Abstract:
A partially shorted microstrip antenna configured to wrap around a projectile&#39;s body without interfering with the aerodynamic design of the projectile. The microstrip antenna has three identical conformal antenna elements equally spaced around the circumference of the projectile&#39;s body. The antenna has an operating frequency of 231.0 MHz±400 KHz. Each antenna element includes a plurality of vias which operate as a partial short connecting the radiating element to the ground plane and thereby increase the bandwidth of the antenna element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a microstrip antenna designed for use on a weapons system. More specifically, the present invention relates to a cylindrical shaped microstrip antenna array which operates at a frequency of 231 MHz±400 KHz and which is adapted for use on a weapons system such as a missile or other projectile. 
     2. Description of the Prior Art 
     A microstrip antenna operates by resonating at a frequency. The conventional design uses printed circuit techniques to put a printed copper patch on the top of a layer of dielectric with a ground plane on the bottom of the dielectric. The frequency of operation of the conventional microstrip antenna is for the length of the antenna to be approximately a half-wavelength in the microstrip medium of dielectric below the patch and air above the patch. 
     Another type of microstrip antenna is a quarter-wavelength microstrip antenna which is similar to the half wavelength microstrip antenna except the resonant length is a quarter-wavelength and one side of the antenna is grounded. 
     There is currently a need to provide an antenna which is similar in design and operates in a manner virtually identical to the quarter-wavelength microwave antenna and also provides for a significant increase in bandwidth. 
     This microstrip antenna is to be used on a weapons system or projectile such as a missile. There is also a requirement for a frequency of operation for the antenna of 231 MHz±400 KHz. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes some of the disadvantages of the past including those mentioned above in that it comprises a highly effective and efficient microstrip antenna designed to transmit telemetry data from a HARM missile at a frequency of 231 MHz±400 KHz. The microstrip antenna comprising the present invention is configured to wrap around a projectile&#39;s body without interfering with the aerodynamic design of the projectile. 
     The microstrip antenna of the present invention has three identical conformal antenna elements equally spaced around the circumference of a projectile&#39;s body. The antenna has an operating frequency of 231 MHz±400 KHz, and is designed for use with the HARM missile to transmit Telemetry data. 
     Each of the three identical antenna elements includes a dielectric printed circuit board, a rectangular shaped radiating element mounted on a top portion of the printed circuit board, and a ground plane mounted on the bottom portion of the printed circuit board. 
     A plurality of copper wire electrical shorts, i.e. copper vias are provided along one edge of the radiating element to connect the radiating element to the ground plane. The copper electrical shorts are equally spaced apart and run from the midpoint of radiating element to the one corner of the radiating element. The unique placement and configuration of the vias allows for a substantial increase in the width of the radiating element and an increase in the bandwidth to ±400 KHz about the center frequency of 231 KHz. 
     To achieve the proper polarization, each of the three antenna elements are driven with an equal amplitude signal and a progressive 120 degree phase shift. A three way power divider is used to obtain the equal amplitude signals and the progressive 120 degree phase shift is obtained by proper length of the feed lines from the power divider to each of the three antenna elements. 
     Each antenna element includes a tuning screw which is used to fine tune the operating frequency of each of the antenna elements of the microstrip antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the partially shorted microstrip antenna comprising the present invention which includes the three identical antenna elements of the microstrip antenna; 
         FIG. 2  is an end view of the microstrip antenna of  FIG. 1 ; 
         FIG. 3  is a top view of one of three identical microstrip antenna elements including the radiating patch for one of the three identical microstrip antenna elements for the microstrip antenna of  FIG. 1 ; 
         FIGS. 4A and 4B  are side view illustrating the copper wire electrical shorts, i.e. copper vias which are provided along one edge of the radiating element to connect the radiating element to the ground plane of each the antenna elements of the microstrip antenna of  FIG. 1 ; 
         FIG. 5  is a side view illustrating the stringing technique to fabricate the copper electrical shorts/vias of  FIG. 4 ; 
         FIG. 6  is a bottom view of one of three identical microstrip antenna elements including the ground plane and tuning screw for one of the three identical microstrip antenna elements for the microstrip antenna of  FIG. 1 ; 
         FIG. 7  is a view illustrating the electric fields generated by the radiating element for each of the antennas elements of the microstrip antenna of  FIG. 1 ; and 
         FIGS. 8A and 8B  are antenna performance plots for the one of the antennas elements of the microstrip antenna of  FIG. 1 ; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , there is shown a perspective view of a microstrip antenna array  20  which includes three identical conformal antenna elements  22 ,  24  and  26  which are mounted on the outer surface of a missile  28 , shown in phantom in  FIG. 1 . Each of the three antenna elements  22 ,  24  and  26  are positioned every 120 degrees around the outer surface of missile  28  in the manner illustrated in  FIG. 1 .  FIG. 2  is an end view of the microstrip antenna  20  of  FIG. 1  illustrating the three identical antenna elements  22 ,  24  and  26  of  FIG. 1 . 
     The present invention which comprises antenna array  20  includes the three antenna elements  22 ,  24 , and  26 , shown in  FIGS. 1 and 2  is designed for use with the HARM missile. The HARM missile is a supersonic air-to-surface missile designed seek and destroy enemy radar-equipped air defense systems. The Navy and Marine Corps F/A-18 and EA-6B have the capability to employ the AGM-88 HARM (high-speed anti-radiation missile). The Harm missile operates in the P band. 
     Referring to  FIGS. 1 ,  2 ,  3 , and  4 A, each of the antenna elements  22 ,  24  and  26  includes a dielectric printed circuit board  30  fabricated from a plurality of high frequency laminates  32  and  34  (shown in  FIG. 4A ), part number RT/duroid 6002, commercially available from Rogers Corporation of Rogers, Connecticut. The dielectric laminates/layers  32  and  34  selected for each element antenna  20  has overall dimensions of 9.171 inches by 7.312 inches. The thickness of circuit board  30  is about 0.210 inches. RT/duroid 6002 is a microwave material with low loss and a low dielectric constant providing for excellent electrical and mechanical properties at microwave frequencies. It should be understood that the circuit board  30  can be fabricated from three or more layers of a dielectric laminate material such as RT/duroid 6002. 
     Each microstrip antenna element  22 ,  24  and  26  of antenna  20  also has an outer cover  36  which is an environment protection laminate fabricated from Rogers Corporation Duroid 5870 high frequency laminate. The thickness of the outer cover  36  is about 0.125 inches. 
     Each of the microstrip antenna elements  22 ,  24  and  26  of antenna  20  includes a generally rectangular shaped copper radiating element or patch  40  which has overall dimension of 8.176 inches in length and a width of 5.304 inches. The copper radiating patch  40  for each microstrip antenna element  22 ,  24  and  26  of antenna  20  is mounted on the upper surface of the circuit board  30  for each antenna element  22 ,  24  and  26 . Copper plating is used to fabricate the copper radiating patch  40 . 
     Each of the microstrip antenna elements  22 ,  24  and  26  of antenna  20  also includes a generally rectangular shaped copper ground plane  42 . The copper ground plane  42  for each element  22 ,  24  and  26  is mounted on the bottom surface of the circuit board  30  for each antenna element  22 ,  24  and  26 . 
     A plurality of copper wire electrical shorts  44  shown in  FIG. 3 , i.e. copper vias are provided lengthwise along one edge  46  of radiating element  40  to connect the radiating element  40  to the ground plane  42  of each antenna element  22 ,  24  and  26 . The copper wire electrical shorts/vias  44  are equally spaced apart and run from the midpoint of radiating element  40  to the one corner of the radiating element  40 . 
     As seen in  FIG. 3 , the radiating element  40  has sixteen vias  44 , with each via  44  being spaced apart wire center to center by 0.271 inches from an adjacent via. The placement of vias  44  along lower edge  46  of the radiating element  40  is from the midpoint of radiating element  40  to lower right corner of radiating element  40 . 
     As shown in  FIG. 3 , current flow in the radiating element is from the upper or opposite edge  48  and left side edge  50  of radiating element  40  to through the vias  44  to the ground plane  42 . A plurality of arrows  52  indicating the direction and pattern of current flow on the radiating element  40 . The electrical feed  51  for the radiating patch  40  each antenna element  22 ,  24  and  26  is located near the lower edge  46  of radiating patch  40  at the center of the radiating patch  40 . 
     Antenna  20  receives three equal amplitude RF electrical signals which are provided to the feeds  50  for the microstrip antenna elements  22 ,  24  and  26 . The RF electrical signals are obtained from a commercially available three way power divider(not illustrated). The power divider is electrically connected to each of the three antenna elements  22 ,  24  and  26  by electrical transmission lines. The electrical transmission lines, which are electrical cables having different lengths, are configured to provide for a 120 degree progressive phase shift. Thus, when the signal to antenna element  22  is 0 degrees, the signal to antenna element  24  will be 120 degrees and the signal to antenna element be 240 degrees. 
     Referring to  FIGS. 1 and 6 , there is shown a tuning screw  54  which is used to fine tune the operating frequency of each antenna elements  22 ,  24  and  26  of microstrip antenna  20 . The tuning screw  54  for each antenna element  22 ,  24  and  26  is located within the ground plane  42  in proximity to the corner  56  of ground plane  42  where edges  48 A and  50 A of ground plane  42  meet. A slot  58  is provided within the tuning screw  54 . The slot  58  within each antenna element  22 ,  24  and  26  allows a user to use a screw driver to fine tune the antenna element  22 ,  24  and  26  to the desired operating frequency. The use of tuning screw eliminates the tuning tabs within each antenna element  22 ,  24  and  26  which have also been used to fine tune antenna elements to a desired operating frequency. 
     Referring to  FIG. 7 , there is shown a general directional pattern for the electric field generated by each of the antenna elements  22 ,  24  and  26  of antenna  20 . This electric field is represented by electric field vectors  59  generated along edge  50  and electric field vectors  60  generated along edge  62 . 
     Referring to  FIGS. 3 ,  4 A and  4 B, there is shown a plurality of copper wire electrical shorts  44 , i.e. copper vias  44  which are provided along one edge of the radiating element and through the dielectric printed circuit board  30  (as shown in  FIGS. 4A and 4B ) to connect the radiating element  40  to the ground plane  42  (shown in  FIG. 6 ). The copper electrical shorts  44  are equally spaced apart and run from the midpoint of radiating element  40  to the one corner of the radiating element  40 . The unique placement and configuration of the vias  44  allows for a substantial increase in the width of the radiating element  40  and an increase in the bandwidth to ±400 KHz about the center frequency of 231 KHz. 
     As shown in  FIG. 5 , a single wire copper wire  64  is strung through a plurality of openings  66  in the dielectric printed circuit board  30  is used to fabricate the copper electrical shorts/vias  44  for connecting the radiating element  40  to the ground plane  42  of each of the antenna elements  22 ,  24  and  26 . The copper wire  64  is then pulled through the openings  66  until the copper wire  64  is flush and in contact with radiating element  40  and the ground plane  42  (as shown in  FIG. 4A ). Solder can then be used to secure the radiating element  40  and copper wire  64  to the ground plane  42 . 
     It should be noted that there are openings drilled into the radiating element  40  and the ground plane  42  which align with the openings  66  drilled into the dielectric printed circuit board  30 . 
     Utilizing the stringing technique illustrated in  FIGS. 4A ,  4 B and  5 , saves a considerable amount time and money in fabricating the vias  44  for each of the antenna elements  22 ,  24  and  26  of antenna  20 . Using the prior technique of fabricating each separately by placing a separate copper wire in each opening  66  in the dielectric printed circuit board  30  and then soldering the wire to the ground plane and the radiating element required several hours of intensive labor and substantially raised the fabrication cost of the antenna. 
     Referring to  FIGS. 8A and 8B , there is shown antenna performance plots for one of the antenna elements of the microstrip antenna  20  of  FIG. 1 . The antenna performance plots  70 ,  72  and  74  of  FIG. 8A  illustrate horizontal polarization for one antenna element  22 ,  24  or  26  of the microstrip antenna  20  of  FIG. 1 . The antenna element was mounted on a ten inch diameter tube which simulated a missile, and measurements were made looking perpendicular into the missile. The antenna performance plots  80 ,  82  and  84  of  FIG. 8A  illustrate horizontal polarization for one antenna element  22 ,  24  or  26  of the microstrip antenna  20  of  FIG. 1 . The plots of  FIGS. 8A and 8B  show that the microstrip antenna has excellent cross polarization performance. 
     From the foregoing, it is readily apparent that the present invention comprises a new, unique, and exceedingly useful microstrip antenna adapted for use on projectiles such as the harm missile, which constitutes a considerable improvement over the known prior art. Many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.