receivers are absolutely essential in radio astronomy work. The
need has driven major advances in the state of the art of cryogenically
cooled front ends with noise temperatures near absolute zero. Antenna
technology has also benefitted from radio astronomy due to the need
for precision steering and narrow beam widths. Phased arrays (aka
interferometers) for interstellar targets requires that element
spacing be large enough to require separate antennas as the elements,
which creates a very large effective aperture, hence greater angular
resolution. Networks located continents apart are synchronized with
the use of atomic clocks to allow signal time of arrival and therefore
phase to be accurately measured. This story gives some of the early
By Dr. F. G. Smith
Astronomers are using new tools and techniques
to provide the answers to some age-old riddles of the universe.
1. One of the parabolic reflectors being used with the interferometer
now in operation at Cambridge, England (right)
1942 radar operators in England began to report a new kind of jamming
observed on their meter-wavelength receivers. Weak radar echoes
became lost in the "grass" on the (radar) screen, as if swamped
by "noise" from a powerful transmitter. In the Army Operational
Group, a scientist named J. S. Hey - later to be known as one of
the pioneers of the new science of radio astronomy - examined the
reports. He established that the source of the "jamming" was no
enemy station, but the sun, and he noticed that at that time an
exceptionally large sunspot was crossing the sun's face.
Radio amateurs can detect this radiation from the sun during
periods of sunspot activity, and even television screens are affected
by it, but few people know that the sun and some other celestial
objects are radiating short radio waves continuously.
first observations of this steady radiation were made in 1932 by
an American radio engineer, Jansky, who was investigating the level
of noise picked up by a sensitive receiver on a frequency of 20
mc. He found that a directional antenna gave a greater noise signal
when pointing at the constellation of Sagittarius, in the brightest
part of the Milky Way, than in directions away from this high concentration
of stars. Ten years later, a radio amateur, Reber, built a parabolic
reflector antenna 30 feet in diameter, in his own yard, and used
this to make a map of received signal strength on frequencies up
to 500 mc. over a large part of the sky.
Fig. 2. Array of full-wave dipoles at 3.7 meters.
This array is one-half of an interferometer for detecting
radio stars. See text for full details.
The 600-inch "radio telescope" installation at the Naval
Research Laboratory which is being used to study radio "signals"
from the sun, moon, and stars. Scientists use this research
tool to extend man's knowledge of the universe and to assist
in forecasting the conditions for radio communications work.
Fig. 3. Interferometer records of "quiet sun" radiation.
Fig. 4. Part of interferometer recording showing the
presence of several of the minor "radio stars."
Fig. 5. Section of record with high sensitivity showing
intense source in Cassiopeia. See text for details.
Fig. 6. (A) How two similar antennas, spaced several
wavelengths apart, are used to detect radiation from the
sun. (B) Use of a phase-sensitive detector to eliminate
receiver noise. (C) Improved version of the circuit shown
in (B) in which the antenna noise is continuously compared
with the noise generated from a controllable local source.
a noise diode. (D) A pair of antennas connected in a radio
interferometer with a device for reversing phase of the
signal from one antenna periodically.
Fig. 7. Parabolic reflectors used in an interferometer
for accurate direction finding.
Then came one of the most startling discoveries, again first hinted
at by J. S. Hey. Workers in Australia and England found that the
radio waves picked up by Jansky and Reber came, not only from the
Milky Way but also from some quite definite points in the sky, as
though individual stars were transmitting to us. But there were
no bright stars at these points, and it was not until 1952 that
these "radio stars" were identified with visible objects in the
sky; even then the objects were so faint and inconspicuous that
it needed the 200-inch Hale telescope at Mt. Palomar to find them.
Many astronomers have become interested in this new science as an
extension of astronomical techniques, and radio-astronomy is now
being put to use in many parts of the world extending our knowledge
of the solar corona, interstellar gas, nebulae, and even of our
own ionosphere. In this article we shall be concerned less with
the results than with the methods, since the problems of technique
are of great interest and are not well known.
The two main
problems facing the radio-astronomer wishing to study radio waves
from some object or region in the sky are simply stated. First,
the power available in his antenna is usually not greater than about
watt. Second, the beam width of his antenna is
usually vastly greater than the angular size of the object, and
the radiation picked up may well have come from many other objects
in this region. Both these difficulties, of signal strength and
resolving power, clearly call for large antenna systems and the
radio astronomers are, in fact, building large antennas for this
work. In Manchester, England, there is now under construction a
very remarkable parabolic reflector antenna. This will be 250 feet
in diameter, and it will be so mounted that it can be directed towards
any part of the sky. The reflector will be made of wire mesh, and
the accuracy of its surface will be such that it can be used on
wavelengths as short as a half meter or less. But many observations
can be made with much smaller antennas by using the principle of
the radio interferometer.
If two similar antennas spaced
several wavelengths apart and both directed towards the sun, are
connected to the same receiver, as in Fig. 6A, it is possible to
distinguish the radiation received from the sun against a background
of radio waves from the stars behind it, although this background
may be several times more intense than the solar radiation. The
records of total power received from such a radio interferometer
as the sun moves slowly across the sky would be like those in Fig.
3, showing some actual records on various wavelengths. In each the
sinusoidal variation of signal is due to the sun passing in and
out of the interference zones of the spaced antennas, whereas the
steady signal, most evident on the longer wavelengths, is from the
extended source of the Milky Way background. An improved method
of recording recently used makes a record of only the sinusoidally
varying signal, giving the intensity of the solar radiation without
any confusion from the background radiation. The method of achieving
this, known as phase-switching, will be described after we have
examined more closely the problem of detecting these exceedingly
The character of the signals received from
the sun and the stars is exactly the same as that of "receiver noise."
If we connect the input of a receiver first to an antenna and then
to a dummy load, the difference in signal may be demonstrated as
a change in the output of a detector circuit, but this change may
be only a few percent of the output, most of which is due to the
receiver noise. It is necessary to record this difference without
including receiver noise, and this is achieved in the schematic
of Fig. 6B. The use of a phase-sensitive detector enables a long
time constant to be used in the output circuit, and the smoothed
output records the difference between the two levels of noise. An
improvement is again made in Fig. 6C, where the antenna noise is
continuously compared with the noise generated in a controllable
local source, in practice, a noise diode. The output from the local
source is automatically adjusted to equality with the antenna noise,
and a record of the current in the diode gives a direct record of
antenna noise unaffected by the characteristics of the receiver.
The records in Fig. 3 were made in this way.
of detecting small noise signals have been widely used in the measurement
of the total noise power received at an antenna. But in radio astronomy
it is often necessary to select only that part of the noise which
is coming from a small source in the sky, perhaps a radio star or
a sunspot, and to disregard a large proportion coming from a diffuse
background of other sources. A new method of detection is then used.
In the schematic of Fig. 6D a pair of antennas is connected
in a radio interferometer with a device for reversing the phase
of the signal from one antenna periodically. The lobes of the interferometer
radiation pattern then shift by a half lobe width, due to the phase
shift, and the signal from a source smaller than the lobes of this
pattern will change periodically by an amount depending on its position
in the pattern. Again a phase-sensitive detector is used to measure
this periodic change in output. In Fig. 4 we see the recorded output
of such a phase-switching receiver connected to a large interferometer
operating at a wavelength of 3.7 meters, shown in Fig. 2. The output
is centered on zero, and the groups of oscillations each record
the passage of a radio star through the antenna receptivity pattern
as the earth rotates. This method of recording radio stars has been
used in the accurate location of some of the most intense radio
stars. A record from the intense radio star in Cassiopeia using
part of the same interferometer is shown in Fig. 5.
interferometer in Fig. 2 is located along an east-west line so that
each radio star is detected as it crosses the meridian, a line from
the zenith to the south point. The time of this crossing, found
from the record, gives the position of the star in the sky. The
timing may often be carried out to an accuracy of about 0.1 second,
but unfortunately the actual position of the star cannot be determined
quite as accurately as this. For one thing, the position of the
interferometer axis must be known, and with the antennas of Fig.
2 this cannot be defined to better than about 2 minutes of arc.
The interferometer in Fig. 7 was specially built for such work,
and the line joining the bearings of the two parabolic reflectors
was determined to 10 seconds of arc. These reflectors are two of
the antennas of the "Wurzburg" radar set much used by the Germans
during the war. They are 27 feet in diameter, and the two are mounted
900 feet apart, 200 wavelengths at 1.4 meters, the wavelength used
in the most accurate direction finding experiment yet made. With
this interferometer, a radio star in the constellation of Cassiopeia
was located within an area only 10 seconds by 30 seconds of arc.
The position was given to astronomers at Mt. Palomar, who found
with the 200-inch telescope a new type of nebula exactly in the
new branch of science is certainly providing new tools for the astronomer
in his survey of the heavens, but it may also prove to be a useful
approach to some studies of the ionosphere. When Hey first detected
radiation from a radio star, he distinguished it from the background
because the signal was fluctuating in a peculiar way. This effect
we now know to be very similar to the scintillation, or "twinkling,"
of ordinary stars. It is caused by refraction in irregularities
in the earth's ionosphere, through which the radio waves pass, and
by studying the fluctuations in signal it has been found that the
irregularities are in the upper part of the F-region, inaccessible
to pulse-sounding methods. It appears that the top of the F-region
occasionally becomes corrugated, to an extent of about one percent
of its total depth, the wavelength of the corrugations being about
5 km. The whole structure is drifting across the earth at a speed
of several hundred miles-per-hour, and the effect on the ground
is similar to the moving pattern of sunlight on the bottom of a
swimming pool when waves disturb the surface. The cause of this
ionospheric disturbance is still unknown.
way of investigating the ionosphere has been suggested. As the radio
waves from a radio star pass through the ionosphere they may be
refracted in such a way as to make the star appear in the wrong
position. The amount of this displacement may be measured, and depends
primarily on the total number of electrons in a vertical column
right through the ionosphere. Pulse-sounding methods are not suitable
for this measurement, and it is likely that understanding of the
ionosphere, still full of mysteries, will be helped by these new
The most exciting discoveries of radio astronomy
have been in the search for sources of radio waves in our galaxy
and in extragalactic nebulae, and this search is being pursued with
great vigor in several places. The new Manchester antenna will be
used in this work. Recently some details were published on a new
at the Ohio State University designed to carryon
the search. There is, however, a large interferometer antenna now
in operation at Cambridge, England, which may well be called the
largest radio-telescope in the world. Its parabolic reflectors cover
an area close to 50,000 square feet. Results from a survey of radio
sources in the Northern sky should be available in a few months'
time. No description of this instrument has yet been published,
and a picture of one of the reflectors in Fig. 1 is the only one
available as yet. It is hoped that this instrument will provide
some further clues to the solutions of the great problems "What
are radio stars?"; "How many are there in our galaxy?"; "Do other
galaxies have radio stars like ours ?" - questions we may hope to
have answered in only a few years from now.
1. Kraus , J. D. and Ksiazek,
E.; "New Techniques in Radio Astronomy," Electronics, September