May 1967 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
The first thing I learned (or re-learned) in reading this article
is that in 1967, "Hertz" had only recently been assigned as
the official unit of frequency. According to Wikipedia, International
Electrotechnical Commission (IEC) adopted it in in 1930, but
it wasn't until 1960 that it was adopted by the General Conference
on Weights and Measures (CGPM) (Conférence Générale des Poids
et Mesures). Hertz replace cycles per second (cps).
The next thing that happened was that I was reminded of
how images such as the op-art tracing of antenna oscillation
that are routinely generated today by sophisticated software,
required huge amounts of setup time and trials to yield just
a single useful and meaningful image using actual hardware.
The third thing was, wow, 1967 was 45 years ago, and that
was nine years after I was born. Ouch.
By Joseph H. Wujek, Jr.
Scheduled for an early launch is a satellite to be used for
radio astronomy purposes only. An array of space antennas having
750-foot elements will be used.
This unusual op-art tracing was made by a portion of an antenna
designed for the Radio Astronomy Explorer satellite. The photo
was made in a thermal test chamber with a small light bulb attached
to the end of about 35 feet of the antenna. The antenna was
allowed to swing free in the chamber to give engineers an insight
into what the normal deployed pattern would be after the momentum
When the brilliant Scots physicist James Clerk-Maxwell (1831-1879)
published his classic "A Treatise on Electricity and Magnetism"
in 1873, very little was known about the nature of electromagnetic
(EM) radiation. Although Maxwell predicted the existence of
EM waves, it was not until after 1885 that high-frequency EM
waves were generated in the laboratory. Heinrich Hertz (1857-1894)
is generally acknowledged to be the first to generate these
waves and was recently honored by having the unit of frequency
- "hertz" - named for him. The theoretical work of Maxwell and
the subsequent experimental research of Hertz thus paved the
way for the technology which we now know as radio. We use the
term "radio" here to include that region of the EM spectrum
which extends from a few hertz to the edge of the infrared region,
which is about 1000 gigahertz (1 million megahertz or 1 terahertz
With the development of radio communications in the twentieth
century, major emphasis was placed on gaining a better understanding
of the nature of radio propagation and noise. Measurements of
radio propagation and noise characteristics were, and continue
to be, made with international cooperation. The National Bureau
of Standards (NBS) of the U.S. Department of Commerce guides
this effort in the United States with technical coordination
maintained among NBS, other government agencies, universities,
Scale model of the Radio Astronomy Explorer
satellite, world's first satellite devoted exclusively to radio
astronomy.A natural outgrowth of propagation and noise studies
was the detection of radio-frequency noise from deep space.
Until the recent advancements in space technology, measurement
of space r.f, signals was confined to the ground or to those
altitudes accessible to aircraft. This was, of course, also
true of r.f. propagation studies. While ground-based and aircraft
measurements have contributed much to our understanding of these
phenomena, measurements from space vehicles enhance these results.
Since the earth's atmosphere acts to severely attenuate certain
r.f, frequencies, a measurement of r.f. signal strength taken
above the atmosphere provides added information regarding the
source, strength, and character of these signals.
The science of radio astronomy has also benefited from space
r.f. measurements. It has been known for some time that stars,
galaxies, and some planets emanate EM waves. The star nearest
earth, our sun, exhibits increased flare, or sunspot activity,
on a somewhat regular basis. In particular, the occurrence of
these flares increases to a maximum every eleven years (Fig.
1). Radio communications in certain frequency bands are severely
affected during such increased solar activity.
Fig. 1. Graph of noise from solar activity
at 2.8 GHz showing the last complete eleven-year cycle. Right
now solar activity is on upswing and new peak should occur around
By studying the nature of the r.f. emanations of the sun
and other stars, scientists are able to better understand the
energy processes which occur in these bodies. The solar flares,
which are believed to be reactions similar to those of a fusion
or hydrogen bomb, release enormous amounts of energy. Swarms
of charged particles and EM waves are discharged from these
reactions. The earth is about 93 million miles or 8 light-minutes
from the sun, yet some of these particles and waves find their
way through the atmosphere and ultimately reach the earth. In
an earlier article ("Radiation Measurements in Space", August
1966) we showed how energetic particles are detected and measured.
Here we will discuss systems used to measure r.f. energy in
Instruments used to measure radiation in the EM spectrum
are called "radiometers". Many different kinds of radiometers
exist; the type used will depend on the portion of the spectrum
to be measured. In this article we shall be concerned only with
Radiometers have been used in space experiments from the
very beginnings of space exploration. These systems generally
consist of an antenna, an amplifier, and a telemetry readout
system. The amplifiers are usually of the frequency-selective
variety so as to amplify and pass only those frequencies of
interest, while all other frequencies are rejected. Some systems
use several amplifiers and/or antennas which are shared by means
of automatic switching controlled by a programmer subsystem.
Ground commands may also be used to select a particular channel
when the payload is traversing a given region of space.
As in the case of ground-based systems, antenna design depends
on the range of signal frequencies to be gathered. Space radiometers
have been developed which have input sensitivities as low as
0.1 microvolt per meter. For some perspective, remember that
in order to obtain a good-quality TV picture on most commercial
receivers, a signal strength of 100 microvolts per meter is
required with a signal-to-noise ratio of at least 30 dB. Space
systems can yield higher sensitivities because they are far
removed from high-level man-made signals and interference. These
higher sensitivities cannot, in general, be verified experimentally
in the laboratory due to the high level of surrounding interference.
Radio Astronomy Explorer Satellites
The first Radio Astronomy Explorer (RAE) satellite has been
tentatively scheduled for launch this year. This will mark the
first time a satellite has been designed and developed for radio
astronomy purposes exclusively. Due to be another first in space
technology is the array of antennas, each of which is 750 feet
These antennas were first developed by The de Havilland Aircraft
of Canada, Limited. In addition to functioning as antennas,
the long tubular sections provide gravity gradient stabilization
of the spacecraft. The principle by which these rods are fabricated
is designated STEM, from the name Storable Tubular Extendable
Member. STEM devices have been used successfully on such space
missions as Gemini (16-foot antenna), the Canadian Topside satellite
(60-foot antennas), and the TRAAC satellite (60-foot gravity
The STEM device consists of a strip of thin material, usually
stainless steel or beryllium-copper alloy, which has been preformed
to a tubular configuration. The strip is then wound on a drum
or compressed in telescope fashion into a canister. In the case
of the longer element lengths, a drive motor rotates the drum
to unfurl the STEM device (Fig. 2). The canister-version boom
is expanded by removing the canister lid, resulting in a jack-in-the-box
unfurling. An explosive bolt or squib is usually detonated by
an electrical signal to shear a pin or latch and thus open the
Fig. 2. The STEM (Storable Tubular Extendable
While the principles of antennas are familiar to all of us,
the notion of gravity gradient stabilization is perhaps not
so familiar. The physics involved here is not too different
from the tightrope walker who carries a long pole for balance.
In the case of spacecraft stabilization, the small difference
in gravity over the length of the rod produces a torque which
tends to align the rod parallel to the gravitational field,
as shown in Fig. 3. The addition of more long rods to the spacecraft
produces more torque which yields a spacecraft attitude which
is stable with respect to earth.
Fig. 3. The principles of gravity gradient
Because of the great length and thin walls of STEM devices,
several problems appear with their use. The vacuum of space
is a cold void except when matter is present to be heated by
the sun's radiations. As a result, that side of the STEM device
which faces the sun is much warmer than the side which looks
away from the sun. Due to contraction and expansion of materials
with heating, the element tends to bend under these temperature
conditions. Thus, the tip of such an element of 300-foot length,
with 1/2-inch diameter and 0.002-inch walls, may deflect more
than 100 feet. The deflection may be reduced by using thicker
walls in the tubing, but if this is done, weight is also increased
- which is a great disadvantage in a good many space applications.
Testing of long STEM devices is also a problem since a low-gravity
environment is required. This is particularly a problem for
the longer elements. How does one create a low-gravity, high-vacuum,
sun-simulating environment for testing? The mechanical forces
which act during unfurling are quite complicated and testing
is demanded. Engineers at NASA's Goddard Space Flight Center
have provided at least a partial solution by using cameras and
photographing the trace created by a small lamp attached to
the tip of the antenna. Some very interesting light patterns
are produced during such tests. One of these is illustrated
in the lead photograph on page 46.
The RAE will probably be assigned a three-stage improved
Delta launch vehicle with an over-all length of 91 feet. The
first stage Thor rocket develops 346,000 pounds of thrust. Recall
that jet engines, as used in commercial transports today, typically
develop 16,000 pounds of thrust. The second stage develops approximately
7000 pounds of thrust, with the third stage (which carries the
spacecraft) producing about 2000 pounds thrust. It is anticipated
that an orbital altitude of about 300 kilometers (186 miles)
will be used.
The RAE Mission
The mission of the RAE satellite may be categorized by five
1. To observe low-frequency radio storms on earth. These
storms are believed to be interactions between particles emanating
from the sun and earth's radiation belts.
2. To monitor large radio noise sources, such as the constellation
3. To study Jupiter, which is the only planet other than
the earth which is known to occasionally emit low-frequency
4. To obtain an EM map of our galaxy (the Milky Way) in the
frequency range from 400 kHz to 10 MHz.
5. To gather data on low-frequency bursts of EM energy which
emanate from the sun. This data should provide added insight
into the nature of the sun's reactions.
In order to achieve orbit and deploy the four 750-foot antennas,
a sequence which will require about two weeks will be initiated
by ground command from Goddard Space Flight Center, Greenbelt,
The data gathered by RAE satellites and their successors
may provide space scientists with enough information to formulate
new theories concerning the earth and its surroundings.