Abstract:
An open Yaggi antenna array is disclosed wherein the reflector element and parasitic director elements of the antenna array are opened in line with the feed point of the driven element so that the reflector and director elements do not cause a shunting effect on the driven element of the antenna.

Description:
STATEMENT OF GOVERNMENT INTEREST 
   The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalty thereon or therefore. 

   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The invention relates to antennas and is directed more particularly to a design for an open Yaggi antenna array. 
   (2) Description of the Prior Art 
   Most prior art Yaggi antennas consist of a driven element and two or more non-driven elements. The driven element is often a half-wave dipole. It is arranged in front of and parallel to a non-driven element that serves as a reflector. The driven element is also arranged behind and parallel to an array of one or more other parasitic elements that serve as directors. The reflector reflects radiation from the dipole back toward the dipole. The directors narrow the dipole radiation along the director side of the dipole. Both the driven and non-driven elements are all parallel on an axis along the same spatial plane. 
   The resultant radiation pattern of the Yaggi antenna as described above is a relatively narrow unidirectional beam along the direction of the director elements away from the dipole. The narrow beam effect produced by the reflector and directors occurs over approximately a 15% bandwidth about the half wavelength frequency of the dipole. 
   There are certain problems with the Yaggi antenna as described above. In particular, the reflector and directors have various undesirable effects on the original impedance of the dipole. The reflector and directors cause a “shunting effect” on the dipole, resulting in reduced antenna impedance in the region where the antenna operates, (at or near 0.5 wavelengths resonance). In addition, the reflector and directors also cause a decrease in the impedance bandwidth of the antenna. Since the directors are parasitic elements, they introduce undesirable resonance/anti-resonance loops in the original impedance of the dipole. What is needed, therefore, is a Yaggi antenna array design that avoids the shunting effect caused by the reflector element and parasitic director elements on the driven element. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is, therefore, to provide an antenna with the performance of a traditional Yaggi array antenna but without any reduced antenna impedance and decreased bandwidth 
   With the above and other objects in view, a feature of the present invention is an open Yaggi array antenna wherein the non-driven elements (reflector and directors) are opened in line with the feed point of the driven element (dipole) so that they do not shunt the driven element of the antenna. In this way the parasitic elements should only add the resonance/anti-resonance loops in the dipole impedance. The basic impedance of the dipole should remain the same. 
   The above and other features of the invention, including various novel details of construction and combinations of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular assembly embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is made to the accompanying drawings in which is shown an illustrative embodiment of the invention, from which its novel features and advantages will be apparent, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: 
       FIG. 1  illustrates an assembly for a traditional prior art yaggi antenna; 
       FIG. 2  illustrates a first embodiment of the present invention, an open yaggi antenna; 
       FIG. 3  is a radiation pattern plot for the first embodiment of the present invention; 
       FIG. 4  is an impedance plot for the first embodiment of the present invention; 
       FIG. 5  illustrates a second embodiment of the present invention, an open yaggi antenna; 
       FIG. 6  is a radiation pattern plot for the second embodiment of the present invention. 
       FIG. 7  is an impedance plot for the second embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , there is shown an assembly for a traditional prior art Yaggi antenna  10 . Yaggi antenna  10  includes a driven element  12 , which is a 0.5 wavelengths dipole at 1 GHz, positioned vertically. The driven element  12  is a conducting rod having a radius of 0.0025 wavelengths, and having a feed point  14  at the center. Yaggi antenna  10  also includes a reflector element  16  having a length of 0.515 wavelengths, positioned parallel to the driven element  12 , and several director elements  18  each having a length of 0.43 wavelengths, positioned parallel to and on an opposite side of the driven element  12 . All of the antenna elements are arranged on the same plane of the antenna axis an equal distance apart 0.1 wavelengths at 1 GHz. Whereas this prior art Yaggi antenna  10  has many useful attributes, one drawback of the antenna  10  is that the non-driven elements, the reflector  16  and the directors  18  create a shunting effect on the driven element, the dipole  12  resulting in reduced antenna impedance, most importantly in the region where the antenna operates, at or near 0.5 wavelengths resonance. 
   Referring to  FIG. 2 , there is shown a first embodiment of the present invention, an “open Yaggi” antenna  20 . The open Yaggi antenna  20  also includes a driven element  22 , which is a dipole having a feed point  24 , a two piece reflector element  26  and one or more two piece director elements  28  all arranged on the same plane of the antenna axis an equal distance apart. For purposes of illustration the open Yaggi antenna  20  has three director elements  28 , however, the invention is not limited to this number. The open Yaggi antenna  20  is designed to avoid the shunting effects of the non-driven elements on the dipole by opening up the reflector element  26  and the director elements  28  in line with the feed point  24  of the dipole  22  thereby creating a gap along the axis of the feed point  24 . 
   By arranging the reflector element  26  and parasitic director elements  28  with a gap in line with the dipole feed point  24 , the reflector element  26  and director elements  28  will only add to the resonance/anti-resonance loops in the dipole impedance. The basic impedance of the dipole will remain the same. To maintain the reflective properties of the reflector element  26 , and the directive properties of the director elements  28 , both types of parasitic elements are designed in two separate parts of equal length. The combined length of each two piece parasitic element is twice the length of the single piece element of the prior art Yaggi antenna  10  as illustrated in  FIG. 1 . For example reflector element  26  is a combination of elements  26   a  and  26   b  whose combined length is equal to twice that of reflector element  16 . 
   In comparison to Yaggi antenna  10 , the open Yaggi antenna  20  has the following dimensions. The driven element  22  dipole is positioned vertically. The maximum length of the dipole  22  is 2.0 wavelengths at 2 GHz or 1.0 wavelengths at 1 GHz. The diameter of the dipole  22  is 0.005 wavelengths at 1 GHz. Each of the two piece non-driven elements is approximately the same size as the driven element. The gap between the two pieces of each non-driven element is 0.025 wavelengths at 1 GHz. All of the open Yaggi antenna elements are arranged on the same plane of the antenna axis an equal distance apart 0.1 wavelengths at 1 GHz. 
   Referring to  FIG. 3  it can be seen from the illustrated radiation pattern plot that the open Yaggi antenna  20  patterns near 1 wavelength at 1 GHz behave similarly to the Yaggi antenna  10 . Unidirectional patterns exist over a small bandwidth. Referring to  FIG. 4  it can be seen from the illustrated impedance plots that the basic dipole impedance locus remains the same with the addition of reflectors and directors. Only the parasitic resonance/anti-resonance loops are added. The desired objective of eliminating the shunting effects of the reflectors and directors is achieved. 
   One concern with this embodiment of the open Yaggi  20  is that the desired patterns where the parasitic resonance/anti-resonance loops occur, in the area where the reflector and directors are near 0.5 wavelengths long, occur where the impedance of the dipole is large at a one wavelength anti-resonance. Normally, a dipole is used where its impedance is at 0.5 wavelengths resonance, where its impedance is near a usable 50 ohms. With this in mind, a second embodiment of open Yaggi antenna is presented herein. 
   Referring to  FIG. 5 , there is shown a second embodiment of the present invention, an “open Yaggi” antenna  40 . The open Yaggi antenna  40  also includes a driven element  42 , which is a dipole having a feed point  44 , a two piece reflector element  46  and one or more two piece director elements  48  all arranged on the same plane of the antenna axis an equal distance apart. For purposes of illustration the open Yaggi antenna  40  has three director elements  48 , however, the invention is not limited to this number. The open Yaggi antenna  40  is also designed to avoid the shunting effects of the parasitic elements on the dipole by opening up the reflector element  46  and the director elements  48  in line with the feed point  44  of the dipole  42  thereby creating a gap along the axis of the feed point  44 . To maintain the reflective properties of the reflector element  46 , and the directive properties of the director elements  48 , both types of elements are designed in two separate parts of equal length. The combined length of each two piece reflector or director element is twice the length of the single piece element of the prior art Yaggi antenna  10  as illustrated in  FIG. 1 . For example reflector element  46  is a combination of elements  46   a  and  46   b  whose combined length is equal to twice the length of reflector element  16 . 
   In comparison to open yaggi antenna  20 , the open yaggi antenna  40  has the following dimensions. The driven element  42  dipole is positioned vertically. One difference, however, is that the length of the driven element has been reduced in length from 1.0 wavelengths at 1 GHz to 0.5 wavelengths at 1 GHz. Using this design, the dipole can now be at 0.5 wavelengths resonance when the reflector and directors are near 0.5 wavelengths long. 
   Referring to  FIG. 6  it can be seen from the illustrated radiation pattern plot that the open Yaggi antenna  40  patterns near 1 wavelength at 1 GHz behave similarly to the Yaggi antenna  10 . Unidirectional patterns exist over a small bandwidth. 
   Referring to  FIG. 7 , the impedance plots illustrate the desired unidirectional patterns about 0.5 wavelengths at 1 GHz occur with an impedance near the original 0.5 wavelength resonance impedance of the dipole. Only resonance/anti-resonance loops are added to the impedance locus. 
   It will be understood that many additional changes in the details, materials, and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principles and scope of the invention as expressed in the appended claims.