WO1993010571A1

WO1993010571A1 - Ferroelectric-scanned phased array antenna

Info

Publication number: WO1993010571A1
Application number: PCT/US1992/009939
Authority: WO
Inventors: Donald C. Collier; Kevin J. Krug; Brittan Kustom
Original assignee: United Technologies Corporation
Priority date: 1991-11-14
Filing date: 1992-11-12
Publication date: 1993-05-27

Abstract

An arrangement of adjacent blocks (52-70) of ferroelectric material is disposed in the path (72) of electromagnetic radiation. A plurality of conductive electrodes (88-108) are provided, each pair of adjacent blocks having a corresponding electrode disposed therebetween. The electrodes are provided with voltage levels in a selected pattern. A bias electric field results across each block such that the electric field is in a direction both normal to the propagation and parallel to the polarization direction of the radiation. A change in the bias electric field can produce a change in the extraordinary wave propagation constant (i.e., the refractive index, ne) of the ferroelectric blocks. Such change produces a phase shift in the radiation which is varied across the face of the aperture, resulting in a controllable alteration of the emanating radiation direction.

Description

Description Ferroelectric-Scanned Phased Array Antenna

Technical Field

This invention relates to phased array antennas, and more particularly to a ferroelectric-scanned phased array antenna.

Background Art

The use of ferroelectric materials in scanning devices such as antennas for radar applications has been the subject of recent interest. This is because certain dielectric properties of such materials change under the influence of an electric field. In particular, an electrooptic effect can be produced by the application of a bias electric field to some ferroelectric materials. By electrooptically varying the refractive indices of such material, a phase shift will occur in electromagnetic radiation passing therethrough.

Regions of ferroelectric materials have a non-zero electric dipole moment in the absence of an applied electric field. For this reason,

ferroelectric materials are regarded as spontaneously polarized.

A suitably oriented polarized ferroelectric medium changes the propagation conditions of passing electromagnetic radiation. A bias electric field of sufficient magnitude in the appropriate direction may change the refractive index of the medium, thereby further altering the propagation conditions. Upon incidence with a uniaxial ferroelectric medium having a suitably aligned optic axis,

radiation divides into two components (i.e., double refraction). A first component exhibits polarization of the electric field perpendicular to the optic axis, and refracts in the medium according to Snell's

Law (the ordinary ray). A second component exhibits polarization orthogonal to that of the first, with some constituent of the electric field parallel to the optic axis (the extraordinary ray). The

extraordinary ray is refracted in a different manner, and may not behave according to Snell's Law.

The refractive indices of the ferroelectric material for the two wave components, no and n_e respectively, determine the different velocities of propagation of the components' phase fronts. The applied bias electric field typically changes the refractive indices, which causes phase shifts in the propagating radiation.

Examples of radar scanning devices which

purported to take advantage of the foregoing

principles of ferroelectric materials are disclosed and claimed in U.S. Patent Nos. 4,636,799 and

4,706,094, both to Kubick, both assigned to the assignee of the present invention, and both of which are hereby incorporated by reference. Each patent describes and illustrates a monolithic piece of ferroelectric material disposed in front of a source of electromagnetic radio frequency ("RF") radiation. The material has a row of electrically conductive wires disposed on each side of the material and spanning the material from top to bottom. A DC voltage applied to the wires in a pattern produces a voltage gradient across the antenna aperture from one end to the other. Such a voltage gradient

purportedly causes a gradient in the refractive index of the material, with a resulting shift in the radiation direction, thereby effectuating

ferroelectric scanning.

Further, the ferroelectric material in Kubick patent 4,706,094 (the "electrooptic scanner patent") has an initial domain orientation parallel to the direction of propagation ("c-poled"), such c-poling being perpendicular to the surface of the

ferroelectric material. With such c-poling, the radiation is affected only by the ordinary index of refraction, n_o. However, it has been found

experimentally that the electrooptic effect manifests itself more commonly in the extraordinary wave refractive index, n_e. Thus, to achieve wave phase shifting, the polarization must be parallel to the optic axis, and, thus, to the bias electric field. Disclosure of Invention

Objects of the present invention include

overcoming the shortcomings of the aforementioned prior art by providing an electric field in an orientation with respect to a radar scanner comprised of ferroelectric material so as to affect a changing of the direction of radiation emanating from the material, the orientation being such that the optic axis is orthogonal to the direction of radiation propagation and parallel to the radiation's

polarization.

According to the present invention, an

arrangement of adjacent blocks of ferroelectric material is disposed in the path of electromagnetic radiation. A plurality of conductive electrodes are provided, each pair of adjacent blocks having a corresponding electrode disposed therebetween. The electrodes are provided with voltage levels in a selected pattern. An electric field results across each block such that the electric field is in a direction both normal to the propagation direction of the radiation and parallel to the polarization direction of the radiation. A change in the electric field varies the extraordinary wave propagation constant (i.e., the refractive index, n_e) of the ferroelectric blocks. Such change produces a phase shift in the radiation which is varied across the face of the aperture, resulting in a controllable alteration of the radiation direction.

These and other objects, features and advantages of the present invention will become more apparent in light of the detailed description of a best mode embodiment thereof, as illustrated in the

accompanying drawings.

Brief Description of Drawings

Fig. 1 is a perspective view of an arrangement of a radar scanner comprised of ferroelectric

material as found in the prior art;

Fig. 2 is a perspective view of a radar scanner comprised of an arrangement of ferroelectric material according to the present invention;

Fig. 3 is a perspective view of the arrangement of ferroelectric material of Fig. 3; and

Fig. 4 is a perspective view of a portion of the arrangement of the ferroelectric material of Fig. 3. Best Mode For Carrying Out The Invention

In Fig. 1 is illustrated a perspective view of a prior art radar scanner (13) comprised of a

monolithic piece of ferroelectric material (17). The scanner scans a beam (22) of millimeter wavelength radiation. The scanner illustrated is that described and claimed in the aforecited Kubick patents. (The parenthetical reference numerals are those

illustrated in Fig. 1 of each of the Kubick patents). The scanner includes parallel input and output sides with impedance matching layers (44). Adjacent and opposite parallel wire grid electrodes (31) are excited with voltage levels across the face of the material (17). Such excitation modifies the

refractive index distribution of the scanner and thereby effectively steers the radiation direction.

As described hereinbefore, the electric field in the Kubick electrooptic scanner patent is parallel to the propagation direction of the beam (22). However, it has been found experimentally that the

electrooptic effect manifests itself more commonly in the extraordinary wave index, n_e. Thus, to achieve wave phase shifting, the wave polarization must be parallel to the optic axis, and, thus, to the bias electric field. The present invention implements this configuration in a manner that provides for very uniform application of bias field, as well as a simplified voltage divider network.

Referring to Fig. 2, there illustrated is a perspective view of a radar scanner 50 according to the present invention. The scanner is similar in some respects to that of Fig. 1 of the Kubick

patents. The differences between the Kubick patents and the present invention lie in the novel structure of the ferroelectric material arrangement used to redirect the electromagnetic radiation passing therethrough.

The scanner 50 comprises an arrangement or stack 51 of blocks 52-70 of ferroelectric material arranged adjacent to one another in a vertical orientation. The material may comprise barium strontium titanate, or any other material, either ferroelectric or non-ferroelectric, having refractive index (e.g., extraordinary wave refractive index, n_e) properties which vary in the presence of an applied electric field. As described in detail hereinafter, the scanner redirects a wave 72 of electromagnetic radiation emanating from a source of RF energy, such as a flared horn 74. As viewed in Fig. 2, the RF wave illustrated is one having its electric field polarization in a horizontal orientation with respect to the scanner 50 and the blocks 52-70. The

polarization is orthogonal to the direction of propagation of the RF wave 72.

The ferroelectric blocks 52-70 are distributed over the aperture of the horn 74 in the form of a planar layer of substantially uniform thickness "d". The thickness is selected to establish at least a single wavelength (i.e., 2τr radian) phase delay under a selected electric field excitation level.

Referring also to Fig. 3, there illustrated in greater detail are the individual ferroelectric blocks 52-70 disposed adjacent to one another in the stack 51. Each block has an electrode comprising a corresponding thin layer of conductive material, e.g., silver, deposited in a known fashion on an inner surface. The electrode surface contacts, or is closely disposed next to, a similar surface on an adjacent block. Each block also has a uniform width "w". As described in detail hereinafter, each block acts as an RF wave phase shifting unit.

The conductive-coated surfaces can better be seen in Fig. 4. There illustrated are two blocks 60,62 in the stack 51 shown apart from one another. Block 60 has, on two surfaces 80,82, the conductive coating deposited thereon. Similarly, block 62 has, on two surfaces 84,86, the conductive coating

deposited thereon. Surfaces 82,84 may be in

electrical contact if blocks 60,62 are brought together in physical contact. Similarly, surface 80 of block 60 may contact a coated surface of block 58, while surface 86 of block 62 may contact a coated surface of block 64. Such capacitor-like parallel plate electrode arrangement provides for a very uniform bias electric field.

Corresponding electrical conductive wires 88-108 are provided in electrical contact with the

conductive-coated surfaces. Such electrical contact is accomplished by soldering or any other known electrical contacting means. Fig. 4 better

illustrates the conductive wires 96,98 attached to the conductive layers 80,84, respectively.

Adjacent to the front and back sides of the stack 51 of ferroelectric blocks 52-70 are disposed impedance matching layers 110,112. Each layer

110,112 comprises material, e.g., magnesium calcium titanate having a dielectric constant in the range of 15-140. The refractive index is the square root of the dielectric constant, or relative permittivity. The layers are required because of the impedance mismatch between free space and the high dielectric constant (e.g., >500) of the ferroelectric material. Without these layers, the RF wave impinging upon the ferroelectric blocks would be reflected off the face of the blocks. The resulting stack 51 comprising the blocks 52-70 together with the layers 110,112 has parallel front and back sides which are perpendicular to the propagation direction of the RF wave.

The magnesium calcium titanate is chosen to have a dielectric constant which equals the square root of the dielectric constant of the corresponding

ferroelectric material comprising the blocks. Such characteristic of the impedance matching layers provides for wide matching bandwidth. The impedance matching layers are preferably prefabricated into thin sheets or layers having a selected thickness. The layers are attached to each side of the stack 51 using adhesive or other known bonding techniques.

Assuming a dielectric constant of 625 for the ferroelectric blocks 52-70, the permittivity of each layer is 25 (i.e., the square root of 625). Low-loss microwave ceramics comprised of varying compositions of magnesium and calcium titanates are commercially available with dielectric constants in the range of 10 to 140, measured at the X frequency band (8.2 GHz to 12.4 GHz). As these materials show no dispersion in the X band, it is expected that their dielectric properties will remain constant as the frequencies increase into the Ku frequency band (12.4 GHz to 18.6 GHz). To achieve optimal radiation coupling, the impedance matching layers must be a quarter

wavelength thick at the operating frequency. Such characteristic of the impedance matching layers may reduce reflections of the radiation by nearly 100%. For a permittivity of 25, the layer thickness is 0.159 cm (about 59 mils) for operation at 10 GHz.

Through use of impedance matching layers, the

thickness, d, of the ferroelectric blocks can be freely varied, limited only by structural

considerations and insertion loss.

The conductive wires 88-108 individually connect to a voltage source 116 through a known electronic circuitry switch/addressing ("S/A") function 118.

The S/A function 118 controls the application of a sustained voltage to the individual wires 88-108.

The S/A function may comprise, e.g., a number of parallel switches each independently controllable and in series with variable resistances (not shown), thereby applying variable voltage levels to the wires.

In accordance with the present invention, the voltage on each wire creates an electrical field across each block in an orthogonal orientation with regard to the direction of the RF wave 72 radiating from the horn 74. The magnitude of the voltage on each wire is chosen so that a pattern of ascending voltage differences results across the blocks in one direction. For example, wire 88 may provide an outer surface 120 of block 52 with zero volts DC (0VDC, ground), while wire 90 may have 1VDC applied thereto. Thus, the voltage across block 52 equals +1VDC.

Next, wire 92 may have -1VDC applied thereto, for a voltage drop across block 54 of 2VDC. Wire 94 may have +2VDC applied thereto, for a voltage drop across the block of 3VDC. Such pattern of voltage application is repeated until an ascending pattern of voltage drops results across all blocks 52-70 in one direction. This method of bias field application provides for a much simplified voltage divider network. The magnitude of applied voltages described above is purely exemplary; the only constraint imposed on the voltage magnitude is that it be sufficient to cause changes in the extraordinary refractive index of the ferroelectric material comprising each block.

The RF wave radiating from the horn divides into components upon incidence with the ferroelectric blocks. According to the present invention, the phase shift of the RF wave is modified spatially by electrooptically varying the refractive index of the ferroelectric blocks from one side of the antenna to the other. This is accomplished by applying the sustained bias electric field of sufficient magnitude in an appropriate direction. Accordingly, the RF wave component 72 polarized orthogonally to the blocks generally travels through a block of

ferroelectric material at a speed determined by an extraordinary refractive index, n_e(O), if that block is not subject to a bias electric field excitation. However, if the block is subject to a selected level of bias electric field excitation, E, then the refractive index of the ferroelectric material is at a selected value, n_e(E)) which can be set.

The magnitude of the bias electric field applied across any block is determined by the voltage

difference, V, applied across that block. The corresponding electric bias field across any block is: E = V/w (Eq. 1) where w is the width of the block.

Consider an RF wave 72 radiating from the horn 74 and having its polarization perpendicular to the electrode surfaces, e.g., 80-86 (i.e., horizontal polarization). If no bias electric field is applied across the leftmost block 52 (Fig. 3), the portion of the RF wave passing therethrough exits with a phase shift Φ₁ (relative to its phase upon entering the layer) given by: Φ₁ = 2π*(d/L)*n(O) (Eq. 2) where L (lambda) is the free space wavelength, d is the physical distance through which the wave passes

(i.e., the block thickness), and n(O) is the

refractive index of the ferroelectric block for horizontal polarization (extraordinary wave) with zero electric field bias.

If a voltage is applied across the second leftmost block 54, producing a bias electric field, E,, in the material, the portion of the

electromagnetic wave passing therethrough exits with phase shift:

Φ₂ = 2π*(d/L)*n(E₂) (Eq. 3) where n(E₂) is the refractive index of the

ferroelectric block with an applied electric field of

For a electrooptic ferroelectric, n(E) is less than n(O), and thus Φ₂ is less than Φ₁. Thus, a phase difference δ₁₂ exists between the portion of the wave exiting block 52 and the portion exiting block 54, equal to: δ₁₂ = Φ₁ - Φ₂ = 2π* (d/L) *Δn₁₂ (Eq. 4) where

Δn ₁₂ = n(E₂) - n(E₁) (Eq. 5)

By applying successively greater bias electric fields across the width of the aperture, such that each portion of the RF wave exits a block with a phase difference δ with respect to adjacent blocks (δ₁₂ = δ₂₃ = δ₃₄ = ... = δ), the composite signal emanating from the scanner is an RF wave whose line of constant phase no longer propagates normal to the scanner, but at an angle β given by: sin(β) = (1/2π)*(L/s)*δ (Eq. 6) where s is the separation between block centers.

Combining Eqs. 4 and 6 yields: sin(ø) = (d/s)*Δn (Eq. 7)

Thus the scan angle is independent of wavelength.

If the assumption is made that the refractive index is a linear function of the bias field over some operating region with electrooptic coefficient F(n = FE), then, from Eq. 1: n = (F*V)/w (Eq. 8) The change in refractive index, An, from one block to the next as a result of a change (increase) in voltage difference ΔV from one block to the next is: Δn = (F*ΔV)/w (Eq. 9)

The scan angle can then be expressed as: sin(β) = (d/s)*F*ΔV/w (Eq. 10) or, since the thickness of the conductive layers and medium used to bond the layers to the block surfaces are much less than w, then s ≈ w, and: sin(β) = (d/w²)*F*ΔV (Eq. 11)

In this case, the scan angle is proportional to the scanner (block) thickness, d, the measure of

ferroelectric electrooptic activity, F, the voltage gradient between the blocks, ΔV, and inversely proportional to the square of the block width, w.

In summary, the conductive surfaces apply a bias electric field perpendicular to their faces,

producing a variation of the refractive index in each block, thus varying the RF wave path length along the width of the aperture of the flared horn. The variation in path length "bends" the RF wave

emanating from the blocks, for wave polarizations parallel to the bias electric field, at an angle β to the normal. By dynamically varying the control voltages on the conductive surfaces, the direction of the RF wave can be scanned in azimuth. The total amount of phase shift required across the aperture of width S = (N - 1)s, where N is the number of blocks is: δtotal = (N - 1)δ (Eq. 12) then: sin (β) = (1/2π)*(L/S)*δ_tot (Eq. 13)

Combining Eqs. 3 and 8 results in an expression for the phase shift in an active layer as a function of applied voltage: Φ = 2π* (d/L) *F* (V/w) (Eq. 14)

The maximum operating field [E_max = (V/w)_max] will prroduce a maximum single-block phase shift Φ_max for a given material electrooptic coefficient, F.

There are two methods to achieve the total phase shift, δ_tot, required for the scanner. The first is to choose the block thickness, d, such that any individual layer's maximum phase shift, Φ_max, is large enough to achieve the total shift necessary, δ_tot. This may be many multiples of 2π. By making d/L a large number, large variations in Φ can be achieved. However, a large value for d will also increase whatever material losses exist.

The second method of achieving the phase shift necessary relies on the fact that Φ+n2π (where n is an integer) is equivalent to Φ. Since any particular block, j, will now need only to provide a phase shift:

d need only be chosen large enough such that Φ_max = 2π. That is, each time the accumulated phase

differentials exceed 2π, the voltage is reduced such that the phase shift starts at zero again.

While the first method above allows a simple control scheme utilizing a voltage divider network, the second method requires a complex controller, programmed to change ΔV when a 2π phase shift is exceeded. The second method above does have the advantages of a thinner block structure, reducing losses in the material, and the option of increasing the active layer thickness somewhat (so that Φ_max ≥

2π), reducing the high voltage requirements.

Another consideration is that adjacent pairs of conductive surfaces (located between blocks of the active media), being separated by only a thin bonding layer (≈0.001 inch), are in such close proximity that they must necessarily be biased to the same voltage. As a result, the voltage applied to the right

conductive surface of first rightmost block 70 becomes the base voltage to which is added the voltage difference required for the second block. To avoid progressively increasing high voltages, succeeding layers are biased anti-parallel to each other, as described hereinbefore. Since the

electrooptic phenomena is invariant under field reversal, this will produce an equivalent beam steering effect.

The two aforementioned Kubick patents claimed operation in the millimeter wavelength band. This corresponds to a frequency range of 40-100 GHz.

However, it was discovered experimentally that the present invention is not limited as such; it has been observed that the electrooptic activity involved in the present invention occurred at frequencies in the X and Ku bands ( ≈8-18 GHz). Further, the present invention is not limited to even such a frequency range; the invention may be used at any frequency where the aforedescribed electrooptic effect is observed. This may be anywhere in the microwave or millimeter range, or approximately in a frequency range of 1-100 GHz.

Although the invention has been illustrated and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made without departing from the spirit and scope of the invention.

we claim:

Claims

1. A phased array scanner disposed in the path of electromagnetic radiation for modifying the direction of the radiation emanating therefrom, comprising:

an arrangement of a plurality of blocks of ferroelectric material, said blocks being disposed adjacent to one another, said arrangement having parallel input and output sides in a perpendicular orientation with respect to the propagation direction of the radiation;

electrode means, disposed on surfaces of said blocks, for conducting an electric field therethrough upon the application of an electrical voltage

thereon; and

means for establishing a predetermined

distribution of electrical voltage on said electrode means such that an electrical field is established across each of said blocks in a direction

perpendicular to the propagation direction, and parallel to the polarization, of the radiation in said ferroelectric material, thereby varying the corresponding refractive index of said ferroelectric material so as to effectuate a phase change of the radiation propagating in said ferroelectric material.

2. The scanner of claim 1, wherein said

predetermined distribution of electrical voltage comprises an ascending pattern of voltage magnitudes in a lengthwise direction across said arrangement.

3. The scanner of claim 1, wherein said

ferroelectric material comprises any ferroelectric material having refractive index properties which vary in the presence of an applied electric field.

4. The scanner of claim 1, wherein said

ferroelectric material comprises barium strontium titanate.

5. The scanner of claim 1, further comprising:

impedance matching means, comprising a pair of layers of material disposed adjacent to and on each side of said arrangement of blocks, for passing the radiation into said arrangement on a first side of said arrangement, and for passing the radiation out of said arrangement on a second side of said

arrangement, thereby minimizing any reflections of the radiation when encountering said arrangement.

6. The scanner of claim 5, wherein said impedance matching means material comprises magnesium calcium titanate.

7. Apparatus, comprising:

means for generating electromagnetic radiation; an arrangement of a plurality of blocks of ferroelectric material, said blocks being disposed adjacent to one another, said arrangement having parallel input and output sides in a perpendicular orientation with respect to the propagation direction of said radiation;

thereon; and

means for establishing a predetermined

distribution of electrical voltage on said electrode means such that an electric field is established across each of said blocks in a direction

perpendicular to the direction of propagation of said radiation and parallel to an optic axis of said ferroelectric material, thereby varying a refractive index of said ferroelectric material so as to

effectuate a phase change in said radiation

propagating in said ferroelectric material.

8. The scanner of claim 7, wherein said

9. The apparatus of claim 7, wherein said means for generating electromagnetic radiation comprises a horn antenna.

10. The scanner of claim 7, wherein said

11. The scanner of claim 7, wherein said

ferroelectric material comprises barium strontium titanate.

12. The apparatus of claim 7, wherein said

electromagnetic radiation is in an approximate frequency range of 1 GHZ to 100 GHz.

13. The scanner of claim 7, further comprising:

impedance matching means, comprising a pair of layers of material disposed adjacent to and on each side of said arrangement of blocks, for passing said radiation into said arrangement on a first side of said arrangement, and for passing said radiation out of said arrangement on a second side of said

arrangement, thereby minimizing any reflections of said radiation when encountering said arrangement.

14. The scanner of claim 13, wherein said impedance matching means material comprises magnesium calcium titanate.

15. A phased array scanner disposed in the path of electromagnetic radiation for varying the phase of a polarized electric field component of the radiation emanating therefrom, comprising:

an arrangement of a plurality of blocks

material, said blocks being disposed adjacent to one another, said arrangement having parallel input and output sides in a perpendicular orientation with respect to the propagation direction of the

radiation;

thereon; and means for establishing a predetermined

distribution of electrical voltage on said electrode means such that an electric field is established across each of said blocks in a direction which is both perpendicular to the direction of propagation of the radiation and parallel to a polarization

direction of the radiation, thereby varying a

refractive index of said material so as to effectuate a phase change in the radiation propagating therein.

16. The scanner of claim 15, wherein said

17. The scanner of claim 15, wherein said block material comprises any ferroelectric material having refractive index properties which vary in the

presence of an applied electric field.

18. The scanner of claim 15, wherein said block material comprises any non-ferroelectric material having refractive index properties which vary in the presence of an applied electric field.

19. The scanner of claim 15, wherein said block material comprises barium strontium titanate.

20. The scanner of claim 15, further comprising: impedance matching means, comprising a pair of layers of material disposed adjacent to and on each side of said arrangement of blocks, for passing the radiation into said arrangement on a first side of said arrangement, and for passing the radiation out of said arrangement on a second side of said

21 The scanner of claim 20, wherein said impedance matching means material comprises magnesium calcium titanate.