Technical background

The present invention relates to direction-finder systems s and in particular an antenna arrangement for use in such a system. The resulting antenna structure is extremely stable and gives a very accurate direction to any number of emitters.

Radio frequency emitters (radars, satellite uplink o stations, cell-phone base stations, relay links) can be detected, analysed, and geo-referenced from a remote observation platform. This is achieved using a sensor with an antenna system for detecting the radiation, connected to a receiver and processing system. These systems can be s deployed from satellites, aircraft, UAVs, ships vehicles or mounted in masts.

Typical solutions employ radio receiver systems operating in the frequency bands 1 through 12 GHz. These systems employ multiple receiving antennas and multiple receivers o to derive a course direction to the emitters.

In order to accurately pinpoint the positions of target emitters, such systems are dependent on antenna systems that are very stable. Temperature differences can here pose a problem, as they cause the antenna to bend and warp. If 5 the antenna is rigidly fixed to a support, such mechanical distortions are promoted. Of the same reasons, sub-antennas in an antenna array may come out of alignment, meaning that the focal point of the array is blurred. This is particular true when the direction-finder system is deployed from a 0 satellite. In space, an antenna system is subject to extreme temperature differences when passing from shadow into full sun exposure. Antennas can also be brought out of alignment by purely mechanical influences, e.g. by vibrations in the support structure, or by wind forces acting directly on the antennas or its support structure. Another source of mechanical disturbances is acceleration of the observation platform. This may happen when the observation platform is onboard a ship in heavy sea, or mounted on a car or other vehicle.

Summary of the invention

It is an object of the present invention to provide an antenna panel for use in direction-finding systems, with optimal mechanical and thermal stability.

Another object is to provide an improved mounting arrangement for such an antenna panel, in which the mechanical impact on the panel is diminished.

Still another object of the invention is to provide an arrangement for actively compensating temperature effects and mechanical disturbances in an antenna array comprising several antenna panels.

These objects are achieved in an antenna panel, an arrangement for mounting an antenna and an arrangement for compensation of an antenna array, as defined in the appended patent claims.

Brief description of the drawings

The invention will now be described in detail, in reference to the appended drawing, in which:

Fig. 1 is a general overview of a direction-finder system,

Fig. 2 is an illustration of an antenna panel according to the invention, Fig. 3 is a view in cross section, of an embodiment of the inventive antenna panel,

Fig. 4 shows an alternative embodiment of the antenna panel in cross section,

Fig. 5 shows how an antenna according to the invention will deform when subjected to temperature changes,

Fig. 6 shows actual measurements of the distortions of an antenna subjected to a temperature swing of 67 degrees,

Fig. 7 shows a mounting arrangement for the antenna panel,

Fig. 8 is a schematic diagram showing an arrangement for compensating mechanical disturbances in an array including several antenna sub-panels, by mechanically adjusting the position of each sub-panel.

Detailed description of the invention

Initially, we will give an overview of a direction-finder system in which the present invention may find its application. As shown in Fig. 1, the direction-finder system includes an antenna unit 1 at left receiving RF signals from a number of emitter sources. The signals are delivered to an RF unit 2, center, where they are amplified, transposed down to baseband and demodulated. The demodulated signals are delivered to a processing unit 3 for processing and analysis.

Briefly, the antenna arrangement includes four antenna panels mounted in a 2X2 relationship. Typical dimensions of the antenna sub-panels are 1-5 wavelengths (15cm - 75cms at 2GHz) .

To minimize the impact of thermal and mechanical distortions, e.g. warping due to sun heating, the antenna has been designed as a layered composite structure with a stiff support layer.

This solution is illustrated in Fig. 2. The outer layer consists of a number of microstrip patches 21 on a s dielectric substrate 22 and a conductive groundplane 23.Below the patch layer, is a stripline feed layer composed of a number of narrow flat conductors 25 lying between upper 23 and lower 27 groundplanes. The upper groundplane 23 is also acting as the groundplane for the o microstrip patches 21.

Fig. 3 shows this antenna in cross section. The antenna includes a lower layer 31 of a structural skin material, e.g. a fibreglass/cyanate ester product marketed as Neltec N8000. The layer may be 1-2 mm thick. Above this layer is a s panel structural layer 32 of a honeycomb material, e.g. a product known as HRH 327, which in this example is about 25 mm thick. The layer 32 provides structural strength to the antenna, and is responsible for its stable mechanical properties. The structural layer 32 is conductive forming o the lower groundplane of the stripline layer (layer 27 in Fig. 2) . Above this is a second structural skin layer 34. This may also be made from Neltec N8000 in about 1 mm thickness. The structural skin layers 32, 34 are included to close the cells of the honeycombs. Then there is a layer 5 of double sided circuit board with the conductor pattern (25 in Fig 2) printed on its lower side, and the upper conductive layer acting as upper groundplane of the stripline layer with the apertures (23 in Fig. 2) . The layer may be produced in 1 mm thickness and from the 0 commercial stock board Roger's 5880. Above this is an alumina enhanced thermal barrier layer 35 of 1-2 mm thickness. This layer forms the dielectric layer for the microstrip patches, and is given a low dielectric constant to increase its thickness. A thin dielectric sheet 36 of Kapton (0,025 mm) forms a support for the microstrip patch radiators, here made from copper.

In order to provide stable thermal properties of the panel, the layers in Fig. 3 should be duplicated in a balanced structure symmetrical with the layers shown. This means that a mirror image of the layers shown in Fig. 3 should be glued onto the lower layer 31. This ensures an identical thermal expansion/contraction in the two structures, which balances each other and prevents the panel from bending.

An alternative embodiment of the invention is shown in Figure 4. Compared with the embodiment shown in Fig. 3, the structural skin layers 31, 33 has been replaced with layers 41, 43 of graphite cyanate ester, e.g. the commercial material known as T300/CE3. In addition, the structural support layer 42 is made of ribs of "SNAPSat", a material marketed by COI.

The antenna will when exposed to temperature stress be exposed to the following sources of deformation:

• Antenna stretching (in-plane homogenous dimension change)

• Antenna bending (out-of-plane structural change)

• Antenna surface irregularities (out-of-plane)

In addition, depending on the mechanical interface to the surrounding structure, the antenna may experience Out-of-plane multi-axis bending (Warping)

• In-plane irregularities (multiple local in-plane dimension changes)

The various forms of distortions are illustrated in Fig. 5.

Fig. 6 shows the resulting out-of-plane distortion measured on a sample antenna according to the present invention. The measurements relates to a temperature swing of 61 degrees (from 295 K to 228 K) . Peak distortion at the edges is some 30 urn, but the symmetrical design and the window function of the antenna illumination brings the actual displacements of the active aperture to well below 10 um.

Fig. 7 shows how an antenna panel may be mounted in a manner that does not deteriorate thermal stability ("window frame mounting") The example relates to a satellite platform, similar conditions exists on earth.

The antenna panel is exposed to sun radiation, with the following factors:

• Absorption from sun radiation. The sun will generate radiation with peak intensity near 1 um.

• Absorption from earth objects radiation. The Earth's radiation will peak at lOum (300K) .

Absorption from solar reflections on Earth. The magnitude of these reflections will depend on earth reflectivity and clouds. • Radiation/convection to antenna back from supporting structure.

• Outgoing radiation. The antenna panel will radiate into the environment with a peak wavelength given by its resulting surface temperature. The outgoing radiation should balance the other factors mentioned, establishing a thermal equilibrium.

In order to minimise the thermal impact, the panel is mounted "floating", i.e. with a resilient gasket between the rim and the mounting frame or box. This gasket will also prevent mechanical vibrations in the support structure from being conducted into the antenna structure, in some degree. In addition, the panel may be covered with a sun- reflecting film ("anti-glare material") that reflects light in the visible and infrared range. The panel may also be mounted behind a window covered with this material.

Fig. 8 illustrates an arrangement for compensating mechanical displacements between the individual sub-panels in an antenna setup, by active feedback control. The individual antenna panels 81, 81' are provided with several sensors detecting temperature (e.g. thermistors 82, 82'), acceleration (e.g. accelerometers 83, 83') and displacement (e.g. straingauges 84, 84'). The sensors are connected to a processing unit 85. The processing unit is adapted to use the signals from the sensors to compute the change in alignment between the sub-panels due to thermal and mechanical effects in accordance with a model of the sub- panels. The processing unit 85 will then try to adjust the position of each sub-panel 81, 81' in order to re-establish a proper relationship betwen the sub-panels. The sub-panels are mounted on position transducers 87, e.g. stacks of piezo-electric elements, which may change the position of a panel by displacing it outward or inward. The position is controlled by signals from the processing unit 85; the signals being amplified in servo amplifiers 86. By using position transducers 87 mounted at each corner of a panel, the processing unit 85 may also tilt the panel in order to restore a proper focus direction.