Abstract:
A broad band antenna uses a bicone antenna configuration as a feed transformer and exponentially tapered reflector fins to radiate the antenna&#39;s energy. The bicone antenna design is used to match the antenna&#39;s impedance to coaxial cable impedance. The fins function to reduce the traditional bicone antenna diameter. Reflection between the cones and the attached reflector fins as well as return loss are reduced by wrapping the reflector fins with metallic foil.

Description:
BACKGROUND OF THE INVENTION 
     The following invention relates to antennas and more specifically to broad band antennas of bicone design. 
     It is desirable to provide an omnidirectional vertically polarized antenna that operates over a wide frequency band. If this goal is met, programmable radios that operate over a wide frequency band can then be connected to a single antenna. The antenna could also be used for several radio systems, thereby reducing the number of antennas required. 
     It is typical to provide dedicated antennas for each radio frequency band used. These bands are typically 10 to 50% of the center frequency of operation. For example, military ultra high frequency (UHF) radios typically operate from 225 to 400 MHz. The antenna used for these radios operate only over this band. For radios operating over other bands, an antenna will be used for each of the radios. 
     Such dedicated antennas are commonly used on shipboard systems, resulting in an “antenna farm” on the ship&#39;s topside and/or many antennas mounted on the ship&#39;s mast. In those instances where broad band antennas are used, such as with electronic counter measure equipment, the antennas are usually based on class bicone antenna designs that are very large in size. 
     There is thus a need for a broad band antenna that can be used for a number of communication systems while at the same time is of minimal size. 
     SUMMARY OF THE INVENTION 
     The invention is a broad band antenna that uses a bicone antenna configuration as a feed transformer and exponentially tapered reflector fins to radiate the antenna&#39;s energy. The fins function to reduce the traditional bicone antenna diameter. Reflection between the cones and the attached reflector fins as well as return loss are substantially reduced by wrapping the reflector fins with metallic foil. 
    
    
     Other objects, advantages and new features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanied drawings. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a side view of an exemplary embodiment of the invention. 
     FIG. 2 is a perspective view of an exemplary embodiment of the invention. 
     FIG. 3 is a detailed side view of an exemplary embodiment of the invention shown with foil layers in place. 
     FIG. 4 illustrates return loss for an embodiment of the invention. 
     FIGS. 5A-G are azimuth and elevation patterns for an embodiment of the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The most critical part of a broadband antenna is its feed. In narrow band antennas a matching section is used to match the impedance of the input coaxial cable feed to the impedance of the antenna. In broad band antennas, matching sections are not feasible as matching sections will function similarly to bandpass filters. The matching sections will limit the bandwidth of the antenna operation. 
     In order for a broadband antenna to operate over a broad bandwidth, the impedance of the antenna must closely match the impedance of the antenna feed. Such an antenna feed is typically an input from a 50 Ohm coaxial cable. 
     In the  Antenna Engineering Handbook , Second Edition, by Richard Johnson and Henry Jasik, it can be learned that a bicone antenna with cone angles of 65 degrees will have an input impedance of approximately 50 Ohms. Such an antenna will be inherently broadband because of this impedance match. The disadvantage of this antenna is that with a 65 degree cone angle, an antenna designed for operation at 200 MHz will be 56 inches in diameter and have a height of 27 inches. The size of this antenna is too large for most shipboard and land mobile applications. 
     Referring to FIG. 1, antenna  10  incorporates a bicone antenna configuration  12  as an antenna feed to match the impedance of the antenna to the impedance of coaxial cable input  14 . Cable  14  has a first conductor that is connected to first cone  16  and a second conductor that is connected to second cone  16 ′. 
     The bicone feed establishes a transverse electromagnetic (TEM) wave mode on the antenna surfaces and is shown to include two opposing hollow cones that each have straight-line profiles. Though substantially identical straight-line profile cones are disclosed, a skilled artisan will realize that cones of other profiles and combinations of profiles may also be employed and still be confined within the spirit of this invention. 
     Once the TEM mode is established, the generated energy is radiated into space by way of first and second exponentially tapered radiators  17  and  17 ′. These radiators may include first and second pluralities  18  and  18 ′ of exponentially tapered conductive fins. The fins are attached at base regions  20  and  20 ′ of cones  16  in a conventional way, not shown, wherein the fins each include an outer surface  22 / 22 ′ that are substantially contiguous with the outer surfaces of the cones. For an antenna feed  14  that equates with the 50 Ohm antenna input impedance, cone angle  24  will be 65 degrees. One skilled in the art will realize however that other cone angles may be used to adjust impedance matching where necessary. 
     When used in conjunction with a 65 degree cone angle, antenna  10  is approximately 23 inches in diameter and 25 inches in height and is suitable for radiating energy from 130 MHz to 8.0 GHZ. FIG. 2 illustrates a perspective of the invention wherein it can be seen in this example two sets of reflector fins  18  and  18 ′ wherein each set includes six fins. In this embodiment the fins are arranged with equal 60 degree fin spacing. It can be envisioned that other number of fins and fin spacing are possible while staying within the scope of this invention. Also shown in this figure are bicone supports  25  that are of a dielectric material. 
     Referring again to FIG. 1, optimization of the antenna described above is furthered by utilizing a feed gap  26  of 0.095 inches and the installation of an anti-arcing washer  28 . For the specifications of the invention described above, the washer was made 1.5 inches in diameter and was of Dupont trademarked Teflon material. Besides minimizing arcing between the cones of the invention, the spacer also promotes a desired distance or spacing between the cones. 
     Referring now to FIG. 3, further optimization of the invention is achieved by covering the reflector fins with a thin metal foil  30 / 30 ′ such as aluminum foil. When a prototype of the invention was tested without the foil for return loss, a large reflection occurred at approximately 1.0 Ghz. This reflection was considered caused by the reflection from the transition from the bicone feed to the exponentially tapered fins. When the exponentially shaped fin reflectors were covered with aluminum foil, the reflection at 1.0 GHz was significantly reduced and the return loss was reduced over a wide frequency band. This foil may be further supported by axially aligned hoops disposed over the reflector fins. 
     FIG. 4 shows the broad band return loss of the antenna of the invention described above. The graph depicts frequency along its horizontal axis starting left-to-right at 0.05 Ghz and extending to 10.5 Ghz. The graph shows the antenna radiating at a return loss less than 10 dB (1.9:1 voltage standing wave ratio (VSWR) from 130 MHz to 9.0 Ghz. 
     Azimuth and elevation patterns were taken at 200 MHz, 400 MHz, 500 MHz, 1.0 GHz, 1.5 GHz, and 2.0 GHz. for the specific embodiment of the invention described above. This data showed a typical gain over this band of 1.0 dBi, an omnidirectional antenna pattern in azimuth at the horizon and an elevation pattern similar to that of a dipole antenna. 
     FIGS. 5A-G show the azimuth and elevation patterns of the example antenna. Azimuth readings were taken at 200 MHz and 1.0, 4.0 and 8.0 GHz. The azimuth pattern in the 1.0 GHz region has azimuth ripple caused by the bicone foil interface and bicone supports. Elevation patters are shown for 0.2, 1.0 and 2.0 GHz. 
     The invention provides numerous advantages. It permits a transmit and receive capability over a broad frequency band (130 MHz to 9.0 GHz., for example) in a single antenna, instead of several antennas and coupling devices or switches. By using the invention, the number of antennas and required mast antenna space on a ship can be reduced. The invention can be used with broad band programmable radios. The invention also provides a reduced size compared to a classical bicone antenna. 
     By using a bicone antenna configuration as an antenna feed, the frequency band of operation of the antenna is increased. The invention also utilizes an exponentially taped reflector which reduces the antenna diameter over that of the classical bicone antenna. 
     Though an example of the invention has been described this example is not intended or to be implied that this example is the only implementation of the invention. 
     While the bicone and radiating elements of the invention described above may lend themselves to be made of metal, it can be envisioned that these elements could be fabricated of fiberglass that is metallized for radiating and conducting purposes. It can also be envisioned that the antenna can be fabricated using a “spun” aluminum structure such as one with holes in the exponential reflector. Similarly, a radome could be built into the bicone feed area as well over the exponential reflector. Additionally, a polarizer could be added into the radome to produce slant, circular or horizontal polarization. 
     Obviously, many modifications and variations of the invention are possible in light of the above description. It is therefore to be understood that within the scope of the claims the invention may be practiced otherwise than as has been specifically described.