GB2283369A

GB2283369A - Shield for radar antennae

Info

Publication number: GB2283369A
Application number: GB9419913A
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: GB9419913D0; DE4336841C1; IT1271694B; GB2283369B; FR2711847B1; ITMI942164A1; ITMI942164A0; FR2711847A1

Description

a 2283369 1 SHIELD FOR RADAR ANTENNAE The present invention relates to a

shield for radar antenna or antennae, in particular for flat-plate and array antennae in aircraft and missiles, which forms part 5 of the radome surrounding the antenna.

Modern combat aircraft are increasingly insensitive to radar beams illuminating them, i.e. enemy radar beams, as is known in particular from the American stealth bomber. A weak spot in the shield against illuminating radar beams is still the radar antenna of the aircraft itself, since because of the regular arrangement of backscatter centres, i.e. slits or radiation elements, the antenna is itself visible to radar from every direction with suitable frequency matching. Hence, an effective shield for the craft's radar antenna must constitute effective protection against the backscatter from enemy radar.

In the case of airborne radar systems both in guided missiles and in combat aircraft, antennae in flat-plate assembly are predominantly used for a variety of reasons.

In this type of antenna, the radar wave to be radiated is composed of a plurality of individual waves generated in the correct phase and amplitude by a regular arrangement of individual radiators. The phase angle of the 2 individual radiators is not adjustable in the case of the flat-plate antenna. However, in the case of the phasedarray antenna, these values may be changed in such a way that the direction of radiation may be controlled over a large angle range.

If a flat-plate or phased-array antenna is illuminated by enemy radar, then the incident wave is radiated back by each individual radiator in the array. The resultant of these individual stray fields is obtained by vectorial addition of the field intensities in the complex plane. In the case of a regular spatial arrangement of the individual radiators, there is a specific radar frequency for each direction of view, at which the reflecting fields of the individual radiators add up to an overall field, which is comparable in intensity to the backscatter field of the vertically illuminated, i.e. irradiated, plate. It is also impossible to decisively reduce this critical problem for the stealth bomber aircraft by means of an irregular arrangement of the individual radiators. Hence, having its own radar antenna is an Achilles heel for the stealth bomber aircraft.

If the radar antenna is covered by a radome, which is transparent to radar beams on narrow band, then it is only visible in a small frequency range. Another possibility is to cover the radar antenna with a roll-up a #i 3 metal foil, the electrically conductive part of which may be rolled up out of the way prior to activation of the radar device. In neither of the solutions is it possible to fully shield the array antenna with respect to frequency or operating time, the residual visibility in the f ormer case being dependent on the width of the effective band and the filter quality of the radome and in the second solution on the relative duration of radiation of the radar and the roll speed of the shield foil.

US Patent 4570166 describes a shield for radar antennae in aircraft and missiles, which forms part of the-radome surrounding the antenna. The absorption capacity of the shield in this case cannot be changed in a time-dependent manner. The absorption is merely dependent on the angle of incidence of the electromagnetic radiation.

DE-PS 39 20 110 describes an electromagnetic window for aircraft and missiles with a light source and a photosensitive layer, which changes reversibly from the electromagnetically transparent state into a reflective state when illuminated by the light source. The chafige in conductivity of the photosensitive layer is utilised thereby on illumination.

A structure, in particular for aircraft, is known from 4 DE-PS 40 07 986 which is exposed to radar waves on radar detection and has one or several cavities which are selectively filled with material for reflecting radar waves or absorbing radar waves. The cavities may be incorporated into the front edges of the wings or tail plane or may be integrated into the skin. In this way, the radar image of the craft may be selectively changed, in which case the change in covering material together with the thus changed conductivity values, dielectric constants and other physical values influences the absorption capacity and transparency.

DE-OS 41 40 944 describes an absorber arrangement with variable absorption characteristics for electromagnetic radiation, which is composed of a support layer provided with embedded ferroelectric 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 control electrodes, thus allowing the absorption profile of the absorber to be changed within specific limits.

It is known from the older but not pre-published EP 568 25 511 to shield an antenna and ultimately also influence i k its ef f ect on the overall radar backscatter cross-section by disposing a movable metal layer in f ront of the antenna, as mentioned above, which may be opened if necessary.

Finally, EP 554 847 describes a switchable resonance f ilter for optical radiation, by means of which the effective dielectric constant may be purposely influenced in the environment of a sensor. The change in dielectric constant is achieved by changing the spacing between the sensor and an electrode located below it.

The object of the present invention i s to provide a shield f or radar antennae f or missiles or other craf t, which may be connected or disconnected within a f ew microseconds, and when connected provides protection for its own radar antenna, which is preferably in no way inferior to the protection of the rest of the craft.

Accordingly, the present invention provides a shield for a radar antenna, in particular for flat-plate and array antennae in aircraft and missiles, which forms part of a radome surrounding the antenna, wherein at a distance in f ront of the antenna and parallel thereto, two plane plates transparent to radar beams are disposed, the dimensions of which are at least equal to the dimensions of the antenna; the cavity between the plates is filled 6 with an ionisable gas; and a cathode or an anode is arranged in the cavity along two opposing edges.

Advantageous embodiments are described in the sub-claims.

A significant advantage of the shield according to the invention is that the electron stream from the cathode to the anode and thus the reflective effect of the shield to shield the missile itself may be switched on and of f within a few microseconds by decreasing or increasing the anode voltage. In this way, the gas discharge tube adapted to the requirements of the craft acts as an ultra-quick plane closure for the radar antenna. The electron stream is thus maintained practically during the entire mission of the craft and the shield is briefly opened only for extremely short periods in which a radar pulse is radiated or received.

The invention is explained in more detail below on the basis of the drawing showing an advantageous embodiment.

Figure 1 shows a top view onto a shield according to the invention, and Figure 2 shows a section through the shield according to the invention cut along line A-A in Figure 1.

z r Q 7 In the f igures, in which the same parts are given the same references, the reference 9 is given to the radome of an aircraft, in particular a combat aircraft, which is f itted with an array antenna 5 disposed inside the radome 9 and arranged parallel thereto. In order to protect this array antenna 5 f rom enemy radar beams striking the aircraft prior to reflection and illuminating it, a shield is provided according to the invention in the form of a modified gas discharge tube, said shield comprising two plane plates 3 and 4 disposed at a distance in front of the array antenna 5 and transparent to radar beams, - said plates having equal dimensions greater than the dimensions of the array antenna 5. The two plates 3, 4 def ine a cavity between them, which is f illed with a noble gas of low molecular weight (e.g. helium) or with hydrogen. An anode 2 connected to an electric heating means and connected to an oxide- coated cathode 1 or an anode 2 connected to a positive voltage source are arranged along two opposing edges in the cavity between the plates 3 and 4. When the cathode and anode are under corresponding power, electrons are generated by the cathode 1, which shift to the positively charged anode 2 and on their path to the anode collide with some of the atoms of the f iller gas, these then ionise so that an electrically conductive plasma is formed, which uniformly fills the cavity between the two plane plates 3 and 4. If a monochromatic enemy radar wave now strikes the plane 8 plasma layer 7 from outside, then the electric field of this wave sets the electrons of the plasma in sinusoidal vibration, which overlays the thermal movement of the electrons.

The vibrating electrons then generate a stray f ield, which overlays the field of the incident radar waves. In each layer of the plasma parallel to the plates 3, 4 between the plates 3, 4, the amplitude of the electron vibration is constant, whilst the plasma changes linearly along the layer. Hence, each layer generates a closely bunched stray field in the direction of reflection, in which case with a suitable electron density and adequate plasma thickness, the incident radar wave is fully reflected, while the radiation back in the direction of the illuminating radar is relatively low. The electron density in this case is primarily dependent on the anode current and on the density, i.e. the pressure of the filler gas.

An effective plane 8 of reflection may be defined from 20 the phase relation between the incident radar wave and the wave reflected on the plasma layer 7. An electrically conductive foil attached in this plane in place of the gas discharge tube would generate the same reflection field as the plasma.

9 The arrangement of the shield according to the invention in the plane structure of the radome 9 is such that the plane of the electrically conductive surf ace of the radome coincides with the effective plane of reflection 5 of the plasma.

In this way, the radar radiation back from the electrically shielded array antenna is minimised.

The density of electrons in the plasma layer necessary for a complete reflection and the minimum thicknEbss of the plasma are easily established by experiment, as are the necessary surface areas of the cathode and anode as well as the electrical power for connection of the gas discharge tube. The minimum requirement for electron density E, requires that the following equation is fulf illed ED:5 f2 Em/e2, wherein m is the electron mass, e the electron charge, E the vacuum dielectric constant and f the highest frequency of the radar waves to be reflected. At 18 GHz 20 the minimum electron density amounts to 4 x 1012 CM-3. When using helium as filler gas, this value is reached at 1033 Pa, with a current density of 0.2 A/CM2 and a field intensity of 1 V/cm. Moreover, the electron density must be high enough so that the penetration depth of the incident radar wave amounts to only a fraction of this radar wavelength.

The thickness of the two plates 3 and 4 should be 5 selected so that the load resulting from the presture difference between the atmosphere and plasma may be safely borne. The few supports 6 included in Figure 2 serve to keep the thickness of the plates as low as possible. Moreover, the distance between plates 3 and 4 must be selected so that transparency to the radar waves radiated from their own antenna is as high as possible. Depending on the thickness of the plates and the angle of emergence or incidence, the optimum-distance is between 20 and 30 of the radar wavelength.

11

Claims

Patent Claims:

1. A shield for a radar antenna, in particular f or flat-plate and array antennae in aircraft and missiles, which forms part of a radome surrounding the antenna, wherein: at a distance in front of the antenna and parallel thereto, two plane plates transparent to radar beams are disposed, the dimensions of which are at least equal to the dimensions of the antenna; the cavity between the plates is filled with an ionisable gas; and a cathode or an anode is arranged in the cavity along two opposing edges.

2.

A shield according to Claim 1, wherein the plates are spaced from one another at a distance amounting to between 20% and 30% of the radar wavelength radiated by the antenna.

A shield according to any one of the preceding claims, wherein several supports are disposed in the cavity between the plates.

4. A shield according to any one of Claims 1 to 3, wherein the cavity is filled with a noble gas with a low molecular weight.

12 A shield according to any one of Claims 1 to 3, wherein the cavity is filled with hydrogen.

6. A shield according to one of the preceding claims, wherein the cathode is coated with an oxide and is connected to a negative electric voltage source; the anode is connected to a positive electric voltage source; and the parameters of cathode and anode dimensions, gas density, plate spacing and electrical power for the cathode and the anode are dimensioned so that the following equation is fulfilled:

ED:5 f2 EM /e2 ' wherein m is the electron mass, e the electron charge, E the vacuum dielectric constant and f the highest frequency of the radar waves to be reflected.

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

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