1. BACKGROUND OF THE INVENTION

a. Field of the Invention

This invention pertains to omnidirectional antennas. More particularly, this invention pertains to antennas that provide hemispherical omnidirectional coverage or coverage of selected sectors of a hemisphere for use in cellular communication systems.

b. Description of the Prior Art

A simple quarter-wave length vertical conductor mounted on, and feed in opposition to, a ground plane provides an omnidirectional radiation pattern in azimuth. Classical antennas of this type are well known in the art. Such an antenna, however, has a null in the radiation pattern at the zenith, i.e. directly above the vertical conductor. In many applications, the null at the zenith is not important. However, the relatively recent development of cellular communications systems has brought with it a requirement for an omnidirectional pattern with no null at the zenith and in some circumstances for coverage of selected sectors of a hemisphere. For instance, an objective of a cellular communication system may be to provide coverage throughout one room from one antenna mounted on the ceiling of the room or to provide coverage throughout a building from one antenna mounted under the roof of the building. In such instances, in order to provide communications coverage throughout the hemisphere below the antenna, the antenna must not only provide omnidirctional coverage in azimuth, but must also not have a null in the radiation pattern immediately below the antenna.

2. SUMMARY OF THE INVENTION

The present invention is an antenna consisting of four radiating elements mounted on a conducting ground plane, which antenna may be placed "upside down" on the ceiling of a room, or under the roof of a building, to provide a radiation pattern that covers the entire room or building below the antenna. Although the radiation pattern of the present invention has no null directly below the antenna, the radiation pattern directly below the antenna is reduced in amplitude. The amplitude is reduced to compensate for the fact that a mobile unit located on the floor of the room directly below the antenna would normally be closer to the antenna in comparison to other locations in the room. By altering the phasing of the radiating elments, coverage is provided for selected sectors of the room or building.

Each radiating element consists of conductor in the form of one-half of a loop, which half-loop is mounted on the ground plane. One end of the half-loop is grounded to the ground plane and the other end of the half-loop is located adjacent to the ground plane and is "excited" or "fed" in opposition to the ground plane. The ground plane generates images of the half-loops, thus, in effect, providing on the radiating element side of the ground plane a radiation pattern that is equivalent to that of four complete loops without a ground plane. Of course, on the side of the ground plane opposite to the radiating elements, the ground plane shields the radiation from the elements, and the radiation pattern on the shielded side of the ground plane differs substantially from the pattern generated on the element side of the ground plane. When mounted in the ceiling of a room, the antenna is mounted "upside down" in the sense that the radiating elements are located underneath, on the bottom surface of, the ground plane.

3. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the invention.

FIG. 2 is a cross-sectional view of the invention showing two of the radiating elements.

FIG. 3 depicts a system for feeding the four radiating elements of the antenna.

4. DETAILED DESCRIPTION

Referring now to FIG. 1. Radiating elements 1, 2, 3 and 4 are mounted upon a conducting ground plane 5. Radiating element 1 consists of a conductor 6 that is nominally one-half wavelength in length with the ground end 7 of the element being electrically grounded to ground plane 5 and the feed end 8 of the element being located adjacent to ground plane 5 and being electrically excited, fed or driven in opposition to the ground plane.

As depicted in FIG. 2, in the preferred embodiment, the feed end of element 1 is driven by connection to inner conductor 8 of coaxial cable 9 which passes through a hole or insulated passageway through ground plane 5. Outer shield 10 of coaxial cable 9 is electrically connected to ground plane 5. The ground plane, in effect, creates an electrical image of element 1, which image of element 1 together with element 1, act as if element 1 were an electrical loop of nominally one-wavelength in circumference.

In each half-loop element, the largest current flows in the portion of the loop that is normal to the ground plane. The current flowing in the horizontal portion (that is parallel to the ground plane) decreases towards the center of the horizontal portion and undergoes a phase reversal. As a consequence the currents in the vertical portions of the half-loops are nominally in phase. Except for the region near the zenith, the currents flowing in the vertical portions of the half-loops are the major contributors to the radiated field.

Elements 2, 3 and 4 are similar to element 1 and, as depicted in FIG. 1, are located at 90 degree intervals about central point 11 of the ground plane. Elements 2 and 3, however, differ from elements 1 and 4 in that the feed points for elements 1 and 4 are located at the outer ends of the loops, away from central point 11, while the feed points of elements 2 and 3 are located at the inner ends of the loops, near to central point 11. As depicted in FIGS. 1 and 2, the central area 12 of each loop is nominally located approximately one-quarter wavelength in physical distance from central point 11.

The antenna exhibits a wide bandwidth over which it provides a useable radiation pattern and an acceptable input impedance. Because the dimensions of the antenna and the radiating elements have been expressed in terms wavelengths, the dimensions, when expressed in terms of the actual wavelength at which the antenna is being used, will depart substantially, from the nominal values used to describe the preferred embodiment of the antenna. For instance, the length of the radiating element could range from approximately 0.3 wavelengths up to 0.8 wavelengths and the nominal spacing of the centers of the radiation elements from the central point in the ground plane could range in a similar fashion, i.e. from 0.15 wavelength to 0.4 wavelength. Although in the preferred embodiment the radiating elements are depicted as being rectangular in shape, the shape of the loops may depart substantially from that of rectangles, e.g. the loops could be in the form of semi-circles or even some other rather irregular shape. Although in the preferred embodiment the radiating elements have been depicted as lying in planes normal to the ground plane, the elements need not lie entirely in such planes, or in any one plane, nor need the nominal plane of each element be normal to the ground plane. Similarly the angular spacings between the elements need not be exactly equal and may depart somewhat from intervals of ninety degrees. The offsets of the elements from the central point on the ground plane also need not be exactly the same. For that matter, the central point in the ground plane is simply a reference point for use in the description of the invention and need not be located absolutely in the center of the ground plane.

In the preferred embodiment, the four elements are fed from a single source 13 by means of power dividers 14, 15 and 16. Power divider 14 is connected to power dividers 15 and 16 by coaxial cables of equal length and power dividers 15 and 16 are connected to the feed points of elements 1 through 4 by coaxial cables of equal length.

If elements 2 and 3, instead, are fed out of phase to elements 1 and 4, i.e. are fed with a phase shift of 180 degrees relative to elements 1 and 4, the antenna provides a bidirectional pattern, the center of one lobe radiating outward between elements 1 and 4 and the center of the second lobe radiating outward in the opposite direction between elements 2 and 3.

If elements 1 and 3 are fed in phase and element 2 is fed with a phase shift of +45 degrees relative to elements 1 and 3 and element and 4 is fed with a phase shift of -45 degrees relative to elements 1 and 3, then the antenna will generate a pattern having one major lobe having its maximum centered between elements 1 and 3. If, instead, element 2 is fed with a phase shift of -45 degrees relative to elements 1 and 3, and element 4 is fed with a phase shift of +45 degrees relative to elements 1 and 3, the same pattern would be generated, except that the direction of the major lobe will be reversed. Similarly, if elements 2 and 4 are in phase and elements 1 and 3 are fed with phase shifts of +45 degrees and -45 degrees respectively, a pattern will be generated having one major lobe centered between elements 2 and 4.

5. CLAIMS