Multiple feed antenna

The multiple feed antenna comprises a primary focussing element and at least two feeds. The feeds are spaced from each other and have different characteristics such as different frequency bands. The focussing element is moved such that its focal point coincides individually with the feeds. The focussing element may be designed to be offset with respect to its focal axis. In this manner, the antenna may have a common aperture and common boresight for each of the feeds.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates to antennas using a single focussing element 
and a plurality of feeds, and more particularly to such antennas which 
have a common aperture and a common boresight. 
2. Discussion of Related Art 
In high performance aircraft and spacecraft applications, space is usually 
at a premium. Yet modern systems applications frequently call for 
multiple, large aperture antennas with a common boresight but each having 
conflicting requirements (e.g. transmit/receive, widely spaced frequency 
bands, etc.). Consequently, there is a need to find a way to combine 
apertures without compromising such requirements. 
An antenna boresight is defined as the beam maximum direction. For focussed 
antenna systems, the boresight coincides with the direction of the focal 
axis. Aperture is defined as the projection of the area of the focussing 
element on a plane perpendicular to the focal axis. 
The problem of co-location and of co-boresighted apertures has been 
addressed in several ways in the past. Several examples of such apertures 
are discussed below. 
Parabolic reflectors with interchangeable feeds have been suggested. This 
solution is similar to that of a microscope with a turret having lenses 
providing several discrete values of magnification. The chief disadvantage 
of this approach is that there are usually cables or waveguides associated 
with each feed which must flex or bend when a new feed is positioned to 
the focus. Flexing can cause phase errors or arcing problems if high power 
is involved. Furthermore, to minimize loss, transmitters or preamplifiers 
are frequently mounted on the feed which increases weight and complexity 
of the movable feed. 
Lenses with interchangeable feeds have also been suggested. This approach 
is similar to the approach using parabolic reflectors with interchangeable 
feeds and has many of the same problems. 
Frequency selective reflectors have been tried. The use of two or more 
apertures operating at different frequencies permits frequency selective 
surfaces to be used to conserve space. One example of such a reflector 
uses a dichroic surface subreflector positioned in front of a parabolic 
reflector. A first feed, near the parabolic vertex, is in the frequency 
band where the subreflector is reflective and so operates as a cassegrain 
system. A second feed is positioned at the parabolic focus and operates in 
the frequency band where the subreflector is "transparent." The second 
feed therefore operates as a point focus feed. 
Another example of a frequency selective reflector system comprises a 
plurality of frequency selective reflectors stacked coaxially. A separate 
feed is directed at each of the reflectors. The first feed reflects off 
the first reflector, which is a bandpass surface at the frequency bands of 
the other feeds. Each successive feed reflects off its associated surface, 
which is a bandpass surface for each of the next successive feeds. 
Disadvantages of the frequency selective reflector approach are that losses 
are associated with each frequency selective surface, particularly when 
the operating bands of the feeds are closely spaced in frequency. Also, 
losses increase and bandpass characteristics change as the angle of 
incidence varies. This approach trades lateral displacement of apertures 
for coaxial displacement and so is not very conservative of volume. 
Other common boresight antennas are known. For example, U.S. Pat. No. 
3,534,375 to Paine discloses a common boresight antenna for any one of 
several feeds. It performs this function by rotating a subreflector in a 
cassegrain (2 reflector) system. This system suffers from blockage of the 
aperture by the subreflector. This blockage is at the center of the 
aperture where its effect on efficiency is most severe. Also, a cassegrain 
antenna with a tilted subreflector and an offset feed tends to have less 
aperture efficiency (greater phase error) than when the feed and 
subreflector are coaxial and symmetrical with the main reflector, where 
the loss in efficiency depends on the amount of tilt and offset. Although 
this phase error can be compensated for to some extent in subreflector 
design, this correction tends to apply over a narrow frequency band. 
U.S. Pat. No. 3,696,435 to Zucker discloses an antenna having a single 
reflector with multiple feeds; however, the feeds are not co-boresighted. 
Here, each feed is associated with a particular direction. Other 
limitations include the fact that the feeds are displaced laterally from 
the parabolic focal axis. Therefore, only one feed can be at the prime 
focus. All other feeds suffer some measure of scan loss (phase error) 
depending on the amount of displacement off axis. Feed locations are 
selected to minimize these errors, but the errors are not eliminated. 
Also, feed position is a function of frequency as well as lateral 
displacement. Thus, beam scan by rotating the reflector is not feasible. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide an antenna having plural 
feeds, and a single element for focussing the feeds. 
A further object of the invention is to provide a plural feed antenna in 
which each feed is fixed and independent and so requires no flexing of 
cables or motion of large complex electronic equipment. 
Another object of the present invention is to provide an antenna having 
multiple feeds wherein no phase distortion occurs due to a change from one 
feed to another. 
One additional object of the present invention is to provide a multiple 
feed antenna having a common boresight and aperture. 
Another object of the present invention is to provide a multiple feed 
antenna which is light in weight and relatively compact so as to enable 
its use in a space limited environment. 
In accordance with the above and other objects, the present invention is a 
multiple feed antenna, comprising a focussing element having a focal axis 
and at least two feeds. The feeds are spaced from each other. A mechanism 
is provided for rotating the focussing element to a different position for 
each feed such that each feed is individually positioned on the focal axis 
of the focussing element and directed at the focussing element. 
The focussing element may be a reflector which is an offset axis sector of 
a paraboloid of revolution. The sector is defined by the intersection of 
the paraboloid of revolution and a right circular cylinder having an axis 
parallel to the axis of the paraboloid of revolution. 
The rotating mechanism may produce rotation of the reflector about an axis 
parallel to the axis of the paraboloid of revolution whereby the antenna 
has a common boresight in each position of the reflector. In another 
embodiment, the rotating mechanism produces rotation of the reflector 
about an axis perpendicular to the axis of the paraboloid of revolution 
whereby the boresight of the antenna is different in each position of the 
reflector. 
In one embodiment, the right circular cylinder is positioned entirely on 
one side of the axis of the paraboloid of revolution, and the rotating 
mechanism produces rotation of the reflector about the axis of the right 
circular cylinder. In this manner, the antenna has a common boresight in 
each position of the reflector and the feeds are positioned outside the 
projected aperture area. In another embodiment, the right circular 
cylinder is positioned to contain the axis of the paraboloid of 
revolution, and the rotating mechanism produces rotation of the reflector 
about the axis of the right circular cylinder such that the antenna has a 
common boresight in each position of the reflector and the feeds are 
positioned to partially block the projected aperture area. 
In place of a reflector, an electromagnetic lens may be used. An advantage 
of this embodiment of the invention is that feeds are always positioned 
behind the lens so aperture blockage by the feeds will not be produced. 
The invention also includes the method of operating a directive antenna 
comprising a movable focussing element having a focal axis and a plurality 
of feeds. The method comprises positioning the feeds spaced from one 
another and rotating the focusing element such that each of the feeds is 
individually located on the focal line and directed at the focussing 
element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIGS. 1 and 2, the antenna 10 of the present invention 
will now be described in detail. The antenna 10 comprises a reflector 
element 12, a plurality of feeds 14 through 17, and a mount 18. 
Reflector element 12 is an offset parabolic reflector formed from a section 
of a paraboloid of revolution indicated by dash-dot line 19. The 
paraboloid of revolution 19 has a focal axis indicated by dash-dot line 
20. The focal axis 20 passes through the vertex of the paraboloid and 
contains the focal point 22 of the paraboloid indicated by "X" 22. As 
indicated in FIGS. 1 and 2, feed 17 is located at the focal point 22. 
Reflector 12 is an offset sector of paraboloid 19. Accordingly, the center 
of reflector 12 is spaced from axis 20 of paraboloid 19. By way of 
example, reflector 12 can be defined by the intersection of paraboloid 19 
and a right circular cylinder indicated in the elevational view of FIG. 1 
by parallel lines 24. FIG. 2 is an end view of the reflector and, thus, 
the projection of the reflector 12 is circular in FIG. 2. 
Clearly, since feed 17 is positioned directly at focal point 22 in FIGS. 1 
and 2, lines of radiation from feed 22 directed to reflector 12 will 
result in lines of radiation which are all parallel to focal axis 20. 
Conversely, lines of radiation which are parallel to focal axis 20 and are 
directed at reflector 12 will converge at feed 17. Feeds 14-17 may be 
conventional horns or other conventional feeds directed to subtend solid 
angles with reflector 12. Accordingly, with reflector 12 in the position 
shown in FIGS. 1 and 2, reflector 12 and feed 17 comprise an antenna 
having a boresight in the direction of focal axis 26 and having an 
aperture described by projection of reflector 12 on a plane perpendicular 
to axis 26. 
As shown in FIG. 1, cylinder 24 has an axis 26 which is parallel to axis 20 
of paraboloid 19. Accordingly, by rotating reflector 12 about axis 26, 
focal point 22 will describe a circular path, referred to here as focal 
circle 28, shown in FIG. 2. As shown in FIGS. 2 through 5, feeds 14 
through 17 are fixed in position on focal circle 28. Accordingly, if 
reflector 12 is rotated in a counterclockwise direction, as viewed in 
FIGS. 2 through 5, about axis 26, focal point 22 will move from its 
position of coincidence with feed 17 along focal circle 28 and will become 
coincident successively with feeds 16, 15 and 14, as shown in FIGS. 3, 4 
and 5, respectively. As shown in FIG. 1, a motor 30, connected to a motor 
support structure 32 may rotate reflector 12 through a shaft 34, connected 
to the point at which axis 26 intersects reflector 12. 
Clearly, as reflector 12 is rotated about axis 26, focal axis 20 always 
remains parallel to axis 26, although changing its position to describe 
focal circle 28. Consequently, it is clear that the aperture and boresight 
of reflector 12 combined with, respectively, feeds 14 through 17 always 
remains the same. By providing each feed 14-17 with a different 
characteristic, such as, for example, differing frequency bands, a 
plurality of different antennas can be formed having exactly the same 
aperture and boresight. 
While four feeds are shown in the drawings, it should be clear from the 
foregoing description that any reasonable number of feeds can be used, 
depending on the necessary number of differing antenna applications. Motor 
30 could be servo controlled to automatically position reflector 12 in 
each position. It should be noted that reflector 12 may be relatively 
light and, thus, a relatively low horsepower motor is required for its 
rotation. As shown, motor 30 is connected through shaft 34 to the center 
of reflector 12. Of course, other possible connections are also feasible, 
as would be apparent to one of ordinary skill in the art. 
The size of reflector 12 would depend on the application in which the 
antenna is being used. If maximum gain and minimum beamwidth are desired, 
reflector 12 may be designed to fill all available space. It should be 
noted, however, that when the sector of paraboloid 19 which is occupied by 
reflector 12 overlaps the paraboloid axis 20, as is the case in FIGS. 1 
through 5, focal circle 28 is contained completely within the antenna 
aperture defined by cylinder 24. Accordingly, the aperture is partially 
blocked by the feeds in the focal circle. However, this blockage occurs at 
the edge of the aperture where field intensity is usually low, so that 
blockage effects are small. 
If it is desired for a particular application to avoid any blockage of the 
aperture, a reflector 12', as shown in FIGS. 6 and 6A, may be used. As 
with reflector 12, reflector 12' is a sector of paraboloid 19 defined by 
the intersection of a right circular cylinder with paraboloid 19. However, 
this intersection is such that reflector 12' is formed entirely on one 
side of focal axis 20 of paraboloid 19. Accordingly, it can be seen that 
the rays, indicated by solid lines in FIG. 6, which are directed from feed 
17 to reflector 12' result in parallel rays which completely miss feed 17. 
It will also be apparent that the focal circle 28' for reflector 12' is 
therefore completely outside of the aperture of reflector 12' and, 
accordingly feeds 14 through 16 are similarly outside of the aperture. 
As will be understood from the foregoing discussions, the antennas of FIGS. 
1 and 6 are common aperture, common boresight antennas which can be 
confined to very limited space and are able to utilize any one of a 
plurality of feeds without the necessity of phase compensation. Absolutely 
no adjustment or movement of the feeds 14 through 17 is necessary. Simply 
by rotating the reflector 12 or 12', various antenna characteristics can 
be achieved simply and economically. However, applications may occur where 
it is desirable to have a plurality of boresight directions. In this case, 
the embodiment of FIGS. 7 through 9A can be used. FIGS. 7 through 9A show 
a reflector 12" in a left handed 3-dimensional coordinate system having X 
axis 40, Y axis 42 and Z axis 44. In FIGS. 7 and 7A, X axis 40 is the 
focal axis of the paraboloid of which reflector 12" is a sector. 
Accordingly, the position of reflector 12" in FIGS. 7 and 7A corresponds 
to the position of reflector 12 in FIGS. 6 and 6A. However, in FIGS. 7 and 
7A, reflector 12" is to be rotated about Z axis 44 which is perpendicular 
to the focal axis of the paraboloid. Accordingly, the focal circle 46 is 
defined in the X-Y plane as reflector 12" is rotated. Accordingly, the 
parallel rays to or from reflector 12", shown in solid lines, are always 
parallel to the X-Y plane, but revolve around the Z axis. FIGS. 7 and 7A 
show the orientation of reflector 12" when focal point 17 is coincident 
with the first feed 50, and the boresight of reflector 12" is in the +X 
direction. FIGS. 8 and 8A show the case where reflector 12" has been 
rotated such that focal point 17 is coincident with a second feed 52 and 
the boresight is in approximately the -Y direction. FIGS. 9 and 9A show 
the situation where reflector 12" has been rotated such that focal point 
17 is coincident with feed 54 and the boresight is in approximately the -X 
direction. Of course, additional feeds could be added, as desired. It is 
also possible to rotate the reflector 12" about several different axes. 
For example, a focal circle similar to that of FIGS. 6 and 6A could be 
defined in each of the directions shown in FIGS. 7, 7A, 8, 8A, 9 and 9A. 
In this case, reflector 12" could be rotated about an axis parallel to its 
boresight in each direction X, -Y and -X to provide a plurality of feeds 
in each such direction. 
In FIGS. 1-9A, it has been assumed that reflectors 12, 12' and 12" are 
sectors of similar paraboloids. Therefore, even though these reflectors 
may be different sectors, they have the same focal point 17. Of course, if 
a different paraboloid is used, the focal point would vary. 
FIGS. 10 and 11 show an embodiment of the invention where an 
electromagnetic lens 60 is used in place of a reflector. Lens 60 has a 
focal axis indicated by dotted line 62. Lens 60 is asymmetrical and is 
rotated about an axis of rotation indicated by dash-dot line 64. 
Accordingly, as the lens 60 is rotated about axis 64, the focal axis 62 
describes a focal circle indicated by dotted line 66. Two feeds 68 and 70 
are shown with their phase centers positioned on the focal circle. In the 
position shown in FIG. 10, focal axis 62 passes through the phase center 
of feed 70. In FIG. 11, the lens 60 is shown in a second position where it 
has been rotated about rotation axis 64 to a position where the focal axis 
62 passes through the phase center of feed 68. Accordingly, it can be seen 
that the embodiment of FIGS. 10 and 11 is equivalent to the embodiment 
using reflectors for the antenna focussing element. That is, by simply 
rotating lens 60, different antenna characteristics can be achieved by 
causing the lens focal axis to pass through a feed element having the 
desired antenna characteristics. 
It will be noted that axis of rotation 64 is positioned in the center of 
asymmetrical lens 60, as viewed in FIGS. 10 and 11. In this manner, when 
lens 60 is rotated, the aperture and boresight of the antenna remain the 
same. In this embodiment, since the lens is not symmetrical, feeds 68 and 
70 are directed so as to subtend a solid angle which includes the 
asymmetrical lens 60. In this manner, a common aperture, common boresight 
multiple feed antenna is provided. Of course, as with the embodiment of 
the reflector 12" shown in FIGS. 7 through 9A, lens 60 could also be 
rotated about an axis which is perpendicular to the focal axis so as to 
define differing boresights. 
The advantage of the use of a lens in place of a reflector is that the lens 
can be made to fill all of the available space and the feeds will not 
obstruct the aperture in any way. However, an electromagnetic lens is 
generally heavier than a reflector and therefore the reflector version of 
the invention is preferrable unless excessive blockage of the aperture 
results. In both versions, all feeds are fixed on the focal circle so that 
no feed motion is required. 
The foregoing examples are provided for purposes of illustrating the 
invention, but are not deemed to be limitative thereof. Clearly, numerous 
additions, changes or other modifications could be made without departing 
from the scope of the invention, as set forth in the appended claims.