Patent Publication Number: US-2021184330-A1

Title: Iris heater structure for uniform heating

Description:
This application claims benefit of priority from U.S. Provisional Application No. 62/949,361, titled IRIS HEATER STRUCTURE FOR UNIFORM HEATING and filed Dec. 17, 2019, which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The technical field of the present disclosure relates to wireless communications; more particularly, the present disclosure relates to antennas that include heating structures to heat the interior of the antenna. 
     BACKGROUND 
     Certain antenna technologies require heating of the antenna in order to bring the antenna to an operational temperature. For example, certain antennas that utilize liquid crystals must have the liquid crystals heated to a specific temperature in order for the liquid crystal to operate as desired. 
     In prior art related to liquid crystal displays (LCD), resistive heating elements are used to keep the LC above a specific temperature for proper operation, for example in automotive display applications where ambient temperatures can reach −30 C to −40 C. These heating elements are made from transparent conductors, such as Indium Tin Oxide (ITO) on a separate glass substrate from the primary LCD substrate. This substrate is subsequently bonded to the primary LCD substrate to provide thermal conductivity. Because the heating element is transparent to optical frequencies, this is a straightforward and practical way to implement a heater for LCDs, even though the heating element is in the signal path. 
     This approach, however, is not feasible when considering LC-based antennas. Because ITO and similar materials are not transparent at RF frequencies, placing these types of heater elements in the path of the RF signal will attenuate the RF signal and degrade the performance of the antenna. 
     Consequently, prior art embodiments of LC-based antennas use resistive heating elements attached to the metal feed structure or other bulk mechanical structures with good thermal properties to heat an internal portion of the antenna where the LC layer resides. However, because the resistive heating elements are physically separated from the LC layer by a number of layers in the antenna stack-up, including layers of thermal insulators, significantly more heat power must be applied in order to heat the liquid crystal, as compared to the LCD implementation. 
     Other implementations of LC-based antenna heaters attempt to heat the LC layer from the edges of the antenna aperture. These embodiments require 400-500 W of power and require 30-40 minutes at this power to bring the LC layer to operational temperatures. This is an inefficient use of heating power resources. 
     SUMMARY 
     An antenna has radio-frequency (RF) antenna elements and two substrates (e.g., an iris substrate and a patch substrate) with a heater structure connected to at least one of the two substrates, for heating the RF antenna elements. In one embodiment, the antenna comprises: a physical antenna aperture having an array of radio frequency (RF) antenna elements formed with patch and iris substrates, the iris substrate having a plurality of layers including an iris metal layer; and a heater structure coupled to one or more of the plurality of layers of the iris substrate for heating the RF antenna elements. Other aspects and advantages of the embodiments will become apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the described embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The described embodiments and the advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings. These drawings in no way limit any changes in form and detail that may be made to the described embodiments by one skilled in the art without departing from the spirit and scope of the described embodiments. 
         FIG. 1A  illustrates an example of heating elements used to heat the RF antenna elements in an antenna aperture, where the heating wires have equal line lengths and follow gate routing and heater routing between RF elements. 
         FIG. 1B  illustrates an embodiment of the heating wire on an antenna aperture having heating wires of unequal length and their cross sections are unequal to each other. 
         FIGS. 2A-2C  illustrate an example cross section, or side view, of an antenna aperture having iris and patch layers. 
         FIG. 3A  illustrates an example of heater power bus placement integrated on an antenna aperture for heater wires of equal length. 
         FIG. 3B  illustrates an example of heater power bus placement integrated on an antenna aperture for heater wires of unequal length. 
         FIG. 4A  illustrates a heater bus connection scheme for use in connecting the heater bus to a heater power supply. 
         FIG. 4B  is a generic cross section of a heater bus connecting to a heater wire inside the aperture, extending under the seal and coming out to the bond pad structure on the iris overhang. 
         FIG. 5  illustrates on embodiment of a heater power bus electrically crossing over from iris layer to the patch layer inside a border seal. 
         FIG. 6  illustrates one embodiment of a heater bus electrical cross-over from the iris layer to the patch layer within a border seal structure. 
         FIGS. 7A-7C  are typical TFT Voltage vs. Current curves at different temperatures. 
         FIG. 8A  is a flow diagram of one embodiment of a process for determining an estimate of temperature of the LC using a TFT (or other type of transistor). 
         FIG. 8B  illustrates an example of a temperature measurement circuitry. 
         FIG. 8C  is a flow diagram of one embodiment of a process for determining an estimate of temperature of the LC using a TFT (or other type of transistor) configured in a different manner than that of  FIG. 8A . 
         FIG. 8D  illustrates another example of a temperature monitoring circuit for a TFT using the procedure of  FIG. 8C . 
         FIG. 9  illustrates a circuit to determine the capacitance of the LC in order to determine the temperature of the LC in the RF antenna elements. 
         FIG. 10  illustrates an aperture having one or more arrays of antenna elements placed in concentric rings around an input feed of the cylindrically fed antenna. 
         FIG. 11  illustrates a perspective view of one row of antenna elements that includes a ground plane and a reconfigurable resonator layer. 
         FIG. 12  illustrates one embodiment of a tunable resonator/slot  1210 . 
         FIG. 13  illustrates a cross section view of one embodiment of a physical antenna aperture. 
         FIG. 14A  illustrates a portion of the first iris board layer with locations corresponding to the slots. 
         FIG. 14B  illustrates a portion of the second iris board layer containing slots. 
         FIG. 14C  illustrates patches over a portion of the second iris board layer. 
         FIG. 14D  illustrates a top view of a portion of the slotted array. 
         FIG. 15  illustrates a side view of one embodiment of a cylindrically fed antenna structure. 
         FIG. 16  illustrates another embodiment of the antenna system with an outgoing wave. 
         FIG. 17  illustrates one embodiment of the placement of matrix drive circuitry with respect to antenna elements. 
         FIG. 18  illustrates one embodiment of a TFT package. 
         FIG. 19  is a block diagram of one embodiment of a communication system that performs dual reception simultaneously in a television system. 
         FIG. 20  is a block diagram of another embodiment of a communication system having simultaneous transmit and receive paths. 
         FIGS. 21A and 21B  illustrate an example of a superstrate with a heater pattern attached thereto. 
         FIG. 22  illustrates iris metal layer and heater bus metal in an embodiment of a heater for a metamaterial antenna. 
         FIG. 23  illustrates a cross-section of a heater trace near an RF antenna element. 
         FIG. 24  illustrates a uniform iris heater with a single bus plane (iris metal). 
         FIGS. 25A and 25B  illustrate a heater trace underneath the iris metal (left) and above the iris metal (right), respectively. 
         FIG. 26  illustrates a cross-sectional view of a spacer/heater structure. 
         FIG. 27  illustrates heater bundles and heater bus segments. 
         FIG. 28  illustrates a resistive model of the segmented heater bus design. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide a more thorough explanation of the embodiments of the present antennas. It will be apparent, however, to one skilled in the art, that various embodiments may be practiced with variations of, or perhaps even without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention. 
     Embodiments of the antennas include techniques for placing heaters (e.g., heating elements) on the interior of LC-based, radio frequency (RF) antenna apertures. In one embodiment, the heaters and/or heater structures are placed inside the antenna aperture near the RF elements and closer to liquid crystal (LC) that is part of the RF antenna elements. This allows for more direct heating of the aperture, lessens the heater power requirements, and shortens the temperature rise time over techniques that use more indirect heating methods of raising the temperature inside the aperture, e.g., resistive heating elements on the back of the feed structure. 
     In one embodiment, the LC-based, radio frequency (RF) antenna apertures include a pair of substrates (e.g., glass layers) that contain patches and irises of the RF antenna elements (e.g., surface scattering metamaterial antenna elements) and the heater structures are integrated into the metal layers of one or both substrates. In one embodiment, this is done in a way that the heater implementation does not interfere with the RF properties of the aperture. In one embodiment, the heater elements (e.g., heater traces) are located within the antenna aperture at locations that reduce, and potentially eliminate, RF interference while providing more direct heating within the aperture. In one embodiment, this is accomplished by putting heating elements between the RF elements on nearly the same plane as the RF elements. In one embodiment, the location of the heater elements is in the same plane as the iris elements of an iris layer that is part of a patch/iris slotted-array antenna. By moving the heater wiring inside the aperture onto nearly the same plane as the iris metal, the interaction of the heating wires with the RF signal is reduced, and potentially minimized. 
     The techniques disclosed herein also include methods for detecting the temperature within an antenna aperture. In one embodiment, the temperature is detected from a transistor directly on a transistor backplane. In one embodiment, the transistor backplane is a thin-film transistor (TFT) backplane. In one embodiment, if the transistors on the transistor backplane are in contact with an LC or other material, detecting the temperature of a transistor provides an indication of the temperature of the LC/material. 
     Techniques described herein decrease the cost of the heater system, require less power, decrease the rise time of the aperture temperature, and shrink the footprint of the controller board used to control the antenna. More specifically, in one embodiment, techniques described herein require 75-100 watts of power and will bring the LC layer temperature to operational temperature in 20 minutes. 
     Furthermore, temperature is typically sensed on the break-out printed circuit board (PCB), which is substantially physically removed from the glass assembly that includes patch and iris glass layers and an LC layer. On-glass temperature sensing provides much tighter control of the thermal management feedback loop. 
     Overview of Heater Embodiments 
     In one embodiment, the heater structure consists of several parts: the heating elements, heater power buses to supply the heater elements, and connection schemes to connect the heater power buses to the heater power supplies that are located outside and/or inside the aperture. In one embodiment, the heater elements are wires. In one embodiment, the heater power buses are of very low resistance. 
     The implementation of heater wiring, heater buses, and heater connections, depending upon implementation, may require additional depositions of conductor layers, passivation layers, via openings and so on during aperture fabrication. These additional layers may serve to build the heater structures, isolate the heater structure electrically or chemically from other structures, and to provide interfaces for the heater to existing aperture structure as needed. 
     Heating Wires 
     It is desirable that the heating of the aperture occur uniformly. Two configurations of heating wires are described herein that may achieve this objective. 
     In one embodiment, the heating wires are of equal length and the cross sections of these heating wires are the same (or similar) in dimension over the length of the heating wires and from heating wire to heating wire. In the aggregate, this provides the same power dissipation per unit area over the aperture. In one embodiment, the heating wires are evenly distributed over the aperture quality area, with heating wires laying between irises, not crossing or contacting the patches or irises. In one embodiment, the heating wires are close to the same distance apart from each other (the same pitch) across the aperture area. 
       FIG. 1A  illustrates an example of heating elements used to heat the RF antenna elements in an antenna aperture, where the heating wires have equal line lengths and follow gate routing and heater routing between RF elements. In one embodiment, the gate routing is the routing to control gates that turn on and off liquid crystal-based RF antenna elements, which is described in more detail below. 
     Referring to  FIG. 1A , an antenna aperture segment  100  illustrates one quarter of an antenna array of RF antenna elements. In one embodiment, four antenna aperture segments are coupled together to form an entire array. Note that other numbers of segments may be used to construct an entire antenna array. For example, in one embodiment, the segments are shaped such three segments coupled together form a circular array of RF antenna elements. For more information on the antenna segments and the manner in which they are coupled together, see U.S. Application Patent Publication No. US2016/0261042, entitled “ANTENNA ELEMENT PLACEMENT FOR A CYLINDRICAL FEED ANTENNA”, filed on Mar. 3, 2016 and see U.S. Application Patent Publication No. US2016/0261043, entitled “APERTURE SEGMENTATION OF A CYLINDRICAL FEED ANTENNA”, filed on Mar. 3, 2016. Note that the techniques described herein are not limited to operate with antenna aperture segments and may be used on single apertures that contain an entire array of RF antenna elements. 
     Heating wires (elements)  101  are shown on antenna aperture segment  100 . In one embodiment, the heating wires  101  are equal in length. In one embodiment, heating wires  101  are located between RF antenna elements (not shown) in the antenna array. In one embodiment, heating wires  101  follow the gate lines used to control gates to turn on and off individual RF antenna elements in the array. In one embodiment, heating wires  101  are equal distance between the RF elements. 
     In one embodiment, heating wires  101  are equal distance with respect to each other. In other words, the separation between pairs of heating wires is equal. Note that this is not a requirement, though it may help with providing more uniform heating of the antenna aperture. In one embodiment, when the antenna elements in the antenna array are located in rings, individual heating wires in heating wires  101  are equal distance between two consecutive rings of RF antenna elements. In alternative embodiments, the separation between pairs of heating wires is not equal. 
     It should be noted that the heater wiring depicted in  FIG. 1A  indicates the relative position and routing of the wiring, but does not represent the wiring size or number of wires. For example, in one embodiment, every other wire may be removed with the remaining wires providing the necessary heating where the heating is provided uniformly over an area. With respect to the size of the heating wires, their size is based on the material properties of the heating wire itself and the amount of heating the wires are to provide. 
     In one embodiment, the heating wire cross section (height and width) of heating wires  101  is chosen in the following way. First, the required power for heating the aperture area is converted, for a given desired heater supply voltage with the number and length of the heating wires, to a resistance for the heating wires. In turn, this resistance value is used in conjunction with properties of the heating wire materials to determine the required heating wire cross section. Note that other considerations may be used to select the heating wire cross section, including, but not limited to fabrication yields. 
     In another embodiment, the heating wires are unequal in length and their cross sections are not equal. In one embodiment, where the heating wires with unequal lengths are on concentric arcs between RF elements. In one embodiment, the heater wire widths are equal and the wire heights are adjusted radially from the center of the segment to provide uniform power per unit area over the aperture area. 
       FIG. 1B  illustrates an embodiment of the heating wire on an antenna aperture having heating wires of unequal length and their cross sections are unequal to each other. Referring to  FIG. 1B , heating wires  111  are shown on antenna aperture segment  110 , which is the same type of aperture segment as depicted in  FIG. 1A . In one embodiment, a number of antenna apertures are coupled together to form a complete antenna array. As in  FIG. 1A , in one embodiment, the heating wires are routed between the RF elements. In one embodiment, that routing follows the gate routing for the gates that control the antenna elements. 
     In one embodiment, an objective is still to provide nearly uniform power dissipation per unit area. In this case, however, the heights of the heating wire cross sections are varied over the aperture area to control the current and resistance to provide the same power dissipation per area although the heating wires are of unequal length. 
     It should be noted that the heater wiring depicted in  FIG. 1B  indicates the relative position and routing of the wiring, but does not represent the wiring size or number of wires. With respect to the size of the heating wires, their size is based on the material properties of the heating wire itself and the amount of heating the wires are to provide. 
     In one embodiment, the heating wires lie between the iris features and do not cross or contact the patch or iris features in a tunable slotted array of antenna elements having patch/slot pairs. In the illustration example provided in  FIG. 2 , the heating wires lie in rings midway between the rings of iris/patch elements, with an additional inside and outside ring of heating wires. In one embodiment, the rings of heating wiring are on concentric rings at the same radial pitch over the aperture area. In one embodiment, the heater wiring radial pitch is the same radial pitch as the RF elements. In alternative embodiments, the heater wiring radial pitch is not the same as the radial pitch of the RF elements. 
     In one embodiment, the heater wires lie close to equidistant between the RF elements. 
       FIG. 2  illustrates an example cross section, or side view, of an antenna aperture having iris and patch layers. Referring to  FIG. 2 , patch glass layer  201  and iris glass layer  202  are separated with respect to each other and include patch and iris slots, respectively, to form a tunable slotted array. Such an array is well-known and is also described in more detail below. In one embodiment, patch glass layer  201  and iris glass layer  202  are glass substrates. Note that the patch layer and iris layer may be referred to below as a patch glass layer and an iris glass layer, respectively. However, it should be understood that for purposes herein, the embodiments that include “patch glass layer” and “iris glass layer” may be implemented with a “patch substrate layer” and an “iris substrate layer,” respectively, (or patch substrate and iris substrate) when the substrate is other than glass. 
     Patch metal  211 , as portions of a patch metal layer, is fabricated onto patch glass layer  201 . A passivation patch layer  231  is fabricated over patch metal  211  and the patch metal layer. A liquid crystal (LC) alignment layer  213  is fabricated on top of passivation patch layer  231 . Sections of iris metal  212 , of an iris metal layer, are fabricated onto iris glass layer  202 . Passivation layer  232 , which may also be referred to herein as iris passivation layer  1  or a passivation iris layer, is fabricated over the iris metal  212 . Heater wire  240 , which may also be turned heating wire, is fabricated on top of passivation layer  232 . In one embodiment, heater wire  240  is close to equal distance between a pair of iris elements. Other heating wires are also located between iris elements in this fashion. Another passivation layer  233 , which may also be referred to herein as iris passivation layer  2 , or another passivation iris layer, is fabricated over passivation layer  232  and heater wire  240 . LC alignment layer  213  is fabricated on top of passivation layer  233 . 
     Note that the LC alignment layer  213  is used to align the LC  260  so that it is pointing in a single direction in a manner well known in the art. 
     Heater Power Buses 
     Power buses are provided to supply power to the heating wires. Examples of these are illustrated in the figures below. In one embodiment, the power buses are of low resistance when compared to the heater wires, by several orders of magnitude, so that there is a small voltage drop from one end of the bus to the other, so that all of the heating wires may have the same voltage at each bus end of the heating wire. This makes it simpler to manage the power distribution to the network of heating wires. 
     In one embodiment, the heater buses are placed inside the aperture so that the heating wires are able to connect to the proper supply voltages at each end of the heating wire. 
     In one embodiment, the heater buses are separate structures placed into the apertures solely for the purpose of providing power to the heater wire network. 
     In another embodiment, existing structures in the aperture may be used to also act as heater buses. In one embodiment, the heater bus (or buses) are built into the seal structure of the aperture. In another case, the iris metal (e.g., copper) plane may be used as a heater bus to sink or source current for the heating wires. 
       FIG. 3A  illustrates an example of heater power bus placement integrated on an antenna aperture for heater wires of equal length. Referring  FIG. 3A , antenna aperture segment  300 , which represents one of the antenna segments that are coupled together to form an entire antenna array, includes heater bus lines  301  and  302 , which may also be referred to herein as heater power bus lines. Heater bus lines  301  and  302  are electrically connected to and provide power to heating wires  303 . 
       FIG. 3B  illustrates an example of heater power bus placement integrated on an antenna aperture for heater wires of unequal length. Referring to  FIG. 3B , heater buses  304  and  308 , which may also be referred to herein as heater power buses, are electrically connected to heating wires  305  on antenna aperture segment  310 . 
     Heater Bus to Power Supply Connection 
     In one embodiment, heater buses on the inside of the aperture are brought outside of the aperture structure to make connection to the heater power supply. In one embodiment, this can be accomplished by connecting the heater buses through a border seal structure at the outer portion of the antenna aperture to a metallization layer on one of the layers in the aperture outside the aperture border seal. For example, one such metallization layer is on the iris glass layer or on the patch glass layer. This metallization connects to the heater buses inside the seal and extends from inside the seal, through the seal, and out to portions of the patch or iris glass layers that extend beyond each other. These may be referred to as overhang regions. In such cases, portions of the patch or iris glass layers beneath those overhang regions may be referred to as under-hang regions. 
       FIGS. 4A and 4B  illustrate examples of the heater buses coming through the border seal out onto the iris glass layer overhang. In one embodiment, the RF aperture, in this case, is cut so that both the iris glass layer and the patch glass layer have overhang regions (where the substrate has a metallized region not faced by glass layer opposite the metallized face). Note that while the iris and patch layers may be described herein at times as glass layers, they are not limited to being glass and may constitute other types of substrates. 
       FIG. 4A  illustrates a heater bus connection scheme for use in connecting the heater bus to a heater power supply. Referring to  FIG. 4A , in one embodiment, the heater power supply (not shown) is located outside of the antenna element array, such as antenna element array  430 , which includes the heating wires. Antenna aperture segment  400  includes a patched layer and an iris layer as discussed herein. A portion of the iris layer, referred to as iris overhang  401  and  402  extends over portions of the patch layer. Similarly, a portion of the patch glass layer, referred to herein as the patch overhang  403 , extends beyond a part of the iris glass layer. The iris glass layer and the patch glass layer are sealed together with an aperture border seal  460 . Heater power bus  410  crosses border seal  460  at seal crossing  421 . Heater bus  411  crosses border seal  460  at seal crossing  420  and connects to a power supply. In both cases, heater bus  410  and heater bus  411  are able to connect through a power supply by exiting antenna aperture segment  400 . Antenna aperture segment  400  includes heater buses  410  and  411 , which may also be referred to herein as heater power buses, electrically connected to heating wires  481  in the antenna element array  430 . 
       FIG. 4B  is a generic cross section of a heater bus connecting to a heater wire inside the aperture, extending under the seal and coming out to the bond pad structure on the iris overhang. Referring to  FIG. 4B , heater bus metal  443  goes under a border seal, border seal adhesive  450 , on the iris glass layer  431  on top of passivation layer  446 . Thus, heater bus metal  443  is underneath border seal adhesive  450 . Border seal adhesive  450  couples the patch glass layer  430  to iris glass layer  431  including the fabricated layers thereon. 
     Heating wire  444  is deposited on top of passivation layer  446  and a portion of heater bus metal  443 , thereby electrically connecting to heater bus metal  443  with heater wire  444 . Heater wire  444  is fabricated on a portion of passivation layer  441  that is fabricated on top of iris metal  445  and is fabricated onto a portion of heater bus metal  443 . In an alternative embodiment, there is a passivation layer between heater bus metal  443  and heating wire  444 , with a via, through the passivation layer, connecting heater bus metal  443  and heating wire  444 . 
     Passivation layer  441  is fabricated on top of heating wire  444  and at least a portion of heater bus metal  443 . An alignment layer  432  is fabricated on top of passivation layer  441 . Passivation layer  441  is also fabricated on the bottom of the patch glass layer  430 . Similarly, alignment layer  432  is fabricated over a portion of passivation layer  441  on the patch glass layer  430 . Note that while heater wire  444  is shown deposited directly on top of heater bus metal  443  without a passivation layer and via in between, in an alternative embodiment, another layer of passivation is deposited between heater wire  444  and heater bus metal  443  with an electrical connection between the two being made using a via. This layer of passivation protects the heater bus metal while the heater wire metal is being etched. 
     Bond pad/connector structure  442  is a location to electrically connect the power supply to heater bus metal  443 . 
     The power for the heater buses may cross from the patch glass layer side of the aperture to the iris glass layer side of the aperture inside of the border seal, within the border seal itself, or outside of the border seal. Bringing the heater buses out to the patch layer overhang has the advantage of possibly making the heater connection within the connector used for the rest of the interface lines from the controller electronics to the aperture. The following illustrations show methods of doing this inside and within the border seal. 
       FIG. 5  illustrates on embodiment of a heater power bus electrically crossing over from iris layer to the patch layer inside a border seal. Referring to  FIG. 5 , patch glass layer  501  is shown over iris glass layer  502 . There are a number of layers fabricated onto patch glass layer  501  and iris glass layer  502  and a border seal adhesive  521  couples these two substrates together. In one embodiment, patch glass layer  501  and iris glass layer  502  comprise glass layers, though they may be other types of substrates. 
     Iris metal  541  is fabricated on top of iris glass layer  502 . Passivation layer  531  is fabricated on top of iris metal  541  and the iris glass layer  502  where iris metal  541  is absent. Over passivation layer  531  comprises heater bus metal  512 . Over the passivation layer  531  that is over iris metal  541  is passivation layer  550 . Heating wire  510  is fabricated on top of passivation layer  550  and on top of a portion of heater bus metal  512 . In an alternative embodiment, there is a passivation layer between heater bus metal  512  and heating wire  510  with a via through the passivation layer connecting heater bus metal  512  and heating wire  510 . Passivation layer  530  is fabricated over heating wire  510  or at least a portion of heating wire  510 , with alignment layer  540  on top of passivation layer  530 . On patch glass layer  501 , a passivation layer  532  is fabricated. On top of passivation layer  532  is a heater bus supply metallization  511  supplying the heater bus. A passivation layer  530  covers a portion of heater bus supply metallization  511 , while alignment layer  540  covers a portion of passivation layers  530  and is used for aligning LC  560 . A bond/connector structure  513  is located to allow an electrical connection between the heater power bus and an external power supply (not shown). 
     Conductive cross-over  520  electrically connects the heater bus supply metallization  511  to heater bus metal  512  such that the power supply connected to connector structure  513  is able to supply power through heater bus supply metallization  511  through the conductive cross-over  520  to heater bus metal  512  which provides the power to heating wire  510 . 
       FIG. 6  illustrates one embodiment of a heater bus electrical cross-over from the iris layer to the patch layer within a border seal structure. Referring to  FIG. 6 , a conductive cross-over  620  is with the border seal  621  and provides an electrical connection between the heater bus supply metallization  611  that is fabricated on the patch glass layer  601  to heater bus metal  612 , which is on the iris glass layer  602 . Heater wire  615  is fabricated on a portion of passivation layer  650  that is fabricated on top of iris metal  641  and is fabricated onto a portion of heater bus metal  612 . In an alternative embodiment, there is a passivation layer between heater bus metal  612  and heater wire  615  with a via through the passivation layer connecting heater bus metal  612  and heater wire  615 . 
     A patch overhang has no facing iris glass outside the border seal. An iris under hang has no facing patch glass outside the border seal. Metallization on an overhang or under hang is therefore accessible to make a connection to the heater power supply/controller. For example, this connection might be made by an ACF (anisotropic conductive film, a type of adhesive) to a flex cable. This flex cable might connect to the heater power supply/controller. This heater power supply/controller might be on the aperture controller board or might be an independent power supply/controller unit. 
     Note that in the figures the patch glass, especially around the border seal region, has a number of other structures on it besides this heater wiring. The heater connection structures as drawn focus only on a method of supplying the heaters, and do not try to show the integration with other patch structures, for example, the voltage bus that connects from the patch overhang to the iris metal. A layer of passivation above the heater bus supply metallization  511  (in FIGS.  5 ) and  611  (in  FIG. 6 ), isolates this heater bus supply metallization  511  from the rest of the patch circuitry. 
     Placement of the Heater Wiring, Heater Buses and Connections 
     The heater wiring and heater buses might be placed on either the patch glass side of the aperture, the iris glass side of the aperture, or have may have parts on both patch and iris glass (or non-glass) layers of the aperture. The connection for the heaters may come out on the patch glass layer or the iris glass layer side of the aperture. 
     Temperature Sensors Inside the RF Aperture 
     In one embodiment, one or more temperature sensors are located within the aperture. These temperature sensors are used to monitor the internal aperture temperature and to control whether the heater, including the heating elements (wires), heater buses and heater connections need to be engaged to regulate the temperature in the aperture. This may be necessary where the RF antenna elements need to be put in a certain temperature or range of temperatures. For example, when each of the RF antenna elements includes an LC, the antenna element operates more effectively if the LC is at a certain temperature. Therefore, by monitoring the temperature within the aperture and determining that the temperature of the LC is below its optimal temperature range, the heating wires, buses and connections can be used to heat the internal aperture until the LC is at the desired temperature range. 
     Using an Antenna Element Control Transistor (e.g., TFT) for Aperture Temperature Measurement 
     Embodiments of the invention include techniques for using a transistor (e.g., a TFT) integrated onto the patch layer substrate to measure LC temperature. In one embodiment, this technique uses the changing mobility characteristics of the TFT over temperature to indicate the temperature. 
       FIGS. 7A-7C  are typical TFT Voltage vs. Current curves at different temperatures. Referring to  FIGS. 7A-7C , each chart has a plot for two values of Vds where the vertical axis is Id, the horizontal axis is Vgs. 
     Note that Id for a given Vds and Vgs changes over temperature. By using this TFT characteristic and setting the Vgs and Vds to known constant values, the measured Id value can be correlated to the temperature of the TFT. 
       FIG. 8A  is a flow diagram of one embodiment of a process for determining an estimate of temperature of the LC using a TFT (or other type of transistor). The TFT is connected to the LC. Therefore, the temperature of the TFT provides an indication of the temperature of the LC. The process is performed by a temperature control system that includes a temperature monitoring subsystem. 
     Referring to  FIG. 8A , the process begins by adjusting a digital voltage value, referred to as the digital-to-analog converter (DAC) value, until the voltage Vgs measurement analog-to-digital converter (ADC) indicates the predefined Vgs value (processing block  801 ). Next, processing logic in the temperature control system measures the current Id by reading the Id measurement ADC that is monitoring the voltage across a current sense resistor (processing block  802 ). Based on the Vgs voltage value and the Id current value, processing logic correlates the Id value to the calibrated temperature value (processing block  803 ). The correlation may be performed by a correlator/processing unit (e.g., processor) that access a lookup table (LUT) using the values to determine a corresponding temperature value for the TFT. 
       FIG. 8B  illustrates an example of a temperature measurement circuitry. Referring to  FIG. 8B , a voltage value is provided by DAC  861  to a circuit having current sensor resistor  862  coupled in series with a transistor  864 . In one embodiment, a transistor  864  is in contact with liquid crystal (LC) in the RF antenna element. In one embodiment, transistor  864  comprises a thin film transistor (TFT). In one embodiment, the voltage value output from DAC  861  comes from a temperature controller  831 . In one embodiment, a temperature adjustment unit  843  may provide different voltage values based on the type of transistor being monitored. 
     The voltage value across current sensor resistor  862  is monitored using comparator  863  to produce a current measurement that is converted to digital form by ADC  810 . Based on the measurement current and the measured Vgs voltage, correlator  841  determines the temperature  842  of transistor  864  based on a correlation between transistor  864  and the measured current Id and Vgs voltage (processing block  803 ). Since transistor  864  is in contact with the LC, the temperature of transistor  864  is used to indicate or represent the temperature of the LC. 
       FIG. 8C  is a flow diagram of one embodiment of a process for determining an estimate of temperature of the LC using a TFT (or other type of transistor) configured in a different manner than that of  FIG. 8A . As in  FIG. 8A , the TFT is connected to the LC and the temperature of the TFT provides an indication of the temperature of the LC. The process is performed by a temperature control system that includes a temperature monitoring subsystem. 
     Referring to  FIG. 8C , the process begins by adjusting a digital voltage value, referred to as the digital-to-analog converter (DAC) value, until the voltage Vds measurement analog-to-digital converter (ADC) indicates the predefined Vds value (processing block  804 ). Next, processing logic in the temperature control system measures the current Id by reading the Id measurement ADC that is monitoring the voltage across a current sense resistor (processing block  805 ). Based on the Vds voltage value and the Id current value, processing logic correlates the Id value to the calibrated temperature value (processing block  806 ). The correlation may be performed by a correlator/processing unit (e.g., processor) that access a lookup table (LUT) using the values to determine a corresponding temperature value for the TFT. 
       FIG. 8D  illustrates another example of a temperature monitoring circuit for a TFT using the procedure of  FIG. 8C . The circuit in  FIG. 8D  is substantially similar to that of  FIG. 8B  with the exception that transistor  814  is coupled in a different way. Thus, the measuring by the monitoring subsystem and the operation of temperature controller  831  operates in the same way. 
     In one embodiment, multiple test TFTs can be distributed around the RF elements (and their LC) in the antenna array to measure the temperature at various locations and/or for temperature averaging. 
     Using Capacitance Properties of the LC to Measure LC Temperature 
     In one embodiment, LC temperature is measured by using the capacitance properties of the LC. This uses the characteristic of the LC that the electrical capacitance changes as a function of temperature. 
     In one embodiment, an electrical test capacitor is made by placing a conductive surface on the patch glass layer and a matching conductive surface be placed on the iris glass layer, thereby creating a capacitor with the LC acting as the separating dielectric material. These conductive surfaces are connected to circuitry that measures the capacitance (such as a capacitance-to-digital converter (CDC)). Since the capacitance of the LC is a function of temperature, the capacitance of the test capacitor can be correlated to the temperature of the LC directly. 
       FIG. 9  illustrates a circuit to determine the capacitance of the LC in order to determine the temperature of the LC in the RF antenna elements. Referring to  FIG. 9 , an excitation signal  901  is provided to a conductor  910 D that connects iris glass layer  910 E to liquid crystal  910 C. In one embodiment, the excitation is a square wave. In one embodiment, the excitation signal  901  comes from a DAC with an input provided temperature controller  931 . In one embodiment, a temperature adjustment unit  943  may provide different voltage values based on the type of test capacitor being monitored. 
     Patch glass layer  910 A is coupled to liquid crystal  910 C using conductor  910 B. Applying the square wave of signal  901  to conductor  910 D causes a capacitance to be created over liquid crystal  910 C that is measured with Σ-Δ digital converter (CDC)  902 . The output of CDC  902  is provided to temperature controller  931 , which correlates the capacitance measurement, using correlator  941 , to a temperature  942  of the LC of the LC-based test capacitor. This temperature is then used as the temperature of the LCs in the RF antenna elements in the array. 
     In yet another embodiment, the temperature monitoring subsystem is operable to measure decay speed of a liquid crystal and correlate the decay speed to a temperature of the liquid crystal. The decay speed of an LC is well-known in the art and the amount of time an LC is used is easily tracked. In one embodiment, the correlation operation is performed in the same manner as described above in conjunction with  FIGS. 8B, 8D and 9 . 
     In one embodiment, multiple test patches are distributed around the antenna array of RF LC-based antenna elements to measure the temperature various location and/or for temperature averaging. 
     The heater, including the heater elements and heater buses, is operated in conjunction with a temperature sensor to provide feedback to the heater system. The temperature sensor may be in the aperture or on the aperture. Some correlation of the temperature inside the aperture and the temperature measured by the sensor may need to be established by a calibration procedure. 
     In one embodiment, the temperature of the aperture is regulated by a control loop consisting of the temperature sensor and the heater power supply/controller. When the sensor indicates that the aperture is below its operational temperature, the heater power controller causes the heater to turn on to heat the aperture. There are many methods by which the desired aperture temperature can be controlled using the heater structures described herein. 
     In an alternative embodiment, instead of placing the heater inside of the RF aperture, the same types of heater wire patterns, heater wire pattern placement, heater buses and heater bus placements are made on a superstrate. In one embodiment, the superstrate is a substrate directly on the satellite facing side of the RF aperture. In one embodiment, the implementation is the same as is described above for use within the RF aperture (in the RF element/LC plane). 
     In one embodiment, when placing the heater on the superstrate, the superstrate is placed with the heater wire pattern between the top of the patch layer and the bottom of the superstrate, as close to the LC layer as possible. One potential problem with placing the heater on the superstrate is that the interaction of RF coming from the patch layer with the heater wires on the superstrate may have a detrimental effect on the RF pattern being formed by the RF aperture. To reduce the interaction of the RF with the heater wires, in one embodiment, the patch layer is thinned as much as possible, to move the heater as close to the RF element/LC plane as possible. 
       FIGS. 21A and 21B  illustrate an example of a superstrate with a heater pattern attached thereto. Referring to  FIGS. 21A and 21B  superstrate  2101  includes a heater wire pattern  2103  on its bottom side. A heater bus  2102  is also attached to the bottom of superstrate  2101 . Superstrate  2101  is coupled to segment  2100  that includes aperture area  2110  of RF antenna elements as shown in  FIG. 21B , a patch overhang  2104 . 
     Examples of Antenna Embodiments 
     The techniques described above may be used with flat panel antennas. Embodiments of such flat panel antennas are disclosed. The flat panel antennas include one or more arrays of antenna elements on an antenna aperture. In one embodiment, the antenna elements comprise liquid crystal cells. In one embodiment, the flat panel antenna is a cylindrically fed antenna that includes matrix drive circuitry to uniquely address and drive each of the antenna elements that are not placed in rows and columns. In one embodiment, the elements are placed in rings. 
     In one embodiment, the antenna aperture having the one or more arrays of antenna elements is comprised of multiple segments coupled together. When coupled together, the combination of the segments form closed concentric rings of antenna elements. In one embodiment, the concentric rings are concentric with respect to the antenna feed. 
     Overview of an Example of Antenna Systems 
     In one embodiment, the flat panel antenna is part of a metamaterial antenna system. Embodiments of a metamaterial antenna system for communications satellite earth stations are described. In one embodiment, the antenna system is a component or subsystem of a satellite earth station (ES) operating on a mobile platform (e.g., aeronautical, maritime, land, etc.) that operates using either Ka-band frequencies or Ku-band frequencies for civil commercial satellite communications. Note that embodiments of the antenna system also can be used in earth stations that are not on mobile platforms (e.g., fixed or transportable earth stations). 
     In one embodiment, the antenna system uses surface scattering metamaterial technology to form and steer transmit and receive beams through separate antennas. In one embodiment, the antenna systems are analog systems, in contrast to antenna systems that employ digital signal processing to electrically form and steer beams (such as phased array antennas). 
     In one embodiment, the antenna system is comprised of three functional subsystems: (1) a wave guiding structure consisting of a cylindrical wave feed architecture; (2) an array of wave scattering metamaterial unit cells that are part of antenna elements; and (3) a control structure to command formation of an adjustable radiation field (beam) from the metamaterial scattering elements using holographic principles. 
     Examples of Wave Guiding Structures 
       FIG. 10  illustrates an aperture having one or more arrays of antenna elements placed in concentric rings around an input feed of the cylindrically fed antenna. In one embodiment, the cylindrically fed antenna includes a coaxial feed that is used to provide a cylindrical wave feed. In one embodiment, the cylindrical wave feed architecture feeds the antenna from a central point with an excitation that spreads outward in a cylindrical manner from the feed point. That is, a cylindrically fed antenna creates an outward travelling concentric feed wave. Even so, the shape of the cylindrical feed antenna around the cylindrical feed can be circular, square or any shape. In another embodiment, a cylindrically fed antenna creates an inward travelling feed wave. In such a case, the feed wave most naturally comes from a circular structure. 
     Antenna Elements 
     In one embodiment, the antenna elements comprise a group of patch antennas. This group of patch antennas comprises an array of scattering metamaterial elements. In one embodiment, each scattering element in the antenna system is part of a unit cell that consists of a lower conductor, a dielectric substrate and an upper conductor that embeds a complementary electric inductive-capacitive resonator (“complementary electric LC” or “CELC”) that is etched in or deposited onto the upper conductor. As would be understood by those skilled in the art, LC in the context of CELC refers to inductance-capacitance, as opposed to liquid crystal. 
     In one embodiment, a liquid crystal (LC) is disposed in the gap around the scattering element. This LC is driven by the direct drive embodiments described above. In one embodiment, liquid crystal is encapsulated in each unit cell and separates the lower conductor associated with a slot from an upper conductor associated with its patch. Liquid crystal has a permittivity that is a function of the orientation of the molecules comprising the liquid crystal, and the orientation of the molecules (and thus the permittivity) can be controlled by adjusting the bias voltage across the liquid crystal. Using this property, in one embodiment, the liquid crystal integrates an on/off switch for the transmission of energy from the guided wave to the CELC. When switched on, the CELC emits an electromagnetic wave like an electrically small dipole antenna. Note that the teachings herein are not limited to having a liquid crystal that operates in a binary fashion with respect to energy transmission. 
     In one embodiment, the feed geometry of this antenna system allows the antenna elements to be positioned at forty-five-degree (45°) angles to the vector of the wave in the wave feed. Note that other positions may be used (e.g., at 40° angles). This position of the elements enables control of the free space wave received by or transmitted/radiated from the elements. In one embodiment, the antenna elements are arranged with an inter-element spacing that is less than a free-space wavelength of the operating frequency of the antenna. For example, if there are four scattering elements per wavelength, the elements in the 30 GHz transmit antenna will be approximately 2.5 mm (i.e., ¼th the 10 mm free-space wavelength of 30 GHz). 
     In one embodiment, the two sets of elements are perpendicular to each other and simultaneously have equal amplitude excitation if controlled to the same tuning state. Rotating them+/−45 degrees relative to the feed wave excitation achieves both desired features at once. Rotating one set 0 degrees and the other 90 degrees would achieve the perpendicular goal, but not the equal amplitude excitation goal. Note that 0 and 90 degrees may be used to achieve isolation when feeding the array of antenna elements in a single structure from two sides. 
     The amount of radiated power from each unit cell is controlled by applying a voltage to the patch (potential across the LC channel) using a controller. Traces to each patch are used to provide the voltage to the patch antenna. The voltage is used to tune or detune the capacitance and thus the resonance frequency of individual elements to effectuate beam forming. The voltage required is dependent on the liquid crystal mixture being used. The voltage tuning characteristic of liquid crystal mixtures is mainly described by a threshold voltage at which the liquid crystal starts to be affected by the voltage and the saturation voltage, above which an increase of the voltage does not cause major tuning in liquid crystal. These two characteristic parameters can change for different liquid crystal mixtures. 
     In one embodiment, as discussed above, a matrix drive is used to apply voltage to the patches in order to drive each cell separately from all the other cells without having a separate connection for each cell (direct drive). Because of the high density of elements, the matrix drive is an efficient way to address each cell individually. 
     In one embodiment, the control structure for the antenna system has 2 main components: the antenna array controller, which includes drive electronics, for the antenna system, is below the wave scattering structure, while the matrix drive switching array is interspersed throughout the radiating RF array in such a way as to not interfere with the radiation. In one embodiment, the drive electronics for the antenna system comprise commercial off-the shelf LCD controls used in commercial television appliances that adjust the bias voltage for each scattering element by adjusting the amplitude or duty cycle of an AC bias signal to that element. 
     In one embodiment, the antenna array controller also contains a microprocessor executing the software. The control structure may also incorporate sensors (e.g., a GPS receiver, a three-axis compass, a 3-axis accelerometer, 3-axis gyro, 3-axis magnetometer, etc.) to provide location and orientation information to the processor. The location and orientation information may be provided to the processor by other systems in the earth station and/or may not be part of the antenna system. 
     More specifically, the antenna array controller controls which elements are turned off and those elements turned on and at which phase and amplitude level at the frequency of operation. The elements are selectively detuned for frequency operation by voltage application. 
     For transmission, a controller supplies an array of voltage signals to the RF patches to create a modulation, or control pattern. The control pattern causes the elements to be turned to different states. In one embodiment, multistate control is used in which various elements are turned on and off to varying levels, further approximating a sinusoidal control pattern, as opposed to a square wave (i.e., a sinusoid gray shade modulation pattern). In one embodiment, some elements radiate more strongly than others, rather than some elements radiate and some do not. Variable radiation is achieved by applying specific voltage levels, which adjusts the liquid crystal permittivity to varying amounts, thereby detuning elements variably and causing some elements to radiate more than others. 
     The generation of a focused beam by the metamaterial array of elements can be explained by the phenomenon of constructive and destructive interference. Individual electromagnetic waves sum up (constructive interference) if they have the same phase when they meet in free space and waves cancel each other (destructive interference) if they are in opposite phase when they meet in free space. If the slots in a slotted antenna are positioned so that each successive slot is positioned at a different distance from the excitation point of the guided wave, the scattered wave from that element will have a different phase than the scattered wave of the previous slot. If the slots are spaced one quarter of a guided wavelength apart, each slot will scatter a wave with a one fourth phase delay from the previous slot. 
     Using the array, the number of patterns of constructive and destructive interference that can be produced can be increased so that beams can be pointed theoretically in any direction plus or minus ninety degrees (90°) from the bore sight of the antenna array, using the principles of holography. Thus, by controlling which metamaterial unit cells are turned on or off (i.e., by changing the pattern of which cells are turned on and which cells are turned off), a different pattern of constructive and destructive interference can be produced, and the antenna can change the direction of the main beam. The time required to turn the unit cells on and off dictates the speed at which the beam can be switched from one location to another location. 
     In one embodiment, the antenna system produces one steerable beam for the uplink antenna and one steerable beam for the downlink antenna. In one embodiment, the antenna system uses metamaterial technology to receive beams and to decode signals from the satellite and to form transmit beams that are directed toward the satellite. In one embodiment, the antenna systems are analog systems, in contrast to antenna systems that employ digital signal processing to electrically form and steer beams (such as phased array antennas). In one embodiment, the antenna system is considered a “surface” antenna that is planar and relatively low profile, especially when compared to conventional satellite dish receivers. 
       FIG. 11  illustrates a perspective view of one row of antenna elements that includes a ground plane and a reconfigurable resonator layer. Reconfigurable resonator layer  1230  includes an array of tunable slots  1210 . The array of tunable slots  1210  can be configured to point the antenna in a desired direction. Each of the tunable slots can be tuned/adjusted by varying a voltage across the liquid crystal. 
     Control module  1280  is coupled to reconfigurable resonator layer  1230  to modulate the array of tunable slots  1210  by varying the voltage across the liquid crystal in  FIG. 11 . Control module  1280  may include a Field Programmable Gate Array (“FPGA”), a microprocessor, a controller, System-on-a-Chip (SoC), or other processing logic. In one embodiment, control module  1280  includes logic circuitry (e.g., multiplexer) to drive the array of tunable slots  1210 . In one embodiment, control module  1280  receives data that includes specifications for a holographic diffraction pattern to be driven onto the array of tunable slots  1210 . The holographic diffraction patterns may be generated in response to a spatial relationship between the antenna and a satellite so that the holographic diffraction pattern steers the downlink beams (and uplink beam if the antenna system performs transmit) in the appropriate direction for communication. Although not drawn in each figure, a control module similar to control module  1280  may drive each array of tunable slots described in the figures of the disclosure. 
     Radio Frequency (“RF”) holography is also possible using analogous techniques where a desired RF beam can be generated when an RF reference beam encounters an RF holographic diffraction pattern. In the case of satellite communications, the reference beam is in the form of a feed wave, such as feed wave  1205  (approximately 20 GHz in some embodiments). To transform a feed wave into a radiated beam (either for transmitting or receiving purposes), an interference pattern is calculated between the desired RF beam (the object beam) and the feed wave (the reference beam). The interference pattern is driven onto the array of tunable slots  1210  as a diffraction pattern so that the feed wave is “steered” into the desired RF beam (having the desired shape and direction). In other words, the feed wave encountering the holographic diffraction pattern “reconstructs” the object beam, which is formed according to design requirements of the communication system. The holographic diffraction pattern contains the excitation of each element and is calculated by w hologram =w* in w out , with w in  as the wave equation in the waveguide and w out  the wave equation on the outgoing wave. 
       FIG. 12  illustrates one embodiment of a tunable resonator/slot  1210 . Tunable slot  1210  includes an iris/slot  1212 , a radiating patch  1211 , and liquid crystal  1213  disposed between iris/slot  1212  and patch  1211 . In one embodiment, radiating patch  1211  is co-located with iris/slot  1212 . 
       FIG. 13  illustrates a cross section view of one embodiment of a physical antenna aperture. The antenna aperture includes ground plane  1245 , and a metal layer  1236  within iris layer  1232 , which is included in reconfigurable resonator layer  1230 . In one embodiment, the antenna aperture of  FIG. 13  includes a plurality of tunable resonator/slots  1210  of  FIG. 12 . Iris/slot  1212  is defined by openings in metal layer  1236 . A feed wave, such as feed wave  1205  of  FIG. 11 , may have a microwave frequency compatible with satellite communication channels. The feed wave propagates between ground plane  1245  and resonator layer  1230 . 
     Reconfigurable resonator layer  1230  also includes gasket layer  1232  and patch layer  1231 . Gasket layer  1232  is disposed between patch layer  1231  and iris layer  1233 . Note that in one embodiment, a spacer could replace gasket layer  1232 . In one embodiment, iris layer  1233  is a printed circuit board (“PCB”) that includes a copper layer as metal layer  1236 . In one embodiment, iris layer  1233  is glass. Iris layer  1233  may be other types of substrates. 
     Openings may be etched in the copper layer to form iris/slots  1212 . In one embodiment, iris layer  1233  is conductively coupled by a conductive bonding layer to another structure (e.g., a waveguide) in  FIG. 13 . Note that in an embodiment the iris layer is not conductively coupled by a conductive bonding layer and is instead interfaced with a non-conducting bonding layer. 
     Patch layer  1231  may also be a PCB that includes metal as radiating patches  1211 . In one embodiment, gasket layer  1232  includes spacers  1239  that provide a mechanical standoff to define the dimension between metal layer  1236  and patch  1211 . In one embodiment, the spacers are 75 microns, but other sizes may be used (e.g., 3-200 mm). As mentioned above, in one embodiment, the antenna aperture of  FIG. 13  includes multiple tunable resonator/slots, such as tunable resonator/slot  1210  includes patch  1211 , liquid crystal  1213 , and iris/slot  1212  of  FIG. 12 . The chamber for liquid crystal  1213  is defined by spacers  1239 , iris layer  1233  and metal layer  1236 . When the chamber is filled with liquid crystal, patch layer  1231  can be laminated onto spacers  1239  to seal liquid crystal within resonator layer  1230 . 
     A voltage between patch layer  1231  and iris layer  1233  can be modulated to tune the liquid crystal in the gap between the patch and the slots (e.g., tunable resonator/slot  1210 ). Adjusting the voltage across liquid crystal  1213  varies the capacitance of a slot (e.g., tunable resonator/slot  1210 ). Accordingly, the reactance of a slot (e.g., tunable resonator/slot  1210 ) can be varied by changing the capacitance. Resonant frequency of slot  1210  also changes according to the equation 
     
       
         
           
             f 
             = 
             
               1 
               
                 2 
                  
                 π 
                  
                 
                   
                     L 
                      
                     C 
                   
                 
               
             
           
         
       
     
     where f is the resonant frequency of slot  1210  and L and C are the inductance and capacitance of slot  1210 , respectively. The resonant frequency of slot  1210  affects the energy radiated from feed wave  1205  propagating through the waveguide. As an example, if feed wave  1205  is 20 GHz, the resonant frequency of a slot  1210  may be adjusted (by varying the capacitance) to 17 GHz so that the slot  1210  couples substantially no energy from feed wave  1205 . Or, the resonant frequency of a slot  1210  may be adjusted to 20 GHz so that the slot  1210  couples energy from feed wave  1205  and radiates that energy into free space. Although the examples given are binary (fully radiating or not radiating at all), full gray scale control of the reactance, and therefore the resonant frequency of slot  1210  is possible with voltage variance over a multi-valued range. Hence, the energy radiated from each slot  1210  can be finely controlled so that detailed holographic diffraction patterns can be formed by the array of tunable slots. 
     In one embodiment, tunable slots in a row are spaced from each other by λ/5. Other spacings may be used. In one embodiment, each tunable slot in a row is spaced from the closest tunable slot in an adjacent row by λ/2, and, thus, commonly oriented tunable slots in different rows are spaced by λ/4, though other spacings are possible (e.g., λ/5, λ/6.3). In another embodiment, each tunable slot in a row is spaced from the closest tunable slot in an adjacent row by λ/3. 
     Embodiments use reconfigurable metamaterial technology, such as described in U.S. patent application Ser. No. 14/550,178, entitled “Dynamic Polarization and Coupling Control from a Steerable Cylindrically Fed Holographic Antenna”, filed Nov. 21, 2014 and U.S. patent application Ser. No. 14/610,502, entitled “Ridged Waveguide Feed Structures for Reconfigurable Antenna”, filed Jan. 30, 2015. 
       FIGS. 14A-D  illustrate one embodiment of the different layers for creating the slotted array. The antenna array includes antenna elements that are positioned in rings, such as the example rings shown in  FIG. 10 . Note that in this example the antenna array has two different types of antenna elements that are used for two different types of frequency bands. 
       FIG. 14A  illustrates a portion of the first iris board layer with locations corresponding to the slots. Referring to  FIG. 14A , the circles are open areas/slots in the metallization in the bottom side of the iris substrate, and are for controlling the coupling of elements to the feed (the feed wave). Note that this layer is an optional layer and is not used in all designs.  FIG. 14B  illustrates a portion of the second iris board layer containing slots.  FIG. 14C  illustrates patches over a portion of the second iris board layer.  FIG. 14D  illustrates a top view of a portion of the slotted array. 
       FIG. 15  illustrates a side view of one embodiment of a cylindrically fed antenna structure. The antenna produces an inwardly travelling wave using a double layer feed structure (i.e., two layers of a feed structure). In one embodiment, the antenna includes a circular outer shape, though this is not required. That is, non-circular inward travelling structures can be used. In one embodiment, the antenna structure in  FIG. 15  includes the cylindrical feed of  FIG. 10 . 
     Referring to  FIG. 15 , a coaxial pin  1601  is used to excite the field on the lower level of the antenna. In one embodiment, coaxial pin  1601  is a 50Ω coax pin that is readily available. Coaxial pin  1601  is coupled (e.g., bolted) to the bottom of the antenna structure, which is conducting ground plane  1602 . 
     Separate from conducting ground plane  1602  is interstitial conductor  1603 , which is an internal conductor. In one embodiment, conducting ground plane  1602  and interstitial conductor  1603  are parallel to each other. In one embodiment, the distance between ground plane  1602  and interstitial conductor  1603  is 0.1-0.15″. In another embodiment, this distance may be λ/2, where λ is the wavelength of the travelling wave at the frequency of operation. 
     Ground plane  1602  is separated from interstitial conductor  1603  via a spacer  1604 . In one embodiment, spacer  1604  is a foam or air-like spacer. In one embodiment, spacer  1604  comprises a plastic spacer. 
     On top of interstitial conductor  1603  is dielectric layer  1605 . In one embodiment, dielectric layer  1605  is plastic. The purpose of dielectric layer  1605  is to slow the travelling wave relative to free space velocity. In one embodiment, dielectric layer  1605  slows the travelling wave by 30% relative to free space. In one embodiment, the range of indices of refraction that are suitable for beam forming are 1.2-1.8, where free space has by definition an index of refraction equal to 1. Other dielectric spacer materials, such as, for example, plastic, may be used to achieve this effect. Note that materials other than plastic may be used as long as they achieve the desired wave slowing effect. Alternatively, a material with distributed structures may be used as dielectric layer  1605 , such as periodic sub-wavelength metallic structures that can be machined or lithographically defined, for example. 
     An RF-array  1606  is on top of dielectric layer  1605 . In one embodiment, the distance between interstitial conductor  1603  and RF-array  1606  is 0.1-0.15″. In another embodiment, this distance may be λ eff /2, where λ eff  is the effective wavelength in the medium at the design frequency. 
     The antenna includes sides  1607  and  1608 . Sides  1607  and  1608  are angled to cause a travelling wave feed from coaxial pin  1601  to be propagated from the area below interstitial conductor  1603  (the spacer layer) to the area above interstitial conductor  1603  (the dielectric layer) via reflection. In one embodiment, the angle of sides  1607  and  1608  are at 45° angles. In an alternative embodiment, sides  1607  and  1608  could be replaced with a continuous radius to achieve the reflection. While  FIG. 15  shows angled sides that have angle of 45 degrees, other angles that accomplish signal transmission from lower level feed to upper level feed may be used. That is, given that the effective wavelength in the lower feed will generally be different than in the upper feed, some deviation from the ideal 45° angles could be used to aid transmission from the lower to the upper feed level. For example, in another embodiment, the 45° angles are replaced with a single step. The steps on one end of the antenna go around the dielectric layer, interstitial the conductor, and the spacer layer. The same two steps are at the other ends of these layers. 
     In operation, when a feed wave is fed in from coaxial pin  1601 , the wave travels outward concentrically oriented from coaxial pin  1601  in the area between ground plane  1602  and interstitial conductor  1603 . The concentrically outgoing waves are reflected by sides  1607  and  1608  and travel inwardly in the area between interstitial conductor  1603  and RF array  1606 . The reflection from the edge of the circular perimeter causes the wave to remain in phase (i.e., it is an in-phase reflection). The travelling wave is slowed by dielectric layer  1605 . At this point, the travelling wave starts interacting and exciting with elements in RF array  1606  to obtain the desired scattering. 
     To terminate the travelling wave, a termination  1609  is included in the antenna at the geometric center of the antenna. In one embodiment, termination  1609  comprises a pin termination (e.g., a 50Ω pin). In another embodiment, termination  1609  comprises an RF absorber that terminates unused energy to prevent reflections of that unused energy back through the feed structure of the antenna. These could be used at the top of RF array  1606 . 
       FIG. 16  illustrates another embodiment of the antenna system with an outgoing wave. Referring to  FIG. 16 , two ground planes  1610  and  1611  are substantially parallel to each other with a dielectric layer  1612  (e.g., a plastic layer, etc.) in between ground planes. RF absorbers  1619  (e.g., resistors) couple the two ground planes  1610  and  1611  together. A coaxial pin  1615  (e.g., 50Ω) feeds the antenna. An RF array  1616  is on top of dielectric layer  1612  and ground plane  1611 . 
     In operation, a feed wave is fed through coaxial pin  1615  and travels concentrically outward and interacts with the elements of RF array  1616 . 
     The cylindrical feed in both the antennas of  FIGS. 15 and 16  improves the service angle of the antenna. Instead of a service angle of plus or minus forty-five degrees azimuth (±45° Az) and plus or minus twenty-five degrees elevation (±25° El), in one embodiment, the antenna system has a service angle of seventy-five degrees (75°) from the bore sight in all directions. As with any beam forming antenna comprised of many individual radiators, the overall antenna gain is dependent on the gain of the constituent elements, which themselves are angle-dependent. When using common radiating elements, the overall antenna gain typically decreases as the beam is pointed further off bore sight. At 75 degrees off bore sight, significant gain degradation of about 6 dB is expected. 
     Embodiments of the antenna having a cylindrical feed solve one or more problems. These include dramatically simplifying the feed structure compared to antennas fed with a corporate divider network and therefore reducing total required antenna and antenna feed volume; decreasing sensitivity to manufacturing and control errors by maintaining high beam performance with coarser controls (extending all the way to simple binary control); giving a more advantageous side lobe pattern compared to rectilinear feeds because the cylindrically oriented feed waves result in spatially diverse side lobes in the far field; and allowing polarization to be dynamic, including allowing left-hand circular, right-hand circular, and linear polarizations, while not requiring a polarizer. 
     Array of Wave Scattering Elements 
     RF array  1606  of  FIG. 15  and RF array  1616  of  FIG. 16  include a wave scattering subsystem that includes a group of patch antennas (i.e., scatterers) that act as radiators. This group of patch antennas comprises an array of scattering metamaterial elements. 
     In one embodiment, each scattering element in the antenna system is part of a unit cell that consists of a lower conductor, a dielectric substrate and an upper conductor that embeds a complementary electric inductive-capacitive resonator (“complementary electric LC” or “CELL”) that is etched in or deposited onto the upper conductor. 
     In one embodiment, a liquid crystal (LC) is injected in the gap around the scattering element. Liquid crystal is encapsulated in each unit cell and separates the lower conductor associated with a slot from an upper conductor associated with its patch. Liquid crystal has a permittivity that is a function of the orientation of the molecules comprising the liquid crystal, and the orientation of the molecules (and thus the permittivity) can be controlled by adjusting the bias voltage across the liquid crystal. Using this property, the liquid crystal acts as an on/off switch for the transmission of energy from the guided wave to the CELC. When switched on, the CELC emits an electromagnetic wave like an electrically small dipole antenna. 
     Controlling the thickness of the LC increases the beam switching speed. A fifty percent (50%) reduction in the gap between the lower and the upper conductor (the thickness of the liquid crystal) results in a fourfold increase in speed. In another embodiment, the thickness of the liquid crystal results in a beam switching speed of approximately fourteen milliseconds (14 ms). In one embodiment, the LC is doped in a manner well-known in the art to improve responsiveness so that a seven millisecond (7 ms) requirement can be met. 
     The CELC element is responsive to a magnetic field that is applied parallel to the plane of the CELC element and perpendicular to the CELC gap complement. When a voltage is applied to the liquid crystal in the metamaterial scattering unit cell, the magnetic field component of the guided wave induces a magnetic excitation of the CELC, which, in turn, produces an electromagnetic wave in the same frequency as the guided wave. 
     The phase of the electromagnetic wave generated by a single CELC can be selected by the position of the CELC on the vector of the guided wave. Each cell generates a wave in phase with the guided wave parallel to the CELC. Because the CELCs are smaller than the wave length, the output wave has the same phase as the phase of the guided wave as it passes beneath the CELC. 
     In one embodiment, the cylindrical feed geometry of this antenna system allows the CELC elements to be positioned at forty-five-degree (45°) angles to the vector of the wave in the wave feed. This position of the elements enables control of the polarization of the free space wave generated from or received by the elements. In one embodiment, the CELCs are arranged with an inter-element spacing that is less than a free-space wavelength of the operating frequency of the antenna. For example, if there are four scattering elements per wavelength, the elements in the 30 GHz transmit antenna will be approximately 2.5 mm (i.e., ¼th the 10 mm free-space wavelength of 30 GHz). 
     In one embodiment, the CELCs are implemented with patch antennas that include a patch co-located over a slot with liquid crystal between the two. In this respect, the metamaterial antenna acts like a slotted (scattering) wave guide. With a slotted wave guide, the phase of the output wave depends on the location of the slot in relation to the guided wave. 
     Cell Placement 
     In one embodiment, the antenna elements are placed on the cylindrical feed antenna aperture in a way that allows for a systematic matrix drive circuit. The placement of the cells includes placement of the transistors for the matrix drive. 
       FIG. 17  illustrates one embodiment of the placement of matrix drive circuitry with respect to antenna elements. Referring to  FIG. 17 , row controller  1701  is coupled to transistors  1711  and  1712 , via row select signals Row 1  and Row 2 , respectively, and column controller  1702  is coupled to transistors  1711  and  1712  via column select signal Column 1 . Transistor  1711  is also coupled to antenna element  1721  via connection to patch  1731 , while transistor  1712  is coupled to antenna element  1722  via connection to patch  1732 . 
     In an initial approach to realize matrix drive circuitry on the cylindrical feed antenna with unit cells placed in a non-regular grid, two steps are performed. In the first step, the cells are placed on concentric rings and each of the cells is connected to a transistor that is placed beside the cell and acts as a switch to drive each cell separately. In the second step, the matrix drive circuitry is built in order to connect every transistor with a unique address as the matrix drive approach requires. Because the matrix drive circuit is built by row and column traces (similar to LCDs) but the cells are placed on rings, there is no systematic way to assign a unique address to each transistor. This mapping problem results in very complex circuitry to cover all the transistors and leads to a significant increase in the number of physical traces to accomplish the routing. Because of the high density of cells, those traces disturb the RF performance of the antenna due to coupling effect. Also, due to the complexity of traces and high packing density, the routing of the traces cannot be accomplished by commercially available layout tools. 
     In one embodiment, the matrix drive circuitry is predefined before the cells and transistors are placed. This ensures a minimum number of traces that are necessary to drive all the cells, each with a unique address. This strategy reduces the complexity of the drive circuitry and simplifies the routing, which subsequently improves the RF performance of the antenna. 
     More specifically, in one approach, in the first step, the cells are placed on a regular rectangular grid composed of rows and columns that describe the unique address of each cell. In the second step, the cells are grouped and transformed to concentric circles while maintaining their address and connection to the rows and columns as defined in the first step. A goal of this transformation is not only to put the cells on rings but also to keep the distance between cells and the distance between rings constant over the entire aperture. In order to accomplish this goal, there are several ways to group the cells. 
     In one embodiment, a TFT package is used to enable placement and unique addressing in the matrix drive. 
       FIG. 18  illustrates one embodiment of a TFT package. Referring to  FIG. 18 , a TFT and a hold capacitor  1803  is shown with input and output ports. There are two input ports connected to traces  1801  and two output ports connected to traces  1802  to connect the TFTs together using the rows and columns. In one embodiment, the row and column traces cross in 90° angles to reduce, and potentially minimize, the coupling between the row and column traces. In one embodiment, the row and column traces are on different layers. 
     An Example System Embodiment 
     In one embodiment, the combined antenna apertures are used in a television system that operates in conjunction with a set top box. For example, in the case of a dual reception antenna, satellite signals received by the antenna are provided to a set top box (e.g., a DirecTV receiver) of a television system. More specifically, the combined antenna operation is able to simultaneously receive RF signals at two different frequencies and/or polarizations. That is, one sub-array of elements is controlled to receive RF signals at one frequency and/or polarization, while another sub-array is controlled to receive signals at another, different frequency and/or polarization. These differences in frequency or polarization represent different channels being received by the television system. Similarly, the two antenna arrays can be controlled for two different beam positions to receive channels from two different locations (e.g., two different satellites) to simultaneously receive multiple channels. 
       FIG. 19  is a block diagram of one embodiment of a communication system that performs dual reception simultaneously in a television system. Referring to  FIG. 19 , antenna  1401  includes two spatially interleaved antenna apertures operable independently to perform dual reception simultaneously at different frequencies and/or polarizations as described above. 
     Note that while only two spatially interleaved antenna operations are mentioned, the TV system may have more than two antenna apertures (e.g., 3, 4, 5, etc. antenna apertures). 
     In one embodiment, antenna  1401 , including its two interleaved slotted arrays, is coupled to diplexer  1430 . The coupling may include one or more feeding networks that receive the signals from elements of the two slotted arrays to produce two signals that are fed into diplexer  1430 . In one embodiment, diplexer  1430  is a commercially available diplexer (e.g., model PB1081WA Ku-band sitcom diplexer from A1 Microwave). 
     Diplexer  1430  is coupled to a pair of low noise block down converters (LNBs)  1426  and  1427 , which perform a noise filtering function, a down conversion function, and amplification in a manner well-known in the art. In one embodiment, LNBs  1426  and  1427  are in an out-door unit (ODU). In another embodiment, LNBs  1426  and  1427  are integrated into the antenna apparatus. LNBs  1426  and  1427  are coupled to a set top box  1402 , which is coupled to television  1403 . 
     Set top box  1402  includes a pair of analog-to-digital converters (ADCs)  1421  and  1422 , which are coupled to LNBs  1426  and  1427 , to convert the two signals output from diplexer  1430  into digital format. 
     Once converted to digital format, the signals are demodulated by demodulator  1423  and decoded by decoder  1424  to obtain the encoded data on the received waves. The decoded data is then sent to controller  1425 , which sends it to television  1403 . 
     Controller  1450  controls antenna  1401 , including the interleaved slotted array elements of both antenna apertures on the single combined physical aperture. 
     An Example of a Full Duplex Communication System 
     In another embodiment, the combined antenna apertures are used in a full duplex communication system. 
       FIG. 20  is a block diagram of another embodiment of a communication system having simultaneous transmit and receive paths. While only one transmit path and one receive path are shown, the communication system may include more than one transmit path and/or more than one receive path. 
     Referring to  FIG. 20 , antenna  1401  includes two spatially interleaved antenna arrays operable independently to transmit and receive simultaneously at different frequencies as described above. In one embodiment, antenna  1401  is coupled to diplexer  1445 . The coupling may be by one or more feeding networks. In one embodiment, in the case of a radial feed antenna, diplexer  1445  combines the two signals and the connection between antenna  1401  and diplexer  1445  is a single broad-band feeding network that can carry both frequencies. 
     Diplexer  1445  is coupled to a low noise block down converter (LNBs)  1427 , which performs a noise filtering function and a down conversion and amplification function in a manner well-known in the art. In one embodiment, LNB  1427  is in an out-door unit (ODU). In another embodiment, LNB  1427  is integrated into the antenna apparatus. LNB  1427  is coupled to a modem  1460 , which is coupled to computing system  1440  (e.g., a computer system, modem, etc.). 
     Modem  1460  includes an analog-to-digital converter (ADC)  1422 , which is coupled to LNB  1427 , to convert the received signal output from diplexer  1445  into digital format. Once converted to digital format, the signal is demodulated by demodulator  1423  and decoded by decoder  1424  to obtain the encoded data on the received wave. The decoded data is then sent to controller  1425 , which sends it to computing system  1440 . 
     Modem  1460  also includes an encoder  1430  that encodes data to be transmitted from computing system  1440 . The encoded data is modulated by modulator  1431  and then converted to analog by digital-to-analog converter (DAC)  1432 . The analog signal is then filtered by a BUC (up-convert and high pass amplifier)  1433  and provided to one port of diplexer  1445 . In one embodiment, BUC  1433  is in an out-door unit (ODU). 
     Diplexer  1445  operating in a manner well-known in the art provides the transmit signal to antenna  1401  for transmission. 
     Controller  1450  controls antenna  1401 , including the two arrays of antenna elements on the single combined physical aperture. 
     Note that the full duplex communication system shown in  FIG. 20  has a number of applications, including but not limited to, internet communication, vehicle communication (including software updating), etc. 
     Additional Antenna Heater Embodiments 
     In some embodiments, the heating structures (e.g., resistive heaters) generate uniform heating on the iris substrate (e.g., glass layer substrate) of an antenna (e.g., a metamaterial or metasurface antenna). In one embodiment, the metamaterial antenna uses a layer of liquid crystal (LC) material in its antenna element design as a tunable capacitor. Examples of such an antenna are described in more detail herein and are well-known in the art. In one embodiment, the response of LC materials depends on temperature, and the metamaterial antenna design is optimized for response from LC at temperatures 10° C. and above. Because of that, the metamaterial antenna needs a heater structure for its operation at temperatures below 10° C. Additionally, a uniform heat generation mechanism is required to prevent mechanical deformations due to uneven heat generation in the RF antenna segment, where multiple antenna segments are coupled together to form an antenna aperture. See, for example, the antenna described in U.S. Pat. No. 9,887,455, entitled “Aperture Segmentation of a Cylindrical Feed Antenna”. This uniform heater can be incorporated into the glass substrate layer design (e.g., iris substrate or glass layer) of the antenna to provide more efficient and uniform heating since two glass layer substrates (e.g., iris glass layer containing irises/slots and a iris metal layer and a patch glass layer containing patches and a patch metal layer) are in direct contact with the LC layer. A heater incorporated into the glass or other iris substrate layer design for uniform heating is referred to herein as “uniform iris heater”. Heater design concepts described below can decrease heating power requirements by several hundred watts. 
     In one embodiment, the uniform iris heater described herein is a resistive heater on the iris glass. However, in alternative embodiments, the heater is built on the patch glass substrate since it&#39;s also in direct contact with the LC layer. In another design, the uniform iris heater is built as a combination of iris glass layer substrate and patch glass layer substrate. 
     Radial placement of RF elements is a challenge for uniform heating since vertical (or horizontal) heater traces parallel to each other can&#39;t be used to generate uniform heating. Heater traces can&#39;t overlap with openings in the iris metal. In one embodiment, to generate uniform heating, one would place a heater structure (e.g., an identical heater structure) at each RF element. 
     Iris Heater with Parallel Bus Planes 
     In one embodiment, the uniform iris heater design uses two metal layers in the iris glass substrate as heater bus planes. The heater bus planes have very low resistance compared to heater traces (or structures) and heat generation on the bus planes is negligible. This is due to the large area of that the heater bus planes occupy in comparison to the heater traces and the resistive heaters being located at the heater traces. In one embodiment, the iris metal layer, which is also used for forming part of an RF antenna element (e.g., surface scatting metamaterial antenna element), can be used as one of the heater buses and the heater bus metal is used to build the second bus plane as seen in  FIG. 22 . 
       FIG. 22  illustrates iris metal layer  2201  and heater bus metal  2204  in an embodiment of a heater for a metamaterial antenna. Referring to  FIG. 22 , in one embodiment, both layers cover almost the entire surface of the iris glass substrate and their resistance will be few multiples of sheet resistance of the metal used for each layer. There is a keepout area  2202  that includes a distance around the iris opening  2203  in the iris metal layer  2201  that the heater bus metal  2204  does not encroach to prevent coupling. The cross-sectional view of this structure is shown in  FIG. 23 . 
       FIG. 23  illustrates a cross-section of a heater trace  2302  near an RF antenna element. Referring to  FIG. 23 , the heater trace  2302  is defined as a resistive structure between the two bus planes. This heater trace can be placed anywhere except the iris openings  2203  and the heater keepout area  2202  to provide heat to the liquid crystal (LC) of the RF antenna elements between the iris glass layer substrate and the patch glass layer substrate. This flexibility enables placing an identical heater trace at each RF antenna element in the antenna aperture, or segment thereof, or control the density of heat generation by periodically placing an identical heater trace design throughout the aperture. In one embodiment, the heater bus plane is placed between the iris glass  2304  substrate and the iris metal layer  2201 . In another embodiment, the iris metal layer is on top of the iris glass layer substrate and the heater bus plane is above the iris metal layer. 
     In one embodiment, two passivation layers  2306  and  2308  isolate the heater bus plane, the heater trace  2302  and the iris metal layer  2201  as shown in  FIG. 23 . In one embodiment, the passivation layers  2306  and  2308  comprise dielectric layers commonly used in semiconductor industry. In one embodiment, an opening, or via, is generated in each passivation layer to connect the heater trace to bus planes. Other connection schemes may be used in alternative embodiments. 
     Thus, in one embodiment, the heater structure uses a whole (or nearly whole) iris metal layer as one heater bus and uses another heater bus metal that covers nearly the same area as the iris metal layer with a plurality of heater traces/structures between them for uniform heating. 
     The heater concept can also be realized without using iris metal layer as a bus plane. In such a case, in one embodiment, two heater bus plane layers are added to the iris glass layer substrate. In one embodiment, these heater bus plane layers are placed underneath the iris metal layer, over the iris metal layer or they can form a sandwich-like structure with iris metal layer. Note that in one embodiment, in such a case, an additional opening is created in the iris metal layer to connect to the bus plane layers. 
     In one embodiment, the heater operates under a DC voltage. In another embodiment, the heater operates under a switching voltage. In one embodiment, if the iris metal layer is used as a heater bus plane and the voltage on the iris metal is switching due to an RF antenna element driving scheme, the heater structure is driven with a switching waveform to keep a constant differential voltage on the heater trace. For example, in one embodiment, if the iris metal layer is driven with a voltage source switching between V 1  and V 2  every T seconds, the other heater bus plane can be driven with a voltage source switching between V 1 −V heater  and V 2 −V heater  every T seconds to apply V heater  on the heating structure. 
     Iris Metal Layer as a Heater Bus Plane 
       FIG. 24  illustrates a uniform iris heater with a single bus plane (iris metal). In a different uniform iris heater embodiment, the iris metal layer is used as the only heater bus plane, and the other side of the heater trace  2402 , which may also be referred to herein as a heater wire or heating wire, is connected to a heater bus  2406  rather than a bus plane. In one embodiment, this heater bus  2406  and heater traces  2402  are built using the same heater metal layer. In one embodiment, the heater bus  2406  is placed along the edges of the RF antenna segment, and heater traces  2402  are placed along the rings defined by RF antenna element placements such as described above (or in U.S. Pat. No. 9,905,921). In one embodiment, the length of these rings increases as the radius increases. To maintain a common heater trace length, width and heat generation density, shorter traces can be connected, and longer traces can be segmented as shown in  FIG. 24 . In one embodiment, heating traces start from the edges of the RF antenna segment, where heater bus is, but they can be terminated at a termination point  2408  anywhere in the active area and connected to the iris metal  2404  using a via structure as long as they don&#39;t cross iris openings. As shown, one of the heater traces arcs towards the heater bus  2406  on the left side of the antenna segment before arcing back and terminating at a termination point. Note that in one embodiment, in such a case, the heater trace is routed between rings of RF antenna elements, such there are antenna elements  2410  within two parts of the same heater trace. Thus, the iris metal layer is used for one of the heater bus planes and the other heater bus is the frame around the outer periphery of the aperture segment, and one end of the heater traces are connected to the iris metal layer while the other end is connected to the heater bus. 
     Note that in one embodiment, the heater bus and heater traces are maintained at least the keepout distance away from the RF antenna elements. 
     In one embodiment, at each termination point (represented as a circle in  FIG. 24 ) there is a via to contact the heater trace to the iris metal layer. 
       FIGS. 25A and 25B  illustrate heater traces  2502  and  2504  underneath the iris metal  2404  (left) and above the iris metal  2404  (right), respectively. In various embodiments, heater traces  2502  are placed underneath the iris metal  2404  as shown in  FIG. 25A  along with the heater bus (not shown), or heater traces  2504  are placed above the iris metal  2404  as shown in  FIG. 25B  along with the heater bus (not shown). In one embodiment, if the polarity on the iris metal  2404  is switching periodically due to an RF antenna element driving scheme, the heater structure is driven with a polarity switching waveform to keep a constant voltage on the heater trace. 
     In another embodiment, the heater is realized by adding another metal layer other than the iris metal  2404  on the iris glass  2304 . The heater bus plane is formed on this additional metal layer and the heater bus plane on the additional metal layer covers a similar area like the iris metal layer while conserving a keepout distance from iris slots. This additional metal layer and the heater traces can be above the iris metal, below the iris metal or on opposite side of the iris metal like a sandwich structure. 
     Using Spacers as Heaters 
       FIG. 26  illustrates a cross-sectional view of a spacer/heater structure  2606 . In one embodiment, the RF antenna segments are made of two glass layer substrates separated by a spacer/heater structure  2606 , and that spacer/heater structure  2606  is formed using a conductive material and is used and functions as both a spacer, to space apart the patch glass  2604  with associated structures and the iris glass  2304  with associated structures, and a heater in a different design. In one embodiment, the iris metal  2404  in the iris glass  2304  substrate and the patch metal  2602  on the patch glass  2604  substrate are used as heater bus structures. In another embodiment, heat generation occurs only in the spacer/heater structure  2606 . In one embodiment, the iris metal layer used in the RF antenna elements is also used as a heater bus plane here. The heater bus formed using the patch metal layer is independent of the patch electrode above the iris opening. Thus, there is one bus plane in the iris metal plane and there is another bus plane on the patch glass substrate that are coupled together using spacers that provide heat for the RF antenna elements. 
     The heater bus on the patch metal layer can be built in different ways. In one embodiment, the heater bus on the patch metal  2602  layer comprises traces with low resistance built using the patch metal layer or a continuous sheet of patch metal layer similar to the iris metal layer on the iris glass  2304  substrate. In both cases, the bus structure on the patch glass  2604  is isolated from the patch electrode above the iris opening that is used in the RF antenna element structure. A cross-sectional view of the heater structure is shown in  FIG. 26 . 
     While only one spacer/heater structure is shown in  FIG. 26 , in one embodiment, there is one or more spacer/heater structures positioned near and providing heat to each RF antenna element in the RF antenna element array. These spacer/heater structures may be positioned in multiple locations (e.g., random non-interfering placements, rows, columns, etc.) and may have various shapes (e.g., cylindrical posts, rectangular posts, objects of other shapes, etc.). 
     In one embodiment, the spacer/heater structure  2606  is made of a single conductive material. Such a material may be deposited during fabrication. In another embodiment, the spacer/heater structure  2606  includes an inner core with a conductive material around the outside (e.g., plated on the outside of the inner core) that is used to provide heat to one or more RF antenna elements. 
     In one embodiment, if the heater operates under a switching voltage and the voltage on iris metal layer is switching due to RF antenna element driving scheme, the patch side of the heater structure needs to be driven with a switching waveform to keep a constant absolute differential voltage on the heater trace while switching the polarity of the differential voltage every T seconds. This may occur in a similar manner to the description provided above regarding the polarity switching waveform. One aspect of this is that the voltage across the LC has a minimal DC offset. For example, in this case, the patch metal structure would not be involved in the RF driving of the elements but the iris side would be. Even so, it is preferable that the net DC of the routing still be minimized to minimize the net DC across the LC. In another embodiment, the heater is realized using additional metal layers or sheets on each substrate and not using the iris and/or patch metal layers. 
     Segmented Heater Bus 
       FIG. 27  illustrates an alternative heater design heater bundles and heater bus segments. Referring to  FIG. 27 , uniform heating is achieved by grouping heater traces  2704  of heater rings with similar length to heater bundles. The heater bundles are coupled in series and the same current is run through every bundle, thereby creating the same heating per unit length. In one embodiment, the heater traces  2704  with similar length are connected in parallel by heater bus segments  2702  and heater bus  2706  to form heater bundles. The heater bundles are connected in series by heater bus segments  2702  and heater bus  2706 , which in one embodiment are on the same metal layer, to complete the iris heater as seen in  FIG. 27 . In another embodiment, the heater bundles, heater bus segments and heater bus are on different metal layers. 
       FIG. 28  illustrates a resistive model of the segmented heater bus embodiment shown in  FIG. 27 . Referring to  FIG. 28 , groupings of 3 heater rings are shown as an example but the design isn&#39;t limited by number of heater ring groupings. Each bundle is represented as a resistor  2802  in  FIG. 28 . Resistance of the heater bus and segment bus is negligible. In one embodiment, the same current passes through each resistor  2802  in  FIG. 28 . The resistance value is designed in each bundle to keep a constant heat generation per unit area. In one embodiment, within each bundle, the resistance of heater traces is matched by changing the trace width. 
     There is a number of example embodiments described herein. 
     Example 1 is antenna comprising: a physical antenna aperture having an array of radio frequency (RF) antenna elements formed with patch and iris substrates, the iris substrate having a plurality of layers including an iris metal layer; and a heater structure coupled to one or more of the plurality of layers of the iris substrate for heating the RF antenna elements. 
     Example 2 is the antenna of example 1 that may optionally include that the heater structure comprises a resistive heater on the iris substrate. 
     Example 3 is the antenna of example 2 that may optionally include that the heater structure comprises two metal layers on the iris substrate operating as a plurality of heater bus planes. 
     Example 4 is the antenna of example 3 that may optionally include that the plurality of heater bus planes comprises a first heater bus plane and a second heater bus plane, wherein the iris metal layer is used as the first heater bus plane and a heater bus metal is used as the second heater bus plane. 
     Example 5 is the antenna of example 4 that may optionally include a plurality of heater traces coupled between the heater bus metal and the iris metal layer. 
     Example 6 is the antenna of example 5 that may optionally include that the plurality of heater traces are at least a first distance away from iris openings and are outside a heater keepout area around of the iris openings. 
     Example 7 is the antenna of example 2 that may optionally include that the heater structure comprises two metal layers on the iris substrate operating as a plurality of heater bus planes, with the iris metal layer being used as a first heater bus plane and a second heater bus plane extending around an outer edge of a segment of the aperture with heater traces connected to the second heater bus plane. 
     Example 8 is the antenna of example 7 that may optionally include that heater traces have a same length. 
     Example 9 is the antenna of example 1 that may optionally include that the heater structure is driven with a polarity switching waveform. 
     Example 10 is the antenna of example 1 that may optionally include that the heater structure comprises a spacer structure separating the two substrates and coupled to the iris metal layer and the patch substrate, the spacer structure comprising a conductive material used as a heater. 
     Example 11 is the antenna of example 1 that may optionally include that the heater structure comprises a plurality of heater rings forming a plurality of heater bundles of a plurality of heater traces, wherein bundles of the plurality of heater bundles are connected in series with each other. 
     Example 12 is a metamaterial antenna, comprising: a first substrate having an iris metal layer with a plurality of iris openings; a second substrate having a patch metal layer forming patches aligned with the iris openings; and a heater structure attached to the first substrate to heat radio frequency (RF) antenna elements. 
     Example 132 is the antenna of example 12 that may optionally include that the heater structure comprises a resistive heater. 
     Example 14 is the antenna of example 12 that may optionally include that the heater structure attached to the first substrate comprises: a spacer/heater structure attached to a first structure on the first substrate and attached to a second structure on the second substrate to space apart the first substrate and the second substrate with the patches aligned with the iris openings and to heat the RF antenna elements. 
     Example 15 is the antenna of example 14 that may optionally include that the first and second structures are heater bus plane structures. 
     Example 16 is the antenna of example 12 that may optionally include that the heater structure comprises: a heater bus formed by a first metal layer on the first substrate; and a plurality of heater traces formed by a second metal layer on the first substrate and connected to the heater bus, the first metal layer and the second metal layer distinct from the iris metal layer. 
     Example 17 is the antenna of example 12 that may optionally include that the heater structure comprises a plurality of heater traces on the first substrate below the iris metal layer and separated from the iris metal layer by a passivation layer. 
     Example 18 is the antenna of example 12 that may optionally include that the heater structure comprises a plurality of heater traces on the first substrate above the iris metal layer and separated from the iris metal layer by a passivation layer. 
     Example 19 is the antenna of example 12 that may optionally include that the heater structure comprises a plurality of heater traces on the first substrate, each heater trace connected at a first end to a heater bus and connected at a second end to the iris metal layer. 
     Example 20 is the antenna of example 12 that may optionally include that the heater structure comprises a plurality of heater bundles each comprising a plurality of heater traces, on the first substrate; each heater bundle connected at a first end to a heater bus segment; and each heater bundle connected at a second end to a further heater bus segment or a heater bus. 
     Example 21 is the antenna of example 12 that may optionally include that the heater structure comprises a plurality of heater bundles connected in series and each having a plurality of heater traces, on the first substrate; and each heater bundle having a resistance to keep a constant heat generation per unit area. 
     Example 22 is the antenna of example 12 that may optionally include that the heater structure comprises a plurality of heater bundles each having a plurality of heater traces on the first substrate; and each heater trace of each heater bundle having a trace width to match resistance of heater traces. 
     Some portions of the detailed descriptions above are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     The present invention also relates to apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus. 
     The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these systems will appear from the description below. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein. 
     A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium includes read only memory (“ROM”); random access memory (“RAM”); magnetic disk storage media; optical storage media; flash memory devices; etc. 
     Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims which in themselves recite only those features regarded as essential to the invention.