WO2004038452A1

WO2004038452A1 - Adaptive antenna

Info

Publication number: WO2004038452A1
Application number: PCT/SE2002/001944
Authority: WO
Inventors: Lars Josefsson
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2002-10-24
Filing date: 2002-10-24
Publication date: 2004-05-06
Anticipated expiration: 2005-04-24
Also published as: AU2002353684A1

Abstract

A method is disclosed for effectively reducing the radar cross section for external irradiation of an antenna arrangement. The method is particularly applied to an array of radiating elements. It is not necessary to reduce scattering from each element of the array. Instead each element is arranged to scatter with a phase and amplitude such that the combined backscattering from the complete array in a particular direction is minimized. This particular direction is typically towards the external (threat) source. An antenna arrangement is demonstrated in which radar signals are received by an array lobe, while shaping a reduced scattering pattern having a null appearing in the external radar direction and a reduced scattering pattern for any external illumination by introduction of an impedance member in the feed line of a number of the array elements. According to the invention this impedance load may further be systematically varied according to the requirements of each particular situation.

Description

Adaptive Antenna

TECHNICAL FIELD

The present invention relates to antennas, and more specifically how to effectively reduce radar cross section for enabling stealth performance.

BACKGROUND

A well-known problem within antenna techniques is how to effectively reduce the radar cross section of an antenna with a minimum effect on antenna performance to thereby enable a stealth performance. On way to circumvent the problem of course is to use no antennas at all, like for instance the known F117 aircraft. Another possibility is shielding the antenna by means of external frequency selective surfaces (FSS). A third possibility would be to turn away the antenna. However, a turned away antenna can not be used until it is turned back again to the proper direction, which gives an unwanted important negative time factor. Prior art also mentions switched curtains or radomes to be placed in front of an antenna.

A number of documents can be found representing different solutions to this problem. For instance U.S. Patent No. 4,684,952 describes a passive array of resonantly-dimensioned micro-strip antenna radiator patches, which are closely spaced above a ground-plane and individually associated with transmission line segments terminated to cause the overall array to receive an incident RF energy electromagnetic field. The received field is converted into RF electrical currents, which flow along the transmission lines and are absorbed by the termination or reflected therefrom. In the latter case the reflected RF energy is re-transmitted in a predetermined direction as a redirected RF electromagnetic field.

Another recent document WO 02/ 11239 discloses a multiple band re- configurable reflecting antenna array and a method for multiple band operation and beam steering. An array of dipole antennas is disposed on a multiple band high impedance surface. The antenna array is re-configured by changing the length of the dipole elements, to thereby change the resonant frequency of the dipoles. At a given frequency band, small changes in dipole length allow to steer the reflected beam in a selected direction, while large changes in dipole length result in a switch of operating frequency band.

A U.S. Patent No. 3,568, 194 from 1967 discusses a way in which a return signal can be degraded by using a variable transmission load connected to the reflector. The disclosed method and system comprise phase and/ or amplitude modulation of the radar signal by one or more "scattering" sources positioned on a target vehicle, thus causing the return target signal to appear as an incoherent object.

Another U.S. Patent No. 5,036,323 discusses an active radar stealth device for minimizing the radar cross-section of a host platform. A coating, which is essentially transparent for microwaves, is attached to the surface of the host platform and is exposed to an incident microwave field. A plurality of detector/ emitter pairs contained within the coating detect and actively cancel, respectively, the microwave field at each respective detector/ emitter pair.

Finally a U.S. Patent No. 5,153,594 discloses an electronic counter-measure system for installation on an aircraft. The system includes an interferometer transmitter comprising a plurality of repeater amplifier circuits connected in parallel. The plurality of repeater amplifier circuits generate out of phase signals of different amplitudes, which respectively are transmitted by associated spaced transmitting antennas on the aircraft as a response to an incoming radar signal.

It is still a demand of further improved solutions to the above-mentioned problem to obtain a feature providing a decreased radar cross section of an antenna array but still resulting in antenna performance, in terms of gain and sidelobe pattern etc, being only marginally degraded. SUMMARY OF THE INVENTION It is known that the scattering from an antenna element (such as dipoles, small horns etc) depends on the impedance load of the antenna element, i.e. the degree of mismatch (i.e. reflection coefficient in amplitude and phase). According to the invention this impedance load is systematically varied according to the requirements in each particular situation.

The method is particularly applied to an array of radiating elements. It is not necessary for the scattering from each element to be reduced. Instead the loading is controlled such that each element scatters with an amplitude and phase such that the combined backscattering from the complete array in a particular direction is minimized. This particular direction is typically towards the external (threat) source.

In a signal /interference optimization of an antenna array the signal is received by the main beam of the antenna and the sidelobes are shaped such that a null appears in the direction of the interference. According to the present invention we maximize the antenna main beam towards a desired direction (in radar usually towards a desired target) . The sidelobes are of no concern in the present context. Instead a second pattern, namely the scattered pattern, which results from the external illumination from another external source, is shaped such that a null appears in the direction of the external illuminating source.

A method according to the present invention is set forth by the independent claim 1, and further embodiments of the invention are set forth by the dependent claims 2 to 7.

Furthermore an antenna arrangement according to the present invention is set forth by the independent claim 8, and further embodiments are defined by the dependent claims 9 to 12.

SHORT DESCRIPTION OF THE DRAWINGS The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

FIG. 1 illustrates an array antenna with control signals;

FIG. 2 illustrates a radar antenna array irradiated by an external undesired radar source;

FIG. 3 illustrates with a solid line a regular transmit/ receive antenna pattern and with a broken line the backscatter radiation pattern for the array according to the present invention;

FIG. 4 illustrates the principle of the present invention using microstrip feeders each provided with a discontinuity in form of a stub for affecting the backscatter radiation pattern;

FIG. 5 illustrates a wave-guide feeder incorporating a discontinuity in form of an iris at a location corresponding to a phase φ for a scattered signal;

FIG. 6 illustrates an array corresponding to the array of Figure 3 but provided with additional dummy elements for obtaining a controlled scattering for reducing RCS of the array.

DETAILED DESCRIPTION OF THE INVENTION A phased array antenna has typically matched phase shifters (and/ or TR modules) to control the radiation pattern and the beam direction. The scattering pattern is static and independent of the phase shifter settings (provided that the phase shifters and the feeding networks are well matched). According to the invention extra loads or discontinuities are added within the feed lines and controlled in a systematic fashion, thus modifying the reflections in the feed lines of signals from an external source. Thereby a signal scattered off from the array can be controlled. A particular systematic fashion can create a null in the direction of an external source such that the power scattered back towards that source is minimized.

The reduction of the radar cross section (RCS) typically leads to an antenna gain reduction since the mismatch in the feed lines constitutes a loss. However, the mismatch loads are chosen such that the antenna gain (in an intended direction) is maximized and the power scattered back towards the external source is minimized. The procedure can be understood from the analogy with signal/ interference optimization in communication systems.

In Fig. 2 reference 3 represents one radiating element in an array antenna (here shown with four elements altogether). A phase shifter 1 or TR (transmit/ receive) module controls the radiating and receiving patterns of the array antenna. For simplicity the phase shifter is assumed to be fully matched to the feed line. An incoming plane wave 6 emanates from a distant external source 5. Each element 3 will not only receive a part of the incoming signal 6 but also scatter back a portion thereof. Typically for small radiators the received power and the scattered power are of the same magnitude.

A reflected portion 4 of the received signal is reflected from a discontinuity 2 in the feed line. This portion can be controlled in amplitude and phase as indicated in Figure 1. Examples of realizations of the discontinuity 2 will be given later. All elements 3 will scatter equally, assuming those to be identical (thus for simplicity neglecting mutual coupling and other effects). Normalizing to this magnitude the total scattered field from one element can be written

where T is the reflection from the discontinuity 2, as seen at the element port, and ψ is the phase determined by the location of the radiation element in the antenna array relative to the incoming plane wave. For a planar array most of the scattered field will be directed in the specular direction 7 as a concentrated beam. However, this beam is part of the overall scattered pattern, which will have considerable amplitude also in the direction back towards the external source. The inclusion of discontinuities 2 and their control according to the invention provides a means for minimizing this backward radiation and hence also reduces the monostatic radar cross section.

With the scattering sources (the radiating elements) having excitations according to Eq. (1) we can apply known pattern synthesis methods to optimize each r (amplitude and phase) so that a null appears in the scattered pattern towards the external source 5.

The number of (complex) variables is large (equals the number of radiating elements) while only one condition (null towards the external source 5) shall be fulfilled. Thus there are several extra degrees of freedom available that can be used to optimize the antenna system in various ways. One preferred additional condition is to minimize the effect on the antenna performance, thus minimizing the total antenna reflection loss (example with a non- tapered array excitation):

Reflection loss = ∑|Ef (2)

For the procedure to be successful it is necessary to know beforehand the direction towards the external source 5. Common direction of arrival (DOA) methods for determining the direction of arriving signals can be used to achieve this. Since some uncertainty regarding the exact threat direction might exist the extra degrees of freedom can be used to synthesize several closely spaced nulls, or one continuous broader null in the backscattered pattern. This is also effective against several illuminating sources.

Alternatively the threat direction(s) may be roughly known or can be estimated for a certain mission. Then a broad null is aimed towards this direction. The number of degrees of freedom is actually enough for synthesizing very low levels scattered back in almost all directions (except close to the specular direction 7). Thus this could be a standard setting built into the antenna system.

Another embodiment is to make use of the described principle by not using all of the radiating elements in the array. Still another further variation is to add extra (dummy) elements to (the rim of) the array and employ the controlled scattering to those only. The impedance elements used can be of any kind for instance a shunt or series reactance, a stub, an iris, a pin or a varactor diode etc. The control could for instance be effected by biasing the diodes, or by utilizing MEMS (MicroElectroMechanical System) switches. The phase value could be controlled by a phase shifter or by having several switched obstacles along the feed line. Furthermore a controlled reflection can also be realized by a rotating field ferrite device and a polarization sensitive obstacle. Also a static case with fixed irises etc could be used. Some examples are given below to further illustrate the principle.

Figure 3 illustrates schematically an antenna radiation pattern for a transmitted/ received signal in a desired direction, shown as a solid line, and compared to a scattering pattern for an external signal, illustrated by a broken line. The main lobe of the array is controlled according to the state of the art by controlling of the phase φ of each phase shifter 1. The scattering pattern could have a more or less pronouced null in the direction of an external source represented by the arrow 6. To obtain the full beam steering capacity for transmit the phase φ also has to compensate for an influence of each inserted discontinuity member 2 acting to assist in generating a scattering pattern according to the broken line for an external undesired irradiation indicated by the arrow 6. Reference number 5 illustrates a possible amplifier in front of the phase shifter 1. Figure 4 illustrates a method to generate desired discontinuities by means of an arrangement 2 connected to a respective micro-strip feeder. This arrangement may for instance be a variable stub. In a preferred embodiment the value of _T can then be varied according to methods well known by a person skilled in the art. Here the distance of the arrangement seen from the radiator member along the micro-strip corresponding to the factor φ is fixed but generally different for each feeder. The radiator 3 may in this case be for instance a patch antenna element. In Figure 4 a stub is illustrated as the discontinuity arrangement 2 but also other components, such as a pin or a varactor diode could well be utilized for arrangement 2. The arrangement 2 may also be an impedance member presenting a shunt or series reactance. Control of a varactor diode can be effected by biasing the diode in a way well known by a person skilled in the art. Similarly control of a shunt or series reactance can be performed for instance by utilizing MEMS switches.

Figure 5 illustrates still another example of arrangement utilizing a waveguide with an iris inserted at a given distance giving the factor φ along the wave-guide feeding for instance a horn radiator. A number of such waveguides, each with a respective phase shifter 1, will form the array antenna.

In Figure 6 is illustrated still a further embodiment utilizing the present invention. According to Figure 6 extra (dummy) elements are added, for instance to the rim of the array, to create a controlled scattering in those only or combined with all or part of the active elements of the array.

The system according to the present invention enables for instance a fighter or reconnaissance aircraft to operate without being easily observed by other radars. Still full use of the aircraft's own radar may take place.

It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims.

Claims

1. A method for reducing radar cross section of an antenna arrangement, characterized by the steps of: controlling loading of the antenna arrangement containing a number of radiating elements constituting a transmit/ receive antenna, whereby each radiating element scatters with an amplitude and phase such that a combined scattering back from the antenna arrangement of radiation emanating from an external illumination will be minimized.

2. The method according to claim 1, characterized by the further step of: modifying the antenna arrangement by adding at least an extra load in at least one radiator element feed line of the antenna arrangement to thereby minimize backscattering from the external illumination from any direction, while still maximizing a main beam of the antenna arrangement towards a desired direction.

3. The method according to claim 2, characterized by the further step of: applying a pattern synthesis to optimize Y of each element for which a total scattered field can be written (l+r) ^ψ and wherein T represents a reflected signal being a function of the position of the extra load representing a discontinuity along the feed line of the radiating element and ψ represents a phase determined by a location of the element in relation to an external illuminating plane wave.

4. The method according to claim 3, characterized by the further step of: generating the discontinuity in the feed line by means of an impedance member, the impedance member being any of a shunt or series reactance, a stub, an iris, a pin, or a varactor diode or corresponding element.

5. The method according to claim 1, characterized by the further steps of: controlling a phase value by means of a phase shifter or by using several switched obstacles along the feed line.

6. The method according to claim I, characterized by the further step of: implementing a controlled reflection of the antenna array by means of a rotating field ferrite device and at least one polarization sensitive obstacle.

7. The method according to claim 2, characterized by the further steps of: adding further elements as dummy elements to a rim of the antenna array for employing a controlled scattering by the antenna array.

8. An adaptive antenna arrangement for reducing radar cross section characterized in an antenna device containing a number of radiating elements forming an antenna array interconnected by means of feed lines, whereby each radiating element scatters with an amplitude and phase such that a combined scattering back from the antenna array in a desired direction is minimized; and an impedance member forming a load inserted in at least one feed line of a radiating element to thereby minimize backscattering from an external illumination from any direction while still keeping an array main beam at a maximum towards a desired direction using an ordinary phase shifter arrangement for steering the main beam of the antenna array.

9. The adaptive antenna according to claim 8, characterized in that the impedance member being anyone of a shunt or series reactance, a stub, an iris, a pin or a varactor diode or corresponding element.

10. The adaptive antenna according to claim 8, characterized in that a phase value of each radiating element is controlled by means of a phase shifter or by using several switched obstacles along the feed line.

11. The adaptive antenna according to claim 8, characterized in that a controlled reflection is implemented by means of a rotating field ferrite device and at least one polarization sensitive obstacle.

12. The adaptive antenna according to claim 8, characterized in that further elements are added to a rim of the antenna array and operating as dummy elements employing a controlled scattering in those only.