Patent Description:
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.

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 <CIT>. 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 <CIT>. Another example of a currently-available electronically-controlled steerable beam antenna using switchable antenna elements (scatterers) is disclosed in <CIT>.

<CIT> 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.

More specifically, in the antenna disclosed in the above-mentioned '<NUM> 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.

The antenna beam direction ϕ is determined by the period P and the wave propagation speed v in the antenna feed line:
<MAT>
where c is the speed of light, and λ is the free-space wavelength of the beam.

While the above-described antenna of the '<NUM> 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.

Prior art document <CIT>, relates to suppression of parasitic radiation in steerable beam antennas.

Prior art document <CIT>, discloses a surface scattering antenna comprising two separate beams, each propagating in a separate direction, and having different polarizations.

The invention is defined in the independent claims <NUM> (system) and <NUM> (method). Advantageous embodiments are described in the dependent claims. The invention is defined in particular by the essential features prominently marked by the words "according to an essential aspect of the present invention". Other embodiments and/or examples not falling under the claims and not comprising these features are not part of the claimed invention but are useful for understanding the invention. According to an essential aspect of the present invention, 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. According to an essential aspect of the present invention, a steerable beam antenna system comprises a feed line or transmission line defining an axis X; 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 '<NUM> 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 ΔP. The resulting polarization of the antenna beam (from both arrays) depends on the value of ΔP. In embodiments of this disclosure, a linear polarization parallel to the feed line axis occurs when there is no period shift (ΔP = <NUM>); a period shift of ΔP =±P/<NUM> (phase shift = <NUM>°) yields a linear polarization orthogonal to the feed line axis; a left-hand circular polarization is produced when the period shift is ΔP= P/<NUM> (phase shift = <NUM>°) ; and a right-hand circular polarization is produced when the period shift is ΔP = 3P/<NUM> (phase shift = <NUM>°). If a non-zero period shift (ΔP) 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 <NUM>/<NUM> the wavelength λ of the radiated beam, and preferably less than λ/<NUM>.

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 ΔP 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).

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 α with the feed line axis, while the scatterers of the second array are parallel to each other, each forming an angle -α with the feed line axis. The angle α is selected so that each scatterer radiates with a linear polarization of <NUM>° relative to the feed line axis.

<FIG> 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.

In this aspect, an electromagnetic signal feed line <NUM> defines an X axis, with a first linear array <NUM> of scatterers or pixels <NUM> and a complementary linear second array <NUM> of scatterers or pixels <NUM> arranged on opposite sides of the feed line <NUM> parallel to the X axis. Each of the scatterers <NUM> in the first array <NUM> and each of the scatterers <NUM> in the second array <NUM> is switchable (preferably by electronic control) between a high state (H-state, represented by a "<NUM>" in the drawings) and a low state (L-state, represented by a "<NUM>" in the drawings) to scatter a wave propagating through the feed line <NUM> so as to form a steerable antenna beam, in which the beam direction is controlled via the period of the reciprocating patterns of the H-state scatterers and the L-state in the first and second arrays, respectively. More specifically, each of the scatterers <NUM> in the first array <NUM> and each of the scatterers <NUM> in the second array <NUM> 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.

The scatterers <NUM> of the first array <NUM> are parallel to each other, with each scatterer <NUM> forming an angle α relative to the X axis defined by the feed line <NUM>. The scatterers <NUM> of the second array <NUM> are likewise parallel to each other, with each scatterer <NUM> forming an angle -α relative to the X axis, whereby the first and second arrays are mirror images of each other, with the feed line <NUM> as a mirror plane. The magnitude of the angle α is selected so that the output radiation is linearly polarized at <NUM>° relative to the feed line <NUM>, 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 α is nominally <NUM>°, 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 α) to deviate from <NUM>° 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 λ of the radiated beam, and preferably less than λ/<NUM>, such as, for example, λ/<NUM> or λ/<NUM>, or even less.

Each of the scatterers in the first array <NUM> and the second array <NUM> is controllably connectable to ground by a switch <NUM> that may be implemented, for example, by a PIN diode. Although shown schematically as diodes, the switches <NUM> can be implemented as controllable resistors, MEMs, MOSFETSs, or any other suitable switching component. The switches <NUM> 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 <NUM> is associated with each of the scatterers in both the first array <NUM> and the second array <NUM>.

Switching a switch <NUM> to ground (e.g., closing the switch) transitions its associated scatterer from the L-state (<NUM>) to the H-state (<NUM>), while opening the switch (disconnecting its associated scatterer from ground) transitions its associated scatterer from the H-state (<NUM>) to the L-state (<NUM>). The switches <NUM> in each array may advantageously be operated in response to a control signal from a controller <NUM> that, in some embodiments, operates the switches <NUM> in accordance with a software program that is retrieved from memory (not shown) or is otherwise input to the controller <NUM>. The controller <NUM> 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.

Generally, the pattern of H-state scatterers and L-state scatterers in the first array <NUM> will have a first correlation to the pattern in the second array <NUM> 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 <NUM>, by a period shift ΔP that results in a second correlation that produces a beam having a selectable second type of polarization.

By way of specific example, <FIG>, <FIG> illustrate how the polarization of the output beam can be controllably varied by shifting the pattern of H-state and L-state scatterers <NUM> in the first array <NUM> relative to the corresponding pattern of the scatterers <NUM> in the second array <NUM>, or vice versa. The shifting is done by actuating the switches <NUM> of the appropriate scatterers in the either the first array <NUM> or the second array <NUM> 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 H-state scatters (<NUM> in the illustrated example) and d is the separation distance between each adjacent pair of scatterers. While n can be any integer greater than <NUM>, in the illustrated example, each period consists of three successive L-state scatterers or pixels and one H-state scatterer or pixel. Thus, a quarter period (nd/<NUM>) will encompass one scatterer or pixel.

In <FIG>, the pattern of H-state scatterers and L-state scatterers is the same in the second array <NUM> as it is in the first array <NUM>, indicated a "zero-shift" (i.e., ΔP = <NUM>) of the second array <NUM> relative to the first array <NUM>. 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 X-axis (which may be referred to as "horizontal" or "H" polarization).

<FIG> shows the scatterer pattern in the second array <NUM> shifted by P/<NUM> (i.e., ΔP = 2d, where P = 4d) relative to the scatterers in the first array <NUM>. In this configuration, the resultant output beam is linearly polarized in the direction orthogonal to the X-axis (which may be referred to as "vertical" or "V" polarization).

<FIG> shows the scatterer pattern in the second array <NUM> shifted by P/<NUM> (i.e., ΔP = d, where P = <NUM>) relative to the scatterers in the first array <NUM>. In this configuration, the resultant output beam is circularly polarized in the clockwise or left-hand direction ("CL" polarization). Similarly, in <FIG>, the scatterer pattern in the second array <NUM> is shifted by 3P/<NUM> (i.e., ΔP = 3d, where P = <NUM>) relative to the scatterers in the first array <NUM>, yielding circular polarization in the counter-clockwise or anti-clockwise (right-hand) direction ("CR" polarization).

It will be appreciated that any one of the relationships between the respective patterns of the first and second arrays illustrated in <FIG> 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.

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

Where ΔP is not zero, d, or an integral multiple of d, the period shifts (ΔP) will result in polarizations that will deviate from the desired linear or circular polarization. Several such cases are shown in <FIG>. For example, for an antenna with P = 4d, values of ΔP of P/<NUM>, 3P/<NUM>, 5P/<NUM>, or 7P/<NUM> correspond, respectively, to d/<NUM>, 3d/<NUM>, 5d/<NUM>, and 7d/<NUM>, 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 <NUM>, 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 ΔP.

<FIG> is a graphical representation of the relative polarization components in antenna beams formed with different period shifts (ΔP) described above and illustrated in <FIG>. 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 <NUM>-pixel (<NUM>-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 <NUM>-pixel shift; and left-hand and right-hand circular polarizations peak at shifts of <NUM> pixels and <NUM> pixels, respectively.

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 '<NUM> patent, in which the antenna beam direction ϕ is determined by the period P and the wave propagation speed v in the antenna feed line:
<MAT>
where c is the speed of light, and λ is the free-space wavelength of the beam.

Claim 1:
A steerable beam antenna system operable to produce a steerable antenna beam having a plurality of selectable polarizations, the antenna system comprising:
a feed line (<NUM>) defining an axis X;
a first array (<NUM>) of switchable scatterers (<NUM>) disposed along a first side of the feed line (<NUM>) parallel to the axis X; and
a second array (<NUM>) of switchable scatterers (<NUM>) disposed along an opposite second side of the feed line (<NUM>) parallel to the axis X, wherein the second array (<NUM>) of switchable scatterers (<NUM>) is a mirror image of the first array (<NUM>) of switchable scatterers (<NUM>) relative to a mirror plane defined by the feed line (<NUM>);
wherein the first array (<NUM>) of scatterers (<NUM>) is configured to scatter an electromagnetic wave propagating through the feed line (<NUM>) so as to form a first steerable antenna beam portion with a first polarization, and the second array (<NUM>) of scatterers (<NUM>) is configured to scatter the electromagnetic wave propagating through the feed line (<NUM>) so as to form a second steerable antenna beam portion with a second polarization orthogonal to the first polarization;
wherein each of the scatterers (<NUM> and <NUM>) in the first and second arrays (<NUM> and <NUM>) is switchable between a high state and a low state, wherein the scatterers (<NUM> and <NUM>) in the high state and the scatterers (<NUM> and <NUM>) in the low-state in each of the first and second arrays (<NUM> and <NUM>) defines a pattern having a period P = nd, where n is the number of scatterers (<NUM> and <NUM>) between successive scatterers (<NUM> and <NUM>) in the high state and d is the separation distance between adjacent scatterers (<NUM> and <NUM>) that is no greater than one-third the wavelength λ of the propagating electromagnetic wave; and
wherein the scatterers (<NUM> and <NUM>) in the first and second arrays (<NUM> and <NUM>) are switchable to shift the pattern of scatterers (<NUM> and <NUM>) in the high state and scatterers (<NUM> and <NUM>) in the low state in at least one of the first and second arrays (<NUM> and <NUM>) relative to the pattern of scatterers (<NUM> and <NUM>) in the high state and scatterers (<NUM> and <NUM>) in the low state in the other of the first and second arrays (<NUM> and <NUM>) by a selectable period shift ΔP that yields a steerable antenna beam having a selected polarization from the plurality of selectable polarizations.