Abstract:
A directional antenna designed to reduce the occurrence of side lobes, thus reducing the possibility of interference with other radio frequencies is disclosed. The directional antenna includes an antenna member and a reflecting tube. The reflective tube is sleeved over the antenna member. The reflective serves to block unwanted radial side lobes. The directional antenna can also include provisions that assist in suspending the antenna member within the reflective tube. A method for making the directional antenna is also described.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an antenna, and more particularly, to a directional antenna. 
     2. Background of the Invention 
     An antenna is the heart of a wireless communications system. Antennas in transmitters convert electrical signals into airborne radio frequency (RF) waves, and in receivers they convert airborne waves into electrical signals. Without antennas there are no wireless communications. 
     The size of an antenna depends on the radio frequency for which the antenna is designed. The higher the frequency, the smaller the antenna. Therefore, wireless telephones use small antennas to communicate at high frequencies. Because there is a finite range of high frequencies that is allocated for wireless communications, a wireless service provider must reuse some or all of its allocated frequencies to increase call handling capacity, i.e., to enable more customers to use their wireless telephones at the same time in the same service area. 
     To reuse frequencies, a wireless service provider divides its service area into “cells,” and it equips each of the cells with a low-powered antenna system. Antenna systems in two non-adjacent cells may use the same frequency. Cells generally fall into three categories: “macrocells,” “microcells,” and “picocells.” A macrocell covers a relatively large area (e.g., about 50-mile radius), and it is optimized to serve users who are highly mobile such as those in automobiles. A microcell covers a smaller area (e.g., about 10-mile radius), and it is optimized for wireless device users who are less mobile such as pedestrians. A picocell covers an even smaller area (e.g., a tunnel or a parking garage). The antenna system for a picocell requires extremely low output power but it can direct cellular signal into an isolated spot such as a low-lying, tree-covered road intersection. 
     An antenna system at each picocell typically has a donor antenna, a signal-processing device such as an amplifier (for analog signals) or a repeater (for digital signals), and a coverage antenna. These three components are serially connected by coaxial cables. The components are typically mounted on a utility pole that is about 40 to 50 feet tall. The donor antenna receives downlink signals from a macrocell site (also known as the donor cell site) and channels the downlink signals to the signal-processing device. The signal-processing device either amplifies or repeats the downlink signals before the coverage antenna broadcasts the downlink signals to the vicinity of the picocell. Similarly, the coverage antenna receives uplink signals from the vicinity of the picocell and the donor antenna re-transmits the uplink signals to the macrocell site after the amplifier or the repeater has processed the uplink signals. The donor antenna is typically a directional antenna that has a clear line of sight to the donor cell site. On the other hand, the coverage antenna is typically an omnidirectional antenna that has a 360-degree “view” of the picocell. To maximize signal reception and coverage, both antennas must be mounted as high as possible. 
     Each of the donor and coverage antennas has its own RF patterns that are often known as side lobes. The side lobes of the donor antenna often overlap with the side lobes of the coverage antenna, resulting in a signal looping effect. As a result, the signal-processing device is often saturated by signals looping between the two antennas. The saturation situation causes the antenna system to shut down. 
     One solution to reduce the looping effect is to separate the donor antenna from the coverage antenna as far as possible. However, the existing antenna technology still does not offer a satisfactory solution to the looping effect due to the following constraints. First, the antennas cannot be separated more than twenty feet apart on a utility pole that is about 40 to 50 feet high. Second, existing antennas are bulky and heavy, making them difficult to mount at higher locations. Third, existing antennas have large cross-sections that are not desirable at higher altitudes due to wind loading. Fourth, extending the height of the utility pole is not desirable due to cost, environmental, and aesthetic concerns. 
     SUMMARY OF THE INVENTION 
     The present invention is a highly directional antenna. The antenna of the present invention reduces side lobes and thereby minimizing signal looping effect with an adjacent antenna such as a coverage antenna in an antenna system. The antenna of the present invention has an antenna element enclosed in a reflective tube, the interior of which is lined with a reflective material that shields radio frequencies. 
     The reflective tube is generally tubular in shape. The cross-section of the reflective tube may be circular, oval or polygonal. The reflective tube encloses or surrounds the antenna element. In the preferred embodiment, the reflective tube is generally made of a lightweight material, and the reflective material is a layer of metallic paint. In one preferred embodiment, the antenna of the present invention is used as a donor antenna, and it is mounted on a utility pole as part of an antenna system that also comprises a coverage antenna. In another preferred embodiment, the antenna of the invention is used as a donor antenna mounted on a first utility pole, while a coverage antenna is mounted on a second utility pole. 
     It is an object of the invention to provide an antenna that is highly directional. 
     It is another object of the invention to provide a directional antenna with little or no side lobe overlaps with another antenna. 
     It is another object of the invention to provide an antenna that is lightweight. 
     It is another object of the invention to an antenna that has a small wind loading cross section. 
     It is another object of the invention to provide an antenna system that is aesthetic looking and environmentally friendly. 
     It is another object of the invention to mount an antenna system comprising a donor antenna and a coverage antenna on one utility pole without the undesirable signal looping effect. 
     These and other objects of the present invention are described in greater detail in the detailed description of the invention, the appended drawings, and the attached claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an isometric view of a preferred embodiment of the invention. 
     FIG. 2 is a schematic diagram of a cut away view of the preferred embodiment of the invention. 
     FIG. 3 is a schematic diagram of an exploded view of the preferred embodiment of the invention. 
     FIG. 4 is a schematic diagram of an enlarged side view of antenna  300  that is shown in FIG.  3 . 
     FIG. 5 is a schematic diagram of one embodiment of a spacing member. 
     FIG. 6 is a schematic diagram of another embodiment of a spacing member. 
     FIG. 7 is a schematic diagram of an elevation view of the spacing member shown in FIG.  6 . 
     FIG. 8 is a schematic diagram of a prior art antenna without a reflecting tube and the antenna lope shapes produced by the antenna. 
     FIG. 9 is a schematic diagram of an antenna constructed according to the invention and the antenna lope shapes produced by the antenna. 
     FIG. 10 is a flowchart illustrating the steps involved in making reflective tube  102  that has a metallic mesh as reflective material  200 . 
     FIG. 11 is a schematic diagram showing one embodiment of using the invention with a transmission tower. 
     FIG. 12 is a schematic diagram showing a second embodiment of using the invention with multiple transmission towers. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram of an isometric view of a preferred embodiment of the invention. Directional antenna  100  includes a reflective tube  102  and an adapter  104  that is designed to mate with a mast  106 . In one embodiment, adapter  104  preferably includes a curved portion  108  that substantially corresponds to the curve of reflective tube  102 , and a mating portion  110  that is designed to mate with mast  106 . Adapter  104  can be attached to reflective tube  102  by a series of bands  112 . Bands  112  are preferably made of a corrosion resistant material, for example, stainless steel. In another embodiment, adapter  104  and reflective tube  102  are formed as a single, monolithic unit. In other embodiments not shown in the drawings, reflective tube  102  may be any geometrical shape other than the cylindrical shape shown. For example, reflective tube  102  may be a block or an ellipsoid that is substantially tubular with a cross-section of a polygon and an oval, respectively. 
     Preferably, the antenna is sized such that it is large enough to provide reception and transmission, but small enough to reduce wind loading area. Based on these competing considerations, the antenna can be sized accordingly. In an exemplary embodiment of the invention, the antenna has a length of about 33 inches and a radius of about five inches. 
     FIG. 2 is a schematic diagram of a cut away view of reflective tube  102 . A reflective material  200  is preferably disposed on the inside of reflective tube  102 . The reflective material  200  is any material that can block or inhibit any wave or signal on the electromagnetic spectrum. Many materials can be used as the reflective material  200 . Preferably, reflective material  200  is selected so that radio frequencies (RF) are blocked or inhibited. A material that is easy to place inside reflective tube  102  is also preferred. In exemplary embodiments of the present invention, a copper mesh, an aluminum tape, and/or a metallic coating are used as reflective material  200 . The metallic coating is preferably a metallic marine paint, for example, a copper paint. Reflective tube  102 , a housing upon which reflective material  200  is disposed, may be made of any materials. In the preferred embodiment, reflective tube  102  is made of a fiberglass compound. 
     FIG. 2 also shows a weep hole  202 . This hole assists in removing any moisture or water, for example, rain, snow or condensation, that may accumulate inside reflective tube  102 . Weep hole  202  can be disposed in the tube, as shown in FIG. 2, or weep hole  202  can be disposed on end caps  302   a  and  302   b  (see FIG.  3 ). Weep hole  202  can be disposed in any desired location in reflective tube  102 . Preferably, two weep holes  202  are disposed at opposite ends of reflective tube  102 . Or if the reflective tube  102  is mounted in an angled, tilted or vertical position, weep hole  202  is preferably located at a lower portion of reflective tube  102  where moisture would tend to accumulate. 
     FIG. 3 is a schematic diagram of an exploded view of a preferred embodiment of the invention. Reflective tube  102  is designed to surround or enclose antenna  300 . Reflective tube  102  is substantially continuous and it extends along antenna  300  longitudinally. Forward end cap  302   a  and rear end cap  302   b  are attached to opposite ends of reflective tube  102 . End caps  302   a  and  302   b  preferably include provisions to hold antenna  300 . Preferably a female member  304   a  is used to mate with male end portion  306   a  of antenna  300 , and a female member  304   b  is used to mate with male end portion  306   b  of antenna  300 . Female member  304   a  is preferably a hole disposed in forward end cap  302   a , and female member  304   b  is preferably a hole disposed in rear end cap  302   b . After assembly, end caps  302   a  and  302   b  assist in suspending antenna  300  within reflective tube  102  and preventing antenna  300  from contacting reflective tube  102 . Forward end cap  302   a  has an interior side  303   a , and rear end cap  302   b  has an interior side  303   b . In another preferred embodiment, interior side  303   b  may be coated with reflective material  200 . Interior side  303   a  is not coated. 
     FIG. 4 is a schematic diagram of an enlarged side view of antenna  300 . Antenna  300  preferably comprises a backbone  330  with end portions  306   a  and  306   b . Antenna  300  also includes elements  332 . Preferably, antenna  300  includes more than one element. In an exemplary embodiment of the present invention, seven elements are used and the elements increase in size from one end to the other end. In between elements  332  are gaps  334 . 
     For convenient reference, cylindrical coordinate names are used to describe the geometry of antenna  300 . The long axis of backbone  332  is referred to as the axis  402  of antenna  300 . Elements  332  extend in a radial direction  404 , away from axis  402 . 
     The invention preferably includes additional provisions that prevent antenna  300  from contacting reflective material  200  disposed within reflective tube  102 . Additional suspension features, such as spacing members, may be employed to assist in suspending antenna  300  and preventing antenna  300  from contacting reflective material  200 . 
     FIG. 5 a schematic diagram of one embodiment of a spacing member. An expanding foam  502  is disposed inside reflecting tube  102 . Expanding foam  502  encases antenna  300 . Preferably, end portions  306   a  and  306   b  of antenna  300  extend beyond expanding foam  502  to mate with holes  304   a  and  304   b , respectively. Expanding foam  502  surrounds antenna  300  and assists in preventing antenna  300  from contacting reflective material  200  of reflecting tube  102 . Any suitable dielectric materials may be used as expanding foam  502 . Most preferably, expanding foam  502  has a dielectric constant of one. 
     Another embodiment of a spacing member is shown in FIG. 6. A spoked member  602  is used as a spacing member. Any dielectric material may be used as spoked member  602 . The suitable material also preferably has a low expansion/contraction coefficient. Common styrofoam is an example of a suitable dielectric material. Spoked member  602  includes extremities  604 . Extremities  604  are designed to contact the inner surface of reflecting tube  102 . Spoked member  602  also includes a central portion  606  designed to hold antenna  300 . Central portion  606  includes a slot  608  and a hole  610 . Central portion  606  is adapted to receive antenna  300  and engage antenna  300  at a gap  334  (see FIG. 4) between two elements  332 . Spoked member  602  is moved radially towards a gap  334  (see FIG. 4) off antenna  300 . Eventually, slot  608  of spoked member  602  contacts backbone  330  of antenna  300 . Backbone  330  is slid further along slot  608  until backbone  330  reaches the central hole  610 . At that point, the spoked member  602  is in the fully installed condition, shown in FIG.  7 . Hole  610  is shown greatly enlarged for clarity. In the preferred embodiment, hole  610  tightly engages backbone  330 , and no gap would be visible. In an exemplary embodiment, hole  610  is interference fit with backbone  330 . In fact, spoked member  602  is preferably constructed of a resilient material and spokes  604  are interference fit within reflecting tube  102 . In the exemplary embodiment, spoked member  602  is made of a lightweight material such as styrofoam. The degree of interference fitting and the selection of resilient materials can be adjusted so that the holding forces (both between the reflecting tube  102  and spokes  604  and between hole  610  and backbone  330 ) meet desired levels. One or several spoked members  602  may be used at different gaps  334  (see FIG. 4) of antenna  300 . 
     After antenna  300  has been disposed within reflecting tube  102 , dramatic differences in the antenna pattern can be observed. FIG. 8 is a schematic diagram of a prior art antenna without a reflecting tube. Note the regularly shaped lobes, representative of antenna patterns, radiating forwards and backwards along the axis of the antenna. Turning to FIG. 9, an antenna constructed according to the invention, produces very different lobe shapes. The reflecting tube dramatically decreases the size and extent of the side lobes, while, at the same time, dramatically increases the size and extent of the forward and rear lobes. In this way, an antenna according to the present invention, provides a highly directional antenna pattern and reduces the likelihood of interference from side lobes and subsequent saturation of the signal-processing device. 
     Directional antenna  100  has metallic paint as reflective material  200  disposed on reflective tube  102 . Directional antenna  100  may be made using any known methods. For example, directional antenna  100  may be made as follows. First, reflective tube  102  is formed. Any known method of casting reflective tube  102  may be used. In the preferred embodiment in which reflective tube  102  is made of fiberglass, any known method of casting fiberglass articles may be used. Second, reflective tube  102  is coated with reflective material  200 . In one preferred embodiment in which a metallic paint is used as reflective material  200 , the interior side of reflective tube  102  is spray-painted with the metallic paint. Other methods of applying reflective material  200  on reflective tube  102  may be used. Third, one or more weep holes  202  may be created on reflective tube  102 . Fourth, antenna  300  is inserted into reflective tube  102 . Fifth, antenna  300  is suspended by a spacing member. As discussed above, a number of different materials may be used as the spacing member including expanding foam  502  and spoked member  602 . Sixth, end caps  302   a  and  302   b  are attached to reflective tube  102 . 
     FIG. 10 is a flowchart illustrating the steps involved in making reflective tube  102  that has a metallic mesh as reflective material  200 . The metallic mesh is the preferred material for reflective material  200 . The aperture of the metallic mesh grids is a function of the frequency of operation of the antenna, and the aperture is dimensioned such that its reflective characteristics at that frequency are maximized. In step  371 , an appropriate mold is selected. In the preferred embodiment in which reflective tube  102  has a cylindrical shape, PVC pipes may be used as the mold. The diameter of the mold is preferably larger than the longest member of elements  332  that is shown in FIG.  4 . In step  372 , a metallic mesh is wrapped around the mold. As discussed above, any suitable metallic mesh may be used. In step  373 , the mold and the metallic mesh are wrapped with a fabric, preferably a fiberglass fabric. In step  374 , a liquid resin is applied to coat and saturate the metallic mesh and the fabric. In the preferred embodiment, the liquid resin is that of a fiberglass compound. The liquid resin is then allowed to saturate and solidify in step  375 . In step  376 , the mold is removed. One or more weep holes  202  are then created on reflective tube  102 . 
     FIG. 11 is a schematic diagram showing one embodiment of using the invention with a transmission tower. In the embodiment shown in FIG. 11, utility pole  120  along roadway  190  is used as the transmission tower. In this embodiment, donor antenna  100  (a directional antenna), signal processing device  140 , and coverage antenna  150  are mounted on utility pole  120 . Donor antenna  100  is made in accordance with the present invention. Cable  130   a  connects donor antenna  100  to signal processing device  140 . Signal processing device  140  could be an amplifier or a repeater, depending on whether the signals to be processed are analog or digital. Signal processing device  140  is connected to coverage antenna  150  by cable  130   b . Reflecting shield  160  with underside  165  is placed between donor antenna  100  and coverage antenna  150 . Underside  165  is preferably coated with reflective material  200 . In this embodiment, donor antenna  100  is in wireless communication with donor cell site  170  via RF  172 , and coverage antenna  150  is in wireless communication with wireless device  180  via RF  174 . 
     FIG. 12 is a schematic diagram showing a second embodiment of using the invention with multiple transmission towers. In this embodiment, coverage antenna  150  is mounted on first utility pole  120 . Donor antenna  100  and signal processing device  140  are mounted on second utility pole  120   a . Signal processing device  140  may also be mounted on first utility pole  120 . First utility pole  120  and second utility pole  120   a  may be two adjacent poles along roadway  190 . In other embodiments, there may be at least one additional utility pole  120   b  between first utility pole  120  and second utility pole  120   a . Donor antenna  100  is made in accordance with the present invention. Cable  130   a  connects donor antenna  100  to signal processing device  140 . Signal processing device  140  could be an amplifier or a repeater, depending on whether the signals to be processed are analog or digital. Signal processing device  140  is connected to coverage antenna  150  by cable  130   b . In this embodiment, donor antenna  100  is in wireless communication with donor cell site  170  via RF  172 , and coverage antenna  150  is in wireless communication with wireless device  180  via RF  174 . 
     The foregoing disclosure of embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be obvious to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.