Project Cyclops, A Design... - Department of Earth and Planetary ...

SHADOWED
AREA
SHADOWED
AREA
//
//
/
/
/
ROT T TLE
/
f
FOCAL
POINT
\
/
\
/
X
\
\
/
/
/
Figure 9-5a. Rotating turret for axial feeds.
Figure
/ - )\
9-5b.
"x,_
'\ /
FOCAL/
PO, y/
ROTATABLE
X
\
\
/
/
/
TURRET
Ro taring turret for off-axis ]beds.
Second, large feed horns that illuminate the secondary
rather uniformly with little spill can be used
without increasing the total shadowing. These large feed
horns have lower response at large off-axis angles than
the smaller horns that would be needed at the prime
focus to cover the wide subtended angles of the primary
mirror. This reduces the sensitivity of the system to local
interference picked up directly by the feed.
Third, the feed-horn spillover at the secondary is
directed at the sky so radiation received past the rim of
the secondary comes from the sky rather than from the
(hot) ground. Feed-horn spillover thus causes far less
elevation of the antenna temperature than in a prime
focus feed.
Fourth, in a large antenna, the secondary can be
many many wavelengths in diameter and can thus
produce a sharper edged illumination pattern on the
primary mirror than a small horn. This allows the ground
spill to be reduced or a more uniform primary illumination
to be realized, or both.
Finally, a Cassegrainian system having a magnification
rn allows an f/d ratio l/m times as large to be used for
the main reflector for a given feed-horn pattern. (See
Appendix I.) This larger ratio greatly reduces the length
of the legs of the supporting tripod or tetrapod, which in
turn permits a smaller cross section with reduced
shadowing. In fact, with a primary f/d ratio of 114 or
less, one might consider supporting the secondary mirror
with tension members only.
If an isotropic radiator is placed at the prime focus of
a paraboloid or at the secondary focus in a Cassegrainian
system, the intensity of illumination of the primary
reflector falls off as
SHADOWED
AREA
//
/
/
//
/
/ \
]\
i \
@ I- I_ i
IUI2 I o
IUol _
-
Io
- cos 4-
2
(13)
where 0 is the angle subtended at the feed horn by the
ray considered. This ray strikes the primary reflector at a
radius
[
:\
\
x
\
I
Figure 9-5c.
6 )
5 /
MOVABLE i /
TURRET /
IOmeters
XYdisp_ceablereceiverturretfor
]Oeds.
on-axis
0
P = 2mftan m
2
(14)
where f is the focal length of the main reflector and m is
the magnification of the secondary mirror (Appendix I).
If 00 is the angle that the marginal rays subtend at the
feed horn, we see from equation (14) that
Oo d
tan
2
-
4mr
(15)
91