Communications: radio wave antennas – Antennas – Plural antennas
Reexamination Certificate
1999-01-13
2001-01-09
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
Plural antennas
C343S853000, C343S890000, C343S891000, C342S375000
Reexamination Certificate
active
06172654
ABSTRACT:
TECHNICAL FIELD
This invention relates to coaxial cable fed multibeam array antennas and more particularly to antennas employing a conical shaped geometry to effect omni-directional composite coverage when all beams are superimposed.
BACKGROUND
Planar array antennas when imposed to cover multiple direct ions, suffer from scan loss. Since the projected aperture decreases as the beam is steered away from the broadside position which is normal to the ground surface and centered to the surface itself, it follows then that broadside excitation of a planar array yields maximum aperture projection. Accordingly, when such an antenna is made to come off the normal axis, the projected aperture area decreases causing a scan loss which is a function of cosine having a value of with the argument of zero radians (normal) and having a value of 0 when the argument is
π
2
.
Ant
Gain
db
=
10
log
[
4
π
λ
2
*
Area
*
Cos
(
θ
)
]
There are a number of methods of beam steering using matrix type beam forming networks that can be made to adjust parameters as directed from a computer algorithm. This is the basis for adaptive arrays. When a linear planar array is excited uniformly to produce a broadsided beam projection, the composite aperture distribution resembles a rectangular shape. When this shape is Fourier transformed in space, the resultant pattern is laden with high level side lobes relative to the main lobe. The
SINC
=
SIN
(
x
)
(
x
)
function is thus produced in the far-field pattern. In most practical applications these high level side lobes are an undesirable side effect.
Accordingly, a need exists in the art for an antenna system which provides for beam steering without using adaptive techniques.
A further need exists in the art for such an antenna system whereby the beam aperture is relatively constant and broadside to its intended direction without producing undesirable high level side lobes.
These and other objects and desires are achieved by an antenna design which relies on the simple geometry of conical shapes to provide a more natural beam steering.
SUMMARY OF THE INVENTION
In one embodiment of my invention, a transmit antenna is constructed as a series of antenna dipole columns mounted in close proximity to the outer surface of a nearby vertical conical shaped electrical ground surface. The ground surface is constructed circumferentially around a mast and the conical “slope” and is such that the ground surface “faces” downward at an angle, thereby creating on the ground a circumference within which the signal is propagated. This entire structure is contained within a single transparent radome. This same circumferential columnar structure can be used for a receiver antenna array constructed within the same radome on the same mast as the transmit antenna and partitioned therefrom. The ground surface angle, or conical angle can be adjusted to contain or limit the coverage area of the intended radiation pattern.
When a group of columns are excited to create a beam, the positive result from this structure is created by the fact that the reflected “image” energy from the outer columns is dispersed when the radius of the ground surface cylinder is in the range of one &lgr; wavelength. So, when the various parallel ray paths are summed together to make the effective aperture distribution, the shape is close to a cosine function and the spatial transform is similar to a Gaussian shaped far-field pattern. Thus, the antenna system achieves lower side lobes in relation to the main lobe, which in most practical cases, is a desirable effect.
Accordingly, no modifications need be made to the outer array columns to effect side lobe level control as is the case with planar arrays. This is a significant improvement over prior art systems where it is common practice is to remove elements from the outer columns or to dissipate this energy into a resistive load to achieve the same amount of side lobe level control.
In one embodiment, the individual columns can consist of any type of radiator: patch, dipole, helical coil, etc. In the case of dipoles elevated above the grounded surface of the cylinder, the effect can be visualized as a circular patch being projected onto a curved surface where the reflected projection is an ellipse with the major axis of the ellipse being a function of the radius used to make up the cylinder. As that radius increases, the amount of dispersion decreases such that as the radius grows to infinity, the system behaves like the common linear planar array. The first side lobe grows in magnitude converging on the value of that seen with a uniformly excited linear array. So, the level of first side lobe leveling control is a function of the radius of the cylinder. Using this as the design objective, the radius of the preferred embodiment should be limited to a value of
<
3
2
*
λ
.
In some applications, it is desirable to limit the radiation pattern of the antenna system so that a network of such systems can reuse an allocated set of frequencies repeatedly. The cylinder used as an example, could be replaced with a conic section that would be a “frustum of right circular cone”. The larger radius of the two radii of the frustum, would be at the top, when mounted longitudinally. This would accommodate the “down-tilt” required for such a system. Other shapes can be used, such as right circular cones or semi-hemispheres to encompass airborne and space applications as well as terrestrial applications.
Beam width and gain are functions of how many radiator columns are driven at the same time from one excitation source. Any number of columns can be excited to effect the desired beam synthesis. The only requirement is that the active (excited) columns, can “see” the projected wave front that it is supposed to participate in. This would determine the maximum number of columns required to effect a specific beam synthesis. The highest gain, narrowest beam is produced when all Pi radian active elements that are driven together can “see” the wave front that they are each to participate in. In the case of a cylinder, these would be the columns that are Pi apart on the circumference. A line drawn between the most outer and most inner columns, sets up the basis upon which the inner columns are phase retarded in order to produce the desired beam synthesis. However, a simulcast on all beams is possible if all “N” ports are excited at the same time.
The intended beam design objectives are based on the number of available adjacent columns to be excited. The narrower the beam, the more columns must be excited, and the more complex the phase retardation network. The simplest approach, is to disregard the image sources projecting off the ground surface and simply introduce the appropriate amount of phase shift on the inner columns to effect a “coherent” phase front in the direction of beam propagation. In this first approach, this works to create a useful pattern. However, the best gain and side lobe relationship is achieved when image source dispersion is taken into account. After the image sources have been adjusted for dispersion factor and ray trace length, a composite delay is assigned to the inner columns.
Accordingly, it is one technical advantage of my invention to provide an antenna system which relies on conical shaping of its ground surface and radiator positions above this ground to eliminate the effects of scan loss.
A further technical advantage of my invention is to construct an antenna array where dispersion effects of the image sources are used to effect first side lobe level control.
A still further technical advantage of my invention is a methodology for designing antenna radiator feed networks that are used to phase delay specific radiator columns to effect far field pattern synthesis.
An even further technical advantage of my invention is the use of a “frustum of a right circular cone” (a right circular cone with its tip blunted), which allows the system to create “down-tilt” where
Fulbright & Jaworski L.L.P.
Ho Tan
Metawave Communications Corporation
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