Patent Publication Number: US-7907098-B1

Title: Log periodic antenna

Description:
TECHNICAL FIELD 
     The present invention generally relates to the field of antennas, and more particularly to log periodic antennas for software defined radio applications, such as avionic software defined radio applications. 
     BACKGROUND 
     Antennas are utilized for communication, navigation, and surveillance. Aircrafts rely on communication navigation, and surveillance. Special considerations are considered when utilizing an antenna on an aircraft and/or other moving vehicles and devices. 
     Log periodic antennas (also known as log periodic arrays) are multi-element broad band, unidirectional, narrow-beam, antennas with impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequency. Arrays of multielement Log periodic antennas have similar performance but with increased directivity in one plane due to the arraying of individual log periodic antenna with a prescribed phase relationship. The elements may be dipoles within the log periodic array. Log periodic dipole arrays are driven in free space and cannot be close to a local ground and are therefore, non-conformal and/or cannot be mounted flush with a metallic structure. 
     Dipole log periodic antennas are difficult to drive against the ground. A log periodic scaled monopole array may be in an end fire configuration, but the log periodic scaled monopole array is non-conformal due to the height of the individual monopole elements that comprise the array. The height of this array is one quarter of the wavelength being transmitted and/or received (to the first order) at the lowest operating frequency of the array. Log periodic antenna-based phased arrays may be in an end fire configuration, but the log periodic phased arrays are, also, non-conformal. Microstrip broadside log periodic antennas and other types of planar log periodic arrays have broadside rather than end fire performance. 
     SUMMARY 
     The disclosure is directed to a log periodic antenna. 
     The log periodic antenna comprises a plurality of radiating elements in an end fire configuration, each radiating element in the plurality of radiating elements comprises a conductor, and at least two shunt inductances connected to the conductor; a substrate connecting the plurality of radiating elements; and a log periodic stripline feed pathway superimposed on the substrate. The height of each radiating element of the plurality of radiating elements is about one hundredth the size of the wavelength and the length is about 0.14 the size of the wavelength at the lowest operating frequency of the log periodic antenna. The plurality of radiating elements is configured to produce impedance and radiation characteristics that are regularly repetitive as a logarithmic function of an excitation of frequency without performance degradation. 
     The log periodic antenna comprises a plurality of radiating elements in an end fire configuration, each radiating element in the plurality of radiating elements comprises a conductor, at least two shunt inductance tuning elements, and a plurality of switches that control the at least two shunt inductance tuning elements and the conductor; a substrate connecting the plurality of radiating elements; and a log periodic stripline feed pathway superimposed on the substrate. The plurality of switches are capable of activating the at least two shunt inductance tuning elements to optimize a log periodic growth. The height of each radiating element of the plurality of radiating elements is about one hundredth the size of the wavelength and the length is about 0.14 the size of the wavelength at the lowest operating frequency of the log periodic antenna. The plurality of radiating elements is configured to produce impedance and radiation characteristics that are regularly repetitive as a logarithmic function of an excitation of frequency without performance degradation. 
     The log periodic antenna comprises a plurality of radiating elements in an end fire configuration, each radiating element in the plurality of radiating elements comprises a conductor, at least two tuning elements, the tuning element comprising at least one of microelectromechanical variable capacitors, ferroelectric variable capacitors, and switched length transmission line stubs, and a plurality of switches that control the at least two tuning elements and the conductor; a substrate connecting the plurality of radiating elements; and a log periodic stripline feed pathway superimposed on the substrate. The plurality of switches are capable of activating the at least two tuning elements to optimize a log periodic growth. The height of each radiating element of the plurality of radiating elements is about one hundredth the size of the wavelength and the length is about 0.14 the size of the wavelength at the lowest operating frequency of the log periodic antenna. The plurality of radiating elements is configured to produce impedance and radiation characteristics that are regularly repetitive as a logarithmic function of an excitation of frequency without performance degradation. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the claims. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate examples and together with the general description, serve to explain the principles of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous advantages of the disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  is a partial top view illustrating a log periodic antenna and a representation of a log periodic stripline feed pathway; 
         FIG. 2  is a cross-sectional side view illustrating a log periodic antenna; 
         FIG. 3  is a top view illustrating a radiating element; 
         FIG. 4  is a cross-sectional side view illustrating a radiating element; 
         FIG. 5  is a circuit diagram of a switch in the radiating element comprising at least two tunable shunt inductances as illustrated in  FIG. 4 ; 
         FIG. 6A  is an isometric view illustrating log periodic antennas mounted to an aircraft with vertical or horizontal polarizations. 
         FIG. 6B  is a cross-sectional side view of the log periodic antennas mounted to the aircraft with the vertical polarization as illustrated in  FIG. 6A ; and 
         FIG. 6C  is a cross-sectional side view of the log periodic antennas mounted to the aircraft with the horizontal polarization as illustrated in  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2 , a log periodic antenna  100  is shown. The log periodic antenna  100  may be capable of transmitting or receiving radio waves. The log periodic antenna  100  may be small in size and operates over a broad bandwidth. The log periodic antenna  100  may be scaled to operate over the very high frequency band (VHF) (with a frequency of 30 MHz to 300 MHz (with a wavelength of 10 m to 1 m)), the ultra high frequency band (UHF) (with a frequency of 300 MHz to 3,000 MHz (with a wavelength of 1 m to 100 mm)), the super high frequency band (SHF) (with a frequency of 3 GHz to 30 GHz (with a wavelength of 100 mm to 10 mm)), and/or the extremely high frequency band (EHF) (with a frequency of 30 GHz to 300 GHz (with a wavelength of 10 mm to 1 mm)). The log periodic antenna  100  may have extremely broad bandwidth due to the log periodic construct of the array and the high degrees of freedom in setting the resonant frequency and input impedance of the resonant frequency of the individual tuning elements, such as a shunt inductance tuning element. Further, the log periodic characteristic of the log periodic antenna may be optimized by dynamically changing the center frequency (or the resonant frequency at a centered coaxial input). 
     The log periodic antenna  100  may be employed with software defined radio system and/or may be an electronically scanned antenna that scans in azimuth. The azimuth scanning can be accomplishing by changing the relative time delay between different log periodic antennas  100  that comprise the phased array. The log periodic antenna  100  may be an end fire broadband log periodic array for a software defined radio system. The log periodic antenna  100  may be an end fire broadband log periodic azimuth scanned electrical scanned antenna for software defined radio applications. The log periodic antenna  100  may be utilized in a vehicle, in munitions, in an aircraft, or in any suitable object that utilizes an antenna. The log periodic antenna  100  may be utilized in commercial avionics applications. 
     The log periodic antenna  100  may be conformal or have the ability to conform to the surface of the object the log periodic antenna  100  is attached to, even if the surface is non-planar. The log periodic antenna  100  may be aerodynamic and/or have a low drag. The height (which may be measured from the mounted surface of the log periodic antenna) of the log periodic antenna is 0.01 wavelength and the width (which may be the diameter of the log periodic antenna) is 0.14 wavelength of the lowest operating frequency transmitted and/or received by the log periodic antenna  100 . For example, at 30 MHz the width is 140 cm and the height is 10 cm, and at 30 GHz the width is 1.4 mm by 0.1 mm cm. The length of the tunable antenna  100  varies depending upon the desired operating bandwidth. 
     The log periodic antenna  100  may comprise an array of a plurality of radiating elements  102  and a substrate  104  with a superimposed (or printed) log periodic stripline feed pathway  106 . The array  104  of plurality of radiating elements  102  is configured to produce impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation of frequency. 
     The substrate  104  may be any suitable insulating material with sufficient radio frequency properties that may have a pathway superimposed or printed on the substrate, such as conventional RF circuit boards (e.g., Teflon-Glass). This list is not restrictive. It is understood that any suitable substrate that may have a pathway superimposed (or printed) on the substrate and utilized with a periodic log antenna may be utilized without departing from the scope and intent of the disclosure. The compatible radio frequency planar transmission lines may comprise Microstrip, stripline, air stripline, and/or coplanar waveguide. This list is not restrictive. The log periodic stripline feed  106  may be the pathway superimposed or printed on the substrate  104 . 
     The log periodic antenna  100  may comprise a plurality of radiating elements  102  configured to form an array. As used herein “a plurality of radiating elements” refers to the number of radiating elements necessary to form an array for a log periodic antenna  100 . The log periodic antenna (array)  100  may be realized as a passive structure where the diameters, number, and positions of the shunt inductive posts realize the log periodic electrical growth parameters, along with the logarithmic growth of the element-to-element spacing within the log periodic antenna (array)  100 , according to conventional theory. The log periodic antenna  100  in a broadband, end fire configuration may therefore, be produced with extremely low cost. 
     The plurality of radiating elements  102  in the array may be configured to be tunable. The plurality of radiating elements  102  in the array may be actively tuned to optimize the log periodic growth of the log periodic antenna  100 . The tunable radiating element  102  is the same as the tunable radiating element disclosed in U.S. Patent Application with the express mail number EM005738889US filed on Sep. 27, 2007 herein incorporated by reference in its entirety. 
     The plurality of radiating elements  102  in the array are configured to move radiation in one direction and create a log periodic antenna  100 . Therefore, the plurality of radiating elements  102  in the array are configured so that the length between the plurality of radiating elements  102  and the size of the plurality of radiating elements  102  increase logarithmically from one end of the log periodic antenna  100  to the other end of the log periodic antenna as illustrated in  FIGS. 1 and 2 . 
     The array may be in an end fire configuration. The end fire configuration allows for a directive gain over a wide bandwidth. Various end fire array configurations may be utilized in the log periodic antenna, such as a linear end fire array configuration, a two-dimensional end fire array configuration, and switched sector linear end fire array configuration. This list is not restrictive. It is contemplated that other suitable end fire configurations for log periodic antennas utilizing software defined radio applications may be utilized without departing from the scope and intent of the disclosure. 
     Referring now to  FIGS. 3 and 4  a radiating element is shown. The plurality of radiating elements  102  may be in any suitable shape or size. The plurality of radiating elements  102  may be in a circular shape. The plurality of radiating elements  102  may be conformal or have the ability to conform to the surface of the object from with the plurality of radiating elements  102  is attached. The plurality of radiating elements  102  may be aerodynamic and/or have a low drag. The radiating element  102  may be about one hundredth the wavelengths in height, and 0.14 wavelengths in width of the lowest frequency wavelength being transmitted and/or received by the log periodic antenna in the log periodic antenna&#39;s lowest operating frequency. 
     The plurality of radiating elements  102  may comprise a conductor  110  and at least two shunt inductances  108 . The plurality of radiating elements may further comprise a dielectric material  116 , a plurality of switches  112 , a centered coaxial input  114 , a radio frequency cable  122 , a contact  120 , a direct circuit ground  124 , a power source, a local ground  118 , and/or a platform ground  138 . 
     The centered coaxial input  114  extends through the center of the dielectric material  116  with one side of the dielectric material  116  covered, coated, and/or joined to the conductor  110  as illustrated in  FIG. 4 . The centered coaxial input  114  may transmit radio waves though a radio frequency cable  122 . The centered coaxial input  114  may transmit radio waves to and/or from a radio. 
     The dielectric material  116  may be any suitable non-conductive material for an antenna, such as ceramic, glass, and/or plastics. The conductor  110  may be any suitable conductive material for an antenna, such as copper, silver, gold, and/or any other suitable conductor with high radio frequency conductivity. The conductor  110  may be a circular metal plate. 
     The at least two shunt inductances  108  may extend through the dielectric material  116  to the plane of the conductor  110 . The at least two shunt inductances  108  may be fixed or tunable. The fixed at least two shunt inductances  108  are connected to the conductor  110 . However, the tunable at least two shunt inductances  108  do not touch the conductor  110  as illustrated in  FIGS. 3 and 4 . The at least two shunt inductances  108  may be similar in manner to tuning elements of small monopole type antennas. 
     The at least two shunt inductances  108  may be integrated in the dielectric material  116  of radiating element  102 . The radiating element  102  comprising the at least two shunt inductances  108  may form a monopole like radiation pattern. The log period antenna&#39;s  100  radiation pattern is monopole like in a plane perpendicular to the radiating elements by virtue of the monopole-like radiating patterns due to each of the plurality of radiating elements  102 . The radiating element  102  comprising the at least two shunt inductances  108  may produce a monopole type end fire radiation pattern. The dielectric material  116  may support two shunt inductances  108  for each radiating element of the log period antenna  100 . The dielectric material  116  may support four shunt inductances  108  for each radiating element of the log periodic antenna  100 . The dielectric material  116  may support six shunt inductances  108  for each radiating element of the log period antenna  100 . The dielectric material  116  may support eight shunt inductances  108  for each radiating element of the log period antenna  100 . This list is not restrictive. It is contemplated that any suitable number of two or more shunt inductances  108  may be utilized without departing from the scope and intent of the disclosure. 
     The at least two shunt inductance tuning elements  108  may be in any suitable axially symmetric pattern within the dielectric material  118  for increasing the bandwidth and optimizing the log periodic growth of the log periodic antenna  100 . The at least two shunt inductance tuning elements  108  may be positioned in a line radially outward from the centered coaxial input  102 . The at least two shunt inductance tuning elements  108  may be positioned in two different perpendicular lines that extend radially outward from the centered coaxial input  114  as illustrated in  FIGS. 1 and 3 . The log periodic antenna  100  is readily adaptable because the plurality of radiating elements  102  has several degrees of freedom. 
     It is appreciated that the size and shape of the portion of the at least two shunt inductances  108  that is on same plane as the conductor  110  or the heads of the at least two shunt inductance tuning elements  108  may be configured for changing the bandwidth and/or for optimizing the log periodic growth of the log periodic antenna  100 . The heads of the at least two shunt inductances  108  may be in a circular shape. The heads of each of the at least two shunt inductances  108  may be the same size and/or shape or may vary in size as along as the at least two shunt inductances  108  form an axially symmetric pattern. The size of the at least two shunt inductance elements  108  for the radiating element  102  may increase logarithmically from the centered coaxial input  114  in addition to the logarithmic sizing of the radiating elements  102  on the substrate. 
     It is appreciated that the number of, the size of, and the positioning of the at least two shunt inductances  108  may be adjusted to affect the resonant frequency and input impedance of each radiating element, which may optimize the log periodic antenna&#39;s  100  electrical growth parameter, commonly known as Tau (τ), to optimize log periodic performance for desired applications. Repositioning the at least two shunt inductances  108  and increasing the amount of at least two shunt inductances  108  may also be utilized. Additionally, it is contemplated that the number of at least two shunt inductances  108  and the positioning of the at least two shunt inductances  108  may be adjusted to affect the radiation pattern in an azimuth plane. Therefore, the log periodic antenna&#39;s radiated phase in the azimuth plane may be adjusted for desired applications by repositioning the at least two shunt inductances  108  and increasing the amount of at least two shunt inductances  108  within each radiating element  102 . The log periodic antenna  100  may have high gain azimuthally symmetric patterns. 
     The plurality of radiating elements  102  comprising the tunable at least two shunt inductances may comprise a plurality of switches  112 . As used herein the term “the plurality of switches  112 ” refers to a number of switches  112  that is equal to the number of tunable shunt inductances  108  present in the plurality of radiating elements  102 . The plurality of switches  112  may be positioned to connect the conductor  110  and the tunable at least two shunt inductances  108  when closed as illustrated in  FIGS. 3 through 5 . Closing a switch  112  (e.g., establishing a connection within the radiating element  102 ) activates the tunable shunt inductance  108  and changes the bandwidth of the log periodic antenna  100 . Referring to  FIG. 5  a circuit diagram of the switch  112  as illustrated in  FIG. 4  is shown. 
     The plurality of switches  112  may be opened and closed by a power source connected to the plurality of radiating elements  102 . The plurality of the switches  112  may have a common voltage. A contact  120  may open or close the switch  112 . The switch  112  may comprise a micoelectromechanical system (MEMS) switch, a pin diode, and/or a transistor. This list is not restrictive. It is contemplated that any suitable radio frequency switch for a log periodic antenna  100  may be utilized without departing from the scope and intent of the disclosure. Moreover, the plurality of switches  102  may utilize flip chip mounting concepts. 
     The opening and closing of the switches  112  may be selectively chosen in real time for optimizing the log periodic electrical growth (the parameter). The selection may calculated and/or chosen by software in the software define radio and/or in the scanning of the electric scan antenna. Other tuning elements, instead of or in conjunction with the at least two shunt inductances may be utilized in the radiating element, such as microelectromechanical system variable capacitors, ferroelectric variable capacitors, and switched length transmission line stubs for optimizing the log periodic growth of the log periodic antenna  100 . This list is not restrictive. It is appreciated that any suitable mechanism and/or device for selecting and/or opening and closing the plurality of switches  112  of the log periodic antenna  100  for optimizing log periodic electrical growth of the log periodic antenna  100  may be utilized without departing from the scope and intent of the disclosure. 
     As already described a side of the dielectric material  116  is covered, coated, and/or joined to the conductor  110 . A second side of the dielectric material  116  parallel and opposite the conductor  110  may be covered, coated, and/or joined to a local ground  118  as illustrated in  FIGS. 4 and 5 . The local ground  118  may be connected to the direct circuit  124 . The local ground  118  and may be connected to or may be the same structure as a platform ground  138 . 
     The log periodic antenna  100  may further comprise a platform ground  138 , as illustrated in  FIG. 6  if a local ground is present without any performance degradation. The platform ground  138  may be any metallic surface that is larger than the plurality of radiating elements. The log periodic antenna  100  may not require a platform ground  138  because the log periodic antenna  100  is insensitive to platform metallization. The platform ground  138  may be the outer surface of a vehicle, munitions, an aircraft, or any other suitable metal object that may utilize an antenna. 
     The size and/or diameter of the log periodic antenna  100  may be functionally related to the diameter of the optimal wavelength the log periodic antenna  100  may be configured to receive and/or transmit. Each radiating element within the log periodic array  100  has a nominal active operating frequency within the operating bandwidth of the log periodic antenna, and when active has a minimum diameter of approximately 0.14 wavelength of the optimal wavelength being transmitted and/or received by the log periodic antenna  100 . 
     The log periodic antenna  100  may be mounted differently for vertical and horizontal polarizations as illustrated in  FIG. 6 . 
     The periodic antenna  100  may be mounted to the top or bottom of the fuselage  126  and  128  or to the top or bottom of the wing  130  and  132  of an aircraft for a vertical polarization, as illustrated in  FIG. 6A . The vertically polarized log periodic antenna  100  may be in an end fire configuration and may contain four radiating elements  102  attached to the same platform ground  138 , as illustrated in  FIGS. 6A and 6B . It is appreciated that number of radiating elements  102  in the array may vary depending upon the desired utilization of the log periodic antenna  100 . 
     The log periodic antenna  100  may be mounted to the side of the fuselage  136  and  134  of an aircraft for horizontal polarization, as illustrated in  FIGS. 6A and 6C . The horizontally polarized log periodic antenna  100  may be in an end fire configuration and may contain three radiating elements  102  all connected to the same platform ground  138 , as illustrated in  FIGS. 6A and 6C . It is I appreciated that number of radiating elements  102  in the array may vary depending upon the desired utilization of the log periodic antenna  100 . Circular phased arrays of log periodic antenna  100  can be realized in a rectilinear or radial array configuration to enable electric beam scanning in the azimuthal plane. 
     The log periodic antenna  100  may be applicable to highly integrated antenna technology. The log periodic antenna  100  may utilized a monopole-like vertical or horizontal polarization fuselage and wing mount directional end fire arrays, which are usable over a wide variety of frequency bands, e.g., VHF, and Ku band for numerous software defined radio applications and communication, navigation, and surveillance radio systems. The log periodic antenna  100  may utilized a monopole-like horizontal polarization fuselage “side mount” directional end fire array, 
     Furthermore, the application of the log periodic antenna  100  may increase system functionality for phased array technology. 
     The log periodic antenna  100  may be produced at a cost lower than other broadband directional antennas for broadband systems. The cost effective log periodic antenna  100  may also be utilized in electronic warfare (EW), signals intelligence (SIGINT) (e.g., intelligence gathering by the interception of sensitive or encrypted information), military broadband reconfigurable systems, broadband connectivity airborne Ka band commercial satellite communication systems, and surveillance. The periodic antenna  100  is desirable for joint tactical radio systems, the broadband communications, Military International Software Defined Radio (ISDR), and similar radio systems. 
     It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the disclosure or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.