Design and operation of multi-feed cone Cassegrain antenn

Katow, M. S.

Levy, G. S.

Stelzried, C. T.

English

NASA Technical Reports Server

NATWNAL AERONAUTlCS AND SPACE ADMINETRATION 
WAU(IN@TW, D.C, 2OslC 
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G P / O f f i c e  of Assiqtaqt General Counsel 
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In accordance w i t h  the procedures agreed upon by Code GP 
and Code U S I ,  the attached NASA-owned U. S. Patent i s  being 
forwarded for  abstracting and announcement in NASA STAR. ' 
%!he following information i s  ;drrwidedg 
D 
Um Sm Patent NO. .? 8 3.;534,375 
., . 
Government or California Institute of Technology 
Corporate Employee 8 Pasadena, California 
Supplementary Corporate 
Source (if applicable) 8 Jet Propulsion Laboratory 
NASA Patent Case Nom '8 NPO-PO539 
NOTE - If this  patent covers an invention made by a corporate 
employee of a NASA Contractor, the foflowing is applicable: 
Pursuant to Section 305(a) of the National Aeronautics and 
Space A c t ,  the name of the Administrator of NASA appears on 
the firs% page of the patent? howevero the name of the actual 
inventor (author) appear8 a% the heading of Column No. 1 of 
the Specification, following the words with respect ta 
Y e s  a NO n 
https://ntrs.nasa.gov/search.jsp?R=19710001810 2020-03-17T02:35:42+00:00Z
MULTI-FEED CONE CASSEGRAIN ANTENNA 
Filed J u l y  9, 1968 2 Sheets-Sheet L 
\ 
IG. I 
MULTI-FEED CONE C A S S L G R A I N  ANTENNA 
Filed July 9, 1968 2 Sheets-Sheet 2 
n 
M M U  SMOOT KAT0 
I 
the entire Cassegrain feed support structure, an operation 
which requires a considerable amount of time. 
A need therefore exists for an improved Cassegrain 
antenna in which feed cone changing is accomplishable in 
3,534,375 
to ana 
e m ,  G e r m  a minimum of time. 
S. Levy, La Canada, a d  Ndasakm~ S m ~ t  Katow, LOS OBJECT- m~ SUMMARY OF THE W E N T I O N  
It is a primary object of the present invention to pro- 
Another object of the present invention is to provide 
lo a Cassegrain antenna with a multi-feed cone arrangement. 
A further object of the present invention is to provide 
A CT a Cassegrain antenna which is rapidly controllable to 
operate with any one of a plurality of feed cones. 
Still a further object of the present invention is to pro- 
vide a Cassegrain antenna with a relatively simple ar- 
rangement for rapidly switching the antenna to incorpo- 
rate therein any one of a plurality of feed cones, sup- 
ported therein. 
These and other objects of the present invention are 
achieved by providing a Cassegrain antenna in which a 
plurality of feed cones, hereafter referred to  generally 
as the multi-feed cone arrangement, are fixedly positioned 
with respect to the parabolic reflector. The antenna also 
25 incorporates an asymmetrically truncated hyperbolic sub- 
reflector which is rotatable about the antenna’s center 
axis to  focus on one of the feed cones which is to be 
used. The configuration of the hyperbolic subreflector is 
selected so that one of its conjugate foci coincides with 
30 the parabola’s focal point, while its other focus coincides 
with the phase center of the particular feed cone to be 
used. Thus, instead of changing the feed system of the 
antenna by replacing one special purpose cone with an- 
other, feed cone changing is accomplished in the present 
This invention generally relates to antenna design and, 35 invention by rotating the hyperbolic subreflector from one 
feed cone to the other. 
The novel features of the invention are set forth with 
particularity in the appended claims. The invention will 
best be understood from the following description when 
BRIEF DESCRIPTION OF THE DRAWINGS 
vide a new improved Cassegrain antenna. us. c1. 343-779 ‘I Claims 
A Cassegrain antenna with a plurality of feeds, fixed- 
ly positioned with respect to a parabolic reflector about 15 
the antenna’s axis of symmetry, is disclosed. The phase 
centers of all the feeds are in a plane, perpendicular to 
the axis of symmetry. A hyperbolic subreflector which is 
asymmetrically truncated is included. The subreflector is 
rotatable about the axis of symmetry so as to reflect 20 
signals between the parabolic reflector and a selected one 
of the feeds. 
ORIGIN OF INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is subject 
to the provisions of Section 305 of the National Aero- 
nautics and Space Act of 1958, public Law 85-568 (72 
stat. 435; 42 U.S.C. 2457). 
BACKGROUND OF THE INVENTION 
Field of the invention 
more particularly, to a Cassegrain antenna with a plu- 
rality of feed cones. 
Description of the prior art 
grain antenna are well known by those familiar with an- 
tenna design. All known prior art Cassegrain antennas 
consist of a single feed cone which is typically located at FIG. 1 is a diagram which is useful in explaining a 
or near the vertex of a parabolic reflector. A hyperbolic Prior art Cassegrain antenna; 
subreflector, hereafter also referred to simply as the sub- 45 FIG. 2 is a diagram which is useful in explaining the 
reflector, or the hyperbola is located in front of the re- improved Cassegrain antenna of the present invention; 
flector or parabola, between its vertex and its focal point and 
or focus. FIG. 3 is an illustration of an asymmetrically truncated 
parallel rays coming from a target, assumed to be at hyperbolic subreflector actually reduced to practice in one 
infinity, are reflected by the parabola as a converging 50 embodiment Of the invention. 
DESCRIPTION OF THE PREFERRED beam and are rereflected by the hyperbola converging 
EMBODIMENTS at the position of the feed cone or feed. In essence, the 
hyperbolic subreflector images the feed so that it appears 
as a virtual image at the focal point of the parabola. Prior to explaining the novel features of the present 
Typically, a feed cone system is designed to operate invention, reference is made to FIG. 1 which is a simpli- 
at a selected frequency in a selected frequency band. At- fied diagram, useful in explaining the geometry of a con- 
tempts to increase the usefulness of a Cassegrain antenna, ventional signal feed Cassegrain antenna. Therein, nu- 
especially of the type used for space exploration, have meral 11 designates the antenna’s parabolic reflector with 
led to the design of multi-frequency feed cones so that 6o a focal point F located on the antenna’s axis of sym- 
one cone could be used for several missions for experi- metry which is designated by numeral 22. A single feed 
ments in a specified frequency band, such as the S-band. system 13 is shown located with its phase center a t  a 
However, from the use of such a multi-frequency feed point F‘, which together with point F represent the con- 
cone, it has become clear that meaningful improved per- jugate foci of a hyperbolic subreflector 15. The foci are 
formance and the ability to perform experiments at vari- shown aligned on the axis of symmetry 22. 
ous frequencies could only be achieved by the use of spe- 65 Unlike such a prior art single feed Cassegrain antenna, 
cial purpose cones, each one of which is specifically de- in accordance with the teachings of the present invention, 
signed to receive signals in a relatively narrow frequency which may best be explained in conjunction with FIG. 2, 
range. However, as presently constructed, in order t o  a plurality of feeds, which are fixedly supported with re- 
change the feed system of a Cassegrain antenna from one spect to the parabolic reflector PI, are incorporated. In 
special purpose cone to another, it is necessary to remove 70 FIG. 2,pnly two feeds 13a and 1% are diagrammed. I t  
The structure and principles of operation of a casse- 4o read in conjunction With the accomPmYing drawings. 
3,534,375 
should, however, be appreciated that any number of feeds 
was actually reduced to practice, three feeds, spaced 120" 
apart, were incorporated. The phase centers of the two 
Angles 
01 ________________-___---------------- 83"38'57" 
p _______________--------------------- l"21'3" 
may be incorporated. In one specific embodiment which 0 __________---__--_---- - - --  ---- 5"00'00" 
feeds Isa and 13b, designated in FIG. 2 by F', and F'b y _______________------- - ---__---___-_ 13"32'47" 
are located equidistantly from the axis of symmetry 12, 6 .................................... 2"9'6" 
and in a plane which is perpendicular to  the axis 12. E ___________________-_________________  2"50'54' 
Unlike prior art in which the hyperbolic subreflector pz __-___------------------ ----------- 18 "30'00'' 
is fixedly positioned, in the Cassegrain antenna of the /?3 -_______-  ---- --------- _--_ ---_-_-- 19"30'28" 
The subreflector 15, is rotatable about the axis of sym- h .................................... 4"44'27" 
metry 12 by a subreflector rotating unit, which is desig- 71 ................................... 14"39'31" 
nated in FIG. 2 by block 268. 112 ______________-----_---_--_------_- 14"52'48" 
The subreflector which is hyperbolic is truncated asym- -- -- - - - -- -- - - - - -- - - - - - - - --- --- -- _- - 14"48'43" 
present invention a rotatable subreflector 15x is included. PL ..................... -----------_-- 17 ' 27'35" 
TIE 
metrically, a requirement dictated by the off-axis-of- 15 
symmetry location of each of the feeds. The asymmetri- 
cally truncated hyperbolic subreflector 15x is designed SO 
that after the subreflector is rotated and positioned so as 61.7663 
to focus on one of the feeds, its two foci coincide with 70.3839 
the parabolic focal point F and the phase center of the 20 65.7941 
particular feed. In the arrangement diagrammed in FIG. Inches 
2 the subreflector 15x is shown focused on feed 13a3 a _______________----__________________ 178.0286 
while the dashed line 15y represents the Position of the b _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _  47.0642 
subreflector, required for focusing on the second feed 1%. c _______________-_____L_________ 270.0 
From the foregoing it should thus be apparent that 25 f _____________________________________  229.5112 
switching from one feed to another does not require g _____________________________________  253.5112 
changing of a feed system, which as experienced in the h _____________________________________  91.6214 
past is very time consuming. In accordance with the pres- i _____________________________________  446.3239 
ent invention, switching is accomplished by rotating the j _____________________________________  196.2879 
asymmetrically truncated subreflector 15x so that its real 30 k _____________________________________  21.8475 
focal point coincides with the phase center of the de- I _____________________________________  422.8580 
sired feed. It should further be pointed out that the rota- m _ _ _  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _  68.91 18 
tion is performed about the antenna's axis of symmetry IZ _____________________________________  333.0607 
12. In the particular diagrammed arrangement of the two p _____________________L_______________ 5.4611 
feeds, if the two are located 180" apart, all that is re- 35 4 _____________________________________  91.9713 
Wired to switch the antenna from One feed to the Other s _____________________________________  65.9990 
is to rotate the subreflector 15x by 180". Likewise, 120" t _ _ _ _ _ _ _  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _  112.7373 
rotation of the subreflector 15x is required in the pre- _ _ _ _ _  _ _ _ _ _ _ _ _  197.5000 
ViOUSlY referred to m-~bodimen~ in which three feeds, u ----______-____-_____________________ 8.0158 
120" apart, are incorporated. 40 
It should be pointed Out that the particular shape Of r, _____________________________________  127.8347 
the asymmetrically truncated hyperbolic subreflector de- 
pends on the overall antenna dimensions, including the Although particular embodiments of the invention have 
size and dimensions of the parabolic reflector PI, which been described and illustrated herein, it is recognized that 
defines the location of the focal point F, and the distance 45 modifications and variations may readily occur to those 
between the phase center of each feed and the focal point skilled in the art and consequently it is intended that the 
F. It should also be pointed out that any conventional claims be interpreted to cover such modifications and 
means and techniques may be employed in the imple- equivalents. 
mentation of unit 20 (FIG. 2)  to controllably rotate the 
subreflector 15x about axis 12. 
For explanatory and exemplary purposes only, ref- 
erence is made to FIG. 3 which is a useful illustration of 
the truncation of the hyperbolic subreflector 
rated in the three-feed antenna previously referred to. 
In FIG. 3, the letter A designates the parabola's focal 55 
point, corresponding to the previously defined point F. 
Although the parabolic reflector is not shown its vertex is 
designated by the letter H. Numeral 25 represents one of 
the three feeds. The curved surface of the hyperbolic re- 
flector is along line JCDL. Numeral 30 designates a flange 60 
about the subreflector. The function of the flange does 
not form part of this invention and therefore will not be 
discussed in any detail. The aCCOmPanYing legend Pro- 
vides the dimensions and angles of the specific embodi- 
ment illustrated in FIG. 3. 
LEGEND 
COORDINATES IN REF. TO A 
Y X 
....................... 107*1967 
....................... 122*1528 
....................... 
r3 _-_____________-_____________________ 101.3948 
What is claimed is: 
I. In a Cassegrain antenna of the type including a para- 
bolic reflector with a defined focal point and an axis of 
symmetry the improvement comprising: 
a plurality of feed cones, each defining a phase center, 
said feed cones being fixedly positioned about said 
axis of symmetry so that the feed cones' phase cen- 
ters are equidistantly positioned from said axis of 
symmetry in a plane perpendicular thereto; and 
rotatable reflecting means for deflecting signals between 
a selected one of said feed cones and said parabolic 
reflector. 
2. The Cassegrain antenna as recited in claim 1 where- 
in said reflecting means include an asymmetrically trun- 
cated hyperbolic reflector defining first and second conju- 
gate foci, said first foci coinciding with the parabolic re- 
65 flector focal point, said reflecting means including means 
for controllably relating said hyperbolic reflector about 
said axis of symmetry so as to bring the second focal point 
of said hyperbolic reflector into coincidence with the phase 
center of a selected one of said feed cones. 
a main parabolic reflector having a defined focal point 
and a defined axis of symmetry; 
a plurality of feed means through which energy is trans- 
mittable, each feed means defining a phase center, 
said feeds means being fixedly positioned with respect 
50 
Points: 
A-Focus of the paraboloid and the secondary focus 
B-Primary focus of the hyperboloid 
C-Vertex of hyperboloid 
D-Rotation axis of hyperboloid 
E-Geometric center of the hyperboloid's area 
H-Vertex of the paraboloid 
of the hyperboloid 70 3. A Cassegrain antenna comprising: 
75 
3,534,375 
to said parabolic reflector so that their phase centers 
are in a plane perpendicular to said axis of symmetry; 
energy reflecting means defining a first focal point and 
a second focal point; and 
means for controllably rotating said energy reflecting 
means about said axis of symmetry with the first 
focal point of said energy reflecting means fixedly 
coinciding with the focal point of said parabolic re- 
flector, and the second focal point of said energy re- 
flecting means coinciding with the phase center of a 
selected one of said feed means. 
4. The Cassegrain antenna as recited in claim 3 where- 
in said energy reflecting means comprise an asymmetric 
hyperbolic reflector rotatable about said axis of symmetry 
with the distance between its first real focal point and its 
second virtual focal point being equal to the distance be- 
tween the focal point of said parabolic reflector and the 
phase center of any one of said feed means whereby when 
said hyperbolic reflector is focused on one of said feed 
means its virtual and real foci coincide with the parabolic 
reflector focal point and the phase center of said one feed 
means, respectively. 
5. A Cassegrain antenna comprising: 
a main reflector for focusing signals which are directed 
thereto substantially in parallel a t  a focal point there- 
of, said reflector defining an axis of symmetry; 
a plurality of signal feeds fixedly positioned with re- 
spect to said reflector, the phase center of each feed 
equidistantly displayed from said axis of symmetry, 
with all the phase centers in a common plane per- 
pendicular to said axis of symmetry; and 
rotating subreflector means including a subreflector de- 
fining conjugate foci, and means for rotating said sub- 
reflector about said axis of symmetry, with one of 
the foci of said subreflector continuously coinciding 
with the reflector focal point until the other of the 
foci coincides with the phase center of a selected one 
of said feeds. 
6. The antenna as recited in claim 5 wherein said re- 
flector is a parabolic reflector. 
7. The antenna as recited in claim 6 wherein said sub- 
reflector is an asymmetrically truncated hyperbolic sub- 
References Cited 
UNITED STATES PATENTS 
3,164,835 1/1965 Alsberg _ _ _ _ _ _ _ _ _ _ _ _  343-781 
FOREIGN PATENTS 
lo 
15 reflector. 
20 
662,023 4/1963 Canada. 
ELI LIEBERMAN, Primary Examiner 
25 US. C1. X.R. 
343-839,840 


Multi-feed cone Cassegrain antenna Patent

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