Abstract:
A broad-band monopole antenna for high voltage environments is provided. The monopole antenna includes a ground plane, a plurality of flat radiator elements and an electrical conductor. The ground plane has a flat upper surface, a lower surface, a smoothly-radiused outer edge and a hole centrally disposed through the upper and lower surfaces. Each flat radiator element has a thickness, a straight inner edge and a semicircular outer edge. The plurality of flat radiator elements are interconnected along each inner edge and symmetrically arranged about a vertical axis centered on the ground plane hole. The electrical connector extends through the ground plane hole and is coupled to the radiator elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/363,839, entitled “Ultra-Wide Band Monopole Antenna” and filed on Jul. 13, 2010, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to radio frequency (RF) signal antennas. More particularly, the present invention relates to ultra-wideband, omni-directional RF antennas for data acquisition and monitoring systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    Dipole antennas are well known radiators of electromagnetic (EM) signals at radio frequencies (RF). More particularly, dipoles radiators usually include two similar conductive elements, physically oriented oppositely to one another, and are usually excited, at respective nodes positioned near their closest point to one another, by an RF EM signal, with similar signals of opposite polarity applied to the respective nodes. Alternatively, one of the two elements making up the dipole can receive an RF EM signal while the other is held at a constant potential, such as at ground potential. 
         [0004]    Dipoles can have any physical length, from a small fraction of a wavelength of the RF EM excitation signal up to a large multiple of the signal wavelength. A number of dipoles have a size which is a quarter wavelength for each of the two elements, i.e., a half wavelength overall, calculated with reference to the characteristic propagation rate of EM signals along the dipole elements. This size has the property that any signal energy reflected back from the far ends of the elements tends to return to the drive node in phase with the signal arriving from the antenna&#39;s EM signal source at the excitation node at the time of return, and thus to reinforce that signal rather than to degrade it. A slender, rotationally symmetric dipole in an environment similar to free space tends to radiate an EM signal in a pattern of energy density resembling a uniform torus, with the axes of rotational symmetry of the dipole and the torus generally coinciding. 
         [0005]    A type of antenna useful as an alternative to a dipole is a monopole. A monopole is essentially one of the two elements of a dipole. A monopole receives excitation by an RF EM signal, typically at a drive node, such as the bottom end of a vertical conductor, and the signal then propagates away from the drive node. Some part of the applied EM signal energy is coupled to free space, i.e., is radiated from the monopole. A conductive surface proximal to, isolated from, and approximately perpendicular to the drive node typically functions as a reflector so that the signal radiated from the monopole and the reflected signal resemble the emission of a dipole. The reflector configuration varies, and any available conductive surface may serve as a reflector. Specifically, for example, as with an automobile radio, a whip antenna functions as a vertically-oriented monopole, while a metal surface of the automobile approximates a ground plane and thus serves as the reflector. The surface of the earth can also serve as a radiator, as can a metal disk, a wire mesh screen, one or more horizontal radial elements similar in construction and size to the monopole antenna itself, etc. 
         [0006]    Measurement of remote phenomena is increasingly used for control and protection of system components. A recent application of this concept is sensing voltage, current, power, temperature, line sag, tension, and other conditions associated with long-distance, high-voltage, three-phase conductors, e.g., commercial power lines, suspended above the ground from elevated towers or poles. A challenge in this particular application involves transferring the measurement data to a central site. Using copper conductors for telemetering this data is generally not feasible, because, for example, signal conductors leading down from the elevated lines could attract lightning, could provide deadly shock hazards in event of system faults, etc. Fiber optic signal conductors have other limitations, including, for example, unintended conductivity when their coverings become dirty. Coupling telemetry signals from multiple sensor nodes onto the power lines themselves for remote reception has other limitations, such as, for example, link length, modulation-produced line radiation that potentially causes interference to radio receivers nearby, etc. 
         [0007]    An alternative to the above includes attaching a data acquisition and telemetry system to one or more of the power lines, and periodically communicating acquired data to a central site using, for example, an established cellular phone system. In one known system these sensors are roughly toroidal in shape, being split into two C-shapes that can be clamped together to surround one phase wire. However, this known system uses a patch antenna, which is mounted on the toroid such that the radiated signals are highly directional, e.g., along the longitudinal axis of the phase wire. This arrangement reduces the effectiveness of the data acquisition and telemetry system, because the closest cellular tower may be in the opposite direction and thereby shielded by the body of the toroid. Additionally, the high-voltage nature of this environment impacts the effectiveness of the antenna. 
         [0008]    What is needed is an omnidirectional, ultra-wide bandwidth antenna for remote sensing in high-voltage outdoor environments. 
       SUMMARY OF THE INVENTION 
       [0009]    Embodiments of the present invention advantageously provide a broad-band monopole antenna for high voltage environments. 
         [0010]    In one embodiment, the monopole antenna includes a ground plane, a plurality of flat radiator elements and an electrical conductor. The ground plane has a flat upper surface, a lower surface, a smoothly-radiused outer edge and a hole centrally disposed through the upper and lower surfaces. Each flat radiator element has a thickness, a straight inner edge and a semicircular outer edge. The plurality of flat radiator elements are interconnected along each inner edge and symmetrically arranged about a vertical axis centered on the ground plane hole. The electrical connector extends through the ground plane hole and is coupled to the radiator elements. 
         [0011]    In another embodiment, the monopole antenna includes a ground plane, a single flat radiator and an electrical conductor. The ground plane includes a flat upper surface, a lower surface, a smoothly-radiused outer edge and a hole centrally disposed through the upper and lower surfaces. The single flat radiator has a thickness and a circular outer edge, and is arranged about a vertical axis centered on the ground plane hole. The electrical connector extends through the ground plane hole and is coupled to the radiator. 
         [0012]    There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
         [0013]    In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
         [0014]    As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The above and other aspects and features of the present invention will become more readily apparent by describing in further detail exemplary embodiments thereof with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  depicts a perspective view of an embodiment of an antenna disposed on an overhead high voltage data acquisition and monitoring system in accordance with an embodiment of the present invention; 
           [0017]      FIGS. 2A-2L  depict perspective views of monopole antennas in accordance with various embodiments of the present invention; 
           [0018]      FIGS. 3A-3F  depict polar graphs of vertical gain versus azimuth, illustrating propagation patterns for the antennas of  FIG. 2A-2F , in accordance with embodiments of the present invention; 
           [0019]      FIGS. 4 and 5  depict rectangular coordinate graphs of voltage standing wave ratio (VSWR) versus frequency for the antennas of  FIGS. 2A-2F , in accordance with embodiments of the present invention; 
           [0020]      FIG. 6  is a polar graph of vertical gain versus elevation, illustrating propagation patterns for the antenna of  FIG. 2D  in accordance with embodiments of the present invention; and 
           [0021]      FIGS. 7A-7D  shows various radomes according to embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Embodiments of the present invention provide various monopole radiators that advantageously balance azimuth performance and extremely broad band operation in relatively low-power transmitting systems. The antennas perform well over low and high band cellular telephony and the various frequencies used by ZIGBEE®, BLUETOOTH®, Z-WAVE®, and other communication devices in a variety of locations world-wide. 
         [0023]    A monopole with ground plane has a bandwidth over which it can efficiently transmit or receive EM signals. The transmitting efficiency is a characteristic of the monopole&#39;s complex impedance matching to a source/transmission system on the feed side, and to the monopole&#39;s coupling to free space on the radiation side. Impedance matching is commonly measured in terms of voltage standing wave ratio (VSWR), which is a comparison between applied and reflected signal energy measured in terms of voltages from a narrow-band, swept-spectrum transmitter to an antenna. An ideal VSWR is defined as 1.0:1; antennas with VSWR as high as 2.0:1 or considerably greater are usable for some applications, particularly low-power transmitters and high-gain receivers. It is to be understood that energy reflected back from an antenna with a higher VSWR must be diverted from or tolerated by its transmitter. 
         [0024]      FIG. 1  shows an antenna  10  according to an embodiment of the invention, mounted on data acquisition and telemetry system  12  which surrounds a power conductor  14 . Despite being generally toroidal in shape, the data acquisition and telemetry system  12  is discouraged from rotating by a substantial weight bias toward its bottom, and remains fixed along the power conductor  14  by means of a fitting  16  clamped onto the power conductor  14 . 
         [0025]    Any of a variety of sense functions may be incorporated within data acquisition and telemetry system  12  arranged generally like the one shown. The measured data include, for example, voltage, current, temperature, tension, line sag, power factor, electrical noise outside the power line&#39;s nominal spectrum, the presence of broadcast signal energy induced into the power conductor  14 , etc., and are captured by a processor-based, data acquisition and storage subsystem (not shown). Power to operate data acquisition and telemetry system  12  may be extracted from the field gradient present in proximity to the power conductor  14 , and optionally stored in a rechargeable battery subsystem. Voltages at least as high as 750 KV may be present on a representative monitored power conductor  14 , providing an ample gradient. At predetermined intervals, in response to sensing specific types of transmission line problems, in response to polling by a central site, etc., the data acquisition and telemetry system  12  connects to the central site, via a cellular network for example, and transmits the acquired data. 
         [0026]    In a cellular telephony context, each data acquisition and telemetry system  12  acquires at least one unique identity in the form of a Mobile Identification Number (MIN), which is ten decimal digits in the U.S., directly equivalent to a land line telephone number, assigned at least temporarily during the process of manufacturing, distributing, installing, or activating data acquisition and telemetry system  12 . There is likely to be at least a second unique identifier, an Electronic Serial Number (ESN), typically eight hexadecimal digits, embedded in that data acquisition and telemetry system  12  from the time of manufacture, and including manufacturer identity bits as well as production code information. If current types of consumer cellular telephone apparatus are used, there may also be Global Positioning System (GPS) capture capability, allowing the physical location of data acquisition and telemetry system  12  to be verified each time data is acquired. In addition to this, the communication process delivers to the central site at least one cellular tower location datum, which may be used to confirm the GPS data. Thus, from the time of installation, data acquisition and telemetry system  12  can positively affirm its location as well as sensing the condition of the power line  14 . 
         [0027]    In addition to cellular communication, data acquisition and telemetry system  12  can be configured to communicate directly with, for example, a data transceiver operated by a maintenance worker visiting the location of data acquisition and telemetry system  12 . Typical unlicensed radio services for very short range communication include ZIGBEE®, Z-WAVE®, BLUETOOTH®, etc. Any of these and others may be supported by the inventive antenna, which has sufficient bandwidth to support all of them in addition to the low and high band cellular telephone services licensed in the U.S. and the rest of the world. 
         [0028]    As an alternative to cellular telephony, any established commercially licensable radiotelephone service may be preferred for specific applications. Services are feasible on a variety of frequency ranges, and may use an implementer duplicate the combination of towers, antennas, transceivers, tower-to-tower communication links, and data management resources already implemented by cellular providers. 
         [0029]    To the extent that non-cellular services operate in spatial arrangements and frequency domains similar to those of cellular systems, antennas according to the invention may be directly applicable. For services such as some types of satellite-based communications, where a transmitter may be in low earth orbit and thus located at any elevation from horizon to zenith, it may be necessary to adapt antenna geometry as well as size to provide sufficient gain at all elevation angles. For example, satellite-based communication systems are available, such as Iridium, GLOBALSTAR®, ORBCOMM®, SkyWave, BGAN, TDRSS, and the like, capable of providing virtually total world coverage without additional build out. BGAN, TDRSS, and some other services are geosynchronous, and thus at a fixed elevation relative to a specific installation. Since geosynchronous satellites also operate at fixed azimuths and have different gain and signal power requirements than terrestrial systems, directional versions of the invention may be preferred for such applications. 
         [0030]    Generally, antenna  10  includes a monopole radiator disposed over a ground plane and an RF signal connector coupled to the monopole radiator. Antenna  10  is highly effective over all azimuths, while having low weight, simple construction, and exceptional broadband capability. 
         [0031]      FIG. 2A  shows a cylindrical monopole radiator  30 , disposed over a substantially circular ground plane  32 , and an RF signal connector  33  coupled to the monopole radiator  30 , according to an embodiment of the invention. The monopole radiator  30  is terminated with a smoothly-radiused top, while the ground plane  32  has a smoothly-radiused edge  34 . This embodiment exhibits excellent VSWR at optimum frequency. Other embodiments include, for example, a ball on top of monopole radiator  30 , further decreasing curvature at the highest point. 
         [0032]      FIG. 2B  shows a spherical monopole radiator  36 , disposed over a substantially circular ground plane  32 , and an RF signal connector  33  coupled to the monopole radiator  36 , according to an embodiment of the invention. The ground plane  32  has a smoothly-radiused edge  34 . In comparison to the embodiment of  FIG. 2A , the monopole radiator  36  has an increased frequency range over which its VSWR is fairly uniform. 
         [0033]      FIG. 2C  shows a flat, circular monopole radiator  38 , disposed over a substantially circular ground plane  32 , and an RF signal connector  33  coupled to the monopole radiator  38 , according to an embodiment of the invention. The monopole radiator  38  includes a flat circular plate having a certain thickness, an outer edge and two flat faces. The ground plane  32  has a smoothly-radiused edge  34 . In one embodiment, the diameter of monopole radiator  38 , e.g., the height above the ground plane  32 , is equal to the quarter-wave dimension for a frequency near the middle of the antenna&#39;s working range. Generally, the diameter is selected for compatibility with the desired (very broad) bandwidth, allowing the same antenna to be used without alteration for low and high cellular ranges and short-range unlicensed radios, as discussed above. The monopole radiator  38  has very good VSWR over the desired range, and is relatively inexpensive to fabricate. In an alternative embodiment, the flat circular monopole radiator  38  could be formed from two, flat semicircular elements that are joined together. 
         [0034]      FIG. 2G  depicts another embodiment of a single-element monopole radiator  38 ′ in which the outer edge includes a cylindrical rim whose diameter is greater than the thickness of the plate. This embodiment advantageously reduces corona effects and improves radiation and impedance performance. In one embodiment, the cylindrical rim diameter is much greater than the plate thickness, such as, for example, three to five times greater; greater relative dimensions are also contemplated. In an extreme embodiment, the thickness of the plate approaches zero, such that the cylindrical rim governs performance. 
         [0035]      FIG. 2D  shows a monopole radiator  40 , disposed over a substantially circular ground plane  32  with a smoothly-radiused edge  34 , and an RF signal connector  33  coupled to the monopole radiator  40 , according to an embodiment of the invention. The monopole radiator  40  includes three flat semicircular plates, each plate having a certain thickness, a straight inner edge, a semicircular outer edge and two flat faces. The elements form a common vertical axis along their respective inner edges, and are symmetrically arranged about this vertical axis. The monopole radiator  40  advantageously provides a gain variation over all azimuths that is less than 5 dB; in other words, omnidirectionality. Its VSWR is noticeably higher than that of monopole radiator  38  over some parts of its frequency range. Modeling the performance of monopole radiator  40  (discussed below) involved, inter alia, representing each flat plate as a discrete, well-known radiator, such as a curved cylinder. 
         [0036]      FIGS. 2H and 2K  depict another embodiment of a three-element monopole radiator  40 ′ in which the outer edge of each plate includes a cylindrical rim whose diameter is greater than the thickness of the plate. This embodiment advantageously reduces corona effects and improves radiation and impedance performance.  FIG. 2L  depicts a further embodiment in which monopole radiator  40 ″ only includes the cylindrical rim of each plate, i.e., the thickness of the flat portion of each plate has been reduced to zero, leaving three curved cylinders. The three cylindrical rims of monopole radiator  40 ′ are joined at their respective end portions, in a manner suggested by modeling the performance of monopole radiator  40 . Reductions in corona effects and improvements in performance are also provided by this embodiment. 
         [0037]      FIG. 2E  shows a monopole radiator  42 , disposed over a substantially circular ground plane  32  with a smoothly-radiused edge  34 , and an RF signal connector  33  coupled to the monopole radiator  42 , according to an embodiment of the invention. The monopole radiator  42  includes four flat semicircular plates, each plate having a certain thickness, a straight inner edge, a semicircular outer edge and two flat faces. The elements form a common vertical axis along their respective inner edges, and are symmetrically arranged about this vertical axis. This embodiment has a strong azimuthal uniformity and adequate bandwidth. Modeling the performance of monopole radiator  42  (discussed below) involved, inter alia, representing each flat plate as a discrete, well-known radiator, such as a curved cylinder. 
         [0038]      FIG. 2I  depicts another embodiment of a four-element monopole radiator  42 ′ in which the outer edge of each plate includes a cylindrical rim whose diameter is greater than the thickness of the plate. This embodiment advantageously reduces corona effects and improves radiation and impedance performance. 
         [0039]      FIG. 2F  shows a monopole radiator  44 , disposed over a substantially circular ground plane  32  with a smoothly-radiused edge  34 , and an RF signal connector  33  coupled to the monopole radiator  44 , according to an embodiment of the invention. The monopole radiator  44  includes five flat semicircular plates, each plate having a certain thickness, a straight inner edge, a semicircular outer edge and two flat faces. The elements form a common vertical axis along their respective inner edges, and are symmetrically arranged about this vertical axis. This embodiment has a strong azimuthal uniformity and adequate bandwidth. Modeling the performance of monopole radiator  44  (discussed below) involved, inter alia, representing each flat plate as a discrete, well-known radiator, such as a curved cylinder. 
         [0040]      FIG. 2J  depicts another embodiment of a five-element monopole radiator  44 ′ in which the outer edge of each plate includes a cylindrical rim whose diameter is greater than the thickness of the plate. This embodiment advantageously reduces corona effects and improves radiation and impedance performance. 
         [0041]      FIGS. 3A-3F  show gain-vs-azimuth plots  50 ,  58 ,  66 ,  74 ,  82  and  90  for the monopole radiators  30 ,  36 ,  38 ,  40 ,  42  and  44  shown in  FIGS. 2A-2F , respectively. 
         [0042]      FIG. 3A  shows a gain-vs-azimuth plot  50  for monopole radiator  30  including gain pattern  52  for a low-end frequency, 925 MHz, gain pattern  54  for a mid-range frequency, 1795 MHz, and gain pattern  56  for a high-end frequency, 2440 MHz. Both the mid-range and high frequency gain patterns  54 ,  56  are seen to be largely omnidirectional for the monopole radiator  30 , which is anticipated for a radially-symmetric radiator. 
         [0043]      FIG. 3B  shows a gain-vs-azimuth plot  58  for monopole radiator  36  including gain patterns  60 ,  62 , and  64 , for low, mid, and high frequencies, 925 MHz, 1795 MHz, and 2440 MHz, respectively, which closely resemble those of monopole radiator  30 . 
         [0044]      FIG. 3C  shows a gain-vs-azimuth plot  66  for monopole radiator  38  including gain patterns  68 ,  70 , and  72 , for low, mid, and high frequencies, 925 MHz, 1795 MHz, and 2440 MHz, respectively, which are affected by radiator geometry to a greater extent than the gain patterns of monopole radiators  30 ,  36 . 
         [0045]      FIG. 3D  shows a gain-vs-azimuth plot  74  for monopole radiator  40  including gain patterns  76 ,  78 , and  80 , for low, mid, and high frequencies, 925 MHz, 1795 MHz, and 2440 MHz, respectively, which are largely omnidirectional and somewhat triangular. 
         [0046]      FIG. 3E  shows a gain-vs-azimuth plot  82  for monopole radiator  40  including gain patterns  84 ,  86 , and  88 , for low, mid, and high frequencies, 925 MHz, 1795 MHz, and 2440 MHz, respectively, which are largely omnidirectional and somewhat square. 
         [0047]      FIG. 3F  shows a gain-vs-azimuth plot  90  for monopole radiator  42  including gain patterns  92 ,  94 , and  96 , for low, mid, and high frequencies, 925 MHz, 1795 MHz, and 2440 MHz, respectively, which are largely omnidirectional. 
         [0048]    It will be observed that gain patterns at the low end of the analyzed ranges are relatively insensitive to radiator geometry. This is a consequence of the presence of a large metallic mass making up a significant portion of the data acquisition and telemetry system  12 , located beneath the radiator  10 , and oriented in the same way for each embodiment. The patterns may be anticipated to vary for applications not using sensors with comparable magnitude and placement of conductive mass. 
         [0049]      FIG. 4  shows VSWR plots for the embodiments of  FIGS. 2A ,  2 B,  2 C,  2 D,  2 E, and  2 F. From these, the narrow low-VSWR bandwidth  100  of monopole radiator  30 , the very broad low-VSWR bandwidth  104  of monopole radiator  38 , and the similar VSWR bandwidths  106 ,  108  and  110  of the three-element, four-element and five-element radiators  40 ,  42  and  44  (respectively) are apparent. The VSWR bandwidth  102  of monopole radiator  36  is also presented for comparison. While such a plot of VSWR as driven by number of elements is not the only criterion a user may consider in selecting a configuration, it illustrates, like gain vs azimuth, the relative performance of a variety of high-voltage-compatible monopoles. 
         [0050]    The inventive antenna advantageously offers maximum omnidirectionality, maximum VSWR bandwidth, minimum cost, as well as a balance between these performance factors. Each of these factors may also be seen as an optimization parameter, and a manufacturer may further choose to consider tradeoffs in product line complexity when choosing which embodiment to offer for sale. 
         [0051]      FIG. 5  shows performance vs. diameter for a range of disk sizes for monopole radiator  38  (depicted in  FIG. 2C ). Sizes shown are 3 cm (1.2 in)  120 , 4 cm (1.6 in)  122 , 5 cm (2.0 in)  124 , 6 cm (2.4 in)  126 , 7 cm (2.8 in)  128 , 8 cm (3.2 in)  130 , 9 cm (3.5 in)  132 , and 10 cm (4.0 in)  134 . The figure shows that the smallest disk  120  performs poorly below about 1.3 GHz (VSWR=6), while the largest  134  performs at the same VSWR level at 400 MHz.  FIG. 4  suggests that other radiator configurations perform proportionately, albeit with VSWR values that tend to be higher at all frequencies. 
         [0052]    Each disk size also has at least one minimum VSWR within the plotted range. The lowest of all is the smallest disk, the minimum VSWR of which falls at a higher frequency than the range of interest. Thus, low-end VSWR, minimum VSWR, working range, and physical size may be considered in selecting a radiator size, even for the wide-bandwidth antenna disclosed herein. One preferred embodiment is about 6.5 cm (2.6 in) in diameter, with performance falling between that for 6 cm  126  and 7 cm  128 . This embodiment crosses the VSWR=6 threshold around 630 MHz, has a minimum VSWR around 1.3 that falls around 1.3 GHz, and never exceeds a VSWR of 2 below 3.5 GHz, i.e., between 1 GHz and 3.5 GHz. The superior omnidirectionality of the three-element and four-element embodiments may outweigh the superior VSWR of the single disk. 
         [0053]      FIG. 6  is a plot  150  showing gain vs elevation at low  152 , moderate  154 , and high  156  frequencies for monopole radiator  40 ; similar performance is predicted for monopole radiators  42 ,  44 . This shows that signals strength directly below data acquisition and telemetry system  12 , with antenna  10  on its top, is generally very attenuated. Within about 10° to 30°, however, there are lobes even at the highest frequencies of interest that are around −10 dB, which may be ample for ZIGBEE® or other form of communication. Signal strength at the zenith is quite low, which is not a factor for terrestrial communication. 
         [0054]      FIGS. 7A-7D  shows radomes  202 ,  204 ,  206  and  208  according to embodiments of the present invention. In view of the expected environment for the antenna  10 , in which high AC voltage relative to the surrounding space and a high field gradient are permanent conditions, different shapes may be advantageous, such as, for example, a smooth radome  202 , a long creepage length radome  204 , a closely conformal radome  206 , embedment of the conductive components in insulating material  206 , etc. 
         [0055]    Where it is preferred to establish a long creepage length for the radome  204  to the extent practical, a series of smooth, circumferential corrugations  210  increases the length over which contaminants would need to accumulate in order to establish a conductive path. Areas  212  overhung by others would less readily acquire dust. In more extreme configurations, corrugations termed “sheds” (not shown) can overhang sufficiently to block some parts of the surface virtually entirely. A tradeoff in any extent of corrugation is its effect on signal propagation. For example, a simple shape minimizes the amount and variation in the amount of material having a different dielectric constant than air, and thus altering propagation. Very thin insulating coatings or exposure of the radiator itself to air may represent feasible alternatives, at least for short duration use in minimal-contaminant environments such as deserts. 
         [0056]    Materials for radome  46  ( FIG. 2 ) have good resistance to deterioration when subjected to high voltage and to weather, such as sun, rain, salt, and chemical pollutants, for example. The materials should also have low conductivity and reasonably low loss tangent, e.g., energy absorption and dissipation. Thin walls may be used to keep the scale of any losses low as well as to keep any RF signal propagation path distortion associated with the material&#39;s dielectric constant low. Uniform shape with azimuth, i.e., symmetry about a vertical axis of rotation in the portion of radome  46  exposed to the transmitted and received signals, is likewise helpful in maintaining omnidirectionality. A typical useful material for this application is acrylonitrile butadiene styrene (ABS), a thermoplastic copolymer of the named constituents having good electrical and mechanical properties. This material can be reinforced with fiber or other filler and treated with additives that enhance resistance ultraviolet (UV) light, e.g., sunlight, and pollutants. Prudence suggests that a selected combination of polymer, additives, and filler be proved suitable by a directly relevant RF, UHV, and UV history or rigorous analysis and test. 
         [0057]    The monopole radiators can be formed from a variety of conductive materials, such as, for example, copper, aluminum, brass, etc., and shaped and/or joined using a variety of processes, such as, for example, casting, soldering, etc. Cellular telephone antennas for personal mobile use commonly employ a simple circuit-board-style conductive trace on flexible insulating material such as polyimide film, so any material adaptable to a high-voltage environment may be usable. An example is cast zinc, which is sufficiently conductive and durable, easy to manufacture, and inexpensive. Other materials may include molded plastic, either solid or foamed, that can be treated or coated to be conductive, semi-conductive materials such as carbon fiber, etc. Considerations in material choice include long-term stability and voltage withstand. 
         [0058]    The many features and advantages of the invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the invention.