A reflex feed system is described for a dual frequency antenna such as one which transmits and receives both S and X band signals. The dichroic plate, normally employed for directing X band radiation away from the X band horn, is replaced by a flange about the opening of the X band horn

Hartop, R. W.

English

NASA Technical Reports Server

United States Patent [191 
Fletcher et al. 
[111 4,092,648 
[45] May 30, 1978 
[54] REFLEX I%ED SYSTEM FOR DUAL 
FREQUENCY ANTENNA WITH 
FREQUENCY CUTOFF MEANS 
[76] Inventors: James C. Fletcher, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of; Robert W. Hartop, 
South Pasadena, Calif. 
[21] Appl. No.: 780,728 
[22] Filed Mar.24,1977 
[51] Int. ( 3 . 2  ............................................. HOlQ 19/14 
[52] U.S. c1. .................................... 343/782; 343/837; 
343/781 CA 
[58] Field of Search ................ 343/781 CA, 782, 837, 
343/838 
~561 References Cited 
U.S. PATENT DOCUMENTS 
3,763,493 10/1973 Shimada et al. .............. 343/781 CA 
Primary Examiner-Alfred E. Smith 
Assistant Examiner-David K. Moore 
Attorney, Agent, or Firm-Monte F. Mott; John R. 
Manning; Paul F. McCaul 
[571 ABSTRACT 
A reflex feed system for a dual frequency antenna 
which may be, for example, one which transmits and 
receives both S and X-band signals, uses, in place of the 
dichroic plate normally employed for directing X-band 
radiation away from the X-band horn a flange about the 
opening of the X-band horn. 
5 claims, 3 Drawing Figures 
+ 
12 
https://ntrs.nasa.gov/search.jsp?R=19780023378 2020-03-20T13:25:50+00:00Z
U, S. Patent May30,1978 4,092,648 
FIG. I 
34 + 12 
F IG.  3 
FIG.  2 
4,092,648 
1 
REFLEX FEED SYSTEM FOR DUAL FREQUENCY 
ANTENNA WITH FREQUENCY CUTOFF MEANS 
ORIGIN OF INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 USC 2457). 
BACKGROUND OF THE INVENTION 
This invention relates to antennas and more particu- 
larly to an improved reflex feed system for a dual fre- 
quency antenna. 
It has been the custom to employ, with antennas 
transmitting dual frequencies, such as S-band and X- 
band frequencies, a dichroic plate for reflecting the 
S-band signals to the ellipsoidal reflector and cone of 
the S-band horn, while passing X-band signals to the 
X-band horn. It has been found that this dichroic plate 
causes two to three Kelvins noise temperature increase 
at the X-band frequency. Further, the dichronic plate 
must be separately mounted and aligned, requiring spe- 
cial installation and calibration procedures. It would be 
therefore, desirable to eliminate the plate, if possible. 
OBJECTS AND SUMMARY O F  THE 
INVENTION 
An object of this invention is to provide a reflex 
speed system for a dual frequency antenna which does 
not require a dichroic plate. 
Another object of this invention is the provision of a 
reflex speed system for dual frequency antenna wherein 
the function of the dichroic plate is performed without 
an increase in the noise temperature at high frequency. 
Still another object of the present invention is the 
provision of a new and novel reflex feed system for a 
dual frequency antenna. 
The foregoing and other objects of the invention are 
achieved by replacing the dichroic plate with a flange 
which is on, or a part of the aperture assembly of the 
X-band horn, and which is mounted at an angle to re- 
flect the S-band energy to the ellipsoidal reflector of the 
S-band horn. The throat of the X-band horn is below 
cutoff at the S-band frequency and therefore appears as 
a solid reflector plate to the S-band, but is transparent to 
the X-band. 
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 
read in conjunction with the accompanying drawings. 
BRIEF DESCRIPTION O F  THE DRAWINGS 
FIG. 1 is a schematic drawing of the presently em- 
ployed dual frequency antenna, illustrating its reflex 
feed system. 
FIG. 2 is a schematic drawing of a dual frequency 
antenna having a reflex feed system in accordance with 
this invention. 
FIG. 3 is an enlarged sectional view of the flange, 
illustrating choke rings. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENTS 
Referring to FIG. 1, the incoming energy, which is, 
by way of example, at X-band and S-band frequencies, 
is received by the main reflector 10 of the antenna, 
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which redirects it to the subreflector 12 in the casse- 
grainian configuration. The subreflector 12 redirects 
the energy to the feed cone assembly, which includes 
the X-band feed horn, 14, and an S-band feed horn 16. 
The feed horns, respectively 14 and 16 are mounted 
within feed cones respectively 18 and 20, which in turn 
are connected to transmitting and receiving apparatus 
22. 
The X-band and S-band radiation, represented by the 
dotted lines, 24, are directed by the subreflector at the 
dichroic plate 26, which passes X-band radiation to the 
X-band horn 14, and reflects S-band radiation to an 
ellipsoidal S-band reflector 28. The S-band reflector 
redirects the S-band energy to the S-band horn. 
As indicated, the presence of the dichroic plate, 
through which the X-band energy must pass, causes at 
least 2-3 Kelvins noise temperature increase with con- 
sequent reduction in ability to receive or discern same 
signals. 
FIG. 2 is a schematic drawing of the reflex feed sys- 
tem in accordance with this invention. These structures 
which are the same as the ones shown in FIG. 1 bear the 
same reference numerals. The dichroic plate is elimi- 
nated. The feed horn 14 is also omitted since the flange 
structure may be fabricated to include horn structure 
32. A circular flange 30 is mounted on the X-band horn 
32 at an angle to the axis of the X-band horn, which 
serves to reflect the S-band energy to the ellipsoidal 
reflector 28 and from there to the S-band horn 16. Be- 
cause of the required flange angle, one side of the X- 
band feed horn is longer than the other. However, the 
field pattern of the X-band horn is not affected provided 
a dual-mode design is utilized. In an embodiment of the 
invention which was constructed, the flange was made 
to have a diameter of about five feet for X-band reflec- 
tion. It was constructed of the same material as the 
horn, Le., aluminum, or copper. 
Because the throat of the high frequency or X-band 
feed is below cutoff at the S-band or low frequency, the 
flange, which replaces the dichroic plate, appears to be 
shorted (or solid) at the low frequency. Adjusting the 
exact length of the high frequency horn allows the short 
to appear exactly at the aperture. To improve perfor- 
mance at both X and S band frequencies low and high 
frequency rings, which may be called choke rings and 
which comprise circular slots, shown in FIG. 3, may be 
cut into the flange surface. 
FIG. 3 is an enlarged sectional view of a portion of 
the flange 30 mounted on the horn 32. The X and S 
band choke rings 34, may be cut into the top flange 
surface. The technique for making these rings is well 
known by those skilled in the art. 
The use of the flange reflector in place of the dichroic 
plate provides a simpler structure as well as one which 
is less expensive to fabricate, and which is easy to install 
and align. Further, and most importantly, it results in 
improved overall system performance due to reduced 
noise temperature contribution of the antenna. 
I claim: 
1. In a dual frequency antenna system of the type 
which transmits or receives signals at a first high fre- 
quency which is high relative to signals at the second 
lower frequency, and wherein a reflex feed system is 
provided in which the signals are directed in a first path 
toward a high frequency horn, and where it is necessary 
to permit the high frequency energy in said first path to 
reach a high frequency horn, but to redirect the lower 
frequency signals from said first path to a second path to 
4,092,648 
3 4 
be received by a lower frequency horn, the improve-’ flange means mounted on said high frequency horn 
ment comprising: for directing lower frequency energy away from 
a flange means mounted on said high frequency horn said high frequency horn, while blocking said 
for redirecting lower frequency signals to said lower frequency signals from flowing through said 
second path while blocking said lower frequency 5 high frequency horn; and 
signals from flowing through said high frequency a reflector positioned to receive the reflected lower 
horn. frequency energy and direct it at said lower fre- 
2. An antenna as recited in claim 1 wherein there is quency horn. 
included means for supporting said flange means at an 4. An antenna as recited in claim 3 wherein said high 
angle to said first path for deflecting said lower fre- 10 frequency horn has an aperture into which said high 
quency signals into said second path. 
3. A dual frequency antenna of the type wherein a there is provided means for supporting said flange 
cassegrainian main reflector reflects signals at a first means about the opening of said high frequency 
high frequency which is high relative to signals at a horn at an angle for directing said lower frequency 
second lower frequency onto a subreflector, there are 15 energy at said reflector. 
provided in side by side relationship, receiving means 5. An antenna as recited in claim 3 wherein there are 
including a first high frequency horn and a second choke rings cut into the surface of said flange means 
lower frequency horn, and said subreflector directs the which are operative at said respective high and lower 
high and lower frequency signals at said high frequency frequencies. 
frequency energy is directed by said subreflector, and 
* * * * *  horn, the improvement comprising: 20 
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Reflex feed system for dual frequency antenna with frequency cutoff means

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