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

F kmJm
8. ANTENNA ELEMENTS
A tremendous amount of effort has already been
devoted to the design of large antennas for radio
astronomy, radar, satellite tracking and other applications.
The designs that have been developed over the
past two decades reflect an increasing sophistication in
the efficient structural use of materials, more recently as
a result of the use of computer aided design. Each of
these designs has had its own set of specifications as to
surface tolerance, sky coverage, environmental conditions
for operation and survival, and other factors. Thus
intercomparison of designs is extraordinarily difficult.
The limited time and manpower available for the
Cyclops study precluded the possibility of designing in
detail an antenna element for mass production and the
associated fabrication, assembly, and erection tooling
required to substantially reduce the high labor content
of one-of-a-kind designs. Instead, we have been forced to
draw some rough estimates of savings that might result
from the application of these techniques to state-of-theart
designs. The cost estimates arrived at may be
pessimistic, but can only be improved with confidence
by a much larger scale funded study.
CYCLOPS
REQUIREMENTS
At the outset of the Cyclops study some tentative
specifications were set down as guidelines. These were:
Total equivalent antenna diameter _ 10 km (max)
Frequency range _ 500 MHz to 10 GHz
Minimum elevation angle = 20 ° max.
Wind: 20 mph maximum for 10 GHz operation
100 mph minimum for survival
The total effective diameter of 10 km was based on an
early estimate of what might be required to achieve a
detection range of 1000 light-years. It now appears that
this figure may be high and that a diameter as low as 3
km might suffice (Chap. 6).
The frequency range was chosen to cover the microwave
window. If the arguments presented earlier for
favoring the low end of the microwave window are
accepted and if alternative uses of the array do not
require operation at 10 GHz with full efficiency, then
the surface tolerances required will not be excessive.
The minimum elevation angle was chosen on the
following grounds:
1. Operation below 20 ° elevation increases the
system noise temperature appreciably.
2. Operation below 20 ° elevation rapidly increases
the element separation required to prevent selfshadowing.
3. Operation down to about 20° is required to
permit continuous reception with three arrays
spaced at 120 ° around the world or to permit very
long base-line interferometry using pairs of such
arrays.
The wind specifications ultimately would have to
take into account the weather characteristics at the
chosen site. The values used in this study would allow
operation at 10 GHz for over 75% of the time with
survival expectations of over a century at many suitable
locations in the southwest United States.
TYPES
OF ELEMENTS
The usual radio astronomy antenna designed for use
in the microwave region consists of a paraboloidal dish
so mounted that it can be directed at most of the sky
that is visible at any one time. Many other types of
elements have been proposed and some have been built
that sacrifice full steerability or operating frequency
range or beam symmetry to obtain a lower cost per unit
of collecting area. Some examples are:
1. A fixed spherical dish pointed toward the zenith
with tiltable feeds that illuminate a portion of the
dish and allow limited steering of the beam. The
Preceding pageblank 77