Multiple beam antenna system

A multiple beam antenna system may be constructed for reducing a spillover loss n efficiency, improving beam crossover, and reducing undesired sidelobes by the addition of three dielectric lenses between a feed horn cluster connected to a beam forming network and an objective collimator. The system includes a beam forming network including a plurality of feed horns in a feed horn cluster, an objective, and an imaging lens having a lateral magnification less than unity for focusing a reduced image of the feed horn cluster at a predetermined point in space. A field lens is positioned at that predetermined point in space, and an amplitude shaping lens is positioned between the field lens and the objective. The amplitude shaping lens redirects the rays of the image transmitted by the field lens to be denser in the central region of the objective, and reduce the sidelobes of the far field pattern of the transmitted beams.

BACKGROUND OF THE INVENTION 
The present invention relates to multiple beam antenna (MBA) systems, such 
as are useful for communication satellites. Specifically, the present 
invention provides a microwave multiple beam antenna system that 
simultaneously achieves closely spaced beams (high crossover levels) and 
high aperture efficiency (low spillover loss) with a relatively simple 
beam forming network. 
Conventional MBA designs, typically for communication satellites, place the 
feed horn cluster of the antenna at the focal point of an offset reflector 
collimator, as shown in FIG. 1. The feed horns are designed to be 
relatively small for close packaging in the cluster to give reasonably 
high crossover levels (i.e., closely spaced beams). A small feed horn, 
however, produces a broad radiation pattern for illuminating the offset 
reflector. This results in much of the energy not being intercepted by the 
reflector, and gives rise to high spillover loss. On the other hand, if 
the feed horns are designed for more directive beams to reduce the 
spillover loss, the feed horns become larger, yielding wider beam 
separation, and thus lower crossover levels. The result s "holes" in the 
pattern coverage. 
FIG. 1 illustrates a conventional multiple beam antenna configuration. A 
beam forming network (BFN) 11 supplies signals to a feed horn cluster 13. 
which illuminates an offset paraboloid reflector 15. If the feed horns 19 
are made relatively small for close packaging and reasonably high 
crossover levels 17 (as shown in FIG. 2), a significant portion of the 
beam misses the reflector, becoming spillover loss 21. Alternative feed 
horns that produce more directive beams to reduce the spillover loss, 
produce low beam crossover levels 23 in the beams reflected from the 
offset paraboloid reflector, as shown in FIG. 3. 
A partial solution to the spillover loss problem is described by the 
inventor in Wokurka, A Feed Cluster Image Reduction System, Digest, IEEE 
AP-S Symposium, Blacksburg, Virginia, Jun. 1987, pages 199-202. In the 
system there described, an "imaging" lens is used to produce an optically 
reduced image of a large feed horn cluster. The reduced image of the feed 
horns is then used to illuminate the collimating reflector or dielectric 
lens. A field lens is placed between the imaging lens and the objective 
lens to efficiently refract the energy from each feed horn onto the 
objective lens, thereby maintaining low spillover loss for each beam at 
the objective lens. 
Another system that has been suggested is to form overlapping feed horn 
subclusters with a more complex beam forming network. With this approach, 
energy to be radiated in a beam is divided in the BFN and applied to 
several adjacent horns. This approach increases the feed aperture size, 
and narrows the feed radiation pattern, to more efficiently illuminate the 
reflector. Adjacent beams are produced by overlapping these clustered feed 
horns. However, this approach complicates the feed network greatly, 
particularly for millimeter wave length signals and/or systems using a 
large number of beams. This approach also adds significantly to waveguide 
or transmission line losses. Such increased complexity and losses are 
particularly pronounced at higher millimeter wave frequencies, where they 
are least tolerable. 
Another proposed solution to the spillover loss problem is to build several 
antennas, each of which produces widely spaced beams that are a portion of 
the total required. The beams from the separate antennas are then 
interlaced in space to create the full coverage complement. Clearly, this 
approach adds much unnecessary weight and volume to the antenna system by 
adding more antennas. 
SUMMARY OF THE INVENTION 
The present invention is a multiple beam antenna system that includes a 
beam forming network that includes a plurality of feed horns in a feed 
horn cluster and objective. An imaging lens having a lateral magnification 
less than one for focusing a reduced image of the feed horn cluster at a 
predetermined point in space is placed next to the horn cluster. A field 
lens is positioned at that predetermined point in space, and an amplitude 
shaping lens is positioned between the field lens and the objective. The 
amplitude shaping lens redirects the rays of the image transmitted by the 
field lens to be denser in the central region of the objective and 
consequently reduces the sidelobes in the far field pattern of the 
transmitted beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the present invention, spillover loss from individual microwave horns in 
a feed horn cluster used in conventional multiple beam antenna designs is 
reduced by the placement of three dielectric lenses between the feed 
cluster and the final collimating reflector or lens. 
The present invention incorporates a beam forming network 31, which may be 
of the type generally known and understood in the industry. This beam 
forming network transmits beams through a feed horn cluster 33. Such feed 
horn clusters and their attributes are also well understood in the art. 
An imaging lens 35 is placed in the path of the beams 37 from the feed horn 
cluster. This imaging lens 35 has a lateral magnification of less than 
unity, so that an optically-reduced image of the feed horn cluster is 
produced at the field lens 43. The imaging lens can be shaped and 
positioned so that a minimum portion of the beams 37 produced by the feed 
horn cluster bypass the lens. This provides minimum spillover loss 39 from 
the feed horn cluster. 
The imaging lens 35 focuses the reduced image of the feed horns at a point 
in space. The reduced feed horn image can be used to illuminate an offset 
reflector 41. In the embodiment illustrated in FIG. 4, the objective 41 is 
an offset paraboloid reflector. Alternatively, a lens may function as the 
objective. 
The field lens 43 is placed at the feed horn image to efficiently refract 
the energy from each feed horn of the feed horn cluster onto the objective 
reflector 41. By properly refracting the beams from the optically reduced 
image of the feed horn cluster, a maximum of the beams 45 impact the 
objective reflector 41, providing minimal spillover loss 47. 
The imaging lens 35 forms overlapped and clustered feed distributions 
optically in space at the field lens plane, so that the image formed at 
the field lens is a small overlapped replica of the physically larger real 
cluster. The imaging lens may provide a 0.5 lateral magnification (or 
image reduction) factor of the actual feed horn cluster. Focusing the 
reduced image of the feed horn cluster at the field lens 43 causes the 
energy to appear to the objective reflector 41 as though it were coming 
from a more closely spaced feed horn cluster, with correspondingly closer 
horn phase centers. 
By using larger feed horns, with their associated more directive patterns 
as the elements of the feed cluster, and optically reducing the size of 
this cluster with the imaging dielectric lens, spillover loss is reduced. 
The feed horn amplitude taper at the imaging lens edge can be made to be 
-10dB, resulting in low spillover loss 39 at the imaging lens. 
The radiated beams are therefore spaced more closely in space, resulting in 
higher beam crossovers. A given crossover level can be realized by 
properly choosing the lateral magnification of the imaging lens during the 
design of the system. A higher beam crossover level results in a higher 
minimum gain of the composite antenna gain coverage. 
With a uniform amplitude or power density distribution across the objective 
41, the collimated beams 49 reflected from the reflector 41 may contain 
significant sidelobes in the far field pattern due to beam diffraction. To 
reduce the sidelobes in the far field pattern, an amplitude shaping lens 
51 redirects more of the energy rays in the central part of the reflector. 
Thus, the amplitude shaping lens alters the "ray bunching" or power 
density distribution so that the rays of energy from the antenna horns are 
denser in the central region of the system. The amplitude shaping lens 
concentrates the power of the beams in the central part of the collimating 
reflector, giving rise to low sidelobe reflected beams 49. Increasing the 
power density in the central portion of the beam pattern reduces beam 
diffraction and the associated sidelobes in the beam pattern. 
Amplitude shaping is accomplished primarily through refraction at the first 
surface of the amplitude shaping lens 51. The second surface is contoured 
mainly to satisfy the phase constraint. Ordinarily, the chosen shape of 
the lens is sensitive to the central thickness of the lens and the 
distance from the field lens 43 to the amplitude shaping lens 51, and the 
central thickness of the amplitude shaping lens. Some amplitude shaping 
can be done by the objective reflector lens 41. However, such shaping by 
the objective would likely be at odds with the wide-angle "scanning" 
requirement for the multiple beams of a multiple beam antenna system. 
Equations for the paraxial rays (those close to the axis that satisfy the 
small angle approximation) for each lens may be derived, depending on the 
lens material, its dielectric constant, and the lens thickness. 
Geometrical optics computer programs can be used to trace rays through the 
different lenses of the system and determine the aspheric term 
coefficients specifying the surface away from the central axis. A scalar 
defraction theory computer program can be used to determine the amplitude 
and phase distributions on each lens surface and calculate the far field 
radiation patterns. 
The geometrical optics program can be used to successively determine higher 
order coefficients of the lens surface expressions to focus, with the 
imaging lens, the non-paraxial rays at the focused spot images of each 
feed horn in the field lens plane. This helps to insure that the 
non-paraxial rays are not spilled over, but rather fall on the objective 
reflector for each feed horn to realize high aperture efficiency. 
Additionally, the surface coefficients of the objective reflector or 
objective lens can be determined to ensure a low phase error distribution 
(preferably 50 degrees maximum) across the aperture for each beam. 
The lenses for a system for 44 GHz wavelengths may be fabricated of a 
dielectric material, such as alumina having a dielectric constant of 9.72. 
The center of each lens may be approximately one inch thick. The amplitude 
shaping lens 51 in particular should have a center of sufficient thickness 
to ensure that enough dielectric medium is present at the outer rim of the 
lens for the rays to converge and perform the power transformation 
required. 
For such a system for 44 GHz wavelengths, the distance from the edge of the 
feed horns to the objective reflector or the far surface of an objective 
lens may be approximately 32.2 inches. The lenses may be installed in an 
eight inch diameter stainless steel machined tube. The position of the 
imaging lens 35 may be fixed, while the field lens 43, amplitude shaping 
lens 51, and objective lens 53 or reflector 41 may have adjustable 
positions. 
The present invention also increases the "hardness" of the system to 
electromagnetic and particle beam threats by virtue of the hard lens 
material shielding the feed horns and the sensitive receivers connected to 
the antenna feed network ports. The lens surface could also be made 
reflective or diffuse at other threat frequencies, such as in the laser 
optical spectrum. 
The present invention allows the final objective aperture distribution to 
be phase corrected by adjusting higher order coefficients in the lens 
surface equations so as to improve beam distortion resulting from feeds 
progressively farther from the feed cluster access of symmetry. 
The collimator or objective is shown in FIG. 4 as an offset reflector. 
Nevertheless, the collimator could equally be a lens 53, as shown in FIG. 
5. Such a lens would be more appropriate for high millimeter wave 
frequencies (EHF), where the apertures need not be large, and the lens 
weight would not be excessive.