Patent Publication Number: US-11031682-B2

Title: Adaptive polarimetric radar architecture for autonomous driving

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present patent application is a continuation of U.S. patent application Ser. No. 15/842,704, filed on Dec. 14, 2017, which is hereby incorporated by reference in entirety. 
    
    
     BACKGROUND 
     Radio detection and ranging (RADAR) systems can actively estimate distances to features in the environment by emitting radio signals and detecting returning reflected signals that reflect off surfaces in the environment. As a result, distances to radio-reflective features can be determined according to the time delay between transmission and reception. The radar system can emit a signal that varies in frequency over time, such as a signal with a time-varying frequency ramp, and then relate the difference in frequency between the emitted signal and the reflected signal to a range estimate. 
     Some systems may also estimate relative motion of reflective objects based on Doppler frequency shifts in the received reflected signals. Directional antennas (e.g., array antennas) can be used for the transmission and/or reception of signals to associate each range estimate with a bearing. More generally, directional antennas can also be used to focus radiated energy on a given field of view of interest. Combining the measured distances and the directional information allows for the surrounding environment features to be identified and/or mapped. The radar sensor can thus be used, for instance, by an autonomous vehicle control system to avoid obstacles indicated by the sensor information. 
     SUMMARY 
     Examples embodiments involve adjusting the polarization of one or multiple antennas operating on a radar unit. In some examples, adjusting the polarization of a radar antenna involves using a polarization filter that causes the antenna to transmit or receive radar signals radiating in a polarization that differs from the polarization that the antenna was originally designed for. For instance, a polarization filter can be attached to or generated within a radar unit to cause one or multiple antennas to operate within a different polarization. In other examples, adjusting the polarization of a radar antenna involves modifying the inner configuration of the radar unit during development of the radar unit in order to twist (e.g., modify) the polarization of the radar antenna. For instance, the additional twist modification within a radar unit can cause electromagnetic waves traversing inside the radar unit to twist in order to output at a desired polarization. 
     In one aspect, the present application describes a radar unit. The radar unit includes a plurality of transmission antennas configured to transmit radar signals having a first polarization, and a plurality of reception antennas configured to receive radar signals having the first polarization. The radar unit also includes a polarization filter coupled to at least a portion of the plurality of transmission antennas and the plurality of reception antenna. Particularly, the polarization filter causes a subset of the transmission antennas of the plurality of transmission antennas to transmit radar signals having a second polarization, and wherein the polarization filter causes a subset of the reception antennas of the plurality of reception antennas to receive radar signals having the second polarization. 
     In another aspect, the present application describes a radar system. The radar system includes a plurality of transmission antenna configured to transmit radar signals having a first polarization and a plurality of reception antennas configured to receive radar signals having the first polarization. The radar system also includes a polarization filter coupled to at least a portion of the plurality of transmission antennas and the plurality of reception antennas. Particularly, the polarization filter causes a subset of the transmission antennas of the plurality of transmission antennas to transmit radar signals having a second polarization. In addition, the polarization filter also causes a subset of the reception antennas of the plurality of reception antennas to receive radar signals having the second polarization. 
     In yet another aspect, the present application describes a method of signaling with a radar system. The method includes transmitting one or more radar signals using an array of transmission antennas. For instance, the array of transmission antennas is configured to transmit radar signals having a first polarization. Additionally, a polarization filter is coupled to the array of transmission antennas and causes the array of transmission antennas to transmit the one or more radar signals having a second polarization. The method further includes receiving the one or more radar signals using an array of reception antennas. For instance, the array of reception antennas is configured to receive radar signals having the first polarization. Further, the polarization filter is coupled to the array of reception antennas and causes the array of reception antennas to receive the one or more radar signals having the second polarization. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the figures and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates a unit cell of a polarization filter, according to example embodiments. 
         FIG. 2  illustrates a unit cell of a polarization filter and a waveguide, according to example embodiments. 
         FIG. 3  illustrates a unit cell of a polarization filter and two waveguides, according to example embodiments. 
         FIG. 4  illustrates a waveguide, a unit cell of a polarization filter, and a horn antenna, according to example embodiments. 
         FIG. 5  illustrates a unit cell of another polarization filter, according to example embodiments. 
         FIG. 6  illustrates a unit cell of another polarization filter and two waveguides, according to example embodiments. 
         FIG. 7  illustrates a polarization filter layer, according to example embodiments. 
         FIG. 8A  illustrates a wave-radiating portion of an antenna, according to example embodiments. 
         FIG. 8B  illustrates another antenna, according to example embodiments. 
         FIG. 9  illustrates an array of open ended waveguide antenna elements, according to example embodiments. 
         FIG. 10  illustrates another array of open ended waveguide antenna elements and a polarization filter, according to example embodiments. 
         FIG. 11  illustrates an array of waveguide antenna elements, a polarization filter, and an array of waveguide output ports, according to example embodiments. 
         FIG. 12A  illustrates a twisted antenna configuration, according to example embodiments. 
         FIG. 12B  illustrates another twisted antenna configuration, according to example embodiments. 
         FIG. 13  illustrates a twisted antenna, according to example embodiments. 
         FIG. 14  illustrates a method of radiating electromagnetic waves, according to example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying figures, which form a part hereof. In the figures, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, figures, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     As discussed above, a radar system can use one or multiple transmission antennas to emit radar signals in predetermined directions to measure the environment. Upon coming into contact with surfaces in the environment, the radar signals can reflect or scatter in multiple directions. A portion of the radar signals may reflect back towards the radar system and are captured by one or multiple reception antennas. Received reflected signals can then be processed to determine locations of surfaces relative to the radar system. 
     Due to the ability to measure distances to features as well as motion of moving features within an environment (e.g., motion of the moving feature, relative motion of the feature with respect to the radar platform, and/or a combination of motion), radar systems are increasingly used to assist with vehicle navigation and safety. Particularly, vehicles may rely upon a radar system during autonomous or semi-autonomous operation to enable a vehicle control system to detect, for example, nearby vehicles, road boundaries, weather conditions, traffic signs and signals, and pedestrians, among other features within the environment surrounding the vehicle. As the number of vehicle radar systems continues to grow, there is a desire for affordable radar units that can accurately measure the surrounding environment of the radar, e.g., the surrounding environment of a vehicle, and also operate in an environment with multiple vehicles having radar systems. 
     Example embodiments presented herein include low-cost radar units that can mount at various positions and orientations on a vehicle to capture accurate measurements of the vehicle&#39;s environment. Particularly, some example radar units described herein may include one or multiple polarization filters that can manipulate the polarizations in which the transmission and reception antennas transmit or receive radar signals. As such, the position, configuration, and influence that a polarization filter has on a radar unit can vary within examples. 
     To illustrate an example implementation, a radar unit may include a transmission array and a reception array with each array consisting of one or multiple antennas configured to initially transmit or receive radar signals in particular polarizations. Polarization, such as the polarization of a radar signal, represents a property that applies to transverse waves (e.g., electromagnetic radar signals). In particular, polarization specifies the geometrical orientation of the oscillations of a transverse wave. For instance, linear polarization is the confinement of the electric field vector to a given plane along the direction of propagation. 
     To further elaborate, if a radar signal has a vertical orientation during travel (i.e., the radar signal alternates in an up and down path as the signal travels), the radar signal can be described as a vertical linear polarized radar signal. When a radar signal has a horizontal orientation during travel (i.e., the radar signal alternates between side to side along a parallel plane as the signal travels), the radar signal can be described as a horizontal linear polarized radar signal. These polarizations are not the only possible polarizations that radar signals may traverse. Rather, radar signals can also travel along other polarizations, such as slanted polarizations that traverse between horizontal linear and vertical linear polarizations. For example, a radar signal traveling at a slanted polarization may travel at positive or negative forty-five (45) degrees from a horizontal plane. 
     As such, radar units are often designed such that antennas transmit and receive in particular polarizations. For instance, a radar unit may include an array of transmission antennas and an array of reception antennas that both transmit or receive radar signals in a particular polarization (e.g., vertical linear polarization). Radar units can sometimes have a particular design to enable mass production. In these situations, although a radar unit can capture measurements in a particular polarization, the radar unit might be unable to capture measurements in other polarizations. As a result, a radar system operating on a vehicle or other entity may benefit from additional radar units designed to transmit and receive in other polarizations in order to enable the radar system to measure the environment in multiple polarizations. 
     Some examples presented herein include using polarization filters that enable radar units designed to operate in a particular polarization or polarizations to operate in other polarizations. Thus, although the transmission array and/or reception array of a radar unit may include antennas configured to transmit or receive radar signals in a particular polarization or polarizations, the radar unit may further include one or multiple polarization filters that can alter the polarization that all or some of the antennas transmit and/or receive radar signals. 
     A polarization filter can couple to various positions relative to radiating antennas in order to manipulate the polarity of the antennas. For instance, the polarization filter can couple to the radar unit directly under radiating elements or in another layer of the radar unit. Additionally, a polarization filter can also adjust the polarization of a transmitting or receiving antenna to various degrees. For instance, a radar unit can be configured with one or multiple polarization filters to adjust from transmitting (and receiving) multiple signals in a given polarization to another polarization that is orthogonal to original polarization of the radar unit design. 
     When a signal is orthogonal to another signal, this means that each signal is capable of being resolved independently of the other signal due to the way each signal traverses through the environment. For example, when a radar unit transmits both vertically linear polarized and horizontally linear polarized signals, these signals may be orthogonal to each other. In practice, the two orthogonal can be reflected by objects in the environment and received by the radar unit. As such, the vertically polarized reflection signals may be received by a vertically polarized antenna and the horizontally polarized reflection signals may be received by a horizontally polarized antenna. Because a vertically polarized signal is orthogonal to a horizontally polarized signal, a vertically polarized antenna will receive none (or a very small percentage) of a horizontally polarized signal and a horizontally polarized antenna will receive none (or a very small percentage) of a vertically polarized signal. Additionally, this can allow radar to be able to pick up any cross polarized components of a radar signal reflected by the ground or other objects that exhibit cross polarization conversions, such as complex terrain, shrubberies, trees, snow, rain, and complex targets, etc. This can allow a vehicle control system or another system to use radar to analyze the environment more thoroughly, including measuring and analyzing a driving scene with various polarizations. 
     As discussed above, a polarization filter represents a layer of material that can manipulate operation of one or multiple antennas of a radar unit. In some embodiments, a polarization filter can cause a subset of transmission antennas to transmit radar signals in a second polarization despite the transmission antennas having an initial design that causes them to transmit radar signals in a first polarization without the polarization filter. For example, when a polarization filter is positioned on a transmission antenna configured to transmit vertically linear polarized radar signals, the polarization filter can adjust the output of the transmission antenna such that the transmission antenna transmits radar signals that are differently polarized (e.g., horizontal linear polarized). Additionally, in further example embodiments, a radar unit may include one or multiple polarization filters that cause transmission antennas to transmit and reception antennas to receive radar signals radiating at slanted polarizations at approximately positive and negative forty-five degrees from a horizontal plane 
     Some example embodiments may involve using multiple radar polarization filters to adjust polarizations of one or multiple antennas of a radar unit. For example, a radar unit may include a first polarization filter coupled to a first subset of its antennas and a second polarization filter coupled to a second subset of its antennas. In the above implementation, the first polarization filter may cause the first subset of antennas to transmit and receive radar signals traveling in a first polarization (e.g., a vertical linear polarization) and the second polarization filter may cause the second subset of antennas to transmit and receive radar signals traveling in a second polarization (e.g., a horizontal linear polarization). Other examples of radar units may involve additional polarization filters. In further examples, polarization filters can overlap. 
     Some example radar units can have various configurations, including some radar units with antennas designed to transmit and/or receive radar signals in multiple polarizations. For instance, a radar unit may have arrays of antennas designed such that each array transmits or receives radar signals in a particular polarization, such as horizontal linear polarization, vertical linear polarization, and slanted linear polarizations (e.g., approximately positive and negative forty-five degrees from the horizontal plane). As such, these radar units that are capable in using radar signals in multiple polarizations may still include one or more polarization filters to modify the performance of all or a subset of its antennas. 
     Parameters of a polarization filter can vary within examples. For instance, the size, thickness, and design of polarization filters may differ depending on the design and desired performance of the radar units. In some examples, a radar unit may use a single polarization filter that covers all or a set of the antennas on the radar unit. In other examples, a radar unit may include multiple polarization filters that may or may not overlap. A radar unit that includes multiple polarization filters can have gaps in between the polarization filter or the polarization filters may also align within any space in between. 
     In addition, in some embodiments, a polarization filter may be built into the radar unit. For instance, the polarization filter can be generated as a portion (e.g., a layer) of the radar unit. The polarization filter can be the top layer positioned above antennas or as another layer. In other embodiments, a polarization filter may be configurable to attach to a radar unit. For instance, the polarization filter may couple to the radar unit via fasteners or adhesive. As such, some example embodiments can involve polarization filters that are adjustable and can couple to and switch between different portions of the same radar unit. 
     In further examples, one or multiple polarization filters may operate in accordance with one or multiple rotation components. For instance, a radar unit may include a rotation component that can adjust a position or positions of one or multiple polarization filters. Adjusting the position of a polarization filter can cause the polarization filter to modify the polarization of antennas on the radar unit in a manner that dependents on the current position of the polarization filter. A rotation component can adjust the position of a polarization filter using various techniques or sources of power to perform the adjustment. 
     In some examples, the rotation component is a microelectromechanical system (MEMS). MEMS and other potential rotation components may be made using microfabrication techniques. Through the use of MEMS devices, a radar unit may be able to adjust the polarization of one or more radar units in situ. By way of example, an in situ adjustment allows the radar unit (or vehicle) to adjust the polarization of the radar unit without having to physically replace the radar hardware on the vehicle. 
     The configuration, position, and orientation of a transmission or reception antenna as well as the underlying waveguide channel can influence the polarization in which the antenna transmits or receives radar signals, the width and distance of the transmission or reception, and direction of operation of the antenna. As such, different layouts of radar units are presented herein that depict radar units capable of various types of operation, including close range, mid-range, and far-range from a vehicle or other device. These different layouts of radar units can include one or multiple polarization filters that adjust the polarization of transmission as well as reception by given antennas. 
     In some example embodiments, the configuration of a radar unit can further influence the performance of antennas of the radar unit. In particular, a radar unit may be configured internally to additionally modify (e.g., twist) the polarization of electromagnetic waves to an extent desired before antennas transmit the modified electromagnetic waves as desired Likewise, the radar unit may be configured to receive reflections of radar signals and internally modify (e.g., twist) the received signals to a polarization desired. As an example, a radar unit may receive and twist electromagnetic waves to a desired polarization (e.g., slanted polarization). 
     Radar units capable of operating in multiple polarizations can help reduce interference and jamming that can occur when multiple vehicles or devices use radar in the same area. Interference or jamming can cause a radar unit to receive radar signals that do not accurately represent the environment from the perspective of the radar unit. For instance, the radar unit positioned on a vehicle may receive unwantedly receive a radar signal that was transmitted in the same range and polarization by a radar system of a different vehicle. Further, all the different transmissions and reflections of radar signals can produce noise that impacts the performance of radar units. 
     By way of example, if two vehicles are driving toward one another, and both are transmitting radar signals with vertical polarization (or horizontal polarization), each vehicle may receive some radar signals transmitted by the other vehicle. These radar signals from the other vehicle may jam (or otherwise interfere) with the radar units of the vehicle. However, if the radar signals from the two vehicles are orthogonal to each other, then the signals will much less likely jam or interfere with the other vehicle&#39;s radar. 
     Example radar units that can transmit and receive in more than one polarization can potentially circumvent jamming and interference by transmitting and receiving radar signals in polarizations that differ from the polarization used by nearby radar systems. Likewise, a radar unit can selectively transmit and receive radar signals in one or more of multiple polarizations (e.g., all four). The radar system may use the accumulation of measurements from the multiple polarizations to measure the environment. Thus, one or more radar units may be able to image a field of view of the radar unit in one or more polarizations. 
     Further, a radar unit operating in multiple polarizations can enable further analysis of an environment. For instance, the radar system can detect water (e.g., puddles and/or weather conditions) positioned on or near the roadway based on radar measurements in multiple polarizations. Because water has polarization-specific reflection properties, changing polarizations may enhance the ability to detect water on a roadway. A vehicle control system can use enhanced detection from different polarizations to avoid flooded roadways or otherwise navigate around environments unfit for navigation. Likewise, measurements of radar signals in multiple polarizations can assist in detecting metallic traffic signs, such as stop signs and street signs. 
     In some examples, polarimetric measurements can assist with decomposing complex RF reflective objects (e.g., cars, road signs) in the environment into canonical scattering components, such as plates, edges, cylinders, and trihedrals, etc. For example, the radar signals reflecting off the edges of the metallic traffic signs in multiple polarizations can assist the radar system detect the location and estimate the boundaries of a sign. As a result, the measurements can help form a rich and discriminative feature space that can potentially be used for target classification and object identification. Further, in addition to the multi-look nature of polarimetric measurements, a system may use the measurements to characterize the polarimetric spectrum of targets and potentially provide classification capabilities. For instance, the system may identify traffic signs based on detected shapes, road barriers, other vehicles, pedestrians, and other features within an environment using radar measurements and analysis. 
     The following detailed description may be used with an apparatus having one or multiple antenna arrays that may take the form of a single-input single-output single-input, multiple-output (SIMO), multiple-input single-output (MISO), multiple-input multiple-output (MIMO), and/or synthetic aperture radar (SAR) radar antenna architecture. 
     In some embodiments, radar antenna architecture may include a plurality of “dual open-ended waveguide” (DOEWG) antennas. In some examples, the term “DOEWG” may refer to a short section of a horizontal waveguide channel plus a vertical channel that splits into two parts, where each of the two parts of the vertical channel includes an output port configured to radiate at least a portion of electromagnetic waves that enter the antenna. Additionally, a plurality of DOEWG antennas may be arranged into an antenna array. However, this technology is not limited to DOEWG antennas, other antennas may be used within the context of the present disclosure as well. 
     An example antenna architecture may comprise, for example, multiple metal layers (e.g., aluminum plates) that can be machined with computer numerical control (CNC), aligned properly, and joined together. The first metal layer may include a first half of an input waveguide channel, where the first half of the first waveguide channel includes an input port that may be configured to receive electromagnetic waves (e.g., 77 GHz millimeter waves) into the first waveguide channel. The first metal layer may also include a first half of a plurality of wave-dividing channels. The plurality of wave-dividing channels may comprise a network of channels that branch out from the input waveguide channel and that may be configured to receive the electromagnetic waves from the input waveguide channel, divide the electromagnetic waves into a plurality of portions of electromagnetic waves (i.e., power dividers), and propagate respective portions of electromagnetic waves to respective wave-radiating channels of a plurality of wave-radiating channels. 
     Some example automotive radar systems may be configured to operate with frequencies in the IEEE W band (75-110 Gigahertz (GHz)) and/or the NATO M band (60-100 GHz). In one example, the present system may operate at an electromagnetic wave frequency of 77 GHz, which corresponds to millimeter (mm) waves having an electromagnetic wavelength (e.g., 3.9 mm for 77 GHz). These radar systems may use antennas that can focus the radiated energy into beams in order to enable the radar system to measure an environment with high accuracy, such as the surrounding environment around an autonomous vehicle. Such antennas may be compact, efficient (i.e., there should be little 77 GHz energy lost to heat in the antenna, or reflected back into the transmitter electronics), and inexpensive and easy to manufacture. 
     In some example embodiments, a polarization filter may be disposed between two layers of the antenna. One of the two layers may include an array of waveguide antenna elements (e.g., waveguides of DOEWG antennas) used for radiating or receiving signals. The other layer may include waveguide output ports (i.e., ports between the polarization filter and the surrounding environment). 
     The polarization filter may be positioned such that the rounded rectangular polarization-modification channels are positioned with respect to the waveguide antenna elements and/or the waveguide output ports (e.g., rotated at an angle between 44 and 46 degrees with respect to the waveguide antenna elements and at an angle between 44 and 46 degrees with respect to the waveguide output ports). 
     The waveguide antenna elements and/or the waveguide output ports may be rectangular in shape, in some embodiments. In alternate embodiments, the waveguide antenna elements and/or the waveguide output ports may be circular in shape. Other shapes are also possible. 
     The polarization filter may be fabricated using CNC machining or metal-plated plastic molding, in various embodiments. The polarization filter could be fabricated of metal and/or dielectric, in various example embodiments. 
     The rounded rectangular channels may serve as resonant chambers that can alter the polarization of incoming electromagnetic waves. For example, high energy leakage from one polarization to another polarization (e.g., from a horizontal TE 10  polarization to a vertical TE 10  polarization) may occur within the chamber. Unlike alternative methods of changing polarization in waveguides that make use of physical twists in a waveguide occurring over a many wavelength distance, the thickness of the polarization filter can be less than a wavelength (e.g., between a half and a whole wavelength of corresponding input electromagnetic waves) while still achieving sufficient polarization conversion. The rounded rectangular polarization-modification channels may also be designed such that evanescent waveguide modes emanating from the channel die out sufficiently quickly as they propagate away from the channel. Because of both of these factors, less energy loss may occur during the polarization conversion, resulting in increased energy efficiency when compared with alternate methods of rotating/changing polarization. 
     In some examples, a radar system may include one or more radar units configured to adjust the polarization of radar signals. Particularly, a radar unit within the system may be configured to operate as a DOEWG that receives an electromagnetic wave input from a feed guide and divides the electromagnetic waves into multiple channels. The radar unit may further include a configuration that can receive the divided and pre-twisted electromagnetic wave from the neck of the feed waveguide and provide additional twisting (or otherwise modification) of the divided electromagnetic waves to one or multiple desired polarizations within channels. For instance, the additional twisting may cause electromagnetic waves to adjust from horizontal linear polarization to a slanted polarization. 
     As such, the radar unit may include a configuration that allows a sufficient propagation path to stabilize the twisted electromagnetic waves such that all (or a portion) of the parasitic evanescent waves cease to exist and the wanted propagating electromagnetic waves survive in the desired twisted angle (i.e., at the desired polarization). In some examples, the radar unit may further include an impedance match to free space at the end of the guide in order to assist with radiating the electromagnetic waves as radar signals into the environment. Additionally, since twisting electromagnetic waves may occur in the size of radar units capable of attaching to vehicles, the configuration allowing twisting of the electromagnetic waves may be constructed using the CNC machining process. Further, the twisted configuration may be included in various types of radar units, such as radar units configured with different arrays of antennas. For example, a radar unit that internally twists electromagnetic waves to a desired polarization may include a one by ten (1×10) radiating element array. 
     Based on the shape and the materials of the corresponding polarization-modification channels and waveguides, the distribution of propagating energy can vary at different locations within the antenna, for example. The shape and the materials of the polarization-modification channels and waveguides define the boundary conditions for the electromagnetic energy. Boundary conditions are known conditions for the electromagnetic energy at the edges of the polarization-modification channels and waveguides. For example, in a metallic waveguide, assuming the polarization-modification channel and waveguide walls are nearly perfectly conducting (i.e., the waveguide walls can be approximated as perfect electric conductors—PECs), the boundary conditions specify that there is no tangentially (i.e., in the plane of the waveguide wall) directed electric field at any of the wall sides. Once the boundary conditions are known, Maxwell&#39;s Equations can be used to determine how electromagnetic energy propagates through the polarization-modification channels and waveguides. 
     Maxwell&#39;s Equations may define several modes of operation for any given polarization-modification channel or waveguide. Each mode has one specific way in which electromagnetic energy can propagate through the polarization-modification channel or waveguide. Each mode has an associated cutoff frequency. A mode is not supported in a polarization-modification channel or waveguide if the electromagnetic energy has a frequency that is below the cutoff frequency. By properly selecting both (i) dimensions and (ii) frequency of operation, electromagnetic energy may propagate through the polarization-modification channels and waveguides in specific modes. The polarization-modification channels and/or the waveguides can be designed so only one propagation mode is supported at the design frequency. 
     There are four main types of waveguide propagation modes: Transverse Electric (TE) modes, Transverse Magnetic (TM) modes, Transverse Electromagnetic (TEM) modes, and Hybrid modes. In TE modes, the electromagnetic energy has no electric field in the direction of the electromagnetic energy propagation. In TM modes, the electromagnetic energy has no magnetic field in the direction of the electromagnetic energy propagation. In TEM modes, the electromagnetic energy has no electric or magnetic field in the direction of the electromagnetic energy propagation. In Hybrid modes, the electromagnetic energy has some of both electric field and magnetic field the direction of the electromagnetic energy propagation. 
     TE, TM, and TEM modes can be further specified using two suffix numbers that correspond to two directions orthogonal to the direction of propagation, such as a width direction and a height direction. A non-zero suffix number indicates the respective number of half-wavelengths of the electromagnetic energy equal to the width and height of the respective polarization-modification channel or waveguide (e.g., assuming a rectangular waveguide). However, a suffix number of zero indicates that there is no variation of the field with respect to that direction. For example, a TE 10  mode indicates the polarization-modification channel or waveguide is half-wavelength in width and there is no field variation in the height direction. Typically, when the suffix number is equal to zero, the dimension of the waveguide in the respective direction is less than one-half of a wavelength. In another example, a TE 21  mode indicates the waveguide is one wavelength in width (i.e., two half wavelengths) and one half wavelength in height. 
     When operating a waveguide in a TE mode, the suffix numbers also indicate the number of field-maximums along the respective direction of the waveguide. For example, a TE 10  mode indicates that the waveguide has one electric field maximum in the width direction and zero maxima in the height direction. In another example, a TE 21  mode indicates that the waveguide has two electric field maxima in the width direction and one maximum in the height direction. 
     The antennas may be used on a transmit side, a receive side, or both transmit and receive sides of a radar system. Further, the addition of a polarization filter can allow for antennas with different native polarization orientations to communicate with one another using radio communications. For example, an antenna having a vertical polarization may transmit a signal to a receiving antenna that would natively have a horizontal polarization. However, by including a polarization-modification layer and waveguide output ports, the receiving antenna can receive and convert the vertically polarized signal, thereby enabling communication between the two components. 
     In some applications, the inclusion of one or multiple polarization filters can allow various radars within a radar system to use different polarizations to perform measurements. Such a capability may allow multiple viewpoints (e.g., one of horizontally polarized electromagnetic energy and one of vertically polarized electromagnetic energy) of a single scene. For example, certain types of inclement weather (e.g., snow, rain, sleet, and hail) may adversely affect radar signaling. The use of multiple polarizations could reduce such an adverse effect. 
     Additionally or alternatively, different radars using different polarizations may prevent interference between different radars in the radar system. For example, the radar system may be configured to interrogate (i.e., transmit and/or receive radar signals) in a direction normal to the direction of travel of an autonomous vehicle via the synthetic aperture radar (SAR) functionality. Thus, the radar system may be able to determine information about roadside objects that the vehicle passes. In some examples, this information may be two dimensional (e.g., distances various objects are from the roadside). In other examples, this information may be three dimensional (e.g., a point cloud of various portions of detected objects). Thus, the vehicle may be able to “map” the side of the road as it drives along, for example. 
     If two autonomous vehicles are using analogous radar systems to interrogate the environment (e.g., using the SAR technique described above), it could also be useful for those autonomous vehicles to use different polarizations (e.g., orthogonal polarizations) to do the interrogation, thereby preventing interference. Additionally, a single vehicle may operate two radars units having orthogonal polarizations so that each radar unit does not interfere with the other radar unit. Thus, without having to redesign radar units, polarization filters can cause radar units to operate in different polarizations. 
     In some embodiments, multiple polarization filters could be cascaded together. This could increase the bandwidth of frequencies over which effective polarization conversion can occur using the corresponding antenna. Further, various combinations of cascaded polarization filters and various dimensions of the rounded rectangular polarization channels within the cascaded polarization-modification layers could serve as a frequency filtering mechanism. Thus, the associated antennas could select specific polarizations within specific frequency bands over which to perform measurements, thereby introducing an additional method of reducing interference and providing additional radar channels for use by various different radar components in a radar system. 
     Referring now to the figures,  FIG. 1  illustrates polarization filter  100 , which includes pegs  102 , through-holes  104 , and polarization-modification channel  106 . Polarization filter  100  can have other configurations and may be generated in various types of materials, such as metal fabricated using CNC. Further, polarization filter  100  can be a component of a radar unit, but can have other applications as well. Additionally, polarization filter  100  may be an example of a single polarization unit that can be replicated multiple times over a plurality of antennas in order to rotate the polarization transmitted by said antenna. 
     Multiple polarization filter  100  could further be cascaded to allow for additional rotation of polarization. Further, the cascaded polarization filters could permit an increased bandwidth of frequencies over which polarization conversion can occur. 
     Pegs  102  can enable polarization filter  100  to connect to and/or align with other components. For example, pegs  102  may align polarization filter  100  with alignment holes on other radar components, such as waveguides or antennas (e.g., a horn antenna as illustrated in  FIG. 4 ). In alternate embodiments, there may be more than two pegs  102 , fewer than two pegs  102 , or no pegs  102  at all. 
     Through-holes  104  can perform similar tasks to those performed by pegs  102  (e.g., connect and/or align polarization filter  100  with other components). For example, in some embodiments, through-holes  104  may be threaded, allowing through-holes  104  to be engaged by fasteners to connect polarization filter  100  to other radar components. As illustrated in  FIG. 1 , there are four through-holes  104 . In alternate embodiments, there may be more than four through-holes  104 , fewer than four through-holes  104 , or no through-holes  104  at all. 
     Polarization-modification channel  106  represents a portion of polarization filter  100  in which electromagnetic waves undergo a modification of polarization. The thickness and possibly other parameters of the polarization-modification channel  106 , and therefore in some embodiments the thickness of the main body of the entire polarization filter  100 , may be defined based on one or more wavelengths expected to undergo polarization modification using polarization filter  100  (e.g., if polarization filter  100  is being used in radar applications that utilize 77 GHz electromagnetic waves, the thickness of polarization filter  100  could be around 3.9 mm, or about one wavelength). 
     An angle of polarization-modification channel  106  relative to one or more mounting points (e.g., pegs  102  or the through-holes  104 ) may define how much polarization rotation occurs when polarization filter  100  acts on an electromagnetic wave. In the example embodiment illustrated in  FIG. 1 , polarization-modification channel  106  is at a 45-degree angle relative to a line between pegs  102 . Therefore, if a waveguide aligns with pegs  102  for example, electromagnetic waves passing through polarization-modification channel  106  will undergo a polarization rotation of 45 degrees. This results in the antenna transmitting or receiving radar signals that radiate at a different polarization than the antenna was originally configured for. Other angles are also possible (e.g., 44 degrees or 46 degrees). 
     In some embodiments, polarization-modification channel  106  could be filled or partially filled with a material other than air. For example, a dielectric could be used to fill polarization-modification channel  106  to alter a resonant wavelength inside of polarization-modification channel  106 , thereby altering an input wavelength range over which polarization-modification can occur using polarization filter  100 . 
     In some embodiments, the shape of polarization-modification channel  106  could be changed. For example, polarization-modification channel  106  could be circular or substantially circular, allowing for an alignment of polarization filter  100  with circular waveguides. In the embodiment illustrated in  FIG. 1 , polarization-modification channel  106  has a shape of a rounded rectangle. Geometrically, such a shape can be defined as the shape obtained by taking the convex hull of four equal circles of a given radius and placing the centers of the four circles at the four corners of a rectangle having a first side length and a second side length. 
       FIG. 2  illustrates polarization filter  100  and waveguide  202 . As shown,  FIG. 2  includes polarization filter  100  illustrated in  FIG. 1  (including pegs  102 , through-holes  104 , and polarization-modification channel  106 ), as well as a rounded rectangular waveguide  202  forming system  200 . As such, rectangular waveguide  202  and polarization filter  100  may have features sized to accommodate electromagnetic waves having a frequency of 77 GHz, for example. Other frequencies inside and outside of the radio spectrum are also possible. 
     As illustrated in  FIG. 2 , a long end of a port on waveguide  202  (e.g., the length of waveguide  202 ) may lie parallel to a line between pegs  102  of polarization filter  100 . As a result, polarization-modification channel  106  may couple at a 45-degree angle relative to the orientation of the port on rectangular waveguide  202  (other angles are also possible). This can allow the system  200  to be configured to radiate electromagnetic waves that have a polarization that is rotated by an angle (e.g., between 44 and 46 degrees) relative to an input polarization at a base of waveguide  202  (e.g., a port on a side of waveguide  202  opposite of polarization filter  100 ). 
     In other embodiments, system  200  may receive electromagnetic waves having a particular polarization at polarization-modification channel  106  and rotate or otherwise adjust the polarization of the accepted electromagnetic polarization by an angle between 44 and 46 degrees (i.e., act as a receiver rather than a transmitter). System  200  can enable communication between a component on one end (e.g., the transmit end) of a radar system to communicate with a component on a second end (e.g., a receive end) of the radar system, even if the components have different inherent polarizations. For example, the polarization-modification channel  106  could be tuned to an appropriate angle that corresponds to the difference in polarizations between the two components. 
     In alternate embodiments, rectangular waveguide  202  could instead be replaced by a circular waveguide, an elliptical waveguide, or a rectangular waveguide. In such embodiments, polarization-modification channel  106  may consequently be designed of a different shape (e.g., circular, elliptical, or rectangular). 
     Additionally or alternatively, polarization filter  100  could be used to select specific polarizations or frequencies through filtering. Such filtering considerations could also lead to variations in the shape, size, or filling material used within polarization-modification channel  106 . In some embodiments, polarization-modification channel  106  may be designed to transmit, and possibly alter, electromagnetic waves having circular or elliptical polarization. 
     In addition, polarization filter  100  can act as a corrective iris on top of the rectangular waveguide. For example, if the rectangular waveguide is misshapen (e.g., one side of the rectangular waveguide is bent), polarization-modification channel  106  can have a shape that compensates for the shape of the rectangular waveguide. 
     As discussed above, system  200  may form a component of a radar antenna or a radio communication system, for example. Various other applications for system  200  are also possible. In such alternate applications, dimensions of waveguide  202  or polarization filter  100  can have other configurations that account for a given wavelength corresponding to electromagnetic waves used in the respective application, for example. 
       FIG. 3  illustrates polarization filter  100  and waveguides  202 ,  302 . As illustrated, polarization filter  100  may be polarization filter  100  illustrated in  FIGS. 1 and 2 , and waveguide  202  may be the rounded rectangular waveguide  202  illustrated in  FIG. 2 . 
     In the embodiment depicted in  FIG. 3 , waveguide  202  may be referred to as the lower waveguide  202 , and waveguide  302  may be referred to as the upper waveguide  302 . Polarization filter  100 , the lower rectangular waveguide  202 , and the upper rectangular waveguide  302  can together comprise a system  300 . As illustrated, the system  300  may be similar to the system  200  illustrated in  FIG. 2  with an addition of the upper rectangular waveguide  302  seated on or fastened to a side of polarization filter  100  opposite the side of polarization filter  100  to which the lower rectangular waveguide  202  is seated or fastened. 
     As illustrated, the system  300  can be configured to radiate electromagnetic waves that have a polarization rotation of 90 degrees, for example, relative to an input polarization at the base of the lower rectangular waveguide  202 . Such an arrangement could allow input electromagnetic waves (e.g., at a port on a side of the lower rectangular waveguide  202  opposite of polarization filter  100 ) to be rotated from a horizontal TE 10  polarization to a vertical TE 10  polarization at the output (e.g., a port on a side of the upper rectangular waveguide  302  opposite of polarization filter  100 ), for example. Other angular rotations between input and output are also possible. 
     Alternatively, the system  300  could be used to receive electromagnetic waves of a given polarization at a port of the upper rectangular waveguide  302 , and then rotate the polarization of the electromagnetic waves through an angle (e.g., an angle between 75 and 105 degrees) before emitting the electromagnetic waves having the rotated polarization out of a port in the base of the lower rectangular waveguide  202 . 
     In some embodiments, the upper waveguide  302  may represent a waveguide output port of a radiating antenna, for example. Further, the lower waveguide  202  may represent a waveguide antenna element, connected to an electrical circuit within a radar system for example. 
     In the embodiment illustrated in  FIG. 3 , the upper waveguide  302  and the lower waveguide  202  may be of similar shapes and sizes, but rotated in orientation with respect to one another (e.g., at an angle between 88 and 92 degrees). Also, in addition to or alternatively to rotation with respect to one another about a vertical axis, one or both of the upper waveguide  302  and the lower waveguide  202  could be rotated with respect to an axis that lies parallel to a plane of the surface of polarization filter  100 . In alternate embodiments, the upper waveguide  302  and the lower waveguide  202  may be different lengths, widths, heights, or shapes. Analogous to the system  200  illustrated in  FIG. 2 , regardless of whether the upper waveguide  302  and the lower waveguide  202  are the same shape or size as one another, one or both of the upper waveguide  302  and the lower waveguide  202  could be circular, elliptical, or rectangular waveguides, as opposed to rounded rectangular waveguides. If the respective shapes of the upper waveguide  302  and the lower waveguide  202  are not equivalent, dimensions of the respective waveguides may be altered to accommodate the shape difference (e.g., if the lower waveguide  202  is a rounded rectangle and the upper waveguide  302  is a rectangle, the lower waveguide  202  may be slightly longer or wider to accommodate equivalent modes to those accommodated by the upper waveguide  302 ). In still other embodiments, one or both of the upper waveguide  302  and the lower waveguide  202  could be replaced by other components (e.g., photonic components or electronic components). 
       FIG. 4  illustrates a waveguide  202 , a unit cell of polarization filter  100 , and a horn antenna  404 , according to example embodiments. As illustrated, polarization filter  100  may be the polarization-modification unit cell illustrated in  FIGS. 1, 2, and 3 , and waveguide  202  may be waveguide  202  illustrated in  FIGS. 2 and 3 . Polarization filter  100 , waveguide  202 , and the horn antenna  404  can together comprise a system  400 . Also included in the system  400  illustrated in  FIG. 4 , are two fastening plates  402  used to connect the other components of the system  400  (i.e., waveguide  202 , polarization filter  100 , and the horn antenna  404 ) to one another. As illustrated, the system  400  may be similar to the system  200  illustrated in  FIG. 2  with an addition of the horn antenna  404  fastened to a side of polarization filter  100  opposite the side to which waveguide  202  is fastened. 
     As illustrated in  FIG. 4 , polarization filter  100  may be removably connected to the horn antenna  404  and waveguide  202  using the fastening plates  402 . The fastening plates  402 , for example, may be directly connected to the horn antenna  404  and waveguide  202 , respectively, in a semi-permanent fashion (e.g., welded to the horn antenna  404  and waveguide  202 ). The fastening plates  402  may then be attached to one another, polarization filter  100 , or both, using bolts, as illustrated, for example. The bolts may replace pegs  102  illustrated in  FIG. 1 . Alternatively, the bolts may be threaded through threaded ports or through-holes defined within pegs  102  or through one or more other through-holes, such as the through-holes  104  illustrated in  FIG. 1 . Additionally, the system  400  may employ nuts, washers, or both to secure the fastening plates  402  to one another or to polarization filter  100 . 
     In alternate embodiments, the use of fastening plates  402  within the system  400  may be superfluous. For example, the horn antenna  404 , waveguide  202  or both may be directly connected (e.g., welded or fastened) to a portion of polarization filter  100 , thereby obviating a need to use fastening plates  402 . In still other embodiments, the fastening plates  402  may be shaped differently (e.g., rectangular rather than circular). 
     The horn antenna  404  represents a radiating element of the system  400  illustrated in  FIG. 4 . The horn antenna  404  may be an alternate radiating element used in place of the upper waveguide  302  illustrated in  FIG. 3 . Potential advantages of using the horn antenna  404  could include improved directivity, bandwidth, and standing wave ratio (SWR) when compared with alternate antenna radiating elements such as the upper waveguide  302  illustrated in  FIG. 3 . In alternate embodiments, the horn antenna  404  may have an alternate shape (e.g., a sectoral horn, a conical horn, an exponential horn, a corrugated horn, a dual-mode conical horn, a diagonal horn, a ridged horn, a septum horn, or an aperture-limited horn, as opposed to a pyramidal horn) or be sized in a different way (e.g., a width dimension of an output port of the horn antenna  404  is larger than a length dimension of the output port of the horn antenna  404 ). Such changes to the horn antenna  404  may be made such that the horn antenna  404  radiates electromagnetic waves of different frequencies more efficiently or corresponding to different polarizations, for example. In alternate embodiments, besides those illustrated in  FIGS. 3 and 4 , other radiating elements are also possible (e.g., bowtie antennas or corner reflector antennas). 
     The horn antenna  404  may, analogous to the upper waveguide  302  illustrated in  FIG. 3 , radiate electromagnetic waves that have a rotated polarization from a polarization that was input to a port at a base of waveguide  202 . For example, the polarization could be rotated between 88 and 92 degrees (e.g., from roughly a horizontal TE 10  polarization to roughly a vertical TE 10  polarization, or vice versa). 
       FIG. 5  illustrates a unit cell of another polarization-modification  500 , according to example embodiments. Similar to the embodiment illustrated in  FIG. 1 , the polarization-modification unit cell  500  illustrated in  FIG. 5  includes pegs  502 , through-holes  504 , and a polarization-modification channel  506 . The polarization-modification unit cell  500  may be a plate of metal, fabricated using CNC, for example, with other components defined therein (e.g., the polarization-modification channel  506 ) and/or thereon (e.g., the two pegs  502 ). While the polarization-modification unit cell  500  may be a component of an antenna or a radar system, the polarization-modification unit cell  500  may be used in various other applications. 
     Multiple polarization-modification unit cells  500  could further be cascaded to allow for additional rotation of polarization. For example, nine cascaded polarization-modification unit cells, each being similar to polarization-modification unit cell  500 , could each be cascaded one after another. Each of the nine cascaded polarization-modification unit cells could have successive polarization-modification channels  506  that are offset 10 degrees from the polarization-modification channels  506  of adjacent polarization-modification unit cells. In this way, the nine cascaded polarization-modification unit cells could rotate polarization of input electromagnetic waves to polarization of output electromagnetic waves by 90 degrees. Further, the cascaded polarization-modification unit cells could permit an increased bandwidth of frequencies over which polarization conversion can occur. For example, a set of cascaded polarization-modification unit cells could act as a broadband (in terms of accepted electromagnetic frequencies) polarization rotating device. In some embodiments, such a device could be capable of rotating any electromagnetic wave having a frequency within the “E-band” (i.e., 60-90 GHz), for example. 
     Similar to the embodiment illustrated in  FIG. 1 , pegs  502  can be configured to allow the polarization-modification unit cell  500  to connect to and/or align with other components. For example, pegs  502  may align the polarization-modification unit cell  500  with alignment holes on other radar components, such as waveguides or antennas (e.g., the horn antenna  404  illustrated in  FIG. 4 ). In alternate embodiments, there may be more than two pegs  502 , fewer than two pegs  502 , or no pegs  502  at all. 
     Also analogous to the embodiment illustrated in  FIG. 1 , through-holes  504  can perform similar tasks to those performed by pegs  502  (e.g., connect and/or align the polarization-modification unit cell  500  with other components). For example, in some embodiments, through-holes  504  may be threaded, allowing through-holes  504  to be engaged by fasteners to connect the polarization-modification unit cell  500  to other radar components. As illustrated in  FIG. 5 , there are four through-holes  504 . In alternate embodiments, there may be more than four through-holes  504 , fewer than four through-holes  504 , or no through-holes  504  at all. 
     The polarization-modification channel  506 , in this embodiment, is the component of the polarization-modification unit cell  500  in which electromagnetic waves undergo a rotation of polarization. The thickness of the polarization-modification channel  506 , and therefore, in some embodiments, the thickness of the main body of the polarization-modification unit cell  500 , may be defined based on one or more wavelengths (or fractions of a wavelength) expected to undergo polarization-modification using the polarization-modification unit cell  500  (e.g., if the polarization-modification unit cell  500  is being used in radar applications that utilize 77 GHz electromagnetic waves, the thickness of the polarization-modification unit cell  500  could be around 3.9 mm, or about one wavelength). 
     An angle of the polarization-modification channel  506  relative to one or more mounting points (e.g., pegs  502  or through-holes  504 ) may define how much polarization-modification occurs when the polarization-modification unit cell  500  acts on an electromagnetic wave. Unlike the embodiment illustrated in  FIG. 1 , however, the polarization-modification channel  506  illustrated in  FIG. 5  is rotated between 10 and 15 degrees relative to a line that is perpendicular to a line between the two pegs  502 . Other angles are also possible in alternate embodiments. As stated above, smaller angles may increase the bandwidth of frequencies of incoming electromagnetic waves over which the polarization-modification channel  506  can effectively rotate polarization, especially when multiple polarization-modification unit cells  500  are cascaded. 
     In some embodiments, the polarization-modification channel  506  could be filled or partially filled with a material other than air. For example, a dielectric could be used to fill the polarization-modification channel  506  to alter a resonant wavelength inside of the polarization-modification channel  506 , thereby altering an input wavelength range over which polarization rotation can occur using the polarization-modification unit cell  500 . 
     Still further, in some alternate embodiments, the shape of the polarization-modification channel  506  could be changed. For example, the polarization-modification channel  506  could be circular or substantially circular, allowing for an alignment of the polarization-modification unit cell  500  with circular waveguides. In the embodiment illustrated in  FIG. 5 , the polarization-modification channel  506  has a shape of a rounded rectangle (i.e., the shape is substantially rectangular). Geometrically, such a shape can be defined as the shape obtained by taking the convex hull of four equal circles of a given radius and placing the centers of the four circles at the four corners of a rectangle having a first side length and a second side length. 
     Additionally or alternatively, some embodiments may influence two or more degenerate modes to form a single circularly polarized wave. In such embodiments, it may be possible for the unit cell to launch or radiate a circularly polarized wave upon receiving only linearly polarized waves as inputs. For example, this may occur in embodiments where the shape of the polarization-modification channel is an ellipse having low eccentricity, a trapezoid, or a rectangle having nearly equal side lengths. 
       FIG. 6  illustrates a unit cell of another polarization-modification  500  and two waveguides  602 / 604 , according to example embodiments. As illustrated, the polarization-modification unit cell  500  may be the polarization-modification unit cell  500  illustrated in  FIG. 5 . In the embodiment of  FIG. 6 , the waveguides  602 ,  604  may respectively be referred to as the upper waveguide  604  and the lower waveguide  602 . The polarization-modification unit cell  500 , the lower waveguide  602 , and the upper waveguide  604  can together comprise a system  600 . As illustrated, the system  600  may be similar to the system  300  illustrated in  FIG. 3 . The primary difference, however, is the orientation of the upper waveguide  604  with respect to the polarization-modification unit cell  500  and the lower waveguide  602 . As illustrated in  FIG. 6 , the upper waveguide  604  is angularly offset about a vertical axis from the lower waveguide  602  by roughly 30 degrees (as opposed to roughly 90 degrees, as illustrated in  FIG. 3 ). As described above with regards to other systems and waveguides, the system  600  illustrated in  FIG. 6  could be cascaded multiple times to achieve various other angles of polarization rotation (e.g., three instances of the system  600  could be cascaded to rotate polarization by roughly 90 degrees). 
     As described above, the system  600  can be configured to radiate electromagnetic waves that have polarization rotation of 30 degrees, for example, relative to an input polarization at the base of the lower rectangular waveguide  602 . Such an arrangement could allow input electromagnetic waves (e.g., at a port on a side of the lower waveguide  602  opposite of polarization filter  100 ) to be rotated from one TE 10  polarization to another TE 10  polarization at the output (e.g., a port on a side of the upper waveguide  604  opposite of polarization filter  100 ), for example. Other angular rotations between input and output are also possible. 
     Alternatively, the system  600  could be used to receive electromagnetic waves of a given polarization at a port of the upper waveguide  604 , and then rotate the polarization of the electromagnetic waves through an angle (e.g., an angle between 25 and 35 degrees) before emitting the electromagnetic waves having the rotated polarization out of a port in the base of the lower waveguide  602 . 
     In some embodiments, the upper waveguide  604  may represent a waveguide output port of a radiating antenna, for example. Further, the lower waveguide  602  may represent a waveguide antenna element, connected to an electrical circuit or a feed waveguide within a radar system for example. 
     In the embodiment illustrated in  FIG. 6 , the upper waveguide  604  and the lower waveguide  602  may be of similar shapes and sizes, but rotated in orientation with respect to one another (e.g., at an angle between 25 and 35 degrees). Also, in addition to or alternatively to rotation with respect to one another about a vertical axis, one or both of the upper waveguide  604  and the lower waveguide  602  could be rotated with respect to an axis that lies parallel to a plane of the surface of the polarization-modification unit cell  500 . In alternate embodiments, the upper waveguide  604  and the lower waveguide  602  may be different lengths, widths, heights, or shapes. Additionally, regardless of whether the upper waveguide  604  and the lower waveguide  602  are the same shape or size as one another, one or both of the upper waveguide  604  and the lower waveguide  602  could be circular, elliptical, or rectangular waveguides, as opposed to rounded rectangular waveguides. If the respective shapes of the upper waveguide  604  and the lower waveguide  602  are not equivalent, dimensions of the respective waveguides may be altered to accommodate the shape difference (e.g., if the lower waveguide  602  is a rounded rectangle and the upper waveguide  604  is a rectangle, the lower waveguide  602  may be slightly longer or wider to accommodate equivalent modes to those accommodated by the upper waveguide  604 ). 
       FIG. 7  illustrates a polarization filter  700 , according to example embodiments. The polarization filter  700  illustrated in  FIG. 7  has multiple polarization-modification channels  702  defined therein. The polarization-modification channels  702  may form an array of polarization-modification channels. Each polarization-modification channels  702  may be similar to the polarization-modification channel  106  illustrated in  FIG. 1 . Further, the polarization filter  700  may be designed for used with an antenna (e.g., a radar antenna), such as the antenna  900  illustrated in  FIG. 9 . 
     As illustrated, the polarization-modification channels  702  may be defined in an array-like fashion within the polarization filter  700 . The polarization-modification channels  702  may further be at an angle between 44 and 46 degrees (e.g., 45 degrees) relative to an orientation of the polarization filter  700 , for example. Other angles are also possible. Further, while the embodiment illustrated in  FIG. 7  depicts each of the polarization-modification channels  702  as having a similar orientation with respect to the polarization filter  700 , this need not be the case. In additional embodiments, the polarization-modification channels  702  could be irregularly arranged or have different angles than one another. In some devices or systems, the polarization rotation that may occur using the polarization-modification layer  700  may not be isotropic for all regions within the device/system. 
     As illustrated in  FIG. 7 , the polarization-modification channels  702  have the shape of a stadium. Geometrically, a stadium (i.e., a disco-rectangle or another partially rounded shape) is defined as a rectangle with semicircles at a pair of opposite sides. However, the polarization-modification channels  702  may have various alternative shapes (e.g., an ellipse, a circle, a rounded rectangle, or a rectangle) or sizes (e.g., different radii, lengths, widths, etc.). Further, the polarization-modification channels  702  may not be the same size or shape as one another. As with the angle of rotation relative to the polarization-modification layer  700 , the polarization-modification channels  702  may have varied shapes and sizes, perhaps spaced irregularly about the polarization-modification layer  700 . Still further, the thickness of the polarization-modification layer  700  may vary among embodiments. For example, the thickness of the polarization-modification layer  700  may be between a half wavelength and a whole wavelength of the associated electromagnetic waves for which the polarization-modification layer  700  is designed (e.g., between 1.45 and 3.9 mm for a polarization-modification layer  700  designed to rotate polarization of incoming electromagnetic waves having an associated frequency of 77 GHz). 
     In some examples, the polarization-modification layer  700  may also include one or multiple resonant cavities. For instance, a resonant cavity may be located on a bottom side of the polarization-modification layer  700  and can function to match an impedance of the polarization-modification layer  700  to an impedance of the antenna to which the polarization-modification layer  700  is coupled. 
     Additionally or alternatively, the polarization-modification layer  700  could be used to select specific polarizations or frequencies through filtering. Such filtering considerations could also lead to variations in the shape, size, or filling material used within the polarization-modification channels  702 . In other embodiments, the polarization-modification channels  702  may be designed to transmit, and possibly alter, electromagnetic waves having circular or elliptical polarization. 
       FIG. 8A  illustrates an example wave-radiating doublet of an example antenna, according to example embodiments. The example antenna could be used to radiate or receive radio waves, in example embodiments. More specifically,  FIG. 8A  illustrates a cross-section of an example DOEWG  800 . The DOEWG  800  may include a horizontal feed (i.e., channel), a vertical feed (i.e., a doublet neck), and a wave-directing member  804 . The vertical feed may be configured to couple energy from the horizontal feed to two output ports  802 , each of which is configured to radiate at least a portion of electromagnetic waves out of the DOEWG  800 . In some embodiments, the farthest DOEWG from the input port may include a backstop at location  806 . DOEWGs that come before the last DOEWG may simply be open at location  806  and electromagnetic waves may propagate through that location  806  to subsequent DOEWGs. For example, a plurality of DOEWGs may be connected in series where the horizontal feed is common across the plurality of DOEWGs (as shown in  FIG. 8B ).  FIG. 8A  shows various parameters that may be adjusted to tune the amplitude and/or phase of an electromagnetic signal that couples into the radiating element. 
     In order to tune a DOEWG such as DOEWG  800 , the vertical feed width, vfeed_a, and various dimensions of the step  804  (e.g., dw, dx, and dz 1 ) may be tuned to achieve different fractions of radiated energy out the DOEWG  800 . The step  804  may also be referred to as a reflecting component as it reflects a portion of the electromagnetic waves that propagate down the horizontal feed into the vertical feed. Further, in some examples, the height dz 1  of the reflecting component may be negative. That is, the step  804  may extend below the bottom of the horizontal feed. Similar tuning mechanisms may be used to tune the offset feed as well. For example, the offset feed may include any of the vertical feed width, vfeed_a, and various dimensions of the step (e.g., dw, dx, and dz 1 ) as discussed with respect to the radiating element. 
     In some examples, each output port  802  of the DOEWG  800  may have an associated phase and amplitude. In order to achieve the desired phase and amplitude for each output port  802 , various geometrical components may be adjusted. As previously discussed, the step (reflecting component)  704  may direct a portion of the electromagnetic wave through the vertical feed. In order to adjust amplitude associated with each output port  802  of a respective DOEWG  800 , a height associated with each output port  802  may be adjusted. Further, the height associated with each output port  802  could be the height or the depth of this feed section of output port  802 . 
     As shown in  FIG. 8A , height dz 2  and height dz 3  may be adjusted to control the amplitude with respect to the two output ports  802 . In some embodiments, such as the embodiment of  FIG. 9 , the two output ports  802  (given reference numeral  902  in  FIG. 9 ) may instead be referred to as waveguide antenna elements (e.g., the waveguide antenna elements  902  illustrated in  FIG. 9 ) as they are shaped like and may function as waveguides and further may serve to radiate or receive electromagnetic waves. The adjustments to height dz 2  and height dz 3  may alter the physical dimensions of the doublet neck (e.g., vertical feed of  FIG. 8A ). The doublet neck may have dimensions based on the height dz 2  and height dz 3 . Thus, as the height dz 2  and height dz 3  are altered for various doublets, the dimensions of the doublet neck (i.e., the height of at least one side of the doublet neck) may change. In one example, because height dz 2  is greater than height dz 3 , the output port  802  associated with (i.e., located adjacent to) height dz 2  may radiate with a greater amplitude than the amplitude of the signal radiated by the output port  802  associated with height dz 3 . 
     Further, in order to adjust the phase associated with each output port  802 , a step may be introduced for each output port  802 . The step in the height may cause a phase of a signal radiated by the output port  802  associated with the respective step to change. Thus, by controlling both the height and the respective step associated with each output port  802 , both the amplitude and the phase of a signal transmitted by the output port  802  may be controlled. In various embodiments, the steps may take various forms, such as a combination of up-steps and down-steps. Additionally, the number of steps may be increased or decreased to control the phase. 
     The above-mentioned adjustments to the geometry may also be used to adjust a geometry of the offset feed where it connects to the waveguide. For example, heights, widths, and steps may be adjusted or added to the offset feed in order to adjust the radiation properties of the system. An impedance match, phase control, and/or amplitude control may be implemented by adjusting the geometry of the offset feed. 
       FIG. 8B  illustrates an example offset feed waveguide portion  856  of an example antenna  850 , according to example embodiments. As shown in  FIG. 8B , a waveguide  854  may include a plurality of radiating elements (shown as  852 A- 852 E) and an offset feed  856 . Although the plurality of radiating elements is shown as doublets in  FIG. 8B , other radiating structures may be use as well. For example, singlets, and any other radiating structure that can be coupled to a waveguide may be used as well. 
     The waveguide  854  may include various shapes and structures configured to direct electromagnetic power to the various radiating elements  852 A-E of waveguide  854 . A portion of electromagnetic waves propagating through waveguide  854  may be divided and directed by various recessed wave-directing member and raised wave-directing members. The pattern of wave-directing members shown in  FIG. 8B  is one example for the wave-directing members. Based on the specific implementation, the wave-directing members may have different sizes, shapes, and locations. Additionally, the waveguide may be designed to have the waveguide ends  860 A and  860 B to be tuned shorts. For example, the geometry of the ends of the waveguides may be adjusted so the waveguide ends  860 A and  860 B act as tuned shorts. 
     At each junction of one of the respective radiating elements  852 A-E of waveguide  854 , the junction may be considered a two-way power divider. A percentage of the electromagnetic power may couple into the neck of the respective radiating elements  852 A-E and the remaining electromagnetic power may continue to propagate down the waveguide. By adjusting the various parameters (e.g., neck width, heights, and steps) of each respective radiating element  852 A-E, the respective percentage of the electromagnetic power may be controlled. Thus, the geometry of each respective radiating element  852 A-E may be controlled in order to achieve the desired power taper. Thus, by adjusting the geometry of each of the offset feed and each respective radiating element  852 A-E, the desired power taper for a respective waveguide and its associated radiating elements may be achieved. 
     Electromagnetic energy may be injected into the waveguide  854  via the waveguide feed  856 . The waveguide feed  856  may be a port (e.g., a through-hole) in a bottom metal layer, in some embodiments. An electromagnetic signal may be coupled from outside the antenna unit into the waveguide  854  through the waveguide feed  856 . The electromagnetic signal may come from a component located outside the antenna unit, such as a printed circuit board, another waveguide, or other signal source. In some examples, the waveguide feed  856  may be coupled to another dividing network of waveguides (such as illustrated in  FIGS. 9 and 10 ). 
     In some examples, the present system may operate in one of two modes. In the first mode, the system may receive electromagnetic energy from a source for transmission (i.e., the system may operate as a transmission antenna). In the second mode, the system may receive electromagnetic energy from outside of the system for processing (i.e., the system may operate as a reception antenna). In the first mode, the system may receive electromagnetic energy at a waveguide feed, divide the electromagnetic energy for transmission by a plurality of radiating elements, and radiate the divided electromagnetic energy by the radiating elements. In the second mode, the system may receive electromagnetic energy at the plurality of radiating elements, combine the received electromagnetic energy, and couple the combined electromagnetic energy out of system for further processing. 
     It should be understood that other shapes and dimensions of the waveguide channels, portions of the waveguide channels, sides of the waveguide channels, wave-directing members, and the like are possible as well. In some embodiments, a rectangular shape of waveguide channels may be highly convenient to manufacture, though other methods known or not yet known may be implemented to manufacture waveguide channels with equal or even greater convenience. 
       FIG. 9  illustrates an array of waveguide antenna elements  902 , according to example embodiments. The size and shape of the waveguide antenna elements  902 , as well as the corresponding feed waveguides illustrated in  FIG. 9 , may correspond to a given electromagnetic frequency (e.g., 77 GHz) and/or polarization (e.g., horizontal TE 10  polarization) for which the array of waveguide antenna elements  902  is designed to operate. Along with other components pictured, the waveguide antenna elements  902  may be part of an antenna system  900 . The waveguide antenna elements  902  may be arranged in an array, as illustrated in  FIG. 9 . Further, the array of waveguide antenna elements  902  may be arranged in a group of individual antennas  850  as illustrated in  FIG. 8 . Specifically, the embodiment illustrated in  FIG. 9  includes six instances of the antenna  850  illustrated in  FIG. 8 , resulting in a 6×10 array of waveguide antenna elements  902 . Other numbers of waveguide antenna elements  902  and/or antennas  850  are also possible. The antenna system  900  may be on a transmit end and/or a receive end of a radar or radio communication system, for example. Further, two instances of the antenna system  900  can be used in conjunction with one another to form a transmit/receive system (e.g., a radio communication system). Still further, the antenna system  900  may be designed to radiate and/or receive electromagnetic waves in a TE 10  waveguide mode. 
     In addition to the waveguide antenna elements  902  arranged in a group of antennas  850  illustrated in  FIG. 9 , the antenna system  900  may additionally include a phase adjusting section  910  and a waveguide input  912 . The waveguide input  912  may be connected to an electromagnetic source (e.g., a radar source), in some embodiments. The phase adjusting section  910  may adjust a phase associated with electromagnetic waves input into the waveguide input  912 , for example. This could allow proper phase to be distributed to each of the waveguide antenna elements  902  when transmitting a signal. Further, the phase adjusting section  910  may be configured to divide power of an incoming electromagnetic wave among multiple feed waveguides associated with multiple instances of the antenna  850 . 
     In some embodiments, as described above, antenna system  900  may include a series of independent antennas  850  that are connected to a common waveguide input  912 . Instead of being independent antennas  850 , the antennas  850  may function as a single antenna unit, as illustrated in  FIG. 9 . Whether the antenna system  900  describes independent antennas or a single antenna unit, the waveguide antenna elements  902  can serve to radiate electromagnetic waves and/or receive electromagnetic waves. The electromagnetic waves radiated and/or received may be transmitted down the horizontal and vertical feeds of the corresponding waveguides, as described with regard to  FIG. 8 . 
       FIG. 10  illustrates an array of waveguide antenna elements  902  and a polarization-modification layer  700 , according to example embodiments. In some embodiments, the array of waveguide antenna elements  902  may be designed according to an industry standard (e.g., an automotive industry standard) and the polarization-modification layer  700  may be designed in such a way as to accommodate that industry standard. Alternatively, the array of waveguide antenna elements  902  and the corresponding polarization-modification layer  700  could be designed for one or more specific applications. Collectively, the array of waveguide antenna elements  902  and the polarization-modification layer  700  may comprise an antenna  1000 . In some embodiments, as in the embodiment illustrated in  FIG. 10 , the antenna  1000  may additionally include the phase adjusting section  910  and/or the waveguide input  912  illustrated in  FIG. 9 . In the example embodiment of  FIG. 10 , the thickness of the polarization-modification layer  700  could be less than a wavelength thick (e.g., between a quarter wavelength and a whole wavelength) of the electromagnetic waves which the antenna  1000  was designed to transmit or receive. Other thicknesses are also possible. Further, the antenna  1000  may be designed to radiate or receive electromagnetic waves in a TE 10  waveguide mode. 
     In the embodiment illustrated in  FIG. 10 , the polarization-modification channels  702  defined within the polarization-modification layer  700  may serve to rotate polarization emitted by the waveguide antenna elements  902 . Thus, the electromagnetic waves radiated by the antenna  1000  may be of a polarization that is rotated with respect to a polarization that is output by the waveguide antenna elements  902 . Additionally or alternatively (e.g., if the antenna  1000  is acting as a receiver within a radar system or radio communication system), the polarization-modification channels  702  defined within the polarization-modification layer  700  may serve to rotate a polarization associated with a received electromagnetic wave prior to transmitting the electromagnetic wave to the waveguide antenna elements  902 . 
     As previously discussed with respect to  FIG. 7 , the polarization-modification layer  700  may also include a resonant cavity as part of each of the polarization-modification channels  702 . The resonant cavity may be configured to perform impedance matching between each waveguide antenna elements  902  and the respective one of the polarization-modification channels  702  coupled to the waveguide antenna elements  902 . In some radar systems, for example, a transmitter may be configured like the antenna  1000  illustrated in  FIG. 10 . Such a transmitter may communicate with a receiver, also configured like the antenna  1000  illustrated in  FIG. 10 . 
     As illustrated in  FIG. 10 , in either of the above described cases (i.e., whether the antenna  1000  is acting as a transmitter or a receiver), the polarization radiated by or accepted by the polarization-modification channels  702  is at an angle with respect to the waveguide antenna elements  902 . This corresponding angle may be between 44 and 46 degrees (e.g., 45 degrees), for example. A variety of alternate angles may also be used in various embodiments. In still other embodiments, the polarization-modification channels  702  need not all be disposed at the same angle relative to waveguide antenna elements  902 . This could allow a corresponding antenna to radiate and receive electromagnetic waves having a variety of polarizations, for example. In yet other embodiments, the polarization-modification channels  702  need not be all the same size and shape as one another. This could allow a corresponding antenna to radiate and receive electromagnetic waves having a variety of polarizations (e.g., if the polarization-modification channels  702  were circular rather than stadium-shaped) and/or a variety of frequencies (e.g., if the polarization-modification channels  702  were sized such that they were resonant at different frequencies), for example. Even further, one or more of the polarization-modification channels  702  could be filled with a material (e.g., a dielectric material), thereby further changing one or more of the properties (e.g., resonant frequency) of the associated electromagnetic waves which could propagate through the corresponding polarization-modification channel  702 . 
     In some embodiments, two or more polarization-modification layers  700  could be cascaded on top of the waveguide antenna elements  902 . If there were multiple polarization-modification layers  700  cascaded on top of the waveguide antenna elements  902 , the corresponding polarization-modification channels  702  could provide increased frequency bandwidth over which electromagnetic waves could be radiated or received by the corresponding antenna. Further, cascading multiple polarization-modification layers  700  could permit an angle of polarization radiated or received to be greater or less than the angle illustrated in  FIG. 10 . For example, an alternate antenna may have two cascaded polarization-modification layers. The first layer could be at an angle between 20 and 25 degrees with respect to the array of waveguide antenna elements  902 , and the second layer could be at an between 20 and 25 degrees with respect to the first layer. In this way, the angle of polarization rotation undergone by electromagnetic waves (i.e., 45 degrees) would be the same as in the embodiment of  FIG. 11 , but the bandwidth could be increased. 
     The design of the antenna  1000  illustrated in  FIG. 10  could also serve to reduce interference between two separate antennas. For example, a radar system could employ two antennas having analogous designs, but the polarization-modification channels within the polarization-modification layer of one antenna are rotated at an angle that is orthogonal to the polarization-modification channels within the polarization-modification layer of the other antenna. In an alternative example, two separate antennas could have polarization-modification layers with polarization-modification channels oriented at a parallel angle with one another, but be facing one another (e.g., if the antennas were mounted in the same orientation on vehicles travelling in opposite directions). Either of the above methods could reduce interference because the two antennas employ orthogonal polarizations. Therefore, cross polarization isolation may occur between the two antennas. For example, a signal output by one antenna may be attenuated by as much as 40 dB (decibels) when transmitted through the polarization-modification layer of the other antenna. 
       FIG. 11  illustrates an array of waveguide antenna elements  902 , a polarization-modification layer  700 , and an array of waveguide output ports  1102 , according to example embodiments. As illustrated, the embodiment illustrated in  FIG. 11  may be analogous to the embodiment illustrated in  FIG. 10  with an addition of an array of waveguide output ports  1102 . The array of waveguide antenna elements  902 , the polarization-modification layer  700 , and the array of waveguide output ports  1102 , in addition to the phase adjusting section  910  and the waveguide input  912  may form an antenna  1100 . Further, the antenna  1100  may be designed to radiate or receive electromagnetic waves in a TE 10  waveguide mode. 
     The antenna  1100  could be used to transmit and/or receive electromagnetic waves (e.g., radio waves) for a variety of purposes (e.g., navigation within an autonomous vehicle using radar or radio communication). In further embodiments, the antenna  1100  may have a greater or lesser number of waveguide antenna elements  902 , waveguide output ports  1102 , and/or polarization-modification channels  702 . Additionally or alternatively, the antenna  1100  may not have the phase adjusting section  910  or the waveguide input  912 . For example, one or more of the individual waveguide antenna elements  902  may be fed by photonic or electronic source(s) rather than feed waveguides connected to the phase adjusting section  910  and the waveguide input  912 . 
     In the embodiment of  FIG. 11 , the waveguide antenna elements  902  could output electromagnetic waves, for example. These electromagnetic waves may then propagate to the polarization-modification channels  702 . The polarization-modification channels  702  may then serve to rotate the polarization of the associated electromagnetic waves by a defined angle (e.g., 45 degrees). The electromagnetic waves, now having an intermediate polarization, may then be transmitted to the waveguide output ports  1102 . The waveguide output ports  1102  may be designed of sufficient length so as to assure that any evanescent waves, which are transmitted from the polarization-modification channels  702  to the waveguide output ports  1102 , are sufficiently attenuated before reaching radiation ports located at an end of the waveguide output ports  1102 . Upon entering the waveguide output ports  1102 , the electromagnetic waves may undergo another polarization rotation (e.g., by an additional 45 degrees). The associated electromagnetic waves, now having a polarization rotated by a given angle relative to the waveguide antenna elements  902  (e.g., a polarization rotated by 45 or 90 degrees; the input polarization thus being orthogonal to the output polarization) may then be radiated to the environment upon exiting the waveguide output ports  1102 . This process could also occur in the pseudo-inverse to receive electromagnetic waves using the same antenna  1100  (i.e., electromagnetic waves are received by the waveguide output ports  1102 , the polarization is rotated upon entering the polarization-modification channels  702 , the polarization is rotated again upon entering the waveguide antenna elements  902 , and then the electromagnetic waves are transmitted to one or more devices attached to the antenna having been rotated in polarization twice). 
     In some embodiments, as illustrated in  FIG. 11 , the number of waveguide antenna elements  902  within the array, the number of polarization-modification channels  702  defined within the polarization-modification layer  700 , and the number of waveguide output ports  1102  within the array will all be the same. In some embodiments, there may be greater or fewer waveguide output ports  1102  than polarization-modification channels  702 , which may in turn be greater or fewer than the number of waveguide antenna elements  902 . Further, the arrangement of the array of waveguide output ports  1102  may not correspond to the arrangement of the polarization-modification channels  702 , as illustrated in  FIG. 11 . In some embodiments, for example, the array of waveguide output ports  1102  may be spaced irregularly or differently from the spacing of the polarization-modification channels  702 . 
     As illustrated in  FIG. 11 , each of the waveguide output ports  1102  is rotated the same amount with respect to the underlying polarization-modification channel  702  (e.g., between 44 and 46 degrees). Further, each of the polarization-modification channels  702  is rotated the same amount with respect to the underlying waveguide antenna element  902  (e.g., between 44 and 46 degrees). As such, in the antenna  1100  of  FIG. 11 , each of the waveguide output ports  1102  is rotated an equal amount with respect to the underlying waveguide antenna elements  902  (e.g., between 88 and 92 degrees). Other angles besides those illustrated in  FIG. 11  are also possible. For example, the angle between the polarization-modification channels  702  and the waveguide antenna elements  902  could be 15 degrees, and the angle between the polarization-modification channels  702  and the waveguide output ports  1102  could be 15 degrees, resulting in an angle between the waveguide output ports  1102  and the waveguide antenna elements  902  of 30 degrees. 
     In some embodiments, the rotation of the waveguide output ports  1102  relative to the polarization-modification channels  702  and/or the waveguide antenna elements  902  may vary among the waveguide output ports (e.g., one waveguide output port is rotated 75 degrees with respect to the underlying waveguide antenna element and another is rotated 90 degrees with respect to the underlying waveguide antenna element). Such a variation could leave to multiple polarization angles being emitted by the antenna  1100 , for example. Further, such a variation in angles could cause the corresponding arrangement of waveguide output ports within the array or the corresponding size/shape of various waveguide output ports to change to accommodate such differences. 
     Additionally, as described above, one or more of the waveguide guide output ports  1102  could additionally or alternatively be rotated about an axis parallel to the planar surface of the polarization-modification layer  700  (as opposed to rotated about the vertical axis that is normal to the planar surface of the polarization-modification layer  700 ). This could allow for directionality of the antenna  1100 , for example. 
     As illustrated in  FIG. 11 , the waveguide output ports  1102  are shaped as rounded rectangles. Further, dimensions associated with the output ports  1102  illustrated in  FIG. 11  may correspond to specific wavelengths of electromagnetic waves that are to be transmitted and/or received by the antenna  1100  (e.g., wavelengths associated with electromagnetic waves having a frequency of 77 GHz). However, one or more of the waveguide output ports  1102  could be replaced by alternately shaped and/or sized output ports (e.g., a horn antenna or a substantially circular waveguide). Still further, the waveguide output ports  1102  may additionally or alternatively be wholly or partially filled with a material other than air (e.g., a dielectric material). Any of these factors (e.g., shape, size, or filling of the waveguide output ports  1102 ), as well as other factors, could enhance or reduce filtering characteristics associated with the antenna  1100 . For example, if one or more of the waveguide output ports  1102  were filled with a dielectric, the resonant wavelength associated with the respective waveguide output port(s)  1102  may be altered, thus enhancing or diminishing the transmission of specific wavelengths through the respective waveguide output port(s)  1102 . 
     Described above analogously, multiple layers of waveguide output port  1102  arrays could be cascaded. This could increase the bandwidth of frequencies which could effectively be used with the antenna  1102 , for example. Further, such a cascading could increase or decrease an angle between the waveguide output ports  1102  and the waveguide antenna elements  902 . Additionally or alternatively, alternating layers of polarization-modification layers  700  followed by waveguide output port  1102  array layers could be cascaded to achieve similar effects. For example, an alternate antenna design may include an array of waveguide antenna elements, followed by two polarization-modification layers, followed by an array of waveguide output ports. In such a design, there could be an angle between each successive layer performing additional polarization rotation (e.g., the polarization-modification channels in the first polarization-modification layer are at an angle, e.g. 25 to 35 degrees, with respect to the array of waveguide antenna elements, the polarization-modification channels within the second polarization-modification layer are at another angle, e.g. 25 to 35 degrees, with respect to the polarization-modification channels in the first polarization-modification layer, and the array of waveguide output ports are at yet another angle, e.g. 25 to 35 degrees, with respect to the polarization-modification channels in the second polarization-modification layer). In addition, the angles, sizes, shapes, distributions, or numbers of waveguide output ports  1102  and/or polarization-modification channels  702  within such cascaded layers may vary from layer to layer. 
       FIGS. 12-13  show further configurations of antennas having a polarization rotation integrated in the throat of the DOEWG that can provide desired power division ratios, an impedance match for all (or a subset) of ports using one or multiple steps, ridges, or a combination. The example antennas shown can provide the desired twisted polarization of electromagnetic waves while also having the ability to maintain a compact size and location overall. In some instances, the antennas shown in  FIG. 12-13  may include a rotation portion that is integrated into a top portion of the antenna block. Thus, unlike the previous description, the polarization twist may be achieved within the split-block antenna structure, therefore a polarization filter may not be used to achieve the polarization rotation. 
       FIG. 12A  illustrates example twisted antenna configuration  1200 . As shown, antenna configuration  1200  includes radiating elements  1202 ,  1204 ,  1206 , and  1208  that each may transmit or receive radar signals as part of a radar unit. The configuration of radiating elements  1202 - 1208  can adjust the polarization at which each radiating element  1202 - 1208  transmits or receives radar signals. For example, the inner configuration  1200  may be configured to twist the polarization of radar signals transmitted or received by ninety (90) degrees. 
     In some examples, the twisted configuration of radiating elements  1202 - 1208  relative to the waveguide of antenna configuration  1200  may adjust the polarization of electromagnetic waves transmitted or received by forty-five degrees. For instance, radiating elements  1202 - 1208  may cause radar signals to be transmitted at a slanted, forty-five degrees from horizontal polarization instead of a vertical linear polarization. In other examples, radiating elements  1202 - 1208  can adjust polarization of radar signals to a greater or lesser extent. As an example, the configuration of radiating elements  1202 - 1208  may adjust the polarization by ninety-degrees (e.g., from horizontal linear polarization to vertical linear polarization). In further examples, the extent of polarization modification can differ among the set of radiating elements  1202 - 1208 . 
       FIG. 12B  illustrates another twisted antenna configuration  1210 . As shown, antenna configuration  1210  includes twisted radiating elements  1212 ,  1214 ,  1216 ,  1218 ,  1220 , and  1222 . In other examples, the configuration, quantity or radiating elements, and other parameters of antenna configuration  1210  can differ. 
     Radiating elements  1212 - 1222  connect to an inner serial feeding waveguide of antenna configuration  1210  in a manner that adjusts the performance of antenna configuration  1210 . In, particular, the twisted configuration can modify operation of radiating elements  1212 - 1222  such that each radiating element operates in a different polarization as desired. For example, radiating elements  1212 - 1222  may transmit or receive radar signals in a slanted polarization (e.g., a slanted polarization at positive or negative forty-five degrees from horizontal). In other examples, the extent of modification of polarization of radar signals can vary depending on the configuration of radiating elements  1212 - 1222 . For instance, radiating elements  1212 - 1222  may alter the polarization of electromagnetic waves by ninety degrees (90 degrees) depending on the configuration of radiating elements  1212 - 1222  relative to the waveguide of antenna configuration  1210 . Further, the type of feed (e.g., parallel, serial) directing electromagnetic waves into the waveguide can differ within examples. 
       FIG. 13  illustrates a twisted antenna  1300 . As shown in  FIG. 13 , waveguide  1304  may include multiple radiating elements (shown as  1302 A- 1302 E). Although radiating elements  1302 A- 1302 E are shown as doublets, other radiating structures may be use as well. For example, singlets, and any other radiating structure that can be coupled to a waveguide may be in other example implementations. 
     Waveguide  1304  may include various shapes and structures configured to direct electromagnetic power to the various radiating elements  1302 A- 1302 E coupled to waveguide  1304 . A portion of electromagnetic waves propagating through waveguide  1304  may be divided and directed by various recessed wave-directing member and raised wave-directing members. The pattern of wave-directing members shown in  FIG. 13  is one example for the wave-directing members. Based on the specific implementation, the wave-directing members may have different sizes, shapes, and locations. 
     At each junction of one of the respective radiating elements  1302 A- 1302 E of waveguide  1304 , the junction may be considered a two-way power divider. As such, a percentage of the electromagnetic power may couple into the neck of the respective radiating elements  1302 A- 1302 E and the remaining electromagnetic power may continue to propagate down waveguide  1304 . By adjusting various parameters (e.g., neck width, heights, and steps) of each respective radiating element  1302 A- 1302 E, the respective percentage of the electromagnetic power may be controlled while an impedance match is maintained in all affected ports associated with the power divider. Thus, the geometry of each respective radiating element  1302 A- 1302 E may be controlled in order to achieve the desired power taper. The adjustments of the geometry of each of the offset feed and each respective radiating element  1302 A- 1302 E can cause the desired power taper for waveguide  1304  and associated radiating elements  1302 A- 1302 E may be achieved. 
     Electromagnetic energy may be injected into the waveguide  1304  via a waveguide feed. For instance, the waveguide feed may be a port (e.g., a through-hole) in a bottom metal layer, in some embodiments. An electromagnetic signal may be coupled from outside the antenna unit into waveguide  1304  through the waveguide feed. The electromagnetic signal may come from a component located outside the antenna unit, such as a printed circuit board, another waveguide, or other signal source. In some examples, the waveguide feed may be coupled to another dividing network of waveguides. 
     In some examples, the present system may operate in one of two modes. In the first mode, the system may receive electromagnetic energy from a source for transmission (i.e., the system may operate as a transmission antenna). In the second mode, the system may receive electromagnetic energy from outside of the system for processing (i.e., the system may operate as a reception antenna). In the first mode, the system may receive electromagnetic energy at a waveguide feed, divide the electromagnetic energy for transmission by a plurality of radiating elements, and radiate the divided electromagnetic energy by the radiating elements. In the second mode, the system may receive electromagnetic energy at the plurality of radiating elements, combine the received electromagnetic energy, and couple the combined electromagnetic energy out of system for further processing. 
     As further shown in  FIG. 13 , radiating elements  1302 A- 1302 E are each shown in a rotated position. Particularly, the rotation can adjusts the performance of each radiating element causing radiating elements  1302 A- 1302 E to operate in a different polarization as desired. For example, the twisted configuration of radiating elements  1302 A- 1302 E can cause radiating elements  1302 A- 1302 E to receive the divided and pre-twisted electromagnetic waves from the neck and subsequently adjust the polarization of the electromagnetic waves to a desired polarization (e.g., slanted forty-five degrees from horizontal). The configuration between waveguide  1304  and radiating elements  1302 A- 1302 E includes sufficient propagation paths that can stabilize the electromagnetic waves. The stabilization of the electromagnetic waves can also diminish or even eliminate parasitic evanescent waves associated with twisted antenna  1300 . As a result of the configuration and propagation paths of twisted antenna  1300 , only desired propagating electromagnetic waves may survive in the desired twisted angle. 
     Additionally, waveguide  1304  may be designed to have waveguide ends  1306 A and  1306 B to be tuned shorts. For example, the geometry of the ends of the waveguides may be adjusted so waveguide ends  1306 A and  1306 B act as tuned shorts. In some implementations, waveguide ends  1306 A,  1306 B or other components can provide an impedance match to free space at the end of waveguide  1304 . As such, twisted antenna  1300  may radiate the electromagnetic waves into free space with no reflection. 
     It should be understood that other shapes, configurations, and dimensions of the waveguide channels, portions of the waveguide channels, sides of the waveguide channels, wave-directing members, and the like are possible as well. In some embodiments, a rectangular shape of waveguide channels may be highly convenient to manufacture, though other methods known or not yet known may be implemented to manufacture waveguide channels with equal or even greater convenience. In further examples, radiating elements  1302 A- 1302 E may have a different configuration relative to waveguide  1304  such that radiating elements  1302 A- 1302 E transmit or receive radar signals at different polarizations as desired. 
     In addition, some examples can include a subset of radiating elements  1302 A- 1302 E to have different configurations such that the subset operates in a different polarization compared to other radiating elements of twisted antenna  1300 . For instance, radiating elements  1302 A,  1302 B, and  1302 C may be configured to operate in a first polarization and radiating elements  1302 D,  1302 E may be configured to operate in a second polarization that differs from the first polarization. Antenna  1300  may be part of a radar system that operates in one or multiple polarizations to reduce potential jamming with radar from other systems. Further, the configuration of antenna  1300  can enhance the signal-to-noise ratio (SNR) reducing unwanted noise while antenna  1300  operates. 
       FIG. 14  illustrates a method  1400  of radiating electromagnetic waves, according to example embodiments. The method  1400  may be performed using the antenna  1100  illustrated in  FIG. 11 , in some example embodiments. Further, the method  1400  could be performed pseudo-inversely to receive electromagnetic waves (as opposed to radiate), in some embodiments. The method  1400  may be performed to aid in navigation of an autonomous vehicle using a radar system mounted on the autonomous vehicle, for example. Alternatively, the method  1400  may be performed to communicate using radio communication techniques. 
     At block  1402 , the method  1400  includes emitting electromagnetic waves having a first polarization from a plurality of waveguide antenna elements in a first array. The waveguide antenna elements in the first array may resemble the array of waveguide antenna elements  902  illustrated in  FIG. 9 , for example. As an example, a radar unit coupled to a portion (or built into a portion) of a vehicle may emit electromagnetic waves. 
     At block  1404 , the method  1400  includes receiving, by channels defined within a polarization-modification layer that is disposed between the waveguide antenna elements and a plurality of waveguide output ports arranged in a second array, the electromagnetic waves having the first polarization. The channels may be oriented at a first angle with respect to the waveguide antenna elements. The first angle may be between 44 and 46 degrees (e.g., 45 degrees), for example. Further, the polarization-modification layer and the channels may be the polarization-modification layer  700  and the polarization rotating channels  702 , respectively, illustrated in  FIG. 7 , for example. Still further, the waveguide output ports may be the waveguide output ports  1102  illustrated in  FIG. 11 , for example. 
     At block  1406 , the method  1400  includes transmitting, by the channels defined within the polarization-modification layer, electromagnetic waves having an intermediate polarization. 
     At block  1408 , the method  1400  includes receiving, by the waveguide output ports, electromagnetic waves having the intermediate polarization. The waveguide output ports may be oriented at a second angle with respect to the channels. The second angle may be between 44 and 46 degrees (e.g., 45 degrees), for example. 
     At block  1410 , the method  1400  includes radiating, by the waveguide output ports, electromagnetic waves having a second polarization. The second polarization may be different from the first polarization. The second polarization may also be different from the intermediate polarization. Further, the first polarization may be different from the intermediate polarization. The first polarization, intermediate polarization, and second polarization could be the following, respectively: a horizontal TE 10  polarization, a TE 10  polarization at a 45-degree angle between horizontal and vertical, and a vertical TE 10  polarization. 
     It should be understood that other shapes and dimensions of the waveguide channels, portions of the waveguide channels, sides of the waveguide channels, wave-directing members, and the like are possible as well. In some embodiments, a rectangular shape, or a rounded rectangular shape, of waveguide channels may be highly convenient to manufacture, though other methods known or not yet known may be implemented to manufacture waveguide channels with equal or even greater convenience. 
     In some examples, an antenna configuration may include a plurality of waveguide antenna elements arranged in a first array configured to operate with a first polarization and a plurality of waveguide output ports arranged in a second array configured to operate with a second polarization. Particularly, the second polarization may differ from the first polarization (e.g., orthogonal to each other). The antenna configuration may also include a polarization-modification layer with channels defined therein such that the polarization-modification layer is disposed between the waveguide antenna elements and the waveguide output ports. As such, the channels may be oriented at a first angle with respect to the waveguide antenna elements and at a second angle with respect to the waveguide output ports. For instance, the first angle may be between 44 and 46 degrees and the second angle may be between 44 and 46 degrees. In addition, the channels may be configured to receive input electromagnetic waves having the first polarization and transmit output electromagnetic waves having a first intermediate polarization. Similarly, the waveguide output ports may be configured to receive input electromagnetic waves and radiate electromagnetic waves having the second polarization. 
     In further examples, the waveguide antenna elements and the waveguide output ports may be substantially rectangular in shape. In other examples, the waveguide antenna elements and the waveguide output ports may be substantially circular in shape. Further, the channels may be shaped as round rectangles and the channels may be filled with a dielectric material. In some instances, the thickness of the polarization-modification layer may vary. For example, the thickness of the polarization-modification layer may be between a half and a whole wavelength of the input electromagnetic waves having the first polarization. 
     In addition, some examples may also involve a secondary polarization-modification layer with secondary channels defined therein. Particularly, the secondary polarization-modification layer may be disposed between the polarization-modification layer and the waveguide output ports and the secondary channels may be oriented at a third angle with respect to the waveguide antenna elements and at a fourth angle with respect to the waveguide output ports. Further, the secondary channels may be configured to receive input electromagnetic waves having the first intermediate polarization and transmit output electromagnetic waves having a second intermediate polarization. As such, the first intermediate polarization may differ from the second intermediate polarization. As an example, a bandwidth of usable frequencies associated with the radar antenna may lie within the 77 GHz band. In some instances, the first angle may be between 25 and 35 degrees, the second angle may be between 50 and 70 degrees, the third angle may be between 50 and 70 degrees, and the fourth angle may be between 25 and 35 degrees. Other degrees for the angles are possible. 
     Further, it should be understood that other layouts, arrangements, amounts, or sizes of the various elements illustrated in the figures are possible, as well. For example, it should be understood that a given application of an antenna or antenna system may determine appropriate dimensions and sizes for various machined portions of the polarization-modification unit cells illustrated in the figures (e.g., channel size, metal layer thickness, etc.) and/or for other machined (or non-machined) portions/components of the antenna(s) and antenna system(s) described herein. For instance, as discussed above, some example radar systems may be configured to operate at an electromagnetic wave frequency of 77 GHz, which corresponds to millimeter electromagnetic wave length. At this frequency, the channels, ports, etc. of an apparatus may be of given dimensions appropriated for the 77 GHz frequency. Other example antennas and antenna applications are possible as well. 
     Still further, the word “antenna” should not be limited to applications involving electromagnetic waves solely within radio frequencies of the electromagnetic spectrum. The term “antenna” is used herein broadly to describe a device that is capable of transmitting and/or receiving any electromagnetic wave. For example, any of the antennas or components of the antennas described herein could be capable of transmitting and/or receiving optical light. Even further, any of the antennas or components of the antennas described herein could be capable of being fed by optical sources (e.g., optical fibers or optical lasers). Such example antennas could be used as optical interconnects within a computing devices, for instance. In addition, corresponding shapes and dimensions of components within such antennas may vary depending on the wavelength (e.g., components used in optical embodiments may have feature sizes on the scale of hundreds of nanometers as opposed to millimeter feature sizes in radio embodiments). In addition, radar units may operate as a part of a vehicle radar system. As such, a radar unit may be positioned on a vehicle component, built into a vehicle component, or a combination in some examples. 
     It should be understood that arrangements described herein are for purposes of example only. As such, those skilled in the art will appreciate that other arrangements and other elements (e.g., machines, apparatuses, interfaces, functions, orders, and groupings of functions, etc.) can be used instead, and some elements may be omitted altogether according to the desired results. Further, many of the elements that are described are functional entities that may be implemented as discrete or distributed components or in conjunction with other components, in any suitable combination and location. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the scope being indicated by the following claims.