Patent Publication Number: US-7595765-B1

Title: Embedded surface wave antenna with improved frequency bandwidth and radiation performance

Description:
FIELD 
     Embedded surface wave antenna methods and apparatuses having a relatively wide bandwidth and favorable pattern characteristics are provided. 
     BACKGROUND 
     In designing antenna structures, it is desirable to provide appropriate gain, bandwidth, beamwidth, sidelobe level, radiation efficiency, aperture efficiency, radar cross-section (RCS), radiation resistance and other electrical characteristics. It is also desirable for these structures to be lightweight, simple in design, inexpensive and unobtrusive, since an antenna is often required to be mounted upon or secured to a supporting structure or vehicle, such as high velocity aircraft, missiles, rockets or even artillery projectiles, which cannot tolerate excessive deviations from aerodynamic shapes. It is also sometimes desirable to hide the antenna structure so that its presence is not readily apparent for aesthetic and/or security purposes. Accordingly, it is desirable that an antenna be physically small in volume and not protrude on the external side of a mounting surface, such as an aircraft skin, while yet still exhibiting all the requisite electrical characteristics. 
     One type of antenna that has been successfully used for broadband conformal applications is the Doorstop™ antenna. The Doorstop™ antenna belongs to a class of antennas known as traveling wave antennas. Examples of other traveling wave antennas are polyrod, helix, long-wires, Yagi-Uda, log-periodic, slots and holes in waveguides, and horns. Antennas of this type have very nearly uniform current and voltage amplitude along their length. This characteristic is achieved by carefully transitioning from the element feed and properly terminating the antenna structure so that reflections are minimized. An example of a Doorstop™ antenna is found in U.S. Pat. No. 4,931,808, assigned to the assignee of the present invention, the entire disclosure of which is hereby incorporated herein by reference. 
     A Doorstop™ antenna generally comprises a feed placed over a dielectric wedge, a groundplane supporting or adjacent to the dielectric wedge, and a cover or radome. The Doorstop™ antenna has two principal regions of radiation that affect patterns: the feed region and the lens region. The size and shape of these two regions generally control bandwidth and pattern performance. 
     In a typical Doorstop™ antenna, the measured voltage standing wave ratio (VSWR) improves with increasing frequency. At reduced frequencies the Doorstop™ element is electrically too short and functions more like a bent monopole antenna. The low frequency limit for the Doorstop™ element is set by the electrical depth of the element. More particularly, the maximum wedge depth and wedge dielectric constant determine the lowest frequency of operation. Once the physical depth and dielectric constant of the wedge are established, the lens to feed length ratio of the basic Doorstop™ configuration determines the pattern performance. At low frequencies, the pattern tends to look very uniform and nearly omni-directional, while at high frequencies the pattern becomes quite directional or end-fired. Additionally, at high frequencies the pattern develops a characteristic null at the zenith that moves forward toward the horizon as the frequency increases. For certain applications and greater operating bandwidths, this characteristic pattern performance is undesirable. 
     Within about a 3 to 1 operating bandwidth, the pattern characteristic can be controlled by adjusting the lens to feed length ratio of the antenna. As the frequency increases above the 3 to 1 ratio, the lens becomes electrically long, producing field components that either support or interfere with the radiation from the feed region. This leads to the creation of nulls in the forward portion of the farfield elevation plane pattern. 
     Other aspects of the typical Doorstop™ antenna that degrade performance include the use of an unsupported (not grounded) microstrip line near the coax feed, which adversely affects the element impedance match. Also, the coaxial pin typically used to interconnect the feed to a transmission line and the microstrip line are sources of radiation, that can degrade pattern performance by creating pattern nulls at certain angles. In addition, trapped energy in the dielectric wedge results in large impedance variation at low frequencies. As still another disadvantageous feature, because the element feed of a typical Doorstop™ antenna is on the surface of the device, it is exposed to improper handling and high temperatures that cause variation in radio-frequency (RF) performance. 
     SUMMARY 
     Embodiments of the present invention are directed to solving these and other problems and disadvantages of the prior art. In accordance with embodiments of the present invention, Doorstop™ antenna elements having improved high frequency and/or low frequency performance characteristics are provided. In one aspect, radar absorbing material (RAM) is incorporated to improve low frequency performance. In another aspect, a lens perturbation feature is incorporated into a Doorstop™ antenna element to reduce nulls at angles of interest and at high frequencies. In still another aspect, a buried feed arrangement is provided, improving the low frequency performance characteristics of the antenna element, and improving resistance to adverse effects of high operating temperatures and/or improper handling of the antenna element. 
     The incorporation of a dielectric comprising a RAM or other lossy material in the feed region of the antenna element can reduce low frequency reflections without overly degrading high frequency performance. The lossy material may be combined with a feed mirror to further improve performance of the element at low frequencies, without unduly affecting high frequency performance. 
     Lens perturbation features in accordance with embodiments of the present invention generally include features to control or shape the wave or phase front of a signal. Accordingly, a lens perturbation feature may comprise the inclusion of volumes of differential dielectric material within the lens portion of the antenna element. For example, a wedge of dielectric material having a relatively low dielectric constant may be inserted in a forward portion of the lens region, while the remaining portion of the lens region may incorporate a dielectric material having a relatively high dielectric constant. In accordance with further embodiments of the present invention, a lens perturbation feature may include shaping the ground plane in the lens region of the antenna element to control the shape of the phase front. 
     A buried feed feature in accordance with embodiments of the present invention may include a feed that is covered by relatively low dielectric constant material in a feed region or on a feed side of the feed element. The lens region on a side of the feed element opposite the feed side may incorporate a dielectric material having a relatively high dielectric constant. In addition, an antenna element with a buried feed may provide a coaxial or other connector for interconnecting the feed element to a transmission line that lies under the dielectric material generally filling the volume defined by the ground plane. 
     Additional features and advantages of the present invention will become more readily apparent from the following description, particularly when taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial side view of a vehicle incorporating a number of antenna elements in accordance with embodiments of the present invention; 
         FIG. 2A  is a cross-section of an antenna element in accordance with embodiments of the present invention; 
         FIG. 2B  is a plan view of a portion of an antenna element in accordance with embodiments of the present invention; 
         FIG. 2C  is a plan view of a portion of an antenna element in accordance with other embodiments of the present invention; 
         FIG. 3  is a perspective view of an antenna element in accordance with embodiments of the present invention; 
         FIG. 4  is a cross-section of an antenna element in accordance with other embodiments of the present invention; 
         FIG. 5  is a cross-section of an antenna element in accordance with other embodiments of the present invention; 
         FIG. 6  is a cross-section of an antenna element in accordance with other embodiments of the present invention; 
         FIG. 7  is a cross-section of an antenna element in accordance with other embodiments of the present invention; 
         FIG. 8  is a cross-section of an antenna element in accordance with other embodiments of the present invention; 
         FIG. 9  is a cross-section of an antenna element in accordance with other embodiments of the present invention; 
         FIG. 10  is a cross-section of an antenna element in accordance with other embodiments of the present invention; and 
         FIG. 11  is a flow chart illustrating aspects of a method for framing an antenna element in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are generally directed to providing antenna elements that are particularly suited for conformal applications. More particularly, embodiments of the present invention provide design features that assist in improving the performance of embedded surface wave antenna elements. In general, improving performance refers to providing more favorable bandwidth and radiation performance in areas of interest than would otherwise be available from a comparable embedded surface wave antenna element. Certain of the design features are particularly effective at improving performance at low frequencies, while other design features are particularly effective at improving performance at high frequencies. As used herein, “low frequencies” and “high frequencies” are not limited to any particular frequency ranges. Instead, these terms respectively apply to the low end and the high end of the overall range of operating frequencies of the antenna element. In addition, through the application of features in accordance with embodiments of the present invention, the useful overall operating range of an antenna element can be improved as compared to an element that did not benefit from the use of such features, through improvements to the beam patterns at the low and/or high frequency ends of the overall operating range. 
     With reference to  FIG. 1 , an array  100  comprising a plurality of antenna elements  104  in accordance with embodiments of the present invention are shown incorporated into a vehicle  108 . Although the vehicle  108  is illustrated as a missile, such as an advanced radar tracking air-to-air missile, this is just one example of the type of vehicle that can be associated with one or more antenna elements  104  described herein. Other examples include aircraft, spacecraft, satellites, ships, tanks, trucks, cars and artillery projectiles. Furthermore, embodiments of the present invention are not limited to being associated with a vehicle  108 , and can instead be associated with stationary or man-portable applications. Antenna elements  104  in accordance with embodiments of the present invention are particularly useful in connection with any application that requires or can benefit from a conformal or substantially conformal antenna element. Furthermore, a number of antenna elements  104  having forward-looking and side-looking beam coverage can be arrayed about the periphery of a vehicle  108 , for example to provide a composite hemispherical coverage volume or beam. As can be appreciated by one of skill in the art, the number of antenna elements  104  included in an array  100  can be selected based on considerations such as frequency band of operation and the desired coverage region. 
       FIG. 2A  is a cross-sectional view of an antenna element  104  in accordance with embodiments of the present invention in elevation. In general, the antenna element  104  comprises a ground plane or means for establishing a ground plane  304  and a feed or means for feeding a signal  308 . A connector  312  is provided at or towards a proximal end  314  of the antenna element. Typically, the connector  312  allows the signal line of a coaxial cable or other transmission line to be interconnected to the feed  308 , and the ground to be connected to the ground plane  304 . The region including the proximal end of the antenna element  104  and containing the feed  308  is generally defined as the feed region  316 . The region including the distal end  318  of the antenna element  104  is generally defined as the lens region  320 . A first or supporting dielectric material  324  generally fills all or a portion of a volume  322  defined by the ground plane  304 , and is generally disposed between the ground plane  304  and the feed  308 . The first dielectric material  324 , in accordance with embodiments of the present invention, supports the feed  308  and/or separates the feed  308  from the ground plane  304 , and therefore comprises a means for supporting the feed  308 . A radome  326  can be provided, for example to provide a surface that conforms to the exterior surface of a vehicle  108  incorporating the antenna element  104 , and to protect the feed  308  and other components of the antenna element  104 . In general, the radome  326  encloses or forms a boundary of the volume  322  defined by the ground plane  304 . As can be appreciated by one of skill in the art after consideration of the present disclosure, the volume  322  need not be a closed volume, in that it may be open to volumes associated with antenna elements on either side of the antenna element under consideration, and/or the volume may not be enclosed by a radome  326 . 
     In the embodiment illustrated in  FIG. 2A , a second dielectric material or feed loading dielectric material  328 , in this example comprising a radar absorbing material (RAM) or means for absorbing radio frequency energy, is disposed in the feed region  316 , between the feed  308  and the ground plane  304 . The incorporation of a feed loading dielectric  328  comprising a RAM in this area can improve the low frequency performance of the antenna element  104 . Without wishing to be bound by any particular theory, it is believed that the loading feed dielectric material  328  improves low frequency performance by loading the feed  308  and by absorbing low frequency energy that would otherwise become trapped in the feed region  316 , and which can reflect and destructively interfere with energy at desired wavelengths. In addition, a feed mirror  332  can be provided. The feed mirror  332  can comprise a metallization or other conductive layer that is applied over the RAM  328 . The feed mirror  332  is electrically connected to the groundplane, and generally assists in improving the performance of the antenna element  104  at high frequencies. 
     In  FIG. 2B , the antenna element  104  shown in  FIG. 2A  is illustrated in plan view, with the radome  326  removed, and with the first dielectric  324  treated as a transparent feature (or alternatively with the first dielectric removed) to provide a view of the feed  308  and the feed mirror  332 . More particularly, an antenna element  104  with a conventional feed  308   a  is illustrated. In addition, it can be seen that the feed mirror  332  may have an area that generally follows or is equal to the area of the feed  308 . 
     In  FIG. 2C , another embodiment of the antenna element  104  shown in  FIG. 2A  is illustrated in plan view, again with certain features removed or not illustrated to provide a view of the feed  308  and the feed mirror  332 . More particularly, an antenna element  104  with a crow&#39;s foot type feed  308   b  is illustrated. As can be appreciated by one of skill in the art, the crow&#39;s foot type feed  308   b  can provide a reduced radar cross section (RCS) as compared to the conventional feed  308   a . The feed mirror  332  may have an area that generally follows or is equal to the outline of the area of the feed  308 . Alternatively, the feed mirror  332  may also have a crow&#39;s foot type outline. 
     A perspective view of the embodiment of the antenna element  104  shown in  FIGS. 2A and 2B  is shown in  FIG. 3  with the radome  326  and first dielectric  324  removed (or not illustrated). As shown, the ground plane  304  can comprise a body extending to the sides of the antenna element  104 . Accordingly, the ground plane  304  can comprise a structural component of a vehicle  108  incorporating the antenna element. In addition, the RAM  328  can extend across the lower surface of the ground plane  304 , to cover an area corresponding to the feed region  316 . RAM is generally omitted from the lens region  320  in order to avoid decreasing the gain of the antenna element  104  at high frequencies. 
       FIG. 4  is a cross-sectional view of an antenna element  104  featuring a lens perturbation feature or means for altering a phase front of a signal in accordance with other embodiments of the present invention in elevation. In such embodiments, a second dielectric material or lens perturbation dielectric material  504  is disposed at the distal end of the antenna element  104 , within the lens region  320  of the antenna element  104 . The lens perturbation dielectric material  504  may feature a lower dielectric constant than the first dielectric material  324 . By providing a lens perturbation dielectric material  504  having a dielectric constant that is different than the dielectric constant of the first dielectric material  324 , the velocity of energy through the antenna element  104  can be changed. Furthermore, because the lens perturbation dielectric material  504  is located in the lens region  320  of the antenna element  104 , it can be particularly effective at altering the high frequency performance of the antenna element  104 . In particular, as illustrated by the rays  508  generally depicting paths of high frequency energy radiated by the antenna element  104 , the phase front  512  of the resulting beam can be altered or curved. By altering the phase front  512  so that the energy vectors produced by the different sources within the antenna element add constructively in the far field (or at least so that destructive interference is avoided), nulls within the beam can be avoided. As shown, the lens perturbation dielectric material  504  can be provided as a wedge-shaped volume disposed towards the distal end of the antenna element and adjacent the ground plane  304  that is larger adjacent or near the radome  326  (not illustrated in  FIG. 4 ) than at the opposite end. This general configuration has been determined to be particularly useful in avoiding nulls in the far field at relatively high frequencies. 
     The effect on the phase front  512  can be modified by changing the relative dielectric constants of the dielectric materials  324 ,  504 . Typically, the materials have dielectric constants that differ from one another by about a 2 to 1 ratio. For example, the first dielectric material  324  may have a dielectric constant of about 3.6, and the lens perturbation dielectric material  504  may have a dielectric constant of about 1.8. The effect on the phase front  512  can also be modified by changing the depth of the wedge comprising the lens perturbation dielectric material  504 . This depth can be characterized by the dimensions illustrated as l 1  and l 2  in  FIG. 4 . For most applications, the length of l 2  should be within from about 33 to about 50% the distance l 1  plus l 2 . This relationship has been found to provide a desirable range of modification to the phase front  512  where the first dielectric material  324  has a dielectric constant that is about twice the dielectric constant of the lens perturbation dielectric material  504 . 
     An alternative configuration of an antenna element  104  incorporating a lens perturbation feature in the form of a lens perturbation dielectric material  504  disposed in the lens region  320  is illustrated in  FIG. 5 . The lens perturbation dielectric material  504  can have a dielectric constant that is higher than the dielectric constant of the first dielectric material  324 . The lens perturbation dielectric material  504  also can be provided as a wedge shaped volume at the distal end of the first dielectric material  324 , and can be larger at an end that is within or near the feed region  316  of the antenna element  104 , and smaller adjacent or near the radome  326  (not illustrated in  FIG. 5 ). As depicted in  FIG. 5 , this configuration can alter the velocity of rays  508  to produce a phase front  512  that is altered or curved in a reverse direction as compared to the embodiment illustrated in  FIG. 4 . 
     Another alternative configuration of an antenna element  104  incorporating a lens perturbation feature in the form of a second dielectric material comprising lens perturbation material  504  in order to improve high frequency performance is illustrated in  FIG. 6 . In such embodiments, the lens perturbation dielectric material  504  is deployed within the lens region  320  of the antenna element  304 . The lens perturbation dielectric material may further be configured such that it describes a generally wedge shaped volume with a first surface that would be adjacent a radome  326  (not illustrated in  FIG. 6 ), a second surface that is proximate to the ground plane at or towards a proximate end of the ground plane  304 , and a third surface that extends from the ground plane  304  to or towards the distal end of the feed  308 . The lens perturbation dielectric material  504  has a dielectric constant that is less than the dielectric constant of the first dielectric material  324 . For example, the dielectric constant of the lens perturbation dielectric material  504  may be about one-half the dielectric constant of the first dielectric material  324 . The depth of the wedge shaped volume defined by the lens perturbation dielectric material  504  may be characterized by the dimensions l 1  and l 2 . For most applications, l 2  should be about 33 to 50% the total length of l 1  plus l 2 . 
     The high frequency performance of an antenna element  104  can also be altered by providing a lens perturbation feature or means for altering a phase front of a signal in the form of ground plane  304  having an altered shape within the lens region  320 . For example, as illustrated in  FIG. 7 , the ground plane  304  can be contoured such that it is generally concave in cross section with respect to the volume defined by the ground plane. More particularly, the ground plane  304  can be contoured such that the distance of the ground plane  304  from the distal lens of the feed  308  along at least a first line changes at a non-linear rate along at least a portion of the ground plane  304  in the lens region  320  of the antenna element. For instance, whereas a ground plane might otherwise follow line A-B in  FIG. 7 , by dishing or contouring the ground plane  304 , the phase front of a beam can be altered to improve or adjust far field performance. 
       FIG. 8  depicts an antenna element  104  in accordance with embodiments of the present invention having a feed  904  comprising a buried feed. According to such embodiments, the feed is “buried” within or between a first or supporting dielectric material  324  and second or feed region dielectric materials  908 . For instance, the feed  904  may extend from a point proximate to the ground plane  304  to a point proximate to the radome  326 , effectively dividing the volume  322  defined by the ground plane  304  into two sub-volumes, a distal sub-volume  912  and proximate sub-volume  916 . Furthermore, a “top” surface of the feed  904  may be overlayed by the second dielectric material  908  generally filling the proximate sub-volume  912 , while the “bottom” surface of the feed  904  generally facing the lens region  320  may be supported by or adjacent to the first dielectric material  324 , generally filling the distal sub-volume  916 . In accordance with embodiments of the present invention, the first  324  and second  908  dielectric materials may have different dielectric constants. In general, providing a buried feed  904  allows the feed  904  to transition directly (or more directly) to the feature or connector  312  that comprises an interconnection to the transmission line. In addition, spurious radiation that can couple to neighboring elements  104  (for example within a common array  100 ), launch surface waves, and adversely affect radiation patterns, can be reduced. Moreover, more energy can be directed from the feed region  316  and into the lens region  320 . Also, less energy is trapped in the antenna element  104 , because fewer standing waves are set-up within the antenna element  104 . The use of a buried feed  904  also provides improved protection for the feed  904  from mishandling during manufacture or installation of the antenna element  104 , and from high temperatures during operation of the antenna element  104 , for example in connection with a vehicle  108  traveling through the atmosphere at a high velocity. 
     Many of the improvements in performance obtained through use of a buried feed  904  are seen in the low frequency range. In order to improve high frequency performance, the buried feed  904  configuration can be combined with lens perturbation features of other embodiments, such as the incorporation of a wedge or volume of lens perturbation dielectric material  504  having a relatively low dielectric constant in the lens region  320  of the antenna element  104 . Such an embodiment is illustrated in  FIG. 9 . Accordingly, at least three distinct volumes of dielectric materials  324 ,  504 ,  908  are included in the antenna element  104  in accordance with such embodiments. 
     The advantages of the buried feed configuration can be enhanced by providing another dielectric material in the form of a radar absorbing material or means for absorbing radio-frequency energy  1104  in a volume between the feed  904  and the radome  326 , on a side of the feed  904  opposite the lens region  320  (See  FIG. 10 ). In particular, providing radar absorbing material  1104  above the feed can absorb trapped energy, improving low frequency performance, with only a relatively small adverse effect on high frequency performance. The radar absorbing material  1104  can be separated from the feed  904  by a feed region dielectric material  908 . As shown, this configuration can (but need not) be combined with a volume of lens perturbation dielectric material  504  within the lens region  320  that is different than other dielectric material  324  in the lens region  320 . 
     With reference now to  FIG. 11 , the manufacture of an antenna element  104  in accordance with embodiments of the present invention is illustrated. Initially, at step  1204 , a ground plane  304  is formed. Formation of the ground plane  304  can comprise contouring a flat piece of conductive material to have the desired shape, for example by stamping. Alternatively, forming the ground plane can comprise machining a piece of conducting material. Where a number of antenna elements  104  are used together in an array  100 , forming the ground plane  304  can comprise forming the ground planes  304  for a number of the antenna elements  104  simultaneously or at about the same time. For instance, forming the ground plane  304  can comprise forming a shape of revolution comprising the ground planes  304  for each element  104  within an array  100  from a piece of conductive material forming a structural portion of a vehicle  108 . 
     At step  1208 , determination is made as to whether a feed mirror  332  and/or a feed loading dielectric material  328  is to be included in the antenna element  104 . If such features are to be included, the feed loading material  328  or the feed mirror  332  are placed within the volume defined by the ground plane  304 . For example, the feed loading material  328  comprising a dielectric radar absorbing material may be later placed on a portion of the ground plane  304  corresponding to the feed region  316 , and the feed mirror  332  may be formed on top of the radar absorbing material  328 . 
     At step  1216 , determination is made as to whether lens perturbation features using dielectric materials are to be included in the antenna element  104 . If such lens perturbation features are to be included, supporting dielectric material  324  and lens perturbation material or materials  504  are placed within the volume defined by the ground plane  304 . Furthermore, these materials may be placed in the lens region  320  of the antenna element  104 . If it is determined that lens perturbation features using dielectric materials are not to be included in the antenna element  104 , supporting dielectric material  324  is placed within the volume defined by the ground plane  304 , and in particular within a volume including at least a portion of the lens region  320  of the antenna element  104 . 
     At step  1228 , the feed  308  or  904  is formed on top of the dielectric material  324 . For example, a conductive foil or film may be laid on top of the supporting dielectric material  324  and interconnected to the connector  312 . A determination may then be made as to whether the feed is a buried feed  904 . Where the feed is a buried feed  904 , another dielectric material  908  can then be placed on top of the feed  904  (step  1236 ). After placing feed region dielectric material  908  on top of the feed, a determination may be made as to whether feed region RAM  1104  is to be included (step  1240 ). If feed region RAM is to be included, the feed region RAM  1104  is placed on the feed region dielectric material  908  (step  1244 ). After determining, that the feed is not a buried feed, or after placing feed region dielectric material and/or feed region RAM, a radome  326  may be placed over the antenna element  104  components (step  1248 ). As can be appreciated by one of skill in the art, a radome  326  is not required. Furthermore, radome  326  may be placed over antenna element  104  components after installation of the antenna element  104  in a vehicle  108  or other structure. In addition, after placement of the antenna element  104  in a vehicle  108  or other structure, the connector  312  may be joined to a transmission line. 
     As can be appreciated by one of skill in the art and after consideration of the present disclosure, the required shape of the dielectric materials  324 ,  328 ,  504 ,  908  and/or  1104  may be fairly complex. Accordingly, the material or materials  324 ,  328 ,  504 ,  908  and/or  1104  may be molded into the final shape (or near the final shape), in order to avoid or reduce machining or milling operations. 
     Although various embodiments of the antenna elements  104  are described herein have been illustrated having wedges or volumes of dielectric materials with sharp angles between surfaces, it should be appreciated that other configurations are possible. For example, curved interfaces between adjacent materials can be used to lower the radar cross-section of the antenna element  104 . 
     As can be appreciated by one of skill in the art from the description provided herein, various of the features provided herein can be used in combination to provide improved antenna performance at low and high frequencies. Furthermore, it can be appreciated that combinations in addition to those illustrated are possible. For example, multiple lens perturbation features in the form of multiple volumes of lens perturbation dielectric materials may be provided. As a further example, a lens perturbation feature comprising one or more lens perturbation dielectric materials  504  can be combined with a lens perturbation feature comprising a curved ground plane  304 . As still another example, a buried feed  904  and/or loaded feed  308  or  904  can be combined with any of the lens perturbation features. In addition, although operation of an antenna element incorporating features described herein has at times been described in connection with the transmission of radio frequency or microwave energy, it can be appreciated that embodiments of the present invention also have application in connection with improving the performance of antenna elements operating to receive radio frequency or microwave energy. 
     The foregoing discussion of the invention has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, within the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain the best mode presently known of practicing the invention and to enable others skilled in the art to utilize the invention in such or in other embodiments and with the various modifications required by their particular application or use of the invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.