth a vehicle structure is exemplified by a wing edge. Firstly the elements in the antenna array need to be fairly large in order to be resonant, leading to large separations between antenna elements in the array and many grating lobes at higher frequencies. Grating lobes appear in antenna arrays with a periodic repetition of antenna elements and when the distance between elements in the array is greater than a half wavelength. At a frequency of 1 GHz (Giga Hertz) this critical distance is 15 cm. Secondly the RCS of a straight cylindrical surface is proportional to the local radius of curvature of the surface and to the square of the length divided by the wavelength. Hence the RCS of a wing edge typically increases with frequency. For aero-dynamical reasons the radius of curvature needs to be fairly large with a high RCS as a result, especially at higher frequencies. In order to reduce the RCS of metallic structures, e.g. including antenna elements, they are coated with Radar Absorbing Materials (RAM). Radar Absorbing Materials are characterized by complex permittivity and permeability values that usually vary with frequency. For planar stratified media with several layers with different properties there is a reflection for each transition and an attenuation of the wave inside the media. Using nonmagnetic purely dielectric media, both the reflections and the attenuation is increased with increasing imaginary part of the dielectric constant, hence there is a trade-off between high attenuation, ensuring low reflection from the inner metallic interface and low reflection from the outer interface. If the reduction in RCS is desired in a narrow frequency band, the thickness of a RAM-layer can be chosen in such way, that the attenuated reflection from the metallic surface has the same magnitude but opposite phase compared to the primary reflection, thereby cancelling it out. For wider frequency bands, this is not possible to accomplish but both the primary reflection and the secondary attenuated reflection need to be low. By using several layers with small change in dielectric properties, the reflection from each interface can be maintained low, while the attenuation is gradually increased, thereby reducing the total required thickness compared with the case when using a single layer with low permittivity material. Another way to accomplish low reflection in the first interface is to use a material with magnetic properties as well. However, the frequency behaviour of the permeability must match the frequency behaviour of the permittivity, and the reflections will only be low at incidence angles close to normal if the permittivity and permeability values are high.Commercial RAM materials are generally designed to give a good RCS reduction performance in a wide frequency band and have a slow transition from low attenuation and high reflection at low frequencies to high attenuation and low reflection at high frequencies. When using this kind of material in the intended application, either the antenna losses will be unacceptably high or the RCS at medium range frequency will be too high. Investigations have shown that it is possible to reduce the RCS levels over a frequency band in a threat sector in elevation by optimization of the material parameters and preferably also the shape of the inner profile of a RAM coated wing edge. Figure 1 shows an antenna array 101 integrated in a wing 102 of an aircraft 103. The treat sector 104 defines an area within which threats like an enemy's radar can be present. The shape of the inner edge is variable and smooth and described by a small number of parameters, e.g. control points of NURBS (Non-Uniform Rational B-Spline), that should be optimized. The RCS value is dependent on the frequency, incident angle and has to be evaluated with computationally demanding CEM (Computational Electro Magnetic) software for each incident angle and frequency value. The RCS and the change of RCS can both be calculated from the electromagnetic field obtained by a CEM (Computational Electro Magnetic) simulation software.Hence there is a need to provide a method for manufacturing an antenna or antenna array and an antenna or antenna array with a low RCS value integrated in a structure having a large radius of curvature and at the same time accomplish a low attenuation of electromagnetic energy at low frequencies and a low reflection for incident waves at higher frequencies. SUMMARY OF THE INVENTION It is therefore the object of the invention to provide a method for manufacturing an antenna or antenna array, with an operating frequency band, comprising antenna elements integrated in a vehicle structure as well as an antenna or antenna array manufactured according to the method to solve the problem to achieve an antenna or antenna array with low RCS while at the same time accomplishing a low attenuation of electromagnetic energy at low frequencies and a low reflection for incident waves at higher frequencies. This object is achieved by a method wherein a RAM structure, conforming to the shape of the vehicle structure and comprising at least one layer of RAM material with an inner surface facing the antenna element and an outer surface being an outer surface of the vehicle structure, is mounted in front of the antenna elements, each RAM-layer denoted i being defined by a thickness dj and frequency dependent RAM properties: relative permittivity EJ , relative permeability Uj, the frequency dependency of the RAM properties being tailored and the thickness di and the number of RAM layers being selected such that the RAM structure is substantially transparent in the operating band, reaching a predetermined Farfield pattern requirement, and simultaneously is an effective absorber, reaching a predetermined Radar Cross Section (RCS) requirement RCSth, at frequencies in a threat band comprising frequencies above the operating frequency band of the antenna, and an RCS requirement RCSop in the operating frequency band. The object is also achieved by an antenna or antenna array manufactured according to the method. Normally the antenna or antenna array has a continuous operating frequency band, but the frequency band can also, within the scope of the invention, be divided in a number of bands, e.g. separate transmit and receive bands. In prior art only a single RAM-layer with constant permittivity and permeability and also only incidence in the plane transverse to the wing axis has been considered. Although the wave is scattered in a cone away from the transmitter from an infinite long cylindrical structure for other incidence angles, the finite extent of the wing will introduce side-lobes pointing in the direction of incidence. These side-lobes will be proportional to the specular reflection in the elevation plane, why this reflection has to be considered as well. This is illustrated in figure 2. Figure 2a shows the incident wave 201 with incident angle Oi, and reflected or specular wave 202 with angle Os. The RCS value 203 caused by the side lobes is plotted in figure 2b as a function of angle 0. A high RCS value at s gives an RCS value at Oi being proportional to the RCS at s. By reducing the RCS at s the RCS at the incident angle i.e. within the threat sector can be reduced. This suggests the use of low dielectric multilayer RAM instead, which means that each interface between the separate layers has to be parameterized as well as the frequency behaviour of the permeability. An advantage with the invention is that by tailoring the permittivity E in the RAM layers it will be possible to obtain a faster transition from low attenuation and high reflection at low frequencies to high attenuation and low reflection at high frequencies. This is illustrated in the diagram of figure 3. The horizontal axis shows the frequency and the vertical axis the reflection coefficient y. The antenna or array antenna has an operating bandwidth between frequencies f1 and f2 and at frequency f3, grating lobes are penetrating the threat sector. Those grating lobes are potentially dangerous and have to be reduced. Frequency f3 is the first grating lobe frequency which appears around the double f2 frequency. Curve 301 shows the slow transition of a commercially available RAM material and curve 302 the fast transition of the E-tailored material of the invention. Both materials are PEC (Perfect Electric Conductor) backed, which means that they e.g. are mounted on a metal sheet. The rapid decrease in reflection coefficient in the region between f2 and f3 for curve 302 guarantees that the antenna will function properly at frequencies between f1 and f2, since incident waves here can penetrate the RAM material and is reflected by the PEC, while at the same time the RCS is kept low at frequency f3, since incident waves here are absorbed by the RAM material and the reflections thus becomes low. Figure 4 shows one embodiment of the invention where an antenna array is integrated in a wing edge 401 of an aircraft. The antenna elements are here realized as slots 404 located in two rows 405 and 406 in a wing structure 402. A RAM structure 403, having an inner surface 407 and an outer surface 408, is mounted to the wing structure and covering the slots. In this embodiment the RAM structure comprises only one layer of RAM material. The RAM structure can however also comprise several layers as will be shown in the detailed description, in order to reduce the RCS value further. The invention can advantageously be implemented on wing edges and an outer protective layer can be applied to the RAM structure to increase the mechanical strength of the RAM structure. The invention can be applied on several types of antenna elements (dipoles, crossed dipoles, patches, fragmented patches etc). It is also possible to apply the invention using different feeds (slots, probes, balanced, unbalanced, etc). BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the threat sector Figure 2a schematically shows incident and specular waves Figure 2b schematically shows RCS from side lobes of incident waves Figure 3 schematically shows the reflection coefficient y for RAM-materials as a function of frequency. Figure 4 schematically shows a perspective view of a wing edge with the invention implemented. Figure 5 schematically shows a cross section of a wing edge with the invention implemented. Figure 6 shows a diagram of dielectric properties for a tailored RAM structure with four layers Figure 7 shows a diagram of reflection coefficient of tailored 4-layer RAM structure. Figure 8 shows a diagram of transmission coefficient of tailored 4-layer RAM structure. Figure 9 shows a diagram of dielectric properties for a commercially available RAM structure with four layers.Figure 10 shows a diagram of reflection coefficient of a commercially available 4-layer RAM structure. Figure 11 shows a diagram of transmission coefficient of a commercially available 4-layer RAM structure. Figure 12 shows a flowchart of the method DETAILED DESCRIPTION The invention will in the following be described in detail with reference to the drawings. Figure 1-4 have already been described in connection with Background art and the Summary. A cross section of an upper half of a wing structure 501 with a RAM structure 502, having an inner surface 508 and outer surface 509, is shown in figure 5. The RAM structure 502 comprises RAM layers 504, 505, 506 and 507. An antenna element 503, in this embodiment being a slot, is mounted to the inner surface of the RAM layer 504 with tangential points 511 and 512 to the antenna element surface. A point 510 is defined as an intersection between the inner surface of the RAM structure and the outer profile of the wing structure. Each interface between the different layers is parameterised with a few parameters as well as the dielectric properties of each layer. The position of the antenna element is also parameterised and optimized by replacing the aperture with a line source and calculating the far-field pattern in the elevation plane. When the optimal design is achieved the antenna element is properly designed and matched. Each layer / in a multilayered RAM is described by their material properties; relative permittivity \, relative permeability jm and layer thickness of/. The tangential component of the propagation vector for a plane wave travelling with angle 0 from the normal in vacuum is k0 sin 0 in all layers, where fc0 = — Co is the wave number in vacuum. For each interface, the tangential components of both the E-field and H-field are continuous; leading to that the incident wave is split into a transmitted wave and a reflected wave, travelling the opposite normal direction as the incident wave. The normal component of the propagation vector in layer / is c^c -Sin20 . since the tangential component is the same in each layer. The H-field is perpendicular to the E-field and the direction of propagation, and the E-field is perpendicular to the direction of propagation. The amplitude I £ of the E-field is f/o,/— times, r)o=the characteristic impedance in free space, V r^i the amplitude of the H-field, hence the tangential component of the E-field is - times the tangential component of the H-field, when the E-field is in the plane of incidence. When the E-field is perpendicular to the plane of incidence, the tangential component of the E-field is times the tangential component of the H-field. For other polarisations, the incident wave can be decomposed into a component in the plane of incidence (parallel or TM polarization) and a component perpendicular to the plane of incidence (perpendicular or TE polarization), which can be treated separately. When the incident wave meets the upper interface, one part of the wave energy is transmitted through the interface and the rest is reflected in the so called specular direction. The amplitude of the reflected wave is determined by that the tangential components of both the H-field and E-field are continuous, giving the relation: attenuated before it reaches the next interface. For high frequencies the attenuation of the wave is so high, that it does not reach the next interface, the primary reflection is then dominant and should be kept as low as possible. One way of doing this, is to use a material withµ, =,, making the reflection coefficient zero at normal incidence. One drawback with this approach is that the reflection coefficient increase rapidly with increasing incidence angles, if the magnitude of //, = e, is large. Further, both the permittivity and the permeability are functions of frequency, and it might be difficult to match those over a large frequency band. A commonly used model for describing the frequency dependency of the relative dielectric constant r, or permittivity, is the Lorentz model, having 5 parameters according to: where ex is the high frequency limit, es the value at zero frequency, frel the relaxation frequency, /0 the resonance frequency, EO the value in vacuum and finally ae the conductivity at zero frequency. Letting the resonance frequency approach infinity reduces the model to the Debye model with 4 As an example consider a mixture of two materials, one base material with low dielectric constant close to 1 for all frequencies and the other with £ =!./«/ =4 GHz and /0=8 GHz independently of inclusion material volume fraction and where the other parameters, asS and oe, are a function of the volume fraction according to the Maxwell Garnett mixing formula which is the simplest and most widely used model for description of composite media at comparatively low concentrations of inclusions. By proper choice of the volume fraction, values according to figure 6 can be achieved for a four layer RAM structure with curve 601, representing the RAM-layer closest to the antenna element, having s=2 and ae=0,2, curve 602 having s=1,75 and ae=0,15, curve 603 having s=1,5 and ae=0,1 and curve 604, representing the RAM-layer being part of the outer surface of the vehicle, having s=1,25 and oe=0,05. In this way there will be a gradual increase of the £-value from e=1 in air to =2 in the layer closest to the antenna element. In figure 6 the horizontal axis represents frequency in GHz and the vertical axis the £r-value calculated according to the Lorentz model with  = \,fre, = 4GHz and f0 = 8 GHz. Assuming a planar stratified media with 4 layers with 25 mm thickness each, the reflection coefficient R can be calculated according to figure 7, when the RAM structure is placed upon a Perfect Electric Conductor (PEC). The calculated reflection coefficient R, is represented on the vertical axis and frequency in GHz on the horizontal axis. Five different incident angles 0 are plotted, curve 701 with 0=0°, curve 702 with 0=15°, curve 703 with 0=30°, curve 704 with 0=45° and curve 705 with 0=60°. The incident angles 0 is in figure 7 and following figures defined as the angle between the normal to the RAM surface and the incident wave. The calculated transmission through the layers when the PEC is replaced with vacuum is shown in figure 8 with transmission coefficient T on the vertical axis and frequency in GHz on the horizontal axis. T and R are calculated both for TE (Transverse Electric) and TM (Transverse Magnetic) polarization according to conventional methods well known to the skilled person. The structure according to figure 8 is approximately equal to the maximum available efficiency for an antenna transmitting through the RAM structure. Five different incident angles are plotted, curve 801 with 0=0°, curve 802 with 0=15°, curve 803 with 0=30°, curve 804 with 0=45° and curve 805 with 0=60°. As can be seen in the figures the reflection above 3 GHz is essentially less than -20 dB (see figure 7) and the transmission at 1 GHz is better than 3-4 dB (see figure 8). Another possibility to achieve similar results is to use inclusion of shaped particles of different sizes and volumetric fractions or to use materials with different Debye and Lorentz parameters. In practice, materials with such low dielectric constant as in the outer layer in the example above have poor mechanical properties. In this example the arrangement has to be protected with a thin layer of mechanical stability, often having a larger dielectric constant or permittivity. The material properties of this layer have to be taken into account in the optimization of the structure. As a comparison with what is typically achieved with commercial RAMs, data from a user supplied data sheet is fitted to a Debye model. The data was only available between 5 and 18 GHz and the original data is displayed with solid curves, the fitted data is shown with dashed curves in figure 9 for four different r-values shown in curves 901-904. The vertical axis represents the r-value and the horizontal axis the frequency in GHz. As seen it is excellent agreement between supplied data and the modelled data as the dashed and solid lines more or less coincides after 5 GHz suggesting that the Debye model is a proper description of the materials used. Figure 10 shows the reflection coefficient R on the vertical axis and the frequency in GHz on the horizontal axis for a commercially available RAM structure with four layers and for five different incident angles 0, curve 1001 with =0°, curve 1002 with 0=15°, curve 1003 with 0=30°, curve 1004 with 0=45° and curve 1005 with 0=60°. Figure 11 shows the corresponding transmission coefficient T on the vertical axis and the frequency in GHz on the horizontal axis for a commercially available RAM structure with four layers and for five different incident angles 0, curve 1101 with 0=0°, curve 1102 with 0=15°, curve 1103 with 0=30°, curve 1104 with 0=45° and curve 1105 with 0=60°. When figure 7, having a RAM structure with tailored E-values, is compared to the corresponding curves for a commercially available RAM structure in figure 10, it can be seen that the reflection coefficient is much lower for the E-tailored RAM, typically below 20 dB from 3 GHZ while the commercially available RAM structure has a reflection coefficient around 5-15 dB in the interval 3-10 GHz. This means that the e-tailored RAM structure gives much lower reflections for incident waves and hence a better RCS value. When the curves for the transmission coefficients for -tailored RAM, figure 8, is compared to the corresponding curves for the commercially available RAM structure of figure 11, it can be seen that the transmission coefficient around 1 GHz is around 3-5 dB for -tailored RAM and 12-14 dB for the commercially available RAM structure. Hence the -tailored RAM structure gives an improvement of transmission in the order of 10 dB in the operating band of the antenna array. In summary the result is that the -tailored RAM structure represents curve 302 in figure 3 and the commercially available RAM structure curve 301 in the same figure. The curve shape of the RAM-layers can be calculated using the Continuum Sensitivity Based approach for optimization. This is done by solving the E-field for TM polarization or the H-field for TE polarization for a set of frequencies, incidence angles and parameter values. The character a is conventionally used for denoting RCS. Henceforth a is therefore used for RCS and should not be mixed up with ae used for conductivity. The change daof the radar cross section by a small displacement 3