Abstract:
A microstrip fed log periodic antenna has two spaced dipole strips mounted on a ground plane. Each dipole strip has a trunk with a base, tip and alternating arms extending perpendicular to the trunk. One dipole strip includes an integral transmission feed line that extends from the tip, along the trunk of the other dipole strip at constant distance and along the ground plane at a constant distance. The one piece dipole strip with the integral transmission feed line reduces passive intermodulation and simplifies manufacture.

Description:
TECHNICAL FIELD 
   The present invention relates to antennas and more particularly to a microstrip fed log periodic antenna with a one piece transmission feed line and radiating element. 
   BACKGROUND ART 
   Log periodic antennas operate over a broad frequency range. Generally log periodic antennas have a plurality of dipole elements in a planar spaced array. The length of the elements and the spacing between the elements are selected in accordance with a mathematical formula, with the shortest elements being near the top of the antenna. Feed conductors generally connect at the tip of the antenna. Electrical connections from feed conductors to opposed elements are alternated to provide a 180 degree phase shift between successive elements. 
   U.S. Pat. No. 5,093,670 to Braathen discloses a log periodic antenna formed by printed circuit board manufacturing methods onto an insulative substrate. The dipole elements and one feed conductor are formed on one side of the substrate and a second feed conductor is formed on the opposite side of the substrate. Vias though the substrate connect the second feed conductor to alternating opposed dipole elements. 
   U.S. Pat. No. 5,917,455 to Huynh et al. discloses an array of log periodic antennas mounted on a backplane. Each antenna includes two flat dipole strips of conductive material with bases of the dipole strips mounted to the backplane in a spaced configuration. Each antenna is fed by a coaxial feed line with the center conductor being connected to one dipole strip and the jacket conductor being connected to the other dipole strip. 
   U.S. Pat. No. 6,133,889 to Yarsunas et al. and U.S. Pat. No. 6,243,050 to Powell disclose antennas with log periodic dipole assemblies fed by a microstrip feed line. Each dipole assembly has two flat dipole strips of conductive material with the bases of the dipole strips being mounted to a backplane in a spaced configuration. The feed line extends between the dipole strips of a dipole assembly and is connected to one dipole strip of the dipole assembly with a connector either at the top of the dipole strip or intermediate the top and the base of the dipole strip. The other dipole strip of the dipole assembly is not connected to the feed line. 
   The “diode junction effect” can be caused by metal to metal junctions, such as welded, soldered, riveted or bolted junctions, in electronic circuitry. This “diode junction effect” creates a non-linear voltage-current characteristic that, in radio frequency (RF) signals, can create intermodulation products that are different than the original frequencies. Passive intermodulation (PIM) may manifest as relatively strong interference signals. It is therefore desirable to avoid metal to metal junctions between the feed line and the tip of a log periodic dipole antenna, and in the feed line to the antenna. 
   DISCLOSURE OF THE INVENTION 
   A microstrip fed log periodic antenna includes a first and second dipole strips and a ground plane. The first and second dipole strips each include a trunk with a base and a tip opposite the base, and spaced dipole arms extending from each trunk. The bases of the first and second dipole strips mount to the ground plane in a spaced relationship. The first dipole strip includes a transmission feed line that is integral and one piece with the first dipole strip. The transmission feed line extends from the tip of the trunk of the first dipole strip, bends over and extends in a spaced relationship along the trunk of the second dipole strip to near the ground plane. The transmission feed line may further extend in a spaced relationship to the ground plane. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Details of this invention are described in connection with the accompanying drawings that bear similar reference numerals in which: 
       FIG. 1  is a perspective view of an antenna embodying features of the present invention. 
       FIG. 2  is a front elevation view of the antenna of FIG.  1 . 
       FIG. 3  is a rear elevation view of the antenna of FIG.  1 . 
       FIG. 4  is a side elevation view of the antenna of FIG.  1 . 
       FIG. 5  is a sectional view along line  5 — 5  of FIG.  4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIGS. 1  to  4 , a log periodic antenna embodying features of the present invention includes a ground plane  11 , a first dipole strip  12  and a second dipole strip  13 . The ground plane  11  is a planar rectangular conductive plate with a flat surface  14  and a plurality of threaded studs  15  extending transverse to the flat surface  14 . In the preferred embodiment, the ground plane  11  is made from aluminum, but other conductive materials such as copper or brass can be used. 
   The first dipole strip  12  is formed in one piece from a conductive material with good bending characteristics. In the preferred embodiment, the first dipole strip  12  is made from aluminum, but other materials such as copper, brass or a flexible printed circuit material can be used. The first dipole strip has a first trunk  16  with a plurality of spaced first dipole arms  17  and a transmission feed line shown as microstrip feed line  18 . The first trunk  16  has a flat rectangular shape with a base  19 , a tip  20  opposite the base  19 , and spaced first and second side edges  21  and  22  extending from the base  19  to the tip  20 . The first dipole arms  16  have a flat, generally rectangular shape and extend transversely from the first and second side edges  21  and  22  in a spaced alternating order. The first trunk  16  includes first trunk apertures  23  spaced between the base  19  and the tip  20 , intermediate the first and second side edges  21  and  22 . A flat base first tab  24  extends transversely from base  19  and includes first base apertures  25  extending through the base first tab  24 . 
   In the preferred embodiment, the second dipole strip  13  is made from aluminum, but other materials such as copper, brass or a flexible printed circuit material can be used. The second dipole strip has a second trunk  27  with a plurality of spaced second dipole arms  28 . The second trunk  27  has a flat rectangular shape with a base  30 , a tip  31  opposite the base  30 , and spaced first and second side edges  32  and  33  extending from the base  30  to the tip  31 . The second dipole arms  28  have a flat, generally rectangular shape and extend transversely from the first and second side edges  32  and  33  in a spaced alternating order. The second trunk  27  includes second trunk apertures  34  spaced between the base  30  and the tip  31 , intermediate the first and second side edges  32  and  33 . Flat base second tabs  35  extend transversely from base  30  and each include a second base aperture  36  extending through the base second tab  35 . 
   The first and second dipole strips  12  and  13  mount to the ground plane  11  in spaced, parallel configuration with the first trunk apertures  23  and the second trunk apertures  34  in alignment and with the first dipole arms  17  of the first dipole strip  12  and the second dipole arms  28  of the second dipole strip  13  extending oppositely. The first and second dipole strips  12  and  13  are mounted with the studs  15  through the first and second base apertures  25  and  36  of the first and second base tabs  24  and  35 , and with threaded first nuts  38  threaded onto studs  15  over the first and second apertures  25  and  36 . Other fasteners or other systems of mounting and electrically connecting the first and second dipole strips  12  and  13  to the ground plane  11  may be used such as welding, swaging, riveting, soldering, or capacitive coupling. 
   The microstrip feed line  18  has a first feed line section shown as first microstrip section  40  and a second feed line section shown as second microstrip section  41 . The first microstrip section  40  has a thin rectangular shape and extends from the tip  20 , intermediate the first and second side edges  21  and  22 , of the first trunk  16 . The first microstrip section  40  bends about 180° and extends at a uniform distance along the second trunk  27  from the tip  31  to near the base  30  of second trunk  27 . The second microstrip section  41  has a flat L shape and extends from the first microstrip section  40 , at a uniform distance from the ground plane  11 , transversely away from the trunk  27  of the second dipole strip  13 , turns 90°, and extends sideways. 
   A dielectric spacer  43  having a rectangular shape and a uniform thickness is located between the second trunk  27  and the first microstrip section  40  to maintain the uniform distance between the second trunk  27  and the first microstrip section  40 . The dielectric spacer  43  includes spacer apertures  44  that align with the second trunk apertures  34 . The first microstrip section  40  includes microstrip apertures  45  that align with the spacer aperture  44 . Hollow, cylindrical, nonconductive trunk spacers  48  are located between first trunk  16  and second trunk  27  in alignment with first and second trunk apertures  23  and  34 . Nonconductive threaded bolts  49  extend through first trunk apertures  23 , through trunk spacers  48 , through second trunk apertures  34 , through spacer apertures  44  and through microstrip apertures  45 . Nonconductive threaded second nuts  50  thread onto bolts  49  to secure the first trunk  16 , the second trunk  27  and the first microstrip section  40  at the selected distances. Other fastening systems such as nonconductive rivets or grommets may be used instead of bolts  49  and second nuts  50 . Non-conductive clips may also be used which may reduce or eliminate the need for the first trunk apertures  23 , the second trunk apertures  34 , and the microstrip apertures  45 , for trunk spacers  48  and dielectric spacer  43 . 
   Although, in the preferred embodiment the first and second trunks  16  and  27  have a rectangular shape and are spaced in a uniform, parallel fashion to excite the gap between the first and second trunks  16  and  27  in parallel plate mode, other configurations may be used. By way of example, and not as a limitation, the first and second trunks  16  and  27  can taper inwardly toward tips  20  and  31 , with the spacing between the first and second trunks  16  and  27  decreasing from bases  19  and  30  to tips  20  and  31 . 
   The second trunk  27  is the transmission line ground for the first microstrip section  40  and ground plane  11  is the transmission line ground for the second microstrip section  41 . Although the first microstrip section  40  has a generally rectangular shape and uniformly spaced from the second trunk  27 , other configurations that provide the desired impedance at the tip  20  of the first trunk  16  are suitable. The shape of the second microstrip section  41 , and the spacing between the second microstrip section  41  and the ground plane  11  can vary. In an array of log periodic antennas, the second microstrip section  41  can be common to all of the antennas and can be shaped with transformers and tapers to regulate the power and phase to each antenna. In such an array, with the second microstrip section  41  common to all of the antennas, a single metal to metal junction may be required between the array and an external transmission line, and passive intermodulation may be significantly reduced relative to prior known antennas. 
   The log periodic antenna of the present invention connects to the transmission feed line in the form of first microstrip section  40  without any metal to metal junctions at the tip of the antenna or along first or second trunks  16  and  27 . Transmission line types other than microstrip may be used, with the transmission feed line being integral and one piece with the first dipole strip. By way of example, and not as a limitation, second trunk  27  combined with a spaced second ground with the first feed line section therebetween would form a stripline. 
   Since the first microstrip section  40  connects to tip  20  of the first trunk  16  without any metal to metal junctions, the antenna of the present invention has significantly reduced passive intermodulation relative to prior known log periodic antennas. The microstrip feed line  18  does not require welding, soldering, riveting or bolting to connect to the tip of the antenna, thereby reducing the manufacturing cost of the antenna of the present invention. 
   Although the present invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made by way of example and that changes in details of structure may be made without departing from the spirit thereof.