Note: Descriptions are shown in the official language in which they were submitted.
IMPROVED SHIFTED FOCUS CASSEGRAIN ANTENNA
WITH LOW GAIN FEED
The present invention relates in general to communication
antennas, and more particularly, to a Cassegrain antenna which
may be considerably reduced in size while providing for
efficient operation in the low CHz range without a disabling
increase in antenna blockage through use of a secondary
reflector of special configuration providing a shifted focus in
combination with a low gain feed.
In Cassegrain antennas, a feed element is located in the
general region of the vertex of a paraboloidal primary
reflector for radiating energy toward an intermediate secondary
reflector which is smaller than the primary reflector and is
located between the vertex and the focal point of the primary
lS reflector. The secondary reflector usually has a hyperboloidal
configuration and its focal point is preferably located
coincident with the focal point of the primary reflector.
One of the heretofore unsolved problems in the design of
Cassegrain antennas is how to provide such an antenna which is
small in size~ that is, with a main dish which is less than
fifteen feet in diameter, for use at fre~uencies as low as 4
GHz. Conflicting desi~n considerations relating to the size of
the feed horn required at frequencies in the low GHz range and
the size of the subreflector to be used with such a feed horn
if antenna blockage and spillover problems are to be avoided
have placed a lower limit on the size of the Cassegrain antenna
at approximately fifteen feet. However, there are many
applications in which a Cassegrain antenna of smaller size is
very desirable for use at frequencies as low as 4 GHz.
One of the long-s~anding problems encountered in the
design of the Cassegrain-type antenna relates to antenna
blockage, which is affected both by the size of the secondary
reflector (subreflector) and the size of the feed element
(horn). As one extreme, it is apparent that the subreflector
cannot be as large as the main reflector, since this would
produce total blockage of the main reflectors while, an
extremely-small subreflector produces undesirable spillover
problems resulting in loss of power. Thus, a typical ratio
between the diameter of the subreflector and the diameter of
the main reflector has been selected to be approximately 1:10
in the design cf a Cassegrain antenna. Similarly, a feed horn
will typically produce an aperture blockage which is at lea~t
equal to the diameter of the horn, and in those cases where the
horn is placed in close proximity to the subreflector, the
blockage produced by this combination will increase
significantly at least for that part of the energy directed to
the central portion o the subreflector where the maximum power
is generally concentrated.
When it is desired to operate in the lower G~z frequency
range, in order to bundle a beam towards the ~ubreflector of
the antenna, it is necessary to utilize a high gain horn~
Thus, a flared horn is typically used as the feed element in a
Cassegrain antenna in order to provide a high gain feed.
Unfortunately, as the operating frequency of the antenna is
reduced, to get a high gain horn, the horn becomes bigger and
bigger in size which contributes to an increase in the blockage
of the main reflector. For example, at 10 GHz if 30 degrees is
the required feed angle to bundle all the energy to the
subreflector~ it i~ necessa{y ~o provide at 15 to 25 db gain
horn which may have an aperture size of 5iX to eight inches.
On the other hand, if the operating frequency is reduced to 4
S GHz, the same horn will have to have an aperture si~e of about
fifteen to twenty inches. ~nder these circumstances, the
minimum diameter fGr the subreflector will ~e about eighteen
inches or one foot and a half in diameter if undesirable
blockage of the main reflector by the horn itself is to be
avoided. With a typical ratio of 1:10 between the size of the
subreflector and the si~e of the main reflector, it can be seen
why at present Cassegrain systems are not capable of being
reduced below ifteen feet in diameter for operating at
frequencies below 4 GHz, without incurring significant blockage
proble~s~
In addition, there is an inherent blockaye based on the
offset between the subreflector and the horn. Thus, with a
standard hyperboloidal subreflector, the first ray at the
center of the horn must be transmitted to the main reflector
past the edge of the horn. If the operatiny band of the
antenna requires use of a feed horn which is eighteen inches in
diameter, this will cause an eighteen inch blockage in itself;
however, the optical geometry of the system will probably make
this a two to four foot blockage depending on the spacing of
the feed horn from the subreflector~ With this in mind, it can
be ~een that any increase in the si~e of the horn for operation
at lower frequencies will result in the blockage problem
becoming more accute, effectively blocking the inner part o~
the main reflector, where the maximum power is transmitted.
~`''3~
In order to obviate this blockage problem which prevents
reduction in size of the Cassegrain antenna for low freguency
operation, there has been proposed in British Patent No.
973,583 by J. L. Lee, as well as ln French Patent No. 1,392,013
to G.R.P. Marie, a Cassegrain-type antenna having a subreflec-
tor in the shape of an ellipsoidal surface of revolution with
one focus of the ellipsoid coincident with the phase center of
the feed horn and the other focus o the ellipsoid positioned
adjacent the outer edge of the subreflector so as to produce a
ring focus in space. With such an arrangement, the phase
center of the feed horn effectively shifts to the ring focus so
that the origin of the reflected energy i5 apparently disposed
along the ring extending around the outes edge of the sub-
reflector, thereby totally eliminating blockage by the
subreflector or the feed horn~ In this regard the ring focus
provides a cross-over of the rays from the feed horn with the
result that the center ray representing the maximum power is
deflected to the outer edge oE the main reflector and is not in
any way blocked by the horn.
While the shifted focus system disclosed by J. L. Lee and
G.R.P. Marie effectively has solved the aperture blockage
problem created both by the feed horn and the subreflector, and
thereby has apparently opened the way to the design of small
size Cassegrain antennas~ such a system has a basic drawback in
that it requires the precise location of the foci of both the
feed horn and the subreflector for proper operation.
Unfortunately,, it has been found that problems arise with this
system due to the fact that the phase center of the high gain
--4--
horn which is normally provided in this type of antenna will
move with changes in frequency. Th~ls, at the high end of the
frequency range over which the antenna is to operate, the phase
center will sit deeper in the horn than at the low frequency
end of that range. This movement oE the phase center disrupts
the optical geometry of the antenna resulting in deterioration
of the operation thereof.
It is therefore a principal object of the present
invention to provide a small size Cassegrain-type antenna which
10 is capable of efficient operation in the low GHz range.
It is another object of the present invention to provide a
Cassegrain-type antenna which is capable of being produced at a
small si~e with a low cost while significantly reducing loss
due to shadowing and blockage by the subreflector or the feed
15 element.
It is a further object of the present invention to provide
a Cassegrain-type antenna of the shifted focus t~pe having a
feed element which is least f~equency dependent.
In accordance with the basic feature of the present
20 invention, there is provided a ~assegrain antenna of the type
having a shaped subreflector which produces a shifted ring
focus in combination with a low gain feed source having a
well-def ined phase center which is relatively insensitive to
frequency. In i~s most-basic form, such a low gain feed source
25 can be provided in the form of an open waveguide; however, a
prime focal-type horn, such as a horn having a rectangular waveguide
section by way of a step portion therebetween, the circular waveguide
section having a corrugated section around the periphery and adjacent -to
the end thereof or a simple diagaonaLly fed square horn may be utilized
for this purpose. Sucll a so~lrce is extremely simple ar~d can be made
--5--
sufficiently ~mall to eliminate any aperture blockage ~y the
horn in a shifted focal point system. This result~ in an
extremely-compact antenna feed system which can be manufactured
at very-low cost. ~owever, even more importantly, with such an
arrange~ent, a Cassegrain antenna as small as four feet in
diameter can be effectivel~ designed which has extremely high
gain and excellent wide angle pattern performance and is
capable of operation at 4 GHz.
These and other objects features and advantages of the
present invention will become more apparent from the following
detailed description of a preferred embodiment, which is
illustrated in the accompanying drawings.
Fi~ure 1 is a schematic diagram of a conven~ional
Cassegrain antenna illustrating the problem of aperture
blocking by the subreflector and feed horn;
Figure 2 is a ~chematic diagram of a shifted focus
Cassegrain antenna having a subreflector formed as an
ellipsoidal surface of revolution;
Figure 3 is a schematic diagram of a Cassegrain antenna
embodying the features of the present invention;
Figure 4 is a longitudinal sectional view of one type of
low gain feed horn which may be used with the present
invention; and
Figures SA and 5B are elevational views of the
subreflector utilized by the present invention having
peripheral corrugations in the circumferential and radial
directions, respectively, for reduction of side lobe
scattering.
The inability to produce a Cassegrain antenna smaller than
about fiEteen feet in diameter is directly linked to the
problem of antenna blockage by the subreflector and feed horn.
~igure l is somewhat exaggerated in its illustration of such
blocking in a typical Cassegrain antenna but is useful in
illustrating the problems which have hindered the manufacture
of small-size Cassegrain antennas to the present date.
In Figure 1 a high gain flared horn l having a phase
cen~er at point A directs microwave energy with an aperture
angle ~ toward a subreflector 2 having a hyperboloidal surface
from which the microwave energy is reflected onto a parabolic
main reflector 3 having an axis X and a diameter D to form a
substantially--parallel trans~ission beam in the well-known
manner. As can be seen from Figure l, the subreflector 2
provides a primary aperture blockage of the rays reflected from
the main reflector 3 by an amount equal to the diameter d of
the subreflector 2. Bowever, it is also apparent that only
those rays directed at the subreflector 2 which are capable of
passing the peripheral edge of the feed horn 1 will reach the
main reflector 3. Thus, while the blockage provided by the
subreflector 2 is equal to the diameter d, an even greater
blockage equal to the distance d' is created by the feed horn
l. Unfortunately, this problem is subject to conflicting
design considerations, especially where an antenna small size
~5 is desired. Thus, for a standard Cassegrain antenna that is
designed to transmit or receive signals in a frequency range of
4 GHz and lower, the typical high gain feed horn regulred at
these frequencies is of such a size that significant blockage
of the antenna occurs. Therefore, as the transmission
frequency is reduced on an antenna of small size, ~ limit is
reached beyond which the antenna will no longer operate in a
satisfactory manner. It is for this reason that present
S Cassegrain systems cannot be designed to operate below 4 GHz
a~d that antennas of this type are limited to a diameter no
smaller th~n approximately fift:een feet.
Figure 2 illustrates a Ca~segrain antenna having a shaped
subreflectc)r 2' which serves to shift the phase center of the
microwave energy from the point A within the feed horn 1 to a
ring focus O disposed adjacent the periphery of the
subreflector 2~o This system, which has been disclosed in
British Patent No. 973r583 and French Patent No. 1,392,013,
eliminates the blocka~e of the main reflector 3 by the feed
horn 1 due to the shifting of the phase center to a ring which
is concentric with the axis X of the feed horn 1 and
subreflector 2' ancl has a diameter slightly larger than the
diameter d of the subreflector 2' so as to eliminate any
blockage whatsoever by the feed horn 1. This shifting of the
phase center of the feed horn 1 results from the fact that the
reflecting surface of the subreflector 2' is formed as an
ellipsoidal surface of revolution with one focus of the
ellipsoid being coincident with the phase center A of the feed
horn and the other ocus of the ellipsoid being coincident with
ZS the ring focus O of the subreflector 2'. With a high gain feed
horn 1 havinq a feed angle ~ which includes the subreflector
2', all energy emanating from the phase center A will be
reflected frc>m the ellipsoidal surface of the subreflector 2'
~ t~
through the ring focus O, ~o that the ring focus O effectively
beco~es the origin of the microwave energy directed toward the
parabolic main reflector 3. Since the ring focus O is
positioned outside of the edge of both the feed horn 1 and
subreflector 2l, all of the rays emanating from the feed horn l
will be reflected by the main reflector 3~y thereby reducing
the antenna blockage to substantially zero.
While the antenna system illustrated in Figure 2 serves
quite well to eliminate the problem of antenna blockage and
opens the way for the production of Cassegrain antennas of
~ignificantly-smaller size than heretofore possible, it i6
apparent that this system has rather rigid constraints on the
optical geometry dictated by the specific shape of the
subreflector 2'. Thus, the subreflector 2' has a reflecting
surface wh~ch requires the precise location of the phase center
A and the ring focus O relative thereto in order to produce
proper reflection of the microwave energy from the subreflector
2' to the main reflector 3. Because of the close proximity of
the two foci between the subreflector and the horn, even slight
movements of one of the two focl with respect to the other has
a profound effect on the performance of the antenna.
In this regard, if a typical high gain horn is used as the
feed horrl l, the performance of the antenna system becomes
subject to the dependency of the phase center of the hGrn on
changes in frequency. Unfortunately, as the frequency of the
antenna is varied across the operating bandwidth thereof, the
phase center A of the typical high gain feed horn 1 will shit
along the a~i.s X with the result that the geometry of the
antenna system wlll be changed producing a drastic
deteriora~ion in the operating performance thereof. In effect,
the antenna system illustrated in Figure 2 require~ that the
two ocl A and O be in relatively-close proximity to one
another and be fixed in position relative to one another for
the system ~o operate in a satisfactory manner.
One embodiment of the present invention is illustrated in
Figure 3 in the form of a Cassegrain antenna system similar to
that shown in Figure 2, but with ~ low gain feed horn 1', such
as a prime focal horn, provided in place of the high gain horn
1. First of all, the low gain feed horn 1' presents a
relatively-fixed phase center which is independent of changes
in frequency, eliminating the problem inherent in the system of
Figure 2. Secondly, with the arrangement of Figure 3 there is
no longer any requirement for a feed angle ~ of twenty to
thirty degrees to eliminate spillover at the subreflector 2',
since the feed horn l~ can be placed in very-close proximity to
the subreflector 2', for example, with the phase center A
approximately one-half to one inch from the peak of the sloping
surface, thereby producing a very-compact feed arrangement
which is as effective as a direct feed insofar as antenna
blockage is concerned. So long as the feed horn l is slightly
smaller than the diameter d of the subreflector, there will be
absolutely no blockaye in the antenna of the present invention.
Further, since the phase center of the low gain feed horn 1' is
relatively insensitive to fre~uency, the optical geometry of
the system will be preserved over a wide bandwidth of operating
frequencies.
--10--
The low gain feed horn 1I may in its most-simple form be
provided as an open waveguide of circular or rectangular cross
section which typically has no gain whatsoever. Boweverr a
horn of the type illustrated in Fi~ure 4 represents
one example of a type of low gain horn which may be used in
accordance with the present inven~ion~ This type of horn
typically provides a gain of 5 to 8 db, as compared to the more
conventionally-used flared horn shown in Figure 2, which
provides a gain as high as 25 db. As seen in Figure 4, the
10 tlorn is provided as a waveguide 4 having a rectan~ular
section 4a extending into a circular section 4b via a step 4c.
A corrugated section 5 is provided around the peri2hery of the
circular section 4b adjacent to the end of the waveguide 4.
This type of horn, which has been used heretofore primarily as
a prime feed horn, has the advantages of very simple
construction and is very light in weight compared to the
standard flared horns normally used in Cassegrain antennas,
with the result that the system of the present invention as
seen in Figure 3 represents an antenna of very simplified
construction which can be manuf,actured for low cost in a small
size not heretofore attainable in Cassegraln antennas.
As can be seen from Figure 5A, the subreflectoe 2' is
generally circular, having a peak P from which the ellipsoidal
surface of revolution extends to the outer edge where a
plurality of annular corrugations 8 are provided in the
subreflector surface. These circumferential corrugations 8,
which have a 1/4 wavelength spacing, serve to cut down the
currents which cause edge scattering and are particularly
useful ln a system such as lllustrated in Figure 3 wherein a
low gain feed horn providing a feed angle of 90 degrees or more
is provided. Figure 5B shows a subreflector 2' also having
edge current control in the form of radial corrugations 8 ' ~
While we have shown and described several embodiments in
accordance with the present invention, it is understood that
the invention is not limited to the details shown and described
herein but ls susceptible of n~merous changes and modifications
as obv.ious to one of ordinary skill in the art, and we
therefore do not wish to be limited to the details shown and
described herein but intend to cover all such changes and
modifications known to those skilled in the art.
