Abstract:
Paraboloidal antennas are common for very high frequencies (VHF) and into the cellular telephone systems and personal communication systems (PCS). Paraboloidal antennas are often used at the base station of either cellular telephone antennas, PCS antennas or both. To avoid possible channel drop out because a sharp focal point of the antenna is misaligned by improper installation or harsh weather conditions. For base stations for cellular telephone systems and/or systems, PCS, a generally paraboloidal antenna that has a less sharp focal point so there is a antenna lower gain, but less relative signal degradation because of weather or other misalignment of the antenna. In such cases, the lower gain, but higher immunity to drop-out more than justifies such arrangements.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to antennas and, more particularly, to reflecting antennas with concave reflectors. 
     The use of paraboloidal antennas for microwave transmission and reception is well known. Paraboloidal antennas are used because of directional attributes and high gains that occur at the focal point of the parabola-of-revolution. Omni-directional electromagnetic energy emitted at the focal point of a paraboloidal antenna will be reflected as collimated radiation. Similarly, electromagnetic energy traveling on an axis parallel to the axis of a paraboloidal antenna, such as a far field omni-directional or laser/maser source, impinging upon a paraboloidal antenna will be reflected to the focal point. The incoming electromagnetic energy is focused to a very compact focal point. 
     The general equation for a paraboloid is: z 2 /a 2 +y 2 /b 2 =x. A representation of such a paraboloid is shown in FIG.  1 . Considering the plane where z=0 then y 2 /b 2 =x or y 2 =b 2 x and for such an equation the focus of the parabola in the plane where z=0 equals b/2. This focal point is the same distance for any of the planes containing the x-axis. The x-axis is the axis of symmetry. 
     The concentration of the received energy at the focal point is a good way of achieving high gains. The high gain region is located tightly around the focal point of the paraboloidal antenna. The tightness of that focal point also has some disadvantages. An installation with the axis of symmetry of the paraboloidal antenna not parallel to the incoming signal will cause a sharp signal drop-off if the angle between the axis of symmetry and the incoming signal increases. Similarly, high wind or icy weather can affect the effective gain of a paraboloidal antenna by deflecting the axis of symmetry from the direction of an incoming signal. Electromagnetic energy coming in to a paraboloidal antenna at an angle to the axis can be received just fine, or it can be just barely received depending upon the size of the angle. At approximately 15° from the axis the gain drops from substantially similar to the gain at the focal point, to substantially zero. Such sharp differences in reception over such a relatively small angle is a problem for which antenna designers and antenna installers must allow. Considering that steel structures sway (some of the tallest buildings sway as much as 10 inches) in high winds, such sway alone could rule out use of a parabolic antenna on top of such structures. 
     SUMMARY OF THE INVENTION 
     The above problems are solved, and a number of technical advances are achieved in the art, by a concave antenna that is substantially paraboloidal but has a larger focal point so that the gain of the antenna does not drop so sharply with respect to the angle the incoming wave front makes with the axis of the antenna. 
     In accordance with an embodiment of the invention, a concave antenna having an axis along which at least two focal points are located is provided. Each of the focal points corresponds to a portion of a respective parabolic antenna having an axis along the concave antenna axis and a respective focal point along the concave antenna axis. Each respective axis is skewed with respect to the other axes. 
     In accordance with another embodiment of the invention, a concave antenna having at least two axes along which at least two focal points are located. Each axis is not co-linear with any of the other axes. Each of the focal points corresponds to a portion of a respective parabolic antenna having a respective axis and a respective focal point along the respective axis. Each respective axis intersects with respect to one of the other axes. 
     In accordance with another embodiment of the invention, a concave antenna having at least two axes along which at least two focal points are located. Each axis is not co-linear with any of the other axes. Each of the focal points corresponds to a portion of a respective parabolic antenna having a respective axis and a respective focal point along the respective axis. Each respective axis is parallel with respect to one other axis. 
     In accordance with another aspect of the invention, a concave antenna having at least two axes along which at least two focal points arc located. Each axis is not co-linear with any of the other axes. Each of the focal points corresponds to a portion of a respective parabolic antenna having a respective axis and a respective focal point along the respective axis. Each respective axis is parallel with respect to one of the other axes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing advantageous features of the invention will be described in detail and other advantageous features will be made apparent upon reading the following detailed description that is given with reference to the several figures of the drawings, in which: 
     FIG. 1 shows a perspective view of a concave antenna that is a standard parabolioidal antenna, with an axis of symmetry and a collector located at a focal point thereof. 
     FIG. 2 shows a perspective view of a concave antenna having axial symmetry with a first portion having one focal point along the axis and a second portion having a second focal point along the axis. 
     FIG. 3 shows a perspective view of a concave antenna having axial symmetry with a first portion having one focal point along a first axis and a second portion having a second focal point along a second axis parallel to the first axis. 
     FIG. 4 shows a perspective view of a concave antenna that has four portions each portion being held in a spaced relationship to its closest adjacent portions by non-reflective spacers. 
     FIG. 5 shows a perspective view of a concave support structure supporting a plurality of small paraboloidal reflectors. 
     FIG. 6 shows a perspective view of a support structure supporting a plurality of small paraboloidal reflectors. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a known example of a paraboidal reflector antenna  1  in a perspective view. The antenna  1  has a reflector  10  that is a parabola which is rotated circularly around the x-axis forming a shape of a 3-dimensional paraboloid. The x-axis is an axis of symmetry  14 . Such a reflector  10  has a focal point  12  located along the x-axis. The focal point is where incoming electromagnetic, EM, radiation along the x-axis that is from a relatively far away source, far enough away so that the light waves are all in parallel to each other, is reflected to the focal point  12 . The antenna  10  has supports  16  and  18  which arc made to be small to reduce any shadow effect each will have with respect to incoming EM radiation support member  20  extends along the axis of symmetry  14  from the supports  16 ,  18 . At the support member  20  a small collector  22 , which is located at the focal point to pickup the signal reflected to the focal point  12 . The support  20  and the collector  22  arc also kept as small as practical in order to minimize their shadow effects have on the overall EM radiation that is collected. The antenna  1  is very efficient at collecting and concentrating EM radiation and/or signal directed to it. As mentioned above in the background, the antenna  1  has difficulty with signals that arc not parallel to the axis  14 . Indeed, if the EM signal source is over 15 degree off of the axis, a substantial drop in signal strength occurs. Likewise if, because a wind or other environmental problem, the collector  22  strays too far from the axis of symmetry, there would be a substantial drop in the collected signal strength. 
     Referring now to FIG. 2, one embodiment of the invention, reflector antenna  200 , is shown in a perspective view. The reflector is made up of paraboloidal portion  210  and paraboidal portion  211 . These two portions  210 ,  211  may be sections of a single paraboloid or sections of two paraboloids. Either way, each of the portions  210 ,  211  has a respective focal point  212 ,  213  located along the axis of symmetry  214 . The two paraboloidal portions  210 ,  211  are joined by ring  215  which may be of a cylindrical shape or a truncated conical shape. The width and extent of ring  215  depends on the differences of the two portions  210 ,  211  and the desired differences in focal points  212 ,  213 . When each of the portions is part of a single, larger paraboloidal reflector, as in FIG. 2, the ring  215  is approximately one wavelength of the reflected signal in length. If the reflected signal contains a band of frequencies, the ring  215  is set at one wavelength of the center frequency of the frequency band. 
     At the front of reflector portion  211  are supports  216  and  218 . Connected to the supports  216  and  218  is a support  220 . At a second end of support  220 , a signal collector  222  is connected. This signal collector  222  is of sufficient size to collect signals reflected to focal point  212  and focal point  213 . The collected signal is carried by a conductor (not shown), which either runs through the support  220  or along side of support  220 . Once the conductor gets to support  216  or  218 , it either runs through one support  216  or  218 , or along side one of the supports  216 ,  218 . With a collector  222  collecting at two focal points, the collected signal will be approximately the same as the reflector antenna  1  shown in FIG. 1, except the performance of the antenna  200  will provide less of a drop-off in signal power collected as the signal source moves away from the axis of symmetry  214 . 
     Referring now to FIG. 3, another embodiment of the invention is shown in a perspective view. The reflector antenna  300  is generally a paraboloid in shape, but the paraboloid is bifurcated near the x-y plane. This plane was taken for ease of explanation, but any plane containing a line segment of the x-axis would have similar effects, only the focal points would have different locations. The reflector  300  is divided into two portions  310 ,  311 . The two portions  310  and  311  are then held in a spaced relationship by a spacer  315 . Each of the portions  310  and  311  has a respective focal point  312 ,  313 . These focal points  312  and  313  are similarly maintained in a spaced relationship to each other by spacer  315 . If the reflector antenna  300  is cut perfectly in half, each of the focal points  312 ,  313  will receive half of a far field reflected signal. 
     The reflector antenna  300  has supports  316 ,  318  to which is connected support  320 . Support  320  is connected to a collector  322 , which is sized sufficiently to collect signals reflected to focal points  312  and  313  by their respective portions  310 ,  311 . Supports  316 ,  318  are sized have minimum shadow zones so as not to unnecessarily reduce the gain of the antenna  300 . Supports  316  and  318  may be moved anywhere, such as to the front of the spacer  315 . or to the rear of the spacer  315  (not shown in FIG.  3 ). If the supports  316  and  318  are at the rear, then the support  320  would extend from the rear to support the collector  322 . 
     Bifurcating the antenna  300  into two portions  310 ,  311  held apart by the spacer  315  makes the antenna  300  have a broader sensitivity beam pattern in the vertical plane so any drop off from misalignment or weather related changes in the vertical plane will be less than a non-bifurcated antenna. If the cut were made along the z-axis (not shown) and a similar spacer installed, those of average skill in the antenna art will recognize that then everything in FIG. 3 will be rotated 90 degrees and the broadened beam pattern will be horizontal, instead of vertical. Such a mounting would be advantageous in high surface wind regions where antennas like this tend to oscillate in the horizontal plane. 
     Referring now to FIG. 4, another embodiment of the invention is shown in a perspective view. The reflector antenna  400  shown in FIG. 4 is somewhat of a combination of the antennas shown in FIGS. 2 and 3, as will be described. Reflector antenna  400  is cut into four portions, though any number of sections would work, four makes a good example because of the symmetry with the previous figures. The four portions  404 ,  406 ,  408 ,  410  in this example are equal in size to each other, that is each is a quarter longitudinal portion of a paraboloid. Having them equal makes the description simpler, but one of average skill in this art should be able to expand this example to a more general, less symmetrical portions. The portions  404 ,  406 ,  408  and  410  are held in a spaced relationship with each other by spacer  415 . Spacer  415  is approximately two parabolic strips, each being similar to spacer  315  in FIG. 3, but the two parabolic strips are at 90 degrees from each other and cross at the rear of the antenna  400 . The crossing at the back of the spacer  415  is not completely simple because portions  404  and  408  are advanced in the x-direction by a fraction of a wavelength. Thus, by its geometry, antenna  400  has four separate focal points. Portion  404  has focal point  412 B, portion  406  has focal point  412 A, portion  408  has focal point  413 B and portion  410  has focal point  413 A. 
     Support members  416  and  418  are connected to the front of the antenna  400  and also to support  420 . Support  420  i s connected to collector  422 , which is sufficiently sized to collect signals at focal points  412 A,  412 B,  413 A and  413 B. With four focal points, the antenna  400  will have a sensitivity beamwidth that is broader than either antenna  200  or antenna  300 . The overall gain at the center of the sensitivity beam will be slightly less, but the signal drop off rate because of misalignment by weather or installation will be at a slower rate. 
     Referring now to FIG. 5, another embodiment of the invention is shown. In FIG. 5, an inside surface  510  of a concave antenna  500  is used for supporting a plurality of paraboloidal reflectors  540 . These reflectors may be formed separately and then fastened to the inside surface  510 , or the inside surface  510  and the subsurface below may have the paraboloidal reflectors  540  formed therein. The parabolodial reflectors  540  may be individually oriented to make as sharp or as large a focal point  512  as desired. At the back of th is antenna  500 , a support  517  is connected thereto. At the other end of support  517  is a collector  522  which is sufficiently sized to collect all the signals reflected by the paraboloidal reflectors  540 . As described above, in some conditions a larger focal point is more advantageous for an antenna that maximum gain. 
     Referring now to FIG. 6, an antenna  600  is formed from a plane  610  having a sufficient depth to provide support for paraboloidal reflectors  640 . Since plane  610  is flat, it is necessary to orient each of the paraboloidal reflectors  640  in a different direction in order to form the focal point  612 . As with FIG. 5 above, the paraboloidal reflectors  640  may be made separately and then fastened to plane  610 , or they may be formed in surface  610  and the depth of the support material below the surface  610 . Each of the paraboloidal reflectors  640  is focused to the focal point  612 , which may be as sharp or as broad as necessary. A support  620  is connected to the plane  610  at one end and at the other it is connected to a collector  622 . Collector  622  is only as large as it needs to be to collect the signals reflected by the paraboloidal reflectors  640 . This embodiment of the invention can take many forms depending on the ability to form or etch the reflectors  640 . 
     While the specification in this invention is described in relation to certain implementations or embodiments, many details are set forth for the purpose of illustration. Thus, the foregoing merely illustrates the principles of the invention. For example, this invention may have other specific forms without departing from its spirit or essential characteristics. The described arrangements are illustrative and not restrictive. To those skilled in the art, the invention is susceptible to additional implementations or embodiments and certain of the details described in this application can be varied considerably without departing from the basic principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention are thus within its spirit and scope.