Patent Publication Number: US-11038275-B2

Title: Bicone antenna with logarithmically extending conical surfaces

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     The United States Government has ownership rights in this invention. Licensing inquiries may be directed to Office of Research and Technical Applications, Space and Naval Warfare Systems Center, Pacific, Code 72120, San Diego, Calif., 92152; telephone (619) 553-5118; email: sssc_pac_t2@navy.mil, referencing Navy Case 104087. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure can pertain generally to antennas. More particularly, the present disclosure can pertain to bicone antennas having surfaces that can be shaped with a particular geometry, so that the antenna can act as a traveling wave antenna, to allow for multi-directional operation over a wide frequency range. 
     BACKGROUND OF THE INVENTION 
     Standard bicone antennas can have insufficiently narrow operating frequency ranges. To extend the frequency range and improve gain, antenna arrays have been designed with multiple antennas, which are designed to cover respective multiple frequency ranges. This configuration can require multiple radio frequency cables and complex electronics. Typical antenna designs can also have positioning or rotary joints to allow an antenna to move in order to receive and/or transmit in multiple directions. 
     While antenna arrays and positioning/rotary joints provide a multi-directional extended frequency range, this antenna design increase the power required, resulting in high return loss. Also, the use of positioning or rotary joints can induce noise. As a result, such designs typically suffer from low gain. 
     In view of the above, it can be an object of the present invention to have a stationary antenna design that can have a multi-directional extended frequency bandwidth with improved gain and improved return loss. Another object of the present invention can be to provide a bicone antenna having surfaces that can be shaped with a particular geometry, so that the antenna can act as a traveling wave antenna. Yet another object of the present invention can be to provide a bicone antenna, which can allow for multi-directional operation over a wide frequency range, but with a minimum of moving parts. Still another object of the present invention can be to provide a bicone antenna that can be easy to manufacture, including by additive manufacturing techniques, in a cost-effective manner. 
     SUMMARY OF THE INVENTION 
     A bicone antenna and methods for manufacture therefor can include a feed portion centered on a vertical axis, and a top section and a bottom section that can be attached to the feed portion so that the top and bottom sections are also centered on the vertical axis. The top section and bottom section can each have a respective conical surface, which can each extend radially outward from the vertical axis at a respective inner portion at a constant angle θ 1  with respect to a horizontal axis of the antenna. For both sections, the inner portion can merge into an outer portion that can have a curved surface, with curved surface extending radially outward from the conical surface so that the curved surface has a logarithmic profile when the antenna can be viewed in side profile. 
     The above structure can allow for a multi-directional antenna with a minimum of moving parts, which can be easily manufactured, including by additive manufacturing techniques. These, as well as other objects, features and benefits will now become clear from a review of the following detailed description, the illustrative embodiments, and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the specification, illustrate example embodiments wherein specific reference characters refer to specifically-referenced parts, and further wherein: 
         FIG. 1  can illustrate a side view of a bicone antenna according to several illustrative embodiments; 
         FIG. 2  can illustrate a three-dimensional view of a bicone antenna of  FIG. 1  according to several illustrative embodiments; 
         FIG. 3  can be a graph of return loss versus frequency, which can illustrate an example plot of return loss realized by a bicone antenna according to several illustrative embodiments; 
         FIGS. 4A-4C  can illustrate alternative shapes for a bicone antenna according to illustrative embodiments; and, 
         FIG. 5  can be a flow chart, which can be used to illustrate steps that can be taken to accomplish the methods for providing a bicone antennas, according to several illustrative embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     According to illustrative embodiments, a bicone antenna can be provided with a top section and a bottom section that each can include a conical surface having an inner portion and an outer portion. The outer portion of each of the top section and the bottom section can extend logarithmically outward, as described more fully below. Logarithmically extending the conical surface can result in wideband performance with high gain and low return loss. 
     Referring initially to  FIGS. 1-2 ,  FIG. 1  can illustrate a side view of a bicone antenna according to several illustrative embodiments. As shown in  FIG. 1 , bicone antenna  100  can include a feed portion  105 , a top section  110 , and a bottom section  120 . The feed portion  105  may be fed through the bottom of the bicone antenna  100  via, for example, a small 50 Ohm coaxial cable (not shown). 
     The top section  110  of the bicone antenna  100  can include a conical surface  115 , and the bottom section  120  can include a conical surface  125 . The top section  110  of the bicone antenna may also include a top cap  130  with rounded edges to improve reflections. 
     The conical surface  115  can include a straight inner portion  115 A extending outward from the feed portion  105  at a constant angle θ 1  with respect to a horizontal axis x of the bicone antenna  100 . The conical surface  115  also can include a transition portion  115 B extending from the inner portion  115 A and an outer portion  115 C extending logarithmically outward. 
     Similarly, the conical surface  125  can include a straight inner portion  125 A extending outward from the feed portion  105  at a constant angle θ 1  with respect to a horizontal axis x of the bicone antenna  100 . The conical surface  125  also can include a transition portion  125 B extending from the inner portion  125 A and an outer portion  125 C extending logarithmically outward. 
     As shown in  FIG. 1 , the inner portions  115 A and  125 A each have a shape similar to that of a typical bicone antenna. A typical bicone antenna can allow incoming radio frequency (RF) energy to transfer into the antenna from a given impedance to a given antenna impedance (i.e. 50 Ohms) with a given dielectric ϑ(e.g., ϑ≠1 for air). According to illustrative embodiments, the addition of the outer portions  115 C,  125 C with curved surfaces that can extend logarithmically outward can allow the RF energy to continue travelling through the bicone antenna  100 . This can cause the bicone antenna  100  to act as a travelling wave antenna in all directions. Extending the curved surfaces of the outer portions  115 C,  125 C of the antenna logarithmically outward in a way so that the antenna can act as a travelling wave antenna can increase the antenna gain, which can increase antenna frequency bandwidth and can improve return loss. The curvature of the curved surfaces can be described with more particularity below. 
     With respect to inner portions  115 A,  125 A, the angle θ 1  may be selected based on a desired input impedance of the bicone antenna  100 . To understand how the angle θ 1  is selected, consider an approximation of the input impendence Z in  of an infinite bicone which can be given as:
 
 Z   in =(120/ n )ln(cot θ hc /2)  (1)
 
where θ hc  is the half-angle of each conical surface of the bicone antenna with respect to the vertical axis y, and n is the desired input impedance (e.g., 50 Ohms). According to illustrative embodiments, once the half-angle θ hc  is determined, that can provide an impedance Z in  that can be close to the desired input impedance n, the constant angle θ 1  of the inner portions  115 A,  115 B of the respective conical surfaces  115 ,  125  is selected as θ 1 =90°−θ hc . For example, to achieve an impedance Z in  of 48.3 Ohms, θ hc  may be set at 67.5°, resulting in θ 1 =22.5°.
 
     Referring again to  FIG. 1 , the outer portion  115 C of the conical surface  115  of the top section  110  can have a curved surface with a first end beginning at point P A  and having an angle θ 2A  with respect to the horizontal axis (at P A ), where θ 2A  is less than θ 1 . Similarly, the outer portion  125 C of the conical surface  125  of the bottom section  120  has a first end beginning at point P B  and having an angle θ 2B  with respect to the horizontal axis, where θ 2B  is less than θ 1 . From the points P A  and P B , the logarithmically extending outer portions  115 C,  125 C, when viewed in cross-section, can each have a profile with a shape that is given by:
 
 f ( x )= B *ln( A*X )− B   (2)
 
where x is a radial distance along the horizontal axis x from the points P A , P B , f(x) can be the distance from the x-axis to the curved surface, and A and B can be constants that affect the shape of the logarithmically extending outer portions  115 C,  125 C with respect to the horizontal axis x and the vertical axis y. A and B can be chosen by an antenna designer to shape the logarithmically extending outer portion as desired.
 
     As shown in  FIG. 1 , the outer portion  115 C of the conical surface  115  of the top section  110  also can include a second end having an angle θ 3A  with respect to the horizontal axis, where θ 3A  is greater than θ 2A . Similarly, the outer portion  125 C of the conical surface  125  of the bottom section  120  also can include a second end having an angle θ 3B  with respect to the horizontal axis, where θ 3B  is greater than θ 2B . 
     As noted above, the conical surfaces  115 ,  125  also can include respective transition portions  115 B,  125 B between the respective inner portions  115 A,  125 A and the respective outer portions  115 C,  115 C. The transition portions  115 B,  125 B are indicated in  FIG. 1  by curved dashed lines (the extent of the transition portions  115 B,  125 B is somewhat exaggerated for illustration purposes). The transition portions  115 B,  125 B may be formed by chamfering a portion of each of the conical surfaces  115 ,  125  where the straight inner portions  115 A,  125 A would otherwise meet the logarithmically shaped curved surfaces of outer portions  115 C,  125 C. The transition portions  115 B,  125 B can each have a length and shape represented in  FIG. 1  as a radius r. Each of the transition portions  115 B,  125 B may be chamfered to have a desired length and shape for a given antenna size. 
     In operation, RF energy arrives at the bicone antenna  100  via a cable fed into the feed portion  105 . The RF energy starts transitioning from an input impedance (e.g., 50 Ohms) at the inner portions  115 A,  125 A of the respective conical surfaces  115 ,  125  to a lower impedance at the respective first ends of the outer portions  115 C,  125 C, due to the angles θ 2A  and θ 2B  being less than θ 1 . The RF energy then transitions into a higher impedance at the respective second ends of the outer portions  115 C,  125 C, due to the angles θ 3A  and θ 3B  being greater than the angles θ 2A  and θ 2B , respectively. As the outer portions  115 C,  125 C of the respective conical surfaces  115 ,  125  extend logarithmically outward with respect to the horizontal axis, the RF energy exiting the bicone antenna  100  acts as a travelling wave, thus improving gain and allowing a narrower elevation beam width to be achieved. 
       FIG. 2  can illustrate a three-dimensional view of a bicone antenna according to several illustrative embodiments. For clarity of illustration, some of the reference numerals shown in  FIG. 1  have been omitted from  FIG. 2 . The three-dimensional view of the bicone antenna  100  shown in  FIG. 2  represents the two-dimensional side view shown in  FIG. 1 , rotated by three hundred sixty (360) degrees. As can be seen from  FIG. 2 , the outer portions  115 C,  125 C of the respective conical surfaces  115 ,  125  extend logarithmically in a radial direction from the inner portions  115 A,  125 A. 
     As can be seen from  FIGS. 1-2 and 4A-4C , the top section  110  and the bottom section  120  of the bicone antenna  100  may be asymmetric so that the bicone antenna fits within a desired volume and/or to allow room for components to fit within the antenna. For example, the angles θ 2A , θ 2B , θ 3A , and θ 3B  and the length of the outer portions  115 C,  125 C of the respective conical surfaces  115 ,  125  may be adjusted to shape the bicone antenna  100  to fit within a desired volume. The angles θ 2A  and θ 2B  may be the same or different. Similarly, the angles θ 3A  and θ 3B  may be the same or different. 
     Additionally, the shapes and sizes of the top section  110  and bottom section  120  may be adjusted by adjusting the logarithmically extending outer portions  115 C,  125 C. Also, the length and shape of the transition portions  115 B,  125 B of the top section  110  and the bottom section  120  may be adjusted to accommodate a desired volume. Further, the shape and the roundness of the edges of the top cap  130  of the top section  110  may be adjusted. 
     Adjustments of the size and shape of the top section and bottom section of a bicone antenna are described in more detail below with reference to  FIGS. 4A-4B . 
     As noted above, the bicone antenna with logarithmically extending conical surfaces can provide improved gain. As those skilled in the art will appreciate, the gain G of an antenna can be given by:
 
 G=E·D   (3)
 
where E=efficiency and D=directivity. The efficiency E can refer to the ability of an antenna to transfer energy from an RF feed cable to the antenna, including the energy internally absorbed by the antenna from resistive and dielectric losses. The directivity D refers to the ability of an antenna to focus energy in a particular direction. According to illustrative embodiments, directivity and efficiency can be maximized by allowing the RF energy to act as a travelling wave due to the logarithmically extending outer portions of the conical surfaces. By maximizing the directivity and the efficiency, the gain is maximized.
 
     According to illustrative embodiments, gain can be improved while maintaining return loss. As those skilled in the art will appreciate, return loss is given by:
 
 RL (dB)=10 log 10 ( P   i   /P   r )  (4)
 
where RL(dB) is the return loss in dB, P i  is the incident power and P r  is the reflected power.
 
       FIG. 3  can illustrate an example plot  300  of return loss realized by a bicone antenna according to several illustrative embodiments. In the plot  300 , the return loss can be expressed in dB over a range of frequencies from 10 MHz to 18 GHz. As can be seen from the plot  300 , the bicone antenna described herein realizes a high return loss over a wide frequency range. This means that RF energy is being transferred efficiently from the RF feed cable into the feed portion of the bicone antenna. Referring to the plot  300  and equation (4) above, a −10 dB return loss equates to approximately 90% of energy transferring from the RF feed cable to the feed portion of the bicone antenna for radiation. Referring to equation (3) above, a high efficiency E implies a high gain G. According to illustrative embodiments, this high gain is realized by the logarithmically extending outer portions of the conical surfaces of the antenna, which allow the antenna to act as a multi-directional, traveling wave antenna over a wide frequency range. 
     According to illustrative embodiments, the size and shape of a bicone antenna may be adjusted as desired while improving antenna gain and the electrical size of the antenna. For example, the shapes of the top section and the bottom section of a bicone antenna may be adjusted such that the top section and the bottom portion fit within an available volume (the space constraints could of course be balanced against desired gain and frequency range design criteria). This may be understood with reference to  FIGS. 4A-4C . For simplicity of illustration, some reference numerals are omitted from  FIGS. 4A-4C . However, it should be appreciated that the top section and bottom section of each of the antennas shown in  FIGS. 4A-4C  include an inner portion, a transition portion, and an outer portion as described above with reference to  FIGS. 1 and 2 . 
       FIG. 4A  can illustrate a bicone antenna  100 A with a top section  110 A, a bottom section  120 A, and a feed portion  105 A. The top section  110 A and the bottom section  120 A of the bicone antenna  100 A have shapes similar to the top section  110  and the bottom section  120 , respectively, of the bicone antenna  100  shown in  FIG. 1 . 
       FIGS. 4B and 4C  illustrative bicone antennas having alternative shapes. As shown in  FIG. 4B , a bicone antenna  100 B has a feed portion  105 B, a top section  110 B and a bottom section  120 B that are respectively wider than the feed portion  105 A, the top section  110 A, and the bottom section  120 A of the bicone antenna  100 A shown in  FIG. 4A . In particular, the top section  110 B and the bottom section  120 B extend further logarithmically outward, compared respectively to the top section  110 A and the bottom section  120 A shown in  FIG. 4A . 
     As shown in  FIG. 4C , a bicone antenna  100 C has a feed portion  105 C, a top section  110 C, and a bottom section  120 C that are respectively narrower but taller than the feed portion  105 A, the top section  110 A, and the bottom section  120 A of the bicone antenna shown in  FIG. 4A . In particular, the top section  110 C and the bottom section  120 C can extend less far logarithmically outward, compared respectively to the top section  110 A and the bottom section  120 A shown in  FIG. 4A . 
     There are tradeoffs in adjusting the antenna size and shape to fit within a desired volume. For example, an excessive extension of the logarithmically extending outer portions of a bicone antenna could increase the capacitive reactance on the bicone antenna, diminishing the efficiency and bandwidth. Further, adjustment of the size of the bicone antenna may affect reflections for the edges of the top section. Accordingly, an antenna designer should be careful in adjusting the shape and size of a bicone antenna. 
       FIG. 5  can be a flow chart, which can be illustrative of steps of a method for providing a bicone antenna according to several embodiments. Referring to  FIG. 5 , the method  500  begins at step  510 , at which a feed portion can be provided. 
     At step  520 , a top cone with a top conical surface can be attached to the feed portion. This step can include the steps of extending a top inner portion of the top conical surface outwardly from the feed portion at a constant angle θ 1  with respect to a horizontal axis of the bicone antenna at step  522 . Step  520  can further include merging the top inner portion outwardly into a top outer portion at step  524 , so that the top outer portion of the top conical surface can have a profile like logarithmic graph, when the antenna can be viewed in side profile. Step  520  can optionally include providing a top transition portion at step  526 . As described above, the top transition portion may be provided by chamfering a portion of top conical surface where the top inner portion and the top outer portion would meet. 
     As shown in  FIG. 5 , method  500  can include step  530 , attaching a bottom cone with a bottom conical surface to the feed portion, and more specifically to the opposite of the feed portion end where the top cone is attached. This step can include extending a bottom inner portion of the bottom conical surface outwardly from the feed portion at a constant angle θ 1  with respect to a horizontal axis of the bicone antenna at step  532 , chamfering bottom inner portion outwardly into a bottom outer portion at step  534 , so that the bottom outer portion can have a profile like a logarithmic graph, when the antenna can be viewed in side profile, and optionally providing a top transition portion at step  536 . As described above, the top transition portion may be provided by chamfering a portion of the bottom conical surface where the bottom inner portion and the bottom outer portion would meet. 
     Because of the complex, bulbous curvature of the top cone and bottom cone, one way to accomplish the methods can be to use additive manufacturing techniques to provide the top section (cone), bottom section (cone) and feed portion as a unitary structure, using additive manufacturing techniques. This could result in a single integrated structure, and allows for top and bottom cones with different radii or logarithmic curvature, should such a configuration be desired. Additive manufacture using metal materials could be accomplished, or additive manufacturing of a non-metallic, dielectric materials, followed by coating the dielectric with a metallic material could be used. In sum, additive manufacturing techniques could result in a unitary, integral structure, which would require a minimum of assembly, and which could afford great flexibility in cone geometry, according to the systems and methods of the present invention. It should be appreciated that fewer, additional, or alternative steps may also be involved in the method  500  and/or some steps may occur in a different order and/or that additional or fewer steps may be involved. 
     As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. Additionally, use of the “a” or “an” are employed to describe elements and components of the embodiments herein. This is done merely for convenience and to give a general sense of the invention. This detailed description should be read to include one or at least one and the singular also includes the plural unless it is obviously meant otherwise. 
     The language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. Accordingly, the disclosure of the inventive subject matter is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims. Many modifications and variations of the embodiments disclosed herein are possible in light of the above description. Within the scope of the appended claims, the disclosed embodiments may be practiced otherwise than as specifically described. Further, the scope of the claims is not limited to the implementations and embodiments disclosed herein, but extends to other implementations and embodiments as may be contemplated by those having ordinary skill in the art.