Cassegrain antenna subreflector flange for suppressing ground noise and increasing antenna transmitting efficienc

Potter, P. D.

English

NASA Technical Reports Server

Sept. 28, 1965 JAMES E. WEBB 3,209,361 
ADMINISTRATOR O F  THE NATIONAL AERONAUTICS 
AND SPACE ADMINISTRATION 
CASSEGRAINIAN ANTENNA SUBREFLECTOR FLANGE 
FOR SUPPRESSING GROUND NOISE 
- - 
Filed Jan. 14, 1963 
INVENTOR. 
PW/P D POTTER 
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3,209,36 B United States Patent Ofice pa, ,,,, ,, ,965 
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flector 16 and are reflected into the horn feed to amear 
3,209,161 
CASSEGRAISIAU ANTENNA SUBREFI.ECTOR 
Fl..AYGP;: FOR SUPII'II<ESSYNG GROUND 
NOISE 
Barnes E. Webb, administrator of the National Aeronau- 
tics and Space Adminisbation, with respect to an 
invention of Philip D. Potter 
Filed Jan. 14, 1963, Ser. No. 251,451 
5 Claims. (@I. 343-781) 
A 
as noise which i; added to the received signal. 
In accordance with this invention, a conical reflector 
flange 24 may be mounted on the edge of the subreflector 
where it is desired to prove an already built antenna, or 
may be formed integrally with the subreflector when 
the antenna is made. FIGURES 2 and 3, respectively 
side and front elevation views of the subreflector and 
flange, illustrate the position of the flange with respect 
This invention relates to antennas and more particu- 
larly to an improvement to antennas of the Cassegrainian 
type. 
A Cassegrainian antenna usually comprises a parabolic 
main reflector dish, a smaller subreflector facing the 
parabolic reflector, and a horn feed facing the subreflec- 
tor. When the antenna is used for receiving wavelengths 
which are of significant size as compared to the size of 
the antenna, some energy radiated by the surrounding 
ground strikes the subreflector and enters the horn feed, 
and this adds noise to the received signal. Similarly, 
when the antenna is used to transmit energy some of the 
energy radiated by the horn feed bypasses the subreflec- 
tor or the main reflector dish, and is lost. 
An object of this invention is the provision of struc- 
ture for substantially minimizing noise in signals re- 
ceived by this antenna. 
Another object of this invention is the provision of 
an improved Cassegrainian antenna which is more effi- 
cient than presently known antennas of this type. 
Yet another object of the present invention is the pro- 
vision of a novel and improved Cassegrainian antenna 
structure. 
These and other objects of the invention may be 
achieved by providing a flange around the edge portion 
of the subreflector of a Cassegrainian antenna. The 
flange makes a smaller angle with the plane of the sub- 
reflector than does the edge of the major hyperbolically 
shaped portion of the subreflector. The optional flange 
size depends on the frequency being used. The flange 
not only reduces the noise level of received signals, but 
also improves the efficiency of the antenna when it is 
used for transmitting. 
The novel features that are considered characteristic 
of this invention are set forth with particularity in the 
appended claims. The invention itself, both as to its 
organization and method of operation, as well as addi- 
tional objects and advantages thereof, will best be under- 
stood from the following description when read in con- 
nection with the accompanying drawings, in which: 
FIGURE 1 is a side sectional view of a Cassegrainian 
antenna showing the improvement in accordance with 
this invention. 
FIGURE 2 is a side view of the subreflector and flange 
in accordance with this invention. 
FIGURE 3 is a front elevation view of the subreflector 
and flange in accordance with this invention. 
A Cassegrainian antenna comprises a rotatable pedes- 
tal 10 supporting an adjustable mount 12 which supports 
the main reflector dish 14. The main reflector dish 14 
receives microwaves from space and directs them toward 
a subreflector 16 which reflects them to a horn feed 18, 
positioned at the center of the main dish 14. A subre- 
flector is usually supported by members 20, extending 
from the edges of the main dish 14. 
The Cassegrainian antenna is used for receiving micro- 
waves, some of which have frequencies for which the 
wavelengths are not insignificantly small when compared 
to the sizes of the main dish and the subreflector. Micro- 
waves having these wavelength dimensions which are 
emitted by the warm surrounding ground area 22, bend 
around the edges of the main dish 14, strike the subre- 
10 to the subreflector. su he flange also serves to reflect 
microwaves from the surrounding warm ground 22, into 
the horn feed 18. However, as represented by the dotted 
lines in FIGURE 1, the path 26 from a point 28 on the 
ground to the subreflector 16 and thereafter to the horn 
15 feed 18, is approximately one-half wavelength shorter 
than a direct path 30 from the same ground point 28 
to the flange 24, and then to the horn feed 48. When 
the two waves arrive at the horn feed they are about one 
hundred and eighty degrees out of phase and cancel each 
20 other out, and the amount of noise is accordingly substan- 
tially minimized. 
When the antenna is used for the purpose of trans- 
mitting, some of the energy emitted from the horn feed 
18 which is then reflected from the subreflector 16, bends 
25 around the edges of the main dish 14 and is lost, since 
it is not reflected back by the main dish. The flange 
24 also reflects energy past the edge of the main dish 14 
but this operates to cancel out the bent rays from the 
subreflector 16. This prevents the loss of this energy 
30 to the surrounding ground and increases antenna effi- 
ciency. Also, some of the energy from the horn feed 
18 which passes around the subreflector and would or- 
dinarily be lost, hits the flange 24, and is reflected toward 
the main dish to further increase antenna efficiency. 
35 The size of the flange 24, and its angle with respect 
to the axis of the subreflector 16, are adjusted until the 
noise level is reduced to a small amount, while the ef- 
ficiency of the antenna is still large. In FIGURE 2, 
6 is the angle between the flange 4 and the plane of the 
40 subreflector, and a is the angle between a target to the 
subreflector and the plane of the subreflector. It  is 
found in practice that, with a Cassegrainian antenna 
operating at a given frequency, a flange having an inner 
to outer radius difference of two wavelengths at an angle 
45 6 of about three fourths the size of the angle a will 
reduce the noise level to a low level while maintaining 
a high antenna efficiency. Specifically, it is found that 
a Cassegrainian antenna with a main dish 114 of 8.5 fcet 
in diameter subtending an angle r of one hundred twenty- 
50 two degrees (shown by the dotted lines in FIG. I ) ,  and 
having a subreflector 16 with a diameter of 0.8 feet, 
with an edge making an angle a of 24.4 degrees with 
the plane of the subreflector, and receiving and trans- 
mitting at a frequency of 9.6 kmc., that a flange having 
55 a rim of 0.2 feet between inner and outer radii and mak- 
ing an angle 6 of 18.4 degrees with the plane of the sub- 
reflector 16, will reduce the noise to almost a minimum 
while increasing the efficiency to near its maximum. 
A flange having a shape other than conical may be 
60 used to achieve the same or a somewhat better effect, 
though the conical shape is very simple to manufacture. 
Any of these shapes will involve a departure from the 
hyperbolic shape of the typical Cassegrainian subreflec- 
tor (which is the shape for best results with very small 
65 wavelengths compared to antenna size, as with visual 
light). The main element of the departure, in accord- 
ance with this invention, is a rim or flange portion on 
the subreflector, which makes a smaller angle with the 
plane of the subreflector than a target to the edge of 
70 the major hyperbolically shaped portion of the subre- 
flector. As was previously pointed out, instead of a 
flange, the subreflector may be enlarged to include a rim 
3,2( 
3 
portion of the proper shape to make the subreflector and 
flange comprise a single article. 
Accordingly, there has been shown and described 
herein a novel, and improved Cassegrainian antenna. 
P claim: 
1. The improvement in a Cassegrainian antenna hav- 
ing a subreflector with a curved reflecting face and de- 
fining a plane with the edges of said curved reflecting 
face, said improvement comprising a reflecting flange ex- 
tending radially from the curved reflecting face and mak- 
ing an angle with the plane defined by the edges of said 
curved reflecting face which is smaller than the angle 
made by the portion of the curved reflecting face near 
the edge thereof with said plane, the radial length of 
said reflecting flange being on the order of an integral 
number of wavelengths of the frequency at which said 
antenna is used. 
2. The improvement in a Cassegrainian antenna of 
the type having a subreflector with a curved reflecting 
face and defining a plane with the edges of said face, 
said improvement comprising a flange extending from 
the edge of said curved reflecting face, said flange having 
an inner to outer radius difference on the order of two 
wavelengths at the frequency with which said antenna 
is employed, said flange making an angle with the plane 
defined by the edges of said face which is less than the 
angle made by the material of said subreflector adjacent 
to said edges with said plane. 
3. A Cassegrainian antenna subreflector comprising a 
central region having a curved reflecting face, and a 
peripheral region comprising a flange extending from 
the edge of said curved reflecting face, said curved re- 
flecting face defining a plane with the edges thereof, 
the inner and outer radius of said flange differing by 
substantially two wavelengths of the frequency at which 
said antenna is employed, said flange making an angle 
with the plane defined by the edges of said subreflector 
which is substantially three fourths of the angle made 
39,361 
4 
by the material around the edges of said subreflector 
with said plane. 
4. In a Cassegrainian antenna of the type having a 
parabolic main reflector dish, a smaller subreflector fac- 
ing the parabolic reflector and a horn feed facing the 
subreflector, and wherein signals outside of the target 
of said main reflector dish are reflected by said subre- 
flector to said horn feed as noise signals, the improve- 
ment comprising flange means extending outwardly from 
the edge of said subreflector for reflecting signals received 
from the locations outside of the target of said main 
reflector to said horn feed one hundred and eighty de- 
grees out of phase with said signals reflected to said horn 
feed by said subreflector received from said locations 
15 outside of said main reflector to cause a cancellation 
of said noise signals. 
5. A Cassegrainian antenna comprising a parabolic 
main reflector dish, a smaller subreflector facing the 
parabolic reflector and a horn feed facing said subreflec- 
20 tor, said subreflector including a flange extending from 
the edges thereof, said flange extending radially a dis- 
tance on the order of twice the wavelength at the fre- 
quency of operation of said antenna, said flange making 
an angle with the plane defined by the edges of said 
25 subreflector which is on the order of three fourths of 
the angle made by the material of the subreflector near 
its edges with the plane defined by the edges of said 
subreflector. 
References Cited by the Examiner 
30 UNITED STATES PATENTS 
2,342,721 2/44 Boerner -------------- 343-781 
2,477,694 8/49 Gutton -------------- 343-837 
2,895,13 1 7/59 Butler --------------- 343-840 
35 3,071,770 1/63 Wilkes -------------- 343-840 
HERMAN KARL SAALBACN, Priti7nry Exanlitzer. 
ELI LIEBERMAN, Exanzit7er. 


Cassegrainian antenna subflector flange for suppressing ground noise Patent

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