BACKGROUND

[0001] The present disclosure relates to directional or steerable beam antennas, of the type employed in such applications as radar and communications. More specifically, it relates to leaky-waveguide antennas, of the type including a dielectric feed line (i.e., a potentially leaky waveguide) loaded with scatterers (antenna elements), where coupling between the scatterers and the feed line can be altered by switches, whereby the antenna’s beam shape and direction are determined by the pattern of the switches that are respectively turned on and off.

[0002] Steerable beam antennas, particularly leaky-wave antennas, are capable of sending electromagnetic signals in, and receiving electromagnetic signals from, desired directions. Such antennas are used, for example, in various types of radar, such as surveillance radar and collision avoidance radar. In such antennas, the receiving or transmitting beam is generated by a set of scatterers coupled to the feed line or waveguide. Interacting with the feed line, the scatterers create leaky waves propagating outside of the feed line. If the scatterers are properly phased, they create a coherent beam propagating in a specific direction. The leakage strength and phase caused by each scatterer depend on the geometry and location of the scatterer relative to the feed line or waveguide. The coupling strength can be controlled by changing the geometry of the scattering elements. Correspondingly, the shape and direction of the scattered beam can be controlled by varying the scatterer geometry or topology. The geometry (topology) of the scatterers can be electronically altered by using microwave (or other suitable) switches connecting parts of the scatterers. Thus, the shape and direction of the antenna beam can be controlled electronically by changing the state of the switches. Different ON/OFF switch patterns result in different beam shapes and/or directions.

[0003] Any of several types of switches integrated into the structure of the antenna elements or scatterers may be used for this purpose, such as semiconductor switches (e.g., PIN diodes, bipolar and MOSFET transistors, varactors, photo-diodes and photo-transistors, semiconductor-plasma switches, phase-change switches), MEMS switches, piezoelectric switches, ferro-electric switches, gas-plasma switches, electromagnetic relays, thermal switches, etc. For example, semiconductor plasma switches have been used in antennas described in US Patent 7,151,499, the disclosure of which is incorporated herein by reference in its entirety. A specific example of an antenna in which the geometry of the scattering elements is controllably varied by semiconductor plasma switches is disclosed and claimed in US Patent 7,777,286, the disclosure of which is incorporated herein in its entirety. Another example of a currently- available electronically-controlled steerable beam antenna using switchable antenna elements (scatterers) is disclosed in US Patent 7,995,000, the disclosure of which is incorporated herein its entirety.

[0004] US Patent 9,698,478, the disclosure of which is incorporated herein by reference in its entirety, is assigned to the assignee of this disclosure. That patent discloses an electronically-controlled steerable beam antenna system, of the general type described above, comprising a feed line or transmission line defining an axis x; and first and second arrays of electronically-controlled switchable scatters distributed along the axis x, each of the scatterers in the first and second arrays being switchable between a "high" scattering state and a "low" scattering state to scatter an electromagnetic wave propagating through the feed line so as to form a steerable antenna beam. The high state/low state scatterer pattern of the first array is advantageously quasi-periodic. The output beam position is controlled by varying the period.

[0005] More specifically, in the antenna disclosed in the above-mentioned '478 patent, the scatterers of the first array are configured to scatter an electromagnetic wave propagating through the feed line. The high-state scatterers in the first array follow a quasi-periodic pattern with a period P = nd , where n is the number of scatterers per period (including both low-state scatterers and high-state scatterers), and where d is the spacing between adjacent scatterers along the axis x. The high-state scatterers in the second array follow the similar quasi-periodic pattern, with the same period P , but the pattern of the second array can be shifted along the x axis relative to the pattern of the first array.

[0006] The antenna beam direction f is determined by the period P and the wave propagation speed v in the antenna feed line:

[0007] sirup

[0008] where c is the speed of light, and l is the free-space wavelength of the beam.

[0009] While the above-described antenna of the '478 patent achieves its intended results, it produces a steerable beam with only a single fixed polarization. It would be desirable, for many applications, to allow this type of antenna to provide for multiple controllable polarizations. SUMMARY

[0010] In some aspects, this disclosure relates to a steerable beam antenna, wherein the antenna is controllably operable to produce a steerable beam in any of several selectable polarizations. In accordance with these aspects, a steerable beam antenna system comprises a feed line or transmission line defining an axis A; and first and second arrays of electronically- controllable switchable scatters distributed on opposite sides of the feed line parallel to the axis X , each of the scatterers in the first and second arrays being switchable between a "high" scattering state and a "low" scattering state to scatter an electromagnetic wave propagating through the feed line so as to form a steerable antenna beam. The high state/low state scatterer patterns of the first and second arrays are advantageously quasi -periodic, and the output beam direction is controlled by varying the scatterer period, as in the antenna disclosed in the above- mentioned '478 patent. In accordance with this disclosure, however, with the scatterers of the first array configured to provide an antenna output having a first polarization, the scatterers of the second array are configured so that the portion of the antenna output scattered by the second array has a second polarization orthogonal to the first polarization of the portion scattered by the first array. More specifically, while the high state/low state scatterer patterns of both the first and second arrays have the same period (which, for the purpose of this disclosure, is denoted "P"), the pattern of the second array may be shifted along the axis X defined by the feed line by a period shift DR. The resulting polarization of the antenna beam (from both arrays) depends on the value of DR. In embodiments of this disclosure, a linear polarization parallel to the feed line axis occurs when there is no period shift (DR = 0); a period shift of DR =±P/2 (phase shift = 180°) yields a linear polarization orthogonal to the feed line axis; a left-hand circular polarization is produced when the period shift is DR= P/4 (phase shift = 90°) ; and a right-hand circular polarization is produced when the period shift is DR = 3P/4 (phase shift = 270°) . If a non-zero period shift (DR) is not commensurate with an integral multiple of the spacing d between adjacent scatterers along the axis X, it should be approximated as the closest integral multiple to minimize the deviation from precisely linear or circular polarization, as the case may be. To this end, the distance d between the scatterers should be as small as possible: no greater than 1/3 the wavelength l of the radiated beam, and preferably less than l/4.

[0011] The scatterers in the first and second arrays are switched, preferably under electronic control, between the high state and the low state. The value of DR can be selectively varied in a prescribed sequence, for example, by selectively switching the appropriate scatterers in the second array between their high states and low states by electronic switches that can be actuated, for example, under the control of a suitably programmed processor. This arrangement would yield polarizations that would be varied in accordance with the prescribed sequence. Alternatively, the scatterers in the second array can be operated in their respective high states and low states in a specific period shift yielding a first polarization, until their high states and low states are switched to a different period shift yielding a second polarization. In either case, a polarization state can be selected to optimize performance of the antenna beam in a particular situation or application. If the inter-scatterer spacing d is small enough, this approach allows the generation of radiated beams with the desired mix of linear and circular polarizations (including various elliptical polarizations).

[0012] In accordance with embodiments of this disclosure, the above-described polarization results can be achieved with the scatterers of both the first and second arrays being shaped as either monopoles or dipoles. The scatterers of the first array are parallel to each other, each forming an angle a with the feed line axis, while the scatterers of the second array are parallel to each other, each forming an angle -a with the feed line axis. The angle a is selected so that each scatterer radiates with a linear polarization of 45° relative to the feed line axis.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a schematic diagram of an electronically controllable steerable beam antenna in accordance with aspects of this disclosure, showing first and second scatterer arrays on opposite sides of a feed line defining an axis X , with the scatterers connected to a controller through a switch network for controllably varying the polarization of the radiated beam.

[0014] Figures 2A-2D are schematic diagrams, similar to Fig. 1, showing the scatterers in each array arranged in a high-state/low state configuration with a period of 4d, showing the patterns of high states and low states in the first and second scatterer arrays that yield linear and circular beam polarizations.

[0015] Figure 3 is a graphical representation showing the polarizations yielded by several different values of DR. [0016] Figure 4 is a graphical representation of relative polarization components in the antenna beams generated by an antenna in accordance with this disclosure as functions of pattern shift DR.

DETAILED DESCRIPTION

[0017] Figure 1 diagrammatically illustrates a steerable beam antenna with an arrangement of scatterers or pixels in accordance with aspects of this disclosure that provides multiple controllable polarizations for a steerable radiated beam. In this aspect, the scatterers may be monopoles or dipoles, but, for simplicity of this discussion, they shall be assumed to be monopoles.

[0018] In this aspect, an electromagnetic signal feed line 80 defines an A axis, with a first linear array 82 of scatterers or pixels 90 and a complementary linear second array 84 of scatterers or pixels 92 arranged on opposite sides of the feed line 80 parallel to the X axis. Each of the scatterers 90 in the first array 82 and each of the scatterers 92 in the second array 84 is switchable (preferably by electronic control) between a high state (//-state, represented by a " 1 " in the drawings) and a low state (/.-state, represented by a "0" in the drawings) to scatter a wave propagating through the feed line 80 so as to form a steerable antenna beam, in which the beam direction is controlled via the period of the reciprocating patterns of the //-state scatterers and the .-state in the first and second arrays, respectively. More specifically, each of the scatterers 90 in the first array 82 and each of the scatterers 92 in the second array 84 may be implemented as a short linear segment of a microstrip line, formed as, for example, as a conductive trace on a suitable substrate by known circuit fabrication methods.

[0019] The scatterers 90 of the first array 82 are parallel to each other, with each scatterer 90 forming an angle a relative to the X axis defined by the feed line 80. The scatterers 92 of the second array 84 are likewise parallel to each other, with each scatterer 92 forming an angle -a relative to the X axis, whereby the first and second arrays are mirror images of each other, with the feed line 80 as a mirror plane. The magnitude of the angle a is selected so that the output radiation is linearly polarized at 45° relative to the feed line 80, with the scatterers of the first array providing a first scattered beam portion having a first polarization, and the scatterers in the second array providing a second scattered beam portion having a second polarization orthogonal to the first polarization. The value of a is nominally 45°, but it will depend on, for example, the dielectric constant and geometry of the feed line. In addition, factors such as RF interference between the scatterers and the feed line, ground, and other antenna elements, as well as interference between active and passive scatterers, may require the dipole/monopole orientations (angle a) to deviate from 45° relative to the feed line axis to obtain orthogonal polarizations between the first and second arrays of scatterers. The scatterers in each array are equidistantly spaced from each other by a separation distance d that is as small as possible: no greater than one-third the wavelength l of the radiated beam, and preferably less than l/4, such as, for example, l/8 or l/16, or even less.

[0020] Each of the scatterers in the first array 82 and the second array 84 is controllably connectable to ground by a switch 86 that may be implemented, for example, by a PIN diode. Although shown schematically as diodes, the switches 86 can be implemented as controllable resistors, MEMs, MOSFETSs, or any other suitable switching component. The switches 86 can be implemented as separate lumped elements, or integrated into the substrate, as when the antenna is formed on a semiconductor (e.g., silicon) wafer. They can be controlled electronically, photo-electrically, thermo-electrically, magneto-electrically, or electro-mechanically, depending on the needs of any particular application. As shown, a switch 86 is associated with each of the scatterers in both the first array 82 and the second array 84.

[0021] Switching a switch 86 to ground (e.g., closing the switch) transitions its associated scatterer from the /.-state (0) to the //-state (1), while opening the switch (disconnecting its associated scatterer from ground) transitions its associated scatterer from the //-state (1) to the /.-state (0). The switches 86 in each array may advantageously be operated in response to a control signal from a controller 94 that, in some embodiments, operates the switches 86 in accordance with a software program that is retrieved from memory (not shown) or is otherwise input to the controller 94. The controller 94 itself, in many embodiments, will be implemented as a programmable processor, whereby the processor is configured by instructions in the program to perform the switch operations needed to implement the selectable radiation polarizations in accordance with this disclosure, as explained below.

[0022] Generally, the pattern of //-state scatterers and /.-state scatterers in the first array 82 will have a first correlation to the pattern in the second array 84 that produces a radiated beam having a first type of polarization. The pattern of at least one of the arrays is shifted relative to the pattern of the other array, by appropriate actuation of the switches 86, by a period shift DR that results in a second correlation that produces a beam having a selectable second type of polarization.

[0023] By way of specific example, Figures 2A, 2B, 2C, and 2D illustrate how the polarization of the output beam can be controllably varied by shifting the pattern of //-state and /.-state scatterers 90 in the first array 82 relative to the corresponding pattern of the scatterers 92 in the second array 84, or vice versa. The shifting is done by actuating the switches 86 of the appropriate scatterers in the either the first array 82 or the second array 84 to achieve the pattern shifts corresponding to the desired polarization. In the exemplary embodiment shown in these figures, the period P of each array is nd, where n is the number of scatterers between successive //-state scatters (4 in the illustrated example) and d is the separation distance between each adjacent pair of scatterers. While n can be any integer greater than 1, in the illustrated example, each period consists of three successive /.-state scatterers or pixels and one //-state scatterer or pixel. Thus, a quarter period (nd/4) will encompass one scatterer or pixel.

[0024] In Figure 2A, the pattern of //-state scatterers and /.-state scatterers is the same in the second array 84 as it is in the first array 82, indicated a "zero-shift" (i.e., DR = 0) of the second array 84 relative to the first array 82. In this configuration, the beam polarization provided by both arrays is linear, with the polarization components produced by both arrays in a direction orthogonal to axis X canceling each other. The resultant output beam has a linear polarization in a direction parallel to the A-axis (which may be referred to as "horizontal" or "H" polarization).

[0025] Figure 2B shows the scatterer pattern in the second array 84 shifted by P/2 (i.e., DR = 2d, where P = 4d) relative to the scatterers in the first array 82. In this configuration, the resultant output beam is linearly polarized in the direction orthogonal to the A-axis (which may be referred to as "vertical" or "V" polarization).

[0026] Figure 2C shows the scatterer pattern in the second array 84 shifted by P/4 (i.e., DR = d, where P = 4) relative to the scatterers in the first array 82. In this configuration, the resultant output beam is circularly polarized in the clockwise or left-hand direction ("CL" polarization). Similarly, in Figure 2D, the scatterer pattern in the second array 84 is shifted by 3P/4 (i.e., DR = 3d, where P = 4) relative to the scatterers in the first array 82, yielding circular polarization in the counter-clockwise or anti-clockwise (right-hand) direction ("CR" polarization). [0027] It will be appreciated that any one of the relationships between the respective patterns of the first and second arrays illustrated in Figs. 2A-2D may be considered the initial relationship or correlation that produces a first type of polarization, and any one of the other relationships may be considered the period-shifted relationship or correlation that produces a second type of polarization.

[0028] Figure 3 is a graphical representation showing the types of polarizations obtained with different values of DR. In this graph, P = 4d, for illustrative purposes. From FIG. 3, it can be seen that H polarization is obtained when DR = 0, and when DR = P (i.e., 4d). V polarization is obtained when DR = P/2 (i.e., 2d). CR polarization is obtained when DR = 3P/4 (i.e., 3d); while CL polarization is obtained when DR = P/4 (i.e., d).

[0029] Where DR is not zero, d, or an integral multiple of d, the period shifts (DR) will result in polarizations that will deviate from the desired linear or circular polarization. Several such cases are shown in Fig. 3. For example, for an antenna with P = 4d, values of DR of P/8, 3P/8, 5P/8, or 7P/8 correspond, respectively, to d/2, 3 d/2, 5d/2, and 7d/2, and are thus not equal to an integral multiple of d, thereby yielding the illustrated elliptical polarizations. For an antenna with a period of 4d, such elliptical polarizations will be obtained whenever, for example, n is not divisible by 4, and the resulting elliptical shape will resemble most closely whichever "pure" linear or circular polarization would be produced by the integral multiple of d closest to DR.

[0030] Figure 4 is a graphical representation of the relative polarization components in antenna beams formed with different period shifts (DR) described above and illustrated in Figs. 2A-2D. For each type of polarization, the polarization ratio (in dB) is shown as a function of the scatterer pattern ("hologram") shift in an array having an 8-pixel (8-scatterer) period (i.e., P = 8d). It can be seen from the graph that linear polarization parallel to the X-axis peaks at zero- shift; linear polarization orthogonal to the X-axis peaks at a 4-pixel shift; and left-hand and right- hand circular polarizations peak at shifts of 2 pixels and 6 pixels, respectively.

[0031] The controllably variable polarization provided by the above-described embodiments, as will be readily appreciated, is fully implementable in a steerable beam antenna, of the type described in the aforementioned‘478 patent, in which the antenna beam direction f is determined by the period P and the wave propagation speed v in the antenna feed line:

[0032] sirup [0033] where c is the speed of light, and l is the free-space wavelength of the beam.

[0034] It will thus be appreciated from the foregoing that the controllable polarization feature disclosed herein can be adapted to a wide variety of steerable beam antenna systems, and that antenna systems employing this feature can be operated to provide controlled polarizations in different sequences as will be suitable to different applications and circumstances. It will therefore be readily understood that the specific embodiments and aspects of this disclosure described herein are exemplary only and not limiting, and that a number of variations and modifications will suggest themselves to those skilled in the pertinent arts without departing from the spirit and scope of the disclosure.