Abstract:
A circularly polarized single point feed notch antenna functions on land, sea, air and space vehicles and communicates using a broad frequency range while maintaining stealth during installation on a fighter aircraft by demonstrating a low Radar Cross Section (RCS). The circularly polarized notch antenna (CPNA) employs a phase delay card polarizer to achieve circularly polarized radiation or reception. The CPNA couples non-planar conductive fins to opposing sides of a non-conducting polarizing member. The fins fashion a ninety degree longitudinal fold, tuning slots at one fin end for tuning the antenna, and bifurcated arms at an opposing end formed by a notch. The bifurcated fins possess curved edges running from a part exterior to a part interior and a recession on the fold beginning near the tuning notch(es) and running the fin length until the recession runs out when it meets the curved edge.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a communications antenna. More specifically, this invention relates to a circularly polarized notch antenna with a single point feed, a low radar cross section, and a broad band frequency range suited for land, sea; air and spacecraft use. 
     BACKGROUND OF THE INVENTION 
     Antennas are commonly employed to transmit, receive, enhance or ensure the reception of a signal within a desired frequency range depending upon the particular antenna design and specific application. Such signals are often transmitted and received at frequencies commonly employed to transmit audio and video signals. While present day antennas have generally proven to be satisfactory for their given applications, each often has limitations which limit its use in some manner. 
     One limitation with many present day antennas used on aircraft is their inability to maintain stealth, or in other words, to remain undetectable by radar. Maintaining stealth is particularly important with regard to certain military aircraft applications. In order to help maintain aircraft stealth, the Radar Cross Section (RCS) of an aircraft becomes important. RCS is a measure of the radar reflection characteristics of a target, or comparatively, a measure of the cross section of the sphere that would reflect the same energy back to a radar system irradiating the target if the sphere were substituted. As RCS increases, the integrity of aircraft stealth decreases and the aircraft becomes vulnerable to detection by radar. Therefore, it is highly desirable for certain military aircraft to maintain stealth by having a low RCS. 
     Another limitation of antennas used on most vehicles and with many land installations relates to the physical presence that most antennas must have in order to effectively transmit and receive signals. Most antennas must be exteriorly mounted on a vehicle or land based structure, or otherwise mounted to provide a relatively unobstructed transmission path to a receiver or transmitter. This requirement greatly limits antenna mounting locations, especially for military aircraft applications, and also greatly increases the RCS of the vehicle or aircraft, thus jeopardizing the ability of the vehicle or aircraft to operate undetected by a radar system. 
     Yet another limitation of antenna installations on military aircraft for electronic warfare transmission includes the number of antenna feed points. An antenna feed point is a point on an antenna where an electrical feed line couples to the antenna to transmit and receive RF signals within the frequency band that the antenna is designed to transmit and receive. Many modern antennas have dual feed points. The number of feed points directly contributes to the complexity of the antenna, its overall manufacturing cost and antenna utilization for a given application. 
     Still another limitation of antennas is their inability to transmit in a circularly polarized fashion. Many antennas, by the nature of their design, are capable of transmitting and receiving frequencies in a vertically or horizontally polarized fashion, but not circularly. Circular polarization is desirable in most transmissions related to military aircraft communications. 
     The problem of maintaining an aircraft&#39;s stealth with regard to antennas has been addressed by the prior art by designing antennas capable of providing a low RCS. A crossed notch antenna is a type of antenna that is capable of providing a low radar cross section. However, a crossed notch antenna utilizes multiple feed points. Additionally, the problem of reducing the number of antenna feed points has been addressed by the prior art with the traditional horn antenna which commonly has a single feed point. However, horn antennas traditionally have a high radar cross section which jeopardizes aircraft stealth. Finally, the problem of transmitting a signal in a circularly polarized fashion has been addressed by the prior art with the crossed notch antenna. However, with a cross notched antenna, an additional external phase shift network is required to create the desired circular polarization. 
     What is needed then is an antenna that does not suffer from the above limitations. Ideally, such an antenna will provide for a single feed point that eliminates the problem of high RCS, and will thus provide a device that is capable of maintaining a low RCS thereby permitting an aircraft or other vehicle to maintain stealth. Additionally, such an antenna would be able to communicate using horizontal, vertical and circulating polarized signals without the need for an external phase shift network. This will permit savings with respect to antenna manufacturing assembly time, antenna installation time, and will provide an overall less complex antenna design. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a circularly polarized notch antenna is disclosed. The invention provides an antenna with a planar polarizing member and multiple non-planar fins coupled to opposing polarizer sides. The invention also provides an electrical connection assembly that connects to the fins to communicate RF signals using a broad band of frequencies. 
     In one preferred embodiment, a circularly polarized notch antenna adaptable for use on land, sea, air and space vehicles includes a planar polarizing member, dual non-planar fins that are mechanically coupled to the polarizer, and a connection assembly that provides electrical connection to the antenna. The planar polarizing member is preferably square or rectangular,. manufactured from a dielectric material such as plastic or ceramic, and includes an elongated center slot. The fins are non-planar, preferably formed so as to include a ninety degree angle, and attached to opposing sides of the polarizing member. Additionally, each fin includes at one end at least one tuning slot, and at the other end dual curved edges formed by a notch, beginning at a fin interior and leading to a fin exterior. The connection assembly is typically a coaxial cable with an end coupler. In a dual, opposing fin arrangement, the outer conductor of the coaxial cable makes contact with one fin and the inner coaxial cable wire makes contact with the remaining fin. The cable passes through the elongated polarizer slot to facilitate the connection. 
     When the antenna parts are assembled, part reduction, overall antenna size and part complexity advantages are realized by eliminating the parts associated with an external phase shift network-and multiple feed cables or lines. Additionally, the notch antenna design of the present invention enhances electronic performance by communicating over a broader frequency range than traditional antennas. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a commercial aircraft showing, in phantom, potential locations of the circularly polarized notch antenna of the present invention. 
     FIG. 2 is a perspective view of a military aircraft showing, in phantom, potential locations of the circularly polarized notch antenna of the present invention. 
     FIG. 3 a  is a perspective view of a prior art pyramidal horn antenna. 
     FIG. 3 b  is a perspective view of a prior art sectoral H-plane horn antenna. 
     FIG. 3 c  is a perspective view of a prior art sectoral E-plane horn antenna. 
     FIG. 3 d  is a perspective view of a prior art diagonal horn antenna. 
     FIG. 4 is a perspective view of a prior art notch antenna. 
     FIG. 5 is a perspective view of the circularly polarized notch antenna of the present invention. 
     FIG. 6 is an end view of the circularly polarized notch antenna of the present invention. 
     FIG. 7 is a side view of a polarizer of the circularly polarized notch antenna of the present invention. 
     FIG. 8 is an end view of a polarizer of the circularly polarized notch antenna of the present invention. 
     FIG. 9 is a top view of a fin of the circularly polarized notch antenna of the present invention. 
     FIG. 10 is an end view of a fin of the circularly polarized notch antenna of the present invention. 
     FIG. 11 is a front view of a fin of the circularly polarized notch antenna of the present invention. 
     FIG. 12 is a plot showing a vertically polarized wave. 
     FIG. 13 is a plot showing a horizontally polarized wave. 
     FIG. 14 is a plot showing a circularly polarized wave. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     With reference to FIG. 1 of the drawings, a perspective view of a private or commercial aircraft  10  having a fuselage  12  depicts the potential locations  14 ,  16  and  18 ,  19  and  21  in phantom, of a circularly polarized notch antenna (CPNA)  20  (FIG. 5) in accordance with the teachings of the present invention. In this example, CPNA  20  is shown at location  14 . FIG. 2 is a perspective view of a military aircraft  22  having a body  24  and showing the potential locations  26 ,  28  and  30 , in phantom, of CPNA  20  (FIG. 5) in accordance with the teachings of the present invention. In this illustration, CPNA  20  is shown at location  26 . The CPNA  20  of the present invention may be mounted exteriorly of the aircraft  10  or  22  or vehicle or flush mounted, below the surface of the aircraft or vehicle. Locations  14 ,  16  and  18  of FIG.  1  and locations  26 ,  28  and  30  of FIG. 2 are merely exemplary installation positions of the CPNA  20 . The CPNA  20  of the present invention is mountable virtually anywhere communications are needed, including on, for example, land based vehicles and ships, as well as in fixed locations on ground based structures or on space based vehicles or structures. 
     Previous and current antenna installations on commercial and military aircraft for electronic transmissions include a variety of antenna types. The horn antenna is one type of prior art antenna having multiple embodiments as shown in FIGS. 3 a - 3   d . FIG. 3 a  shows a pyramidal horn antenna  32   a , FIG. 3 b  shows a sectoral H-plane horn antenna  32   b , FIG. 3 c  shows a sectoral E-plane horn antenna  32   c  and FIG. 3 d  shows a diagonal horn antenna  32   d . While horn antennas are a very popular antenna, they exhibit a high radar cross section (RCS) which makes them undesirable for military use. 
     The RCS is a measure of the radar reflection characteristics of a target. RCS is equal to the power reflected back to the radar divided by the power density of the radar signal irradiating the target. Additionally, the RCS is regarded as the cross sectional area of a sphere that would reflect an equivalent amount of energy back to the radar if the sphere could be substituted. It is always desirable to maintain the minimum RCS possible on military aircraft to preserve the stealth, or undetectibility, of the aircraft. 
     In addition to RCS, an antenna feature known as antenna feed points will be explained. An antenna feed point is a mechanical antenna connection point that connects to a corresponding communications link used to transmit communications to and from the antenna. A coaxial cable is a common communications link connected to an antenna feed point. Generally, antennas have single or dual feed points with a single feed point being generally desirable over dual feed points since a single feed point makes an antenna less complicated, less expensive, and easier to install and subsequently troubleshoot than a multiple feed point antenna. 
     Another type of prior art antenna is a notch antenna  34 , also known as a Vivaldi antenna, depicted in FIG.  4 . Compared to the horn antennas of FIGS. 3 a - 3   d , the notch antenna  34 , with bifurcated portions  36  and  38 , formed by a notch, generally has a lower RCS, but requires dual feed points and an external phase shift network to generate and receive circularly polarized signals which is required for many communication applications, especially military applications. External phase shift networks are generally complicated, expensive and may have electronic performance shortcomings for a given antenna application. In comparison to the CPNA  20  of the present invention, horn antennas and notch antennas are complicated, more expensive to manufacture and maintain, and have electronic or communication shortcomings. The CPNA  20  of the present invention will now be described in greater detail. 
     Turning to FIGS. 5-8, a preferred embodiment of the CPNA  20  of the present invention is shown, The CPNA  20  generally includes a polarizer  40 , fin  42 , fin  44 , elongated polarizer slot  46 , coaxial cable  48 , tuning slot cluster  50 , coaxial coupler  52  and feed wire  54 . The typical coaxial coupler  52  contains inner threads (not shown). The feed wire  54  typically inserts into a feed point  56  such as that shown on fin  44  of FIG. 6, which is at an approximate center of the CPNA  20  fin apex. 
     The polarizer  40  is typically a dielectric material such as plastic or ceramic. The polarizer  40  is placed between antenna fins  42  and  44  and serves as both a mounting structure for the opposing fins and a spacer to adjust the fins for antenna tuning purposes. Antenna tuning is necessary in order for the antenna to receive and transmit RF signals at its intended frequencies. The end view of the CPNA, in FIG. 6 shows that polarizer  40  has opposing surfaces  58  and  60  necessary to mount fins  42  and  44 . The fins  42  and  44  are mounted on the polarizer  40  so as to be in generally facing relation to one another but offset laterally from one another. FIGS. 7 and 8 show the polarizer  40  containing the exemplary, elongated polarizer slot  46  which passes through the polarizer  40  proximate to the polarizer center. Those skilled in the art will realize that the size and precise location of elongated polarizer slot  46  will vary depending, upon the location of antenna feed point  56 . The desired tuning of the CPNA  20  may also require adjusting the lengths of the notches of fins  42  and  44 . Furthermore, the polarizer  40  itself may take on a variety of sizes depending upon the sizes of fins  42  and  44  and the tuning requirements of the CPNA  20 . 
     FIGS.  5  and  9 - 11  show a typical fin  42  which will now be explained in greater detail. The fins  42  and  44  are typically manufactured from a piece of conductive metal such as aluminum, although those skilled in the art will realize that any electrically conductive metal will suffice depending upon the structural and physical limitations of the particular metal. The fin  42  is shown with a fold  78  and an included angle “A” of 90 degrees, although those skilled in the art will realize that the included angle “A” may vary depending upon the antenna RF tuning requirements and effect on the antenna RCS. Fin  42  has a first end  62 , a second end  64 , and a fold  78  coincident with a flat  66  that runs the longitudinal length of fin  42 . The first end  62  has a tuning slot cluster  50  cut into it while the second end  64  has a notch  68  which forms bifurcated portions  70  and  72 . The notch  68  creates curved edges  74  and  76  on the bifurcated portions  70  and  72  of fin  42 . The shape and thin metal of notch  68  enable the CPNA  20  of the present invention to maintain its low RCS, making it advantageous for military applications. The ratio of the overall width of one opposing set of fins to the other can be used to adjust the polarization of this invention (i.e., the ratio of vertical to horizontal polarization). 
     Those skilled in the art will realize that the positions of feed point  56  and tuning slot cluster  50  are adjustable and depend upon the CPNA  20  tuning requirements. The flat  66  runs opposite fold  78  about which the fin  42  is a mirror image. When the fold  78  ends and the notch  68  begins, the flat  66  splits between the curved edges  74  and  76  and runs coincident with each edge. The fin  42  also has a first flat surface  80  and a second flat surface  82 . With reference to FIG. 5, a flat surface of fin  42  is used for mounting to the polarizer  40 . Fin mounting is accomplished in the present invention with a plurality of dielectric screws  84  and  86  which extend through holes  84 a and  86 a, respectively, in one of the fins  42  or  44 . However, any traditional fastening means, such as rivets, clip-type fasteners, adhesives, or any other suitable attachment means may suffice to secure the CPNA  20  to a supporting structure. 
     In order to tune the CPNA  20 , several physical attributes of the antenna must be adjusted. For instance, the tuning slot cluster  50  can be shifted toward the first end  62  or the second end  64  of fin  42 . Furthermore, the tuning slot depth  50   a  (FIG. 11) and number of slots may be changed and the feed point  56  location adjusted accordingly. Tuning the CPNA  20  permits the desired bandwidth to be received. Furthermore, the CPNA  20  is tunable for input impedance matching by utilizing the feed line placement. The impedance of the coaxial cable  48  and its coaxial coupler  52  are typically 50 ohms for aircraft and microwave applications and are typically difficult to change. However, the CPNA  20  impedance can be altered to match the impedance of its coaxial coupler  52  and cable  48 . This is accomplished by moving the feed point  56  of the CPNA  20 . Finally, the placement of fins  42  and  44  on polarizer  40  plays a role in tuning the CPNA  20 . That is, varying the location of fins  42  and  44  on polarizer  40  will affect the tuning of the CPNA  20 . 
     An important factor related to antenna tuning is signal transmission. The CPNA  20  is capable of transmitting RF signals at frequencies used in radio, video, microwave, and cell phone transmissions. Actually, the CPNA  20  can be used for any frequency since the CPNA  20  bandwidth is 300%, or 3:1. One distinct advantage of the CPNA  20  is its ability to receive and transmit circularly polarized RF signals. Polarization is known as the direction of the electric field as radiated from a transmitting antenna. Generally, monopole and dipole antennas oriented in a horizontal plane, generate horizontally polarized waves. Conversely, vertically oriented antennas are considered vertically polarized and generate vertically polarized RF signals. FIGS. 12 and 13 show representative examples of a vertically polarized wave  88  and a horizontally polarized wave  90 , respectively. Signals that are vertically polarized are best received by a vertically oriented antenna and horizontally polarized antennas are best suited for reception of horizontally polarized signals. FIG. 14 shows a representative example of a circularly polarized wave  92 . A circularly polarized wave is one whose electric field varies in a circle, as opposed to horizontally or vertically. 
     The CPNA  20  of the present invention not only implements a single point feed  56  but also uses a polarizer  40  to enable the reception and/or transmission of circularly polarized RF signals. The polarizer  40  is one method of causing the rotation of a linear polarized signal as it travels through space, creating the resulting circular polarization of the outgoing wave. The polarizer  40  also allows circularly and linearly polarized incoming signals to be received as linear signals. 
     While the present invention is shown in cooperation with private or commercial aircraft  10  and military aircraft  22 , those skilled in the art will appreciate that the CPNA  20  of the present invention serves multiple applications. For instance, the CPNA  20  is not only suited for private, commercial and military air use, as noted above, but also for all land, sea, air and space use. Of particular benefit is that the CPNA  20  of the present invention enables circularly polarized signals to be received and/or transmitted while still providing a less complex antenna design, in addition to an antenna design which has a low RCS. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.