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February 1964 Radio-Electronics
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
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Carl Sagan's message to Messier 13.
In 2003, the face of
the Old Man of
the Mountain succumbed to the forces of nature, and fell to Earth. It was
sad news. An eons-old relic was suddenly gone, despite man's efforts to sustain
it. Melanie and the kids and I drove to New Hampshire in 1988 to see it. In
2020, the iconic
Arecibo
radio telescope in Puerto Rico also succumbed to the forces of nature, and
fell to Earth. It
was also sad news. Man's efforts (or lack thereof) to sustain it were shameful. Most people, if they have ever seen it at all, probably know
nothing about it. The 1-kilometer diameter dish has been featured in multiple
movies, including James Bond's
Golden Eye (an OK movie),
Contact
(a great movie), and
Species (which I've not seen). In February 1964, this Radio-Electronics
magazine article introduced
the amazing telescope to the world, and announced the "first light" ("first
radio", more accurately) instance
in November of the preceding year. The Arecibo telescope transmitted Carl Sagan's
"Yopp!" digital
message to globular star cluster
Messier 13 (M-13, an
awesome sight in any optical telescope) in 1974.
The Biggest Telescope on Earth is IN the Earth - Cover Story
Transmission lines and electrical cabling are carried on
this 700-foot catwalk (photographed during construction), which also provides access
for personnel. The waveguide and electrical wiring had not been installed when this
photograph was taken.
By Eric Leslie
The world's biggest radar-radio telescope, illustrated on our cover, is a partly
natural, partly artificial spherical hollow in the hills 12 miles south of Arecibo,
Puerto Rico.
Why such a telescope? And why at Arecibo? What is the new instrument expected
to accomplish?
Its chief purpose is to study the ionosphere. Satellites and rocket probes as
well as radar soundings have given us many new facts about this region whose several
layers surround the earth at distances ranging from less than 50 to more than 250
miles. The study has been limited by the small amount of information that could
be obtained from existing instruments.
Prof. William Gordon of Cornell University envisioned a system using an extremely
powerful transmitter and an antenna of much higher gain than any in existence. He
believed that it would be possible to study the changes in the ionosphere by the
back-scattering of free electrons from the various layers at uhf. This would make
it possible to measure electron density and temperature, determine auroral ionization
and detect transient currents in the ionosphere. Prof. Gordon suggested that the
antenna would probably have to be a stationary dish in a natural bowl in the earth.
It should be near the equator, he said. so that the solar system would be included
in the scanning angle.
The Armed Forces are very much interested in all information obtainable about
the ionosphere, as an aid in ICBM detection and decoy discrimination. Therefore,
funds were supplied by the Advanced Research Projects Agency as part of the Project
Defender Program for exploring ICBM defense techniques. The Air Force Cambridge
Research Laboratories was assigned to provide technical management. The laboratories
immediately suggested that instead of the usual parabolic reflector a spherical
one be used, with a phased-line feed.
This would make it possible to direct the beam over an angle of 20° from
the zenith, a much wider angle than would be possible with a parabolic antenna.
The Arecibo site was selected for several reasons. It is within 18° of the
Equator and thus in a favorable position to scan the ecliptic, in which the sun
and the planets move. There was a natural bowl of very nearly the correct size and
shape.
How signals are beamed into the vertical bowl to produce
a parallel beam. The shape and slotting of the line feed control the amplitude and
the phase of the energy radiated at each point along its length. The vertical angle
of the beam can be varied by moving the line feed along the feed arm.
The temperature varies very little, so structural materials would not be greatly
affected by climatic changes. The sheltered area among higher hills is protected
from heavy winds, and the location is relatively remote from sources of man-made
interference.
Even though the bowl was nearly perfect, 300,000 cubic yards of material had
to be blasted from some spots, while 200,000 cubic yards were added in others. The
big reflector was then constructed of sheets of 1/2-inch galvanized steel mesh,
placed on a cable grid that crisscrosses the bowl north-south and east-west. To
maintain its shape, the reflector is contoured with vertical tie-down cables every
6 feet, and loaded when necessary with steel ballast rods. The surface forms part
of a perfect sphere, with a tolerance of only ±1 inch.
Since the radar beam can be steered only 20° from the zenith, a complete
hemisphere is not necessary. The radius of curvature is 870 feet, while the dish
is 1,000 feet across (a total of 18.5 acres).
Signals are beamed at this reflector in a special way: from a 96-foot line feed
made of aluminum and mounted 435 feet (half the radius) above the reflector. The
reflector being spherical rather than parabolic, the signal can be steered 20°
in any direction from the zenith. The line feed is so shaped that signals from different
parts of it reach the reflector bowl with different intensities. The lengths of
its radiating slots are calculated to vary the phase of these signals so that a
beam of parallel rays will be reflected from the bowl. The line feed is suspended
from a crescent-shaped track called the feed arm, so the vertical angle can be varied.
The feed arm in turn is suspended from a circular azimuth track girder, approximately
129 feet in diameter. Thus, the line feed can be positioned in azimuth within 1
minute of arc, and in elevation to within 0.8 minute of arc.
The structure which holds this transmitting and positioning equipment is a triangular
platform, 216 feet on a side, suspended from three concrete towers. Each of these
is 700 feet from the center of the reflector, and rises 468 feet above its upper
edge. The transmission line, 1,300 feet of waveguide, carries power from the transmitter
building just outside the bowl to the line feed.
Two ingenious waveguide joints were necessary to get power to the line feed:
a rotary joint to take care of antenna rotation, and a crescent-shaped one for the
linear joint. This is a piece of waveguide 160 feet long inside the larger curved
waveguide on the lower surface of the feed arm.
The transmitter can be operated as a continuous-wave radar at 150 kw or as a
pulsed radar with a peak power of 2.5 megawatts. The transmitter is now operating
at 430 mc, although it is expected to operate later at 40 mc, and probably also
at a frequency of 900 mc or higher.
Though most of its time will be spent studying the ionosphere, the new telescope
will have other uses. With 40.000 times the power of the Millstone Hill radar in
Massachusetts, which first detected reflected signals from the planet Venus, it
should be able to contact Venus, Mars or Mercury whenever any one of them is in
the field of view. Millstone Hill had to wait till Venus was near its closest approach
to the earth. Moreover, the new telescope will be able to produce directly observable
signals, instead of having to sort them out of background noise with the help of
a computer. It will probably also be able to make contact with Jupiter and Saturn
when they are in favorable positions. It may also improve the accuracy with which
we can determine the astronomical unit of distance, and we may even be able to observe
the atmosphere of the sun with radar.
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