GB2283368A

GB2283368A - Shield for radar antennae

Info

Publication number: GB2283368A
Application number: GB9419887A
Authority: GB
Inventors: Gerhard Lobert
Original assignee: Deutsche Aerospace AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1993-10-28
Filing date: 1994-10-03
Publication date: 1995-05-03
Anticipated expiration: 2014-10-03
Also published as: ITMI942163A0; HK1002741A1; IT1271693B; FR2711846A1; GB2283368B; ITMI942163A1; DE4336840C1; FR2711846B1; GB9419887D0

Abstract

The cover for the radar antennas of missiles consists of a plurality of dielectric hollow bodies which are arranged concentrically with respect to one another, the outermost hollow body being firmly connected to the outer wall of the missile; the distance between the outermost hollow body and at least one of the inner hollow bodies is variable via a control device. <IMAGE>

Description

1k 1 SHIELD FOR RADAR ANTENNAE 2283368 The invention relates to a shield

for radar antenna or antennae in aircraft and missiles, which is integrated into the outer contour of the craft in the form of a radome.

The reduction in radar visibility of military airborne craft, in particular aircraft, is becoming increasingly important. In combat aircraft, the radar equipment as well as the inlet and cockpit contributes significantly to the radar reflection when the aircraft is illuminated by enemy radar. At certain aspect angles and frequencies, the reflection back from the radar antenna may overshoot that of the rest of the aircraft by a multiple.

US Patent 4570166 describes a shield for radar antennae in aircraft and missiles, which is integrated into the outer contour of the craft in the form of a radome. The absorption capacity of the shield in this case cannot be changed in a time-dependent manner, the absorption being merely dependent on the angle of incidence of the electromagnetic radiation.

A shield for radar antennae in aircraft and missiles is known from DE-PS 39 20 110, which is integrated into the 2 outer contour of the craft in the form of a radome. The shield has a photosensitive layer which, when illuminated by a light source provided for this purpose, changes reversibly from the electromagnetically transparent state into a reflective state, the change in conductivity of the photosensitive layer being utilised by the illumination.

A further shield structure for aircraft and missiles is described in DEPS 40 07 986, which has one or several cavities which may be selectively filled with material - for absorbing radar waves. The structure serves to selectively change the radar image of the craft, in which case the change in covering material with the changed conductivity values, dielectric constants and other physical values influence the absorption capacity and transparency.

DE-OS 41 40 944 describes an absorber arrangement with variable absorption characteristics, which is composed of a support layer provided with embedded f erroelectric dipole molecules and of control electrodes disposed on both sides of the support layer, of which at least the control electrode on the radiation incidence side is transparent to radiation. By supplying control voltage to the control electrodes, the dipole molecules are aligned in accordance with the active electric field between the

1 3 control electrodes, thus allowing the absorption prof ile of the absorber to he changed within specific limits.

Finally, it is known from the older but not pre-published EP 568 511 to shield an antenna and thus influence its effect on the overall radar backscatter cross-section by disposing a movable metal layer in front of the antenna, as mentioned above, which may be opened if necessary.

The object of the present invention is to provide a shield for radar antennae e.g. in missiles, which has variable transparency to radar so that the reflection back from the antenna and radar frame when the radar equipment is switched off maybe adjusted to the smallest possible value in a short time.

Accordingly, the present invention provides a shield for 15 a radar antenna or antennae (e.g. in an aircraft or missile) which is integrated into the outer contour of a craft in the form of a radome, wherein the shield is composed of several dielectric hollow parts arranged concentrically to one another; the outer hollow part is f ixed to the outside wall of the craft; and that the distance between the outermost hollow part and at least one of the inner hollow parts may be varied by means of a control device.

4 Advantageous embodiments are described in the sub-claims.

The mechanical adjustment according to the invention of the individual hollow parts in relation to one another has the advantage that when the radar equipment of the combat aircraft is switched off, the cone distance maybe adjusted quickly, simply and effectively according to a predetermined curve in dependence on the illumination frequency of an enemy radar illuminating the aircraft, whereby the illumination frequency may be determined automatically by an accompanying radar warning system. Unless a measured value is output by this radar warning system, the frequency most likely to pose a threat is used to regulate the spacing of the concentric hollow parts. The mechanically operating adjusting device is insensitive to electric or electronic disturbance factors. By rapidly adjusting the distance between the individual concentric hollow parts and possibly filling the cavities between the hollow parts with a dielectric fluid, high attenuation of the antenna reflection in the order of 20 dB is achieved at all illumination frequencies from enemy radar.

The invention is explained in further detail below on the basis of the drawing showing advantageous examples.

Figure 1 is a graph showing the relations between beam path amplitudes and relative phase angle of a twin-plate radome; Figure 2 shows the vectorial addition of the individual beams transmitted for three plate thicknesses and with variable plate spacing; Figure 3 shows the dependence of transparency of a twin-plate radome on the plate thickness and the platespacing; 2 Figure 4 shows the dependence of the attenuation of the antenna reflection through the radome according to the invention on the plate spacing and the radar frequency; Figure 5 is a schematic representation of a first embodiment of the shield; Figure 6 is a graph showing the determined spacing; Figure 7 is a schematic representation of a wideband, adjustable shield.

If a radar beam falls cn a shield in the form of a radome comprising several dielectric layers, the multiple 0 reflections shown in Figure 1 result at the interfaces 6 between the individual layers of the shield, i.e. of the radome. Besides the directly transmit-Ced main beam, component beams emerge from the rear of the radome resulting from the two-fold, four-fold, six-fold etc.

reflection at the interfaces. The amplitude of the resulting transmitted beam and thus the transparency of the radome results from the addition of the individual component beams according to amount and phase. In the case of a rough determination of transparency, this addition may be restricted to the directly transmitted beams, all being reflected two-fold. In the case of the beam path shown in Figure 1, the amplitudes and relative phase angles of a twin-plate radome, the die1ectric constant E is assumed to be 4 (in which case three-fold and higher reflections are not shown).

The amplitudes of the individual component beams are determined by the intensity of the jump in impedance of the radar wave at the interf aces. In the case of non-ferritic radomes, the jump in impedance is determined by the ratio of the dielectric constants in front of and behind the interf ace. This means the following in concrete terms in the case of vertical incidence EReflection V E2 / E1 - 1 1 EIncidence ETransmission E1ncidence 7 E2 / E1 + 1 2 (E2 / E1) 1/4 V E2 / E1 + 1 With an increasing ratio of the dielectric constants c, the amplitude of the reflected beam increases, whereas that of the transmitted beam decreases.

With a radome composed of two parallel, dielectric plates, six two-fold reflected beams emerge from the rear of the radome besides the directly transmitted beam. If the plate thickness amounts to aX and the plate spacing amounts to bX, in which case X relates to the wavelength in the respective dielectric, then in addition to the main beam 0, two beams 1, 2 result with phase delay 4wa, one beam 3 with the phase delay 4wb, two beams 4, 5 with phase 4wa + 4wb + w and one beam 6 with the phase delay 87ra + 87rb (each relative to the main beam 0).

The addition of all the component beams is clearly 20 shown in Figure 2 in the complex plane. on variation of the plate spacing bX from 0 to X/2, the final point of the resultant R describes a circle. The ratio of P,,x to P-inin is firstly determined by the radius of the circle and secondly by the position of the centre 8 point of the circle. The radius of the circle is dependent on the plate thickness as well as on the dielectric constant E of the radome (Figures 2a, b and c). If one wishes to vary the radome transparency in as large a range as possible by changing the plate spacing, this requires a relatively high dielectric constant E. A radome made of glass-fibre reinforced plastic with E = 4 and with variable plate spacing has a transparency range of about 9% to about 100%. The transparency of a radome composed of two ceramic plates with a dielectric constant of E = 9 din be decreased to zero by the correct selection from a and b.

Figure 3 shows the dependence of the transparency of a twin-plate radome on the plate thickness aX and the plate spacing bX by means of a graph. As the plate thickness increases, the range, in which the transparency may be varied by changing the plate spacing, increases. The greatest band width is reached - in this case it amounts to 0.3 dB to -10.2 dB - with a plate thickness of X/4, the first corresponding to a plate spacing of 0 or X/2, and the lowest transparency reached with a spacing of X/4.

Figure 4 shows the dependence of the attenuation of j 9 the antenna reflection of the shield on the frequency of the incident, i.e. illuminated, radar beams for several plate spacings in the case of a dielectric constant Of E = 4. When the radar equipment of the combat aircraft is switched off, the plate spacing is varied in dependence on the illumination frequency in such a way that the radome transparency always assumes a minimum value. In the case of the design frequency, fD(!Signl which generally coincides with the effective frequency, the attenuation of the antenna reflection through the shield according to the invention amounts to more than 20 dB. In the frequency range 0.6 fDesign < f < 1.4 fDeSign I the attenuation is always more than 10 dB.

Figure 5 shows a schematic representation in the form of a cross-section through a first embodiment of a shield according to the invention. This shield is integrated into the radome in the nose of a combat aircraft and is composed of two concentric, dielectric hollow parts in the form of cones, the wall thickness of each cone being a quarter of the radar wavelength calculated on the basis of the dielectric. The outer cone 1 is fixed via quick-release elements to the aircraft structure and the inner cone 2 is guided by means of a guide pin 3 arranged axially to the two cones and moved by means of an adjusting device coaxially to the outer cone 1. The adjusting device comprises several, e.g. 3, connecting members 4 distributed uniformly around the periphery of the inner cone 2 and connected to a corresponding number of hydraulic jacks 5, which are supported on the aircraft structure. operation of the hydraulic jacks 5 causes the inner cone 2 to shift concentrically relative to the stationary outer cone 1 so that the distance between the two cone walls may be varied.

z On operation of the radar equipment 6 of the aircraft itself, the distance between the two cones 1, 2 is set to Design/2. When the radar equipment 6 is switched off, the distance between the two cones 1, 2 changes in accordance with the curve shown in Figure 6 in dependence on the illumination frequency of an enemy radar. The latter is automatically detected by means of the aircraft's radar warning system. Unless a measured value is output by this radar warning system, the frequency most likely to pose a threat is used to determine the distance between the cones 1, 2.

In place of cones, the shield may also use other nonconical, slender hollow parts, in which case actuation 1 11 of the hydraulic jacks should also be selected so that the radar transparency averaged out over the entire radome is at a minimum.

It one wishes to increase the effectiveness of the shield according to the invention above and below the effective radar band according to Figure 4, it is advantageous to also change the effective thickness of the two conical hollow parts dependent on frequency, as shown in Figure 7 as a schematic cross-section through another embodiment of the shield. In this case, each cone is constructed in a sandwich arrangement with variable core thickness, i.e. the shield in this case comprises an outer cone 8 connected to the aircraft structure and three further cones 9, 10, 11 arranged to slide coaxially thereto, the distances between which may each be varied relative to one another by means of three pairs 12 of control cylinders. The cavities between cones 8 and 9 and between cones 10 and 11 are each closed by means of elastic, flexible seals (not shown), and these cavities are each connected via a pipe 13 to a storage container 14 holding a dielectric fluid, e.g. a dielectric liquid. In this case, the dimensions of the cross-sectional surface of the pipes 13 are such that, when the distance between the individual cones is 12 adjusted, the pressure load on the cone walls is not too great and rapid supply or rapid removal of the dielectric fluid located in the cavity between two cones is assured.

9 If the individual control cylinders 12 are now actuated dependent on frequency so that the effective thickness of the radome sandwich arrangements each formed by two cones and their spacing amounts to X/4, then the attenuation of the antenna reflection on illumination by an enemy radar amounts to 20 dB at all the usual illumination frequencies.

13

Claims

Patent Claims:

1. A shield for a radar antenna or antennae which is integrated into the outer contour of a craf t in the f orm of a radome, wherein the shield is composed of several dielectric hollow parts arranged concentrically to one another; the outer hollow part is fixed to the outside wall of the craft; and that the distance between the outermost hollow part and at least one of the inner hollow parts may be varied by means of a control device.

2. A shield according to Claim 1, composed of two hollow parts in the form of cones, of which the inner cone may slide along a guide pin arranged in the axial direction of the two concentric cones at their tip; is and wherein the adjusting device for the inner cone comprises several connecting members distributed uniformly around the cone periphery and hydraulic jacks allocated hereto supported on the inside wall of the craft.

3. A shield according to Claim 1, composed of four hollow parts in the f orm of cones, each cone being arranged to slide along a guide pin arranged in the axial direction of the concentric cones at their tip; 14 and wherein by means of several connecting members and control cylinders the three inner cones may each slide relative to one another and to the stationary outer cone.

v

4. A shield according to any one of the preceding claims wherein the cavities existing between the concentric cones are closed by means of flexible seals; and each cavity is connected via a pipe to a storage container for a dielectric fluid.

5, A shield substantially as any one embodiment herein described with reference to the accompanying drawings.