BACKGROUND OF THE INVENTION The Combat Identification for Dismounted Soldiers (CIDDS) system that is currently in development by the United States government is a system for use in identifying friendly military personnel in combat situations to help prevent, for example, fratricide. The system uses a laser interrogation signal to electronically interrogate individual infantryman on the battlefield to determine whether they are a friend or an unknown. If the unidentified soldier is CIDDS-equipped, infrared (IR) sensors in his helmet detect the laser query signal and a radio frequency (RF) response signal is transmitted from an antenna on the helmet identifying the soldier as a friend. If the unidentified soldier is not CIDDS equipped, then no response signal is delivered to the querying entity. The response to the laser interrogation signal received by the querying entity acts as an aid in making, for example, an engagement decision.

As can be appreciated, the helmet mounted circuitry used in the CIDDS system must be able to handle the intense stresses that may exist within a combat scenario.

For example, the circuitry should be able to absorb and recover from shock forces on the battlefield, such as those generated during hand-to-hand combat. In addition, the circuitry should be able to withstand scrapes, gouges, and other direct impacts caused by, for example, tree limbs, brush, barbed wire, etc. in the battle region. Furthermore, the helmet mounted circuitry should be very light weight and should not protrude very far from the heimet itself where it might get caught on branches, fence wires, and the like. All of the above requirements are especially important for the antenna unit mounted on the helmet as this is normally the largest and most prominent piece of circuitry on the helmet. In addition, the antenna unit on the helmet should be capable of providing a relatively high antenna gain in the direction of the horizon so that power is not wasted transmitting energy in unnecessary directions.

Therefore, there is a need for a rugged, lightweight antenna structure that is suitable for mounting on a helmet for use during combat situations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded isometric view of an antenna in accordance with one embodiment of the present invention; FIG. 2 is a isometric view of the antenna of FIG. 1 after assembly of the constituent parts; FIG. 3 is a diagram illustrating a feed arrangement for the antenna of FIG. 1 in one embodiment of the present invention; FIG. 4 is a diagram illustrating a helmet carrying multiple antennas in accordance with one embodiment of the present invention; FIG. 5 is an assembly layout of an antenna fabricated according to the present invention; and FIG. 6 is a block diagram illustrating circuitry mounted on the helmet of FIG. 4 in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT The present invention relates to a rugged, lightweight antenna structure that is suitable for mounting on a soldier=s helmet for use during combat situations. The antenna structure utilizes a flexible conductive sheet material as a radiating surface that is held in place relative to a ground plane by a resilient dielectric slab. The antenna structure is thus flexible and capable of absorbing large shock forces without suffering a permanent change in shape. In addition, the resilient dielectric material is lightweight compared to the dense, rigid board materials typically used in microstrip antenna applications. In a preferred embodiment, a heimet mounted patch antenna is implemented that has a relatively high antenna gain in the direction of the horizon.

The inventive antenna structure can be advantageously implemented in virtually any antenna application where repeated impacts and/or shocks from an exterior environment are expected or where curvature is desired.

FIG. 1 is an exploded isometric view of an antenna 10 in accordance with one embodiment of the present invention. As illustrated, the antenna 10 includes : a conductive ground plane 12, a resilient dielectric slab 14, and a thin circuit board 16 having a conductive patch 18 disposed upon a surface thereof. The conductive patch 18 forms the radiating surface of the antenna 10. That is, during operation, currents are set up on the patch 18 that cause RF energy to be radiated in a predetermined far field pattern. The dielectric slab 14 is operative for holding the conductive patch 18 a predetermined distance from the ground plane 12 to achieve a predetermined frequency bandwidth for the patch. The frequency range of operation of the antenna 10 and the shape of the corresponding antenna pattern depend upon, among other things, the physical dimensions of the various constituent parts (e. g., the size and shape of the patch 18 and the ground plane 12, and the thickness of the dielectric slab 14).

In accordance with the invention, the dielectric slab 14 is constructed out of a resilient dielectric material that is capable of automatically regaining its original shape after being deformed by a deforming force (e. g., a physical blow) from an exterior environment. The dielectric material will preferably be able to regain its original shape regardless of the size or direction of the deforming force. In a preferred embodiment, the dielectric slab 14 is made out of an elastomer material in the form of a lightweight plastic foam (either open-celled or closed-celled) or honeycomb. One possible open- celled foam that can be used is polyethylene sold by Rubatex under the and is identified by part number PE-2. A closed-cell foam that can be used is sold under the trademark Volara by Boyd Corporation and identified by part number 2E. Flexible honeycomb materials that can be used in accordance with the present invention include, for example, a material by Supracor and identified by part number M498-ISU.

These materials are generally available in a wide variety of thickness'and usually have a dielectric constant near that of air.

As described above, the conductive patch 18 forms the radiating surface of the antenna 10. In the illustrated embodiment, the conductive patch 18 is square in shape, although any shape can generally be implemented (e. g., rectangle, circle, ellipse, etc.).

In accordance with the invention, the conductive patch 18 is formed from a flexible, conductive material that is capable of recovering its initial shape under the restoring force of the slab 14 when the antenna 10 has experienced a physical impact that

deforms the antenna 10. In a preferred embodiment, as illustrated in FIG. 1, the conductive patch 18 is etched upon a very thin, flexible circuit board material 16 that is also capable of recovering an initial shape under the restoring force of the slab 14.

The circuit board material 16 may also have some resilience of its own for providing an addition degree of restorative force.

In one embodiment, a circuit board material that is sold by Rogers Corporation under the trademark BEND/flexTM and identified by part number 2411.0085 is used to carry the conductive patch 18. This material includes an epoxy-based flexible dielectric board material that is covered on side by a thin layer of copper. The board material has a nominal thickness of 0.0085 inch and the copper layer has a nominal thickness of 0.0014". In a preferred approach, the patch 18 is etched into the copper, and faces away from the slab 14, as illustrated in FIG. 1.

In an alternative embodiment, the conductive patch 18 comprises a supple conductive film that is bonded directly to the dielectric slab 14 without an intervening circuit board structure. The film can include, for example, a thin metallic sheet that is bonded to the dielectric slab using a flexible adhesive material. Examples include a product described by 3M as EMI Shielding Tape and sold by various part numbers, including 1183. Alternatively, a conductive paint can be applied to the slab 14 that remains relatively pliable after drying. An example of such a coating is CHO-FLEX 601TM sold by Chomerics, Inc. In another approach, a conductive film is deposited on the surface of the dielectric slab 14 using a well known deposition process, such as electroplating or sputtering. In yet another approach, a thin flexible screen material is bonded to the upper surface of the dielectric slab 14 to provide the flexible patch 18.

In yet another approach conductive cloth, such as a product described by 3M as Metallized Fabric Shielding Tape and sold under part number 1190, may be bonded to the upper surface of the dielectric slab 12 to provide a flexible patch 18. Regardless of the method used to apply the film, the resulting conductive layer should be relatively thin and flexible so that any deformations (e. g., dents) caused by impacts to the antenna 10 will be substantially removed under the restoring force of the dielectric slab 14. As can be appreciated, any of a number of other flexible conductor arrangements can be used to provide the patch 18 of the antenna. The ground plane 12 is a conductive surface that allows the patch 18 to radiate in a desired manner. In this regard, virtually any sheet of conductive material can be used to provide the ground

plane 12, whether flexible or rigid. In a preferred embodiment, a flexible ground plane structure is used. For example, any of the conductive structures discussed above for providing the conductive patch 18 can also be used to provide the ground plane 12 of the antenna 10. In one approach, for example, a thin flexible board material having a thin metallic layer covering one side thereof is used to supply the ground plane 12.

Preferably, the side of the board material having the metallic layer faces the dielectric slab 14. In another embodiment, the ground plane 12 is a rigid or semi-rigid structure.

For example, a conventional rigid circuit board having a conductive layer on one side thereof can be used as the ground plane 12. Alternatively, a rigid metallic plate can be used.

FIG. 2 is an isometric view illustrating the antenna 10 of FIG. 1 having the ground plane 12, the dielectric slab 14, and the circuit board 16 bound together into a single antenna unit. The components can be bound together using virtually any bonding technique that is capable of maintaining the flexible, resilient nature of the antenna 10 and that will not negatively interfere with the electrical characteristics of the antenna. In one embodiment, for example, the ground plane 12, the dielectric slab 14, and the circuit board 16 are bonded together using a non-conductive adhesive material, such as a polyurethane adhesive manufactured by Ciba and sold under the product designation of Araldite 2042.

FIG. 3 is a diagram illustrating a feed arrangement 20 that is used to feed the antenna 10 of FIG. 1 in one embodiment of the present invention. As illustrated, the feed arrangement 20 utilizes two coaxial cables of equal length 22,24 to deliver RF feed signals from an RF transmitter unit 26 to the antenna 10. The coaxial cables 22, 24 each have an outer shield that is coupled to ground at the RF transmitter 26. The outer shields of the two coaxial cables 22,24 are also coupled to the ground plane 12 at the antenna 10 using conductive straps 28,30. Each of the coaxial cables 22,24 also includes a center conductor 32,34 that is coupled near a corresponding edge of the conductive patch 18 at a point of proper impedance match. At the opposite end, each of the center conductors 32,34 is coupled to a 90 degree hybrid coupler 36 (within the RF transmitter unit 26) to provide a 90 degree phase difference between the feed signals on the two center conductors 32,34 (i. e., quadrature signals). Thus, the conductive patch 18 is fed at two orthogonal locations by quadrature signals and a circularly polarized transmit signal is radiated from the antenna 10.

In an alternative embodiment, the function of the 90 degree hybrid coupler 36 can be implemented by other means. By replacing the 90 degree hybrid coupler 36 in Figure 3 with a splitter that produces equal amplitude signals in phase with each other, the necessary quadrature relationship can be produced by lengthening one of the two cables 22 or 24. Based on the center frequency of the antenna 10, by establishing a length difference which induces a quarter wavelength delay in the signal as it passes through the longer of the two cables 22 or 24, the correct quadrature relationship is established at the antenna 10.

FIG. 4 is a side view of a helmet 40 having a plurality of antennas 10 disposed about an outside surface thereof. In a preferred embodiment, the helmet 40 has four antennas 10 disposed approximately orthogonal to each other about the helmet 40 to achieve full 360 degree transmission coverage. An electronics module 42 disposed on the helmet 40 includes, among other things, a controller for controlling system operation and an RF transmitter for generating the feed signals that are delivered to the antennas 10. As discussed previously, in a preferred embodiment, the electronics module 42 is coupled to each of the antennas 10 using two coaxial cables that feed orthogonal locations of the corresponding patch 18 in quadrature to achieve a circularly polarized transmit signal. The electronics module 42 also attaches to IR sensors 44 for detecting laser queries directed at the soldier wearing the helmet 40.

Although not shown, the helmet 40 will typically be covered with a camouflage covering that provides further protection and support to the antennas 10.

FIG. 5 is an exploded isometric view of the antenna in FIG. 1 illustrating assembly of the constituent parts into a curved geometry that doesn't require external forces to maintain said geometry. In a preferred approach, each antenna 10 is held on the helmet 40 by pockets in a cloth camouflage cover, although other attachment methods can also be used such as bonding, or the use of bezels and conventional screw hardware. Although the antenna 10 is flexible, the degree of curvature necessary to conform to the helmet 40 is considerable. In a preferred embodiment, the ground plane 12, the dielectric slab 14, and the circuit board 16 are bonded together while constrained in a curved shape matching or approximating the helmet 40 shape by tooling devices 70,71 and72 which results in an antenna 10 which naturally conforms to the helmet 40 without external forces.

FIG. 5 shows an exploded assembly of such a bonding configuration.

In an alternative embodiment, particularly those involving rigid or semi-rigid ground plane 12, the ground plane 12, the dielectric slab 14, and the circuit board 16 may be bonded together as a flat assembly. A flat platform can be provided on the helmet 40 for attaching each of the antennas 40. Alternatively, a lower surface of the rigid ground plane 12 can be shaped to conform to the curved surface of the helmet 40. Other arrangements are also possible. In one embodiment of the invention, a conductive surface of the helmet 40 is used as a ground plane for some or all of the circuitry disposed thereon. Thus, the dielectric slab 14 of each antenna 10 can be bonded directly to the conductive surface of the helmet 40 which acts as the ground plane for the antenna 10. The conductive surface of the helmet 40 can be part of the helmet structure itself (e. g., a metal helmet) or a conductive coating can be applied to a non-metallic helmet using well known deposition techniques.

FIG. 6 is a block diagram illustrating the circuitry that is disposed on the surface of the helmet 40 of FIG. 4 in one embodiment of the present invention. As illustrated, the circuitry includes: a plurality of IR sensors 44; an electronics module 42 including a decryption/demodulation unit 50, a controller 52, and an RF transmitter 54; and a plurality of antennas 10. During system operation, an IR sensor 44 detects a laser query signal that was generated by a remote entity. The IR sensor 44 converts the signal to an electrical form and delivers it to the decryption/demodulation unit 50 which decrypts and demodulates the interrogation signal. The demodulated interrogation signal is then delivered to the controller 52 which analyzes the signal and determines whether a response is to be sent to the interrogating entity. If a response is to be sent, the controller 52 generates an appropriate response signal. Normally, the response signal will include information identifying the soldier wearing the corresponding helmet 80 as a friendly entity. The response signal is encoded and encrypted within the controller 52 and delivered to the RF transmitter 54 which performs typical transmitter functions on the signal, such as frequency up-conversion and power amplification. In addition, the RF transmitter 54 will split the signal into four equal amplitude feed signals to be delivered to each of the antennas on the helmet 40. In a preferred embodiment, as described above, the RF transmitter 54 will also split each of the feed signals into two equal amplitude quadrature components for use in radiating a circularly polarized transmit signal from each of the antennas 10.

As described above, the feed 60 between the RF transmitter 54 and each of the antennas 10 preferably includes two coaxial cables, one cable feeding each of two orthogonal locations of a corresponding patch 18. It should be appreciated, however, that any of a number of alternative feed arrangements can be used in accordance with the present invention. For example, in one alternative scheme, a flexible microstrip feed arrangement is used where a pair of microstrip transmission lines run along a surface of the helmet 40 to each of the antennas 10 to provide quadrature feed signals thereto. At the antenna 10, a flexible circuit board is used to carry each microstrip feed line up a side of the antenna 10 to the appropriate location of the corresponding patch 18. The microstrip feed line can also include impedance matching circuitry for improving impedance transition into the antenna 10, thus reducing undesired signal reflections in the system. The ground plane of the microstrip feed line will preferably be directly coupled to the ground plane 12 of the antenna 10 using, for example, a conductive strap. Because the circuit board of the microstrip feed line is flexible, it is able to flex when the antenna 10 is physically impacted. The circuit board will then be able to return to its original shape under the restoring force of the dielectric stab 14 after the deforming force has been removed.

In another feed approach, a compressible center feed arrangement is provided through the dielectric slab 14. That is, the patch 18 is fed by a compressible conductor structure (e. g., a braided conductor or flexible conductive ribbon) that extends up through an orifice in the dielectric slab 14 from below. Because the feed conductor is compressible, it will be able to compress or flex should the antenna 10 be impacted and return to its original shape after the deforming force has been removed.

Although the present invention has been described in conjunction with its preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art readily understand. For example, the inventive principles are not limited to use in military applications or applications requiring the location of an antenna on an individual=s person. On the contrary, the inventive antenna structure can be used in any environment where repeated impacts or shock forces may be experienced or in situations where curvature is desired. In addition, the principles of the invention can be used with radiator types other than patch radiators (such as, for example, microstrip dipole radiators and the like). Furthermore, although described above as primarily a

transmit antenna, it should be appreciated that the inventive antenna structure can also be used in applications requiring a receive antenna or a transmit/receive antenna.

Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.