Abstract:
An antenna exhibiting improved transmission and reception capabilities. The antenna does not make use of a solid support tube or solid support rods used by previous antennas to support a sub-reflector or other device above a main reflector of the antenna. Instead, the antenna employs the use of a low dielectric constant, perforated, support tube to support the sub-reflector, patch antenna, or other form of antenna element above the main reflector. The perforated support tube permits radio frequency signals to pass through the tube, thus decreasing signal degradation experienced due to reflection of the signal off the solid support tube or off the solid support rods.

Description:
FIELD OF THE INVENTION 
   The present invention relates to antennas. More specifically, the invention relates to a method and apparatus for providing an antenna exhibiting improved signal reception and transmission due to reduced levels of signal reflection ices and dielectric loss. 
   BACKGROUND OF THE INVENTION 
   Electromagnetic wave antennas, and radio frequency (RF) antennas in particular, are widely used to transmit and receive energy in the form of radio waves. RF antennas are available in many different shapes, sizes and configurations. One type of RF antenna is the Cassegrain antenna. Cassegrain antennas have a hyperbolic shaped sub-reflector. The sub-reflector is coaxially aligned with and aimed at an axial center of a main parabolic reflector. The sub-reflector Is suspended above the main reflector by either a solid support tube extending from a point near the center of the main reflector, one or more support rods extending from a point near the center of the reflector, or one or more support rods extending from a periphery of the main reflector. When the antenna is in the receive mode the sub-reflector directs RF energy received and reflected by the main reflector to a waveguide (i.e., feedhorn) located at the axial center of the main reflector. When the antenna is in the transmit mode, RF energy transmitted from the waveguide is reflected by the sub-reflector onto the main reflector where the energy is radisted from the antenna. 
   While the above described Cassegrain antenna is able to adequately send and receive radio signals, it would be desirable to improve its operating efficiency. Specifically, Cassegrain antennas and all other types of antennas which employ the use of a device suspended above a main reflector, such as a horn antenna, patch antenna, etc., suffer transmission losses due to the RF signal being blocked and reflected by the device support members. Such support members are usually in the form of solid support tubes or support rods that exhibit large dielectric constants. Consequently, there is a need for an improved antenna exhibiting reduced levels of reflection loss and dielectric loss, resulting in enhanced RF signal transmission and reception. 
   SUMMARY OF THE INVENTION 
   The present Invention overcomes prior art deficiencies by providing an antenna exhibiting improved transmission and reception capabilities. Unlike previous antennas, the antenna of the present invention does not make use of a solid support tube or solid support rods to support a sub-reflector or other feed device above a main reflector of the antenna. Instead, the present invention provides an antenna having a sub-reflector or other feed device positioned above a main reflector by a perforated support device (dielectric), or support tube, having walls with a low dielectric constant. The perforated support tube permits RF signals to pass through the tube, thus decreasing the signal degradation which would be experienced due to reflection of the signal off the walls of a solid support tube or solid support rods. The perforations may be in the form of holes, slots, or numerous other arrangements. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a side view of an antenna in accordance with a first preferred embodiment of the present invention; 
       FIG. 2   a  is a perspective view of the perforated support tube of the antenna of  FIG. 1 ; 
       FIG. 2   b  is a side view of an alternative preferred form of the support tube; 
       FIG. 2   c  is a side view of another alternative preferred form of the support tube; 
       FIG. 3  is a perspective view of the attachment ring of the antenna of  FIG. 1 ; 
       FIG. 4  is a perspective view of the support tube cap of the antenna of  FIG. 1 ; 
       FIG. 5  is a perspective view of the sub-reflector of the antenna of  FIG. 1 ; 
       FIG. 6  is a partial side view of an antenna in accordance with a second preferred embodiment of the present invention with a broken away section of the support tube to better show the patch antenna assembly; 
       FIG. 7  is a perspective view of the patch assembly of the antenna of  FIG. 6 ; 
       FIG. 8  is a side view of the patch assembly of the antenna of  FIG. 6 ; and 
       FIG. 9  is a top view of the patch assembly of the antenna of FIG.  6 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
   As seen in  FIG. 1 , an antenna  10  in accordance with a first preferred embodiment of the present invention is shown. The antenna  10  contains a hyperbolic sub-reflector  12  and a parabolic main reflector  14 . The main reflector  14  has a first surface  16  and a second surface  18 . The sub-reflector  12  is mounted to the first surface  16  by a perforated plastic support tube  20 . Electromagnetic wave signals, such as RF signals, received by the first surface  16  are reflected by the sub-reflector  12  to a waveguide in the form of a feedhorn  21 . Electromagnetic wave signals, such as RF signals, transmitted through the feedhorn  21  are reflected by the sub-reflector  12  to the first surface  16  and radiate from the first surface  16  into space. RF signals received by the antenna  10  are carried from the antenna  10  through a suitable conducting device, such as a coaxial cable (not shown). The conducting device may also carry RF signals to antenna  10  to be transmitted by antenna  10 . The conducting device is connected to the antenna  10  by way of a TNC connector  22  disposed on the second surface  18  of antenna  10 . 
   With reference to  FIG. 2 , the perforated plastic support tube  20  will now be described in detail. The perforated tube  20  is comprised of a top portion  23 , a bottom portion  24 , and a mid-portion  26 . The bottom portion  24  contains a series of small holes  28  capable of receiving suitable fastening devices, such as threaded fastening devices or rivets. The top portion  23  similarly contains a first series of small holes  30  and a second series of small holes  32 , both capable of receiving suitable fastening devices, such as the fasteners or rivets described above. Mid-portion  26  contains a plurality of apertures  34 , the apertures  34  being of any suitable size or configuration so as to allow the passage of RF signals easily through the tube  20 . The apertures  34  may be in the form of circular holes as illustrated in  FIG. 2   a . An alternative form of the support tube  20 ′ is shown in  FIG. 2B  wherein the circular holes are replaced by radial slot openings  34 ′. Still another preferred form of the support tube  20 ″ is shown in  FIG. 2C  wherein the circular holes are replaced by longitudinal slot openings  34 ″. In one preferred form the support tube  20  is formed from a suitably strong plastic, although it will be appreciated that other materials such as, but not limited to, steel or aluminum may also be used. A perforated steel or aluminum support tube could function as a frequency selective surface (FSS). 
   The perforated tube  20  is affixed to the first surface  16  of the main reflector  14  by way of an attachment ring  36  shown in FIG.  3 . The attachment ring  36  is a circular ring comprised of a base portion  38  and an annular rim  40 . Formed within the base portion  38  is a plurality of small holes  42  capable of receiving suitable fastening devices such as threaded screws. Similar small holes  44  capable of receiving fastening devices, such as threaded screws, are formed in the annular rim  40 . 
   The small holes  42  of the base portion  38  cooperate with similar holes (not shown) circumscribing the focal point of the first surface  16  of the main reflector  14 . Suitable fastening devices are inserted through small holes  42  and the holes (not shown) of the first surface  16  to secure the base portion  38  to the first surface  16 . The base portion  38  serves as a support to secure the perforated support tube  20  to the main reflector  14 . Specifically, the perforated support tube  20  is secured to the attachment ring  36  through cooperation of small holes  44  of the annular rim  40  and small holes  28  of the support tube  20 . Small holes  28  and small holes  44  are secured to each other by a suitable fastening device such as screws that are inserted through aligned pairs of small holes  28  and  44 . 
   The top portion  23  of the perforated support tube  20  is covered by a support tube end cap  46  as shown in FIG.  4 . The cap  46  is comprised of a flat surface portion  48  and a rim portion  50 . The rim portion  50  contains a plurality of small holes  52  for receiving suitable fastening devices such as threaded fasteners or rivets. The small holes  52  are aligned with the first series of small holes  30  and end cap  46  is secured to the support tube  20  by fastening devices extending through the aligned pairs of small holes  30  and  52 . 
   Referring now to  FIG. 5 , the sub-reflector  12  is shown in detail. The sub-reflector  12  contains a cone portion  54  and a circular peripheral base portion  56 . The peripheral base portion  56  contains a series of small holes  58  that cooperate with the second series of small holes  32 . Suitable fastening elements are inserted through aligned pairs of small holes  58  and small holes  32  to secure the sub-reflector  12  to the perforated support tube  20 . 
   As seen in  FIG. 6 , an antenna  10   a  in accordance with a second preferred embodiment of the present invention is shown. Antenna  10   a , like antenna  10  of the first preferred embodiment, is comprised of a parabolic main reflector  14   a  having a first surface  16   a  and a second surface  18   a . Mounted to the first surface  16   a , by way of an attachment ring  36   a , is a perforated plastic support tube  20   a  having an end cap  46   a . Mounted to the second surface  18   a  is a TNC connector  22   a . As these components of antenna  10   a  are identical to those of antenna  10 , there is no need to describe them again in detail with reference to antenna  10   a.    
   In addition to the antenna elements described above, antenna  10   a  has a patch antenna assembly  60 . The patch antenna assembly  60  is illustrated in detail in  FIGS. 7 ,  8 , and  9 . The patch antenna assembly  60  is generally comprised of a patch antenna  62  and a patch attachment ring  64 . The patch antenna assembly  60  is mounted to the first surface  16   a  by the perforated plastic support tube  20   a.    
   The patch antenna  62  is comprised of a dielectric substrate  66 , a patch element  68  and a ground plane  70 . Both the patch element  68  and the ground plane  70  are preferably made of copper. The copper patch element  68  covers a first end  72  of the dielectric substrate  66 , except for an outer periphery of the first end  72 . At the center of the patch element  68  is hole  74  which is used to receive a suitable conducting device such as coaxial cable  76 . A corresponding hole (not shown) is located in dielectric substrate  66 . 
   The ground plane  70  completely covers and is bonded to a second end  78  of the dielectric substrate  66 . The ground plane  70  is preferably made of copper and includes a hole (not shown) aligned with hole  74  of the patch element  68  and the hole (not shown) of the dielectric substrate  66 . The surface of the ground plane not bonded to the dielectric substrate  66  is bonded to the patch attachment ring  64 . 
   The patch attachment ring  64  is preferably made of metal. The patch attachment ring  64  is comprised of a ring portion  80  and a surface portion  82 . The ring portion  80  contains a plurality of small holes  84 . The plurality of small holes  84  are aligned with the second series of small holes  32   a  of the support tube  20   a  and both are capable of receiving suitable fastening devices, such as fasteners or rivets, to secure the patch antenna assembly  60  to the support tube  20   a.    
   The surface portion  82  of the patch attachment ring  64  contains cross members  86  and  88 . At the intersect point of cross members  86  and  88  is a hole  90 . Hole  90  is sized to receive coax cable  76  and is aligned with hole  74 , the hole of the dielectric substrate  66 , and the hole of ground plane  70 . Either cross member  86  or cross member  88  also has a connector  92  for receiving the coax cable  76 . 
   RF signals received by the main reflector  14   a  of antenna  10   a  are directed from the main reflector  14   a  to the patch antenna  62 . From the patch antenna  62  the RF signals are conducted through the coaxial cable  76  to a TNC connector  94  disposed at the axial center of the first surface  16   a  of the main reflector  10   a . From connector  94  the signals are conducted from the antenna by way of a suitable conductive device, such as a coaxial cable (not shown), that is attached to connector  22   a . Likewise, RF signals to be transmitted by antenna  10   a  are received by the antenna  10   a  through connector  22   a  and are carried to the patch antenna  62  by way of the coaxial cable  76 . The RF signals to be transmitted radiate from the patch antenna  62  where they are reflected by the first surface  16   a  of the main reflector  14   a  into space. It must be noted that antenna  10   a  does not require the use of a feedhorn as antenna  10  does. 
   While  FIGS. 1 ,  2 , and  6  illustrate the second series of small holes  32  being used to support the sub-reflector  12  and the patch assembly  60 , it should be understood that small holes  32  may be configured to support a variety of antenna-related elements called for in a variety of different antennas. It will also be appreciated that other forms of fastening systems, including adhesives, could be used in place of the threaded fastening elements and rivets described herein. 
   The use of perforated tube  20  to support the sub-reflector  12 , patch assembly  60 , or any other device enhances the signal strength of the signal received or transmitted by the antenna  10 . Traditionally, the sub-reflector  12 , patch assembly  60 , or other device has been suspended above the main reflector  14  by a solid support tube or solid support rods. However, such a configuration is undesirable because the RF energy radiated or transmitted from the antenna reflects off the solid support tube or solid support rods due to the high dielectric constant exhibited by such supports. As a result of this high dielectric constant, the signal strength of the RF signal received by, or transmitted from, the antenna is degraded. 
   In contrast to the prior art antennas, perforated support tube  20  exhibits a decreased dielectric constant as the apertures  34  allow RF signals to pass though the support tube  20  with the signals being reflected less frequently. Because the RF signals are reflected less frequently, antenna  10  is more efficient and is able to receive and transmit RF energy with less signal degradation. 
   Thus, an improved antenna exhibiting a perforated support tube with a decreased wall dielectric constant and, consequently, decreased levels of signal degradation due to signal reflection is provided. The decrease in signal degradation is due to the presence of the perforated support tube  20  to support the sub-reflector  12 , patch assembly  60 , or any other desired device above the main reflector  14 . The use of perforated support tube  20  provides an antenna  10  which exhibits a dielectric constant that is significantly lower than prior art antennas. Consequently, RF signal reflection loss is reduced by the perforated support tube and the RF signals received or transmitted are of a greater strength and quality than the signals of prior art antennas. The principles of the present invention are applicable to all support tubes (dielectric) with perforated holes or slots in the wall of the tube to lower the effective dielectric constant. 
   The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.