February 1958 Radio-Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published 1929 - 1948. All copyrights hereby acknowledged.
Radio astronomy has been the motivation for much research work in
the design of low noise, high sensitivity receivers, but also in
determining the characteristics of the Earth's upper atmosphere.
Before sounding rockets could be launched to verify theoretical
proposals, observed versus predicted behavior in radio signals being
reflected off the moon and planets needed to be explained and, if
necessary, corrected for. One notable example of atmospheric perturbation
is the rotation of polarization caused by electrons in the ionosphere
(the Faraday effect). Parametric and cryogenically cooled receiver
front end technology has been primarily driven by the needs of radio
astronomy. It is true that radio astronomy has the advantage of
not needing to wait for clear, dark skies to be useful the way observation
in visible wavelengths does, but it suffers from its own very large
list of handicaps to which classical visual observations are oblivious.
The Jodrell Bank Radio Telescope
An outstanding pioneer in radio astronomy takes us on a guided
tour of the world's largest radio telescope
By Prof. A. C. B. Lovell*
the night of August 2, 1957, the great radio telescope at Jodrell
Bank, in Cheshire, England, received its first signals from outer
space. A few nights later the region of the sky containing the intense
radio source in Cassiopeia was scanned with the telescope in motion.
Data obtained in a few hours equaled that from a month's work with
previous radio telescopes, and these were only the preliminary test
of the instrument.
The telescope is essentially a paraboloidal steel bowl 250 feet
in diameter, with its focus in the aperture plane (straight up from
the center), built so it can be directed toward any part of the
sky. The total weight above ground of the moving structure is 2,000
tons. In principle the motion of the telescope is alt-azimuth (vertical
and horizontal). The bowl, which weighs about 700 tons, is driven
vertically by a
Ward-Leonard speed-control drive system through two 27-foot
racks from the dismantled battleship Royal Sovereign. These are
mounted 170 feet above ground on two towers which rotate on a 350-foot
circular railroad track to provide horizontal movement.
The drive is through two bogies (see photo) under each tower,
again through a Ward-Leonard system. Four additional bogies, which
are not powered, serve as wind carriages on each side of the structure.
The towers are connected near ground level through a heavy pivot
which is the fundamental locating part of the telescope. Power and
instrument cables come through this central pivot into a motor room
situated within the
diametral girder immediately above the central pivot. This room
contains the motor generator sets and controls for the Ward-Leonard
The 17-foot double-gauge railroad track on which the telescope
rotates is mounted on deep-piled foundations which extend 90-feet
underground in some places. The various power, control and instrument
cables are taken into an annular laboratory below the central pivot
and then through an underground tunnel to the control room. This
control room houses the main control racks and console. The computer
system consists of synchro resolvers (which resolve vectors into
two mutually perpendicular components) working in servo loop to
solve the necessary equations so the telescope can track a star's
At night, outlined by the glare of floodlights, the massive
structure scans the skies.
A wide range of movements can be selected at the control desk
- for example, automatic sidereal (star tracking motion at a given
right ascension and declination, motion in galactic latitude and
longitude, straightforward motion in azimuth and elevation, and
various automatic scanning movements with a choice of rasters. Parallax
corrections can also be introduced when it is desired to track a
body in the solar system. There are no slip rings so that the danger
of creating electrical interference is avoided, and the limit of
motion is 420°, after which an automatic reversal takes place.
The telescope has a tracking accuracy of at least 12 minutes
of arc at speeds up to 4° per minute. The maximum slewing speed
is about 22° per minute in azimuth and elevation. The position of
the telescope in azimuth and elevation is repeated back to the control
room through synchros driven independently of the driving system
by accurately machined chain racks. These positions are repeated
to an accuracy of ±1 minute of arc.
The reflecting membrane is of 1/12-inch-thick steel sheet. It
is made from 7,000 individual 3 x 3-foot sections welded to the
purlins of the steel framework. It was essential to insure good
conductivity across these welded sections, otherwise the membrane
would become very lossy at certain wavelengths. The primary antenna
feed is carried at the focus on a steel tower built 62-1/2 feet
up from the parabaloid's apex. The tower's cross-section diminishes
rapidly with height to avoid obscuration and scattering from the
primary feed. However, it was essential to design it with enough
stiffness to avoid displacement as the bowl turns over.
An important scientific requirement is easy access to the primary
feed so that the operational wavelength can be changed readily.
The aerial is mounted in a 50-foot steel tube which slides into
the top of the aerial tower. With the bowl inverted it can be brought
down to ground level and replaced by another 50-foot tube complete
with aerial system. The radio-frequency cables from the aerial run
inside this tube and can be reached from a small platform near the
base of the tower when the bowl is facing toward the zenith.
Calculated power gain and beam width of the 250-foot-aperture
radio telescope as a function of wavelength.
In much of the work the radio-frequency preamplifiers and other
parts of the receiving equipment must be mounted as close as possible
to the aerial. These essential units will be kept in a small laboratory
which swings underneath the bowl. Further laboratory space is available
at the tops of the two towers, but even from these the minimum length
of cable run to the primary feed is about 200 feet. Other scientific
apparatus will be installed on the base girders, but the main recording
apparatus will be in laboratories adjacent to the control room.
In preliminary tests, the smoothness of the motion of the telescope
in azimuth and elevation has exceeded all expectations, and the
power loading has been a small fraction of that available. The theoretical
curves showing the beam width and power gain as a function of wavelength
are shown in the diagram. On the frequencies used (90 and 160 mc)
the experimental values for the beam width and power gain have agreed
well with these calculations. Further preliminary tests on frequencies
of 408 and 1420 mc are now in progress. Distortions in the bowl
are not believed to exceed about 1 inch relative to the focus and
very good performance, even on the important hydrogen-line frequency
of 1420 mc, is anticipated.
The telescope is adaptable for use either as a receiver for the
galactic and extragalactic radio emissions or as a transmitter and
receiver for the investigation by radar of meteors and other bodies
in the solar system. Some of the tasks for which it will be used
The pioneer observations of Jansky, and later of Reber, showed
that the intensity of the radio emission varied markedly with the
direction of the aerial beam, being most intense from the direction
of the galactic center***. The variation was generally what might
be expected if the stars in the Milky Way were responsible for the
emission. However, neither Jansky, Reber nor any subsequent worker
has succeeded in detecting radio emissions from any of the stars
(other than the sun), nor have the localized radio sources since
identified coincided with any typical common star. It is possible
that Reber's original suggestion, that the radiation is emitted
by the interstellar gas, is at least partially true, but the situation
is very complex and the question of the origin of these galactic
radio emissions will form a prominent part in the program of the
Localized radio sources
Towering over the surrounding countryside, the radio
telescope is shown as it looked just before completion.
When some of the radiation from space was discovered emanating
from localized radio sources, it was tempting to conclude that the
background continuum was made up of large numbers of these discrete
radio sources, unresolved by the available radio techniques. This
situation would be analogous to that in which the Milky Way is viewed
by eye or through a low-power telescope, when all faint stars appear
as a continuum of light and only the brightest stars stand out individually.
It is now known that this view of the radio emission is untenable.
Not only have improved techniques failed to reveal the increased
number of appropriately distributed sources, but the spectra of
the background and the sources are not compatible.
About 15 or 20 of the localized sources of radio emission satisfy
the various criteria, such as appreciable angular extent, intensity
and distribution, which makes it highly probable that they are members
of the local galaxy. Seven of these have been satisfactorily identified
with galactic objects. For example, the radio source in Taurus,
which is the third most intense in the sky, is associated with the
Crab nebula. The Crab nebula is the expanding gaseous shell of the
supernova of 1054 AD. Its distance is about 4,000 light years, and
the angular dimensions of both the telescopic and radio object are
about 4 x 6 minutes of arc. The temperature of the gaseous shell,
which is expanding at the rate of about 70 million miles per day,
is 50,000°K, which is far too low to produce the observed radio
intensity of 1.8 x 10-23 watt/m2 per cycle
per second (at 80 mc) by thermal processes.
It is possible that the radio emission can be explained as a
synchrotron** mechanism resulting from the movement of high-energy
electrons in weak magnetic fields. It now seems highly probable
that supernovae like the Crab nebula are both powerful radio emitters
and responsible for the generation of an appreciable fraction of
There are two other well-attested cases of supernovae in the
galaxy, those observed by Tycho Brahe in 1572 and by Kepler in 1604.
Unlike the Crab nebula, these are not spectacular objects. Even
so there seems little doubt that radio sources are associated with
The most intense radio source in the sky lies in Cassiopeia. Its
angular extent is about 4 minutes of arc, and the flux density of
80 mc is 2.3 x 10-22 watt/m2 per cycle per
second. Although this was the first radio star to be discovered
in the Northern Hemisphere, it was not until 1951 that a successful
search for its visible counterpart was initiated with the 200-inch
telescope by Baade and Minkowski.
One of the four powered bogies which rotate the telescope.
Eight non-powered units are also used.
The result of this search was the surprising discovery that this
powerful radio emitter consists of a very faint extended nebulosity
of a type previously unknown. The gaseous filaments of this nebulosity
are in violent motion at thousands of kilometers per second. There
is no satisfactory explanation of the mechanism of the generation
of radio waves nor is there any agreement as to the nature of the
object itself, although opinions have been expressed that it may
be the remains of a very old supernova.
There are only three other agreed identifications of radio sources
with galactic objects. These are the Cygnus loop and the nebulosities
in Auriga and Gemini. These are all extended gaseous nebulosities
of low photographic brightness, with filamentary structure. As with
the Cassiopeia source, there is no agreed opinion as to their nature
or that of the mechanism whereby they emit radio waves.
The only general conclusion which be drawn from the present situation
is that the galactic radio sources appear to represent a very rare
type of celestial object characterized by an appreciable extension
of diffuse gas of low photographic brightness. Whether they are
different manifestations of the same phenomena (supernovae) or vary
in character remains uncertain. Attempts have been made to associate
the radio sources with other rare classes of galactic objects such
as novae, planetary nebulae and globular clusters, but without success.
The new telescope, with its high definition and adaptability
over a wide frequency range, is expected to be a powerful tool in
the investigation of the problem of these galactic sources and of
the continuum. Initially it is hoped to make measurements at a few
selected points in the frequency range of 20-1,400 mc to study the
isophotes (lines of equal brightness) of the continuum and the spectra
of the localized sources.
At present, surveys in England and Australia have revealed between
2,000 and 3,000 localized radio sources. Apart from the galactic
concentration of a small number of the intense and extended sources
discussed above, these sources are distributed isotropically (identically
in all places) and are probably extragalactic. A relatively small
number have been identified with telescopic objects such as the
Andromeda nebula and other similar nebulae, but it seems likely
that the majority of these are quite abnormal and at very great
distances. For example, the second most intense radio source in
the sky lies in Cygnus, and this has been identified as two galaxies
in collision at a distance of 200 million light years. The existence
of this intense radio source (1.4 X 10-22 watt/m2
per cycle per second at 80 mc) associated with a celestial collision
nearly at the limit of penetration of the 200-inch telescope is
one of the most remarkable features of radio astronomy, with far-reaching
During the last few years a further half dozen or so radio sources
associated with unusual extragalactic objects have been discovered.
These include NGC1275 in the Perseus cluster and a source in Hydra,
which Baade and Minkowski consider may be galaxies in collision.
Other peculiar associations include NGC5128, which has a dark and
across it, and M87 from which a jet emanates.
The normal nebulae show a ratio of radio to optical emission of
about 10-6. Compared with this, the peculiar objects
have a much greater ratio, of the order of 10-3, reaching
unity in the case of Cygnus. This represents an extremely high conversion
efficiency, and the mechanism by which such objects generate radio
waves is a challenging problem.
Inside the 250-foot-diameter bowl. A 62.5-foot antenna
feed mast is located in its center.
In a collision of galaxies the stars are too widely separated
for significant collisions to occur, but the dust and gas, which
represent an appreciable fraction of the galactic mass, will certainly
suffer real collisions at velocities of, perhaps, thousands of kilometers
per second. The key to the mechanism that generates the radio waves
probably lies in this highly agitated ionized gas.
The collection of further data on both the normal and abnormal
radio sources is essential to understanding the problem of the extragalactic
radio emissions. The new telescope is well fitted to pursue this
important task over a wide range of wavelengths.
The preceding examples of the new telescope's uses have all concerned
the programs in which the instrument will be used as a receiving
aerial. There are many problems in which it will be used as a combined
transmitting and receiving aerial. (It ha been used, for example,
for tracking artificial satellites and their attendant rocket sections.
- Editor) In these radar or radio-echo aspects of the work, the
moon and the planets will figure prominently.
Radio echoes from the moon were first claimed to have been observed
in 1946 by Z. Bay in Hungary. His recording system was unusual,
and the first certain echoes obtained in the conventional sense
on a cathode-ray tube were by the U. S. Army Signal Corps. Subsequently
echoes were obtained by Kerr and Shain in Australia. These experiments
showed that the moon echoes were subject to deep and rapid fading
- an effect which is now believed to be due to a peculiarity of
the moon's motion with respect to the earth, known as libration.
During this period, apparatus for lunar-echo studies was also
under development at Jodrell Bank, and it appears that this is the
only systematic investigation of the moon by the radio-echo technique
which has yet been carried through. This apparatus works on a frequency
of 120 mc and uses a transmitter giving 10 kw in 30-msec pulses
at a recurrence rate of 0.6 per second. The receiver bandwidth is
30 cycles, and appropriate arrangements have to be made to allow
for Doppler shift in the frequency of the returned signal.
The most important results obtained with this apparatus concern
the long-period fading (20 to 30 minutes), which by cross-polarization
experiments has been shown to be due to the rotation of the plane
of polarization of the radio wave as it traverses the ionosphere
(the Faraday effect). This immediately led to developing a moon-echo
system by which the ionosphere's total electron content could be
The technical difficulties in this work are considerable and,
with the present aerial system, measurements can be made only with
the moon in transit for about 10 periods in each lunation. The new
telescope will immediately remove these handicaps and will enable
systematic data to be collected about the total ionospheric electron
content. This is bound to be of considerable importance to our understanding
of the ionosphere and of solar-terrestrial relationships.
The problem of radio echoes from the planets is vastly more difficult
and, as far as is known, no serious attempts have yet been made
to solve it. The magnitude of the problem relative to the moon is
indicated by the fact that success in detecting radio echoes from
Venus would demand an overall power sensitivity between 1 and 10
million times greater than that required in the case of the moon.
This assumes, of course, that the reflection coefficient of the
planet would not be inferior to that of the moon.
The problem cannot be appreciably eased by increasing the length
of the transmitter pulse with appropriate decrease of receiver bandwidth,
because of the Doppler spread introduced by the rotation of the
planet. The rotation period of Venus is unknown (this would, in
fact, be one of the main scientific results to be expected from
the experiment) but on the basis of current estimates, the Doppler
spread would probably limit the useful pulse width to about 40 msec,
which is only a few times longer than that used in the lunar investigations.
The main factor must therefore be achieved in the gain of the aerial,
by increasing the transmitter power, and possibly by integration
of successful echoes.
At the controller's desk. At night the telescope is illuminated
so the controller can still keep everything in view.
The problem has been carefully considered at Jodrell Bank in
relation to the very great gain of the new telescope, and an attempt
to obtain planetary echoes will be made early in the research schedule.
The complete return journey of the earth-Venus radio signal will
take 4 minutes and success in detecting such a radio echo would
be a spectacular technical accomplishment. Nevertheless, the experiment
could not be justified on this basis, and it is hoped that with
the telescope a systematic program will be possible in which the
rotation period can be determined and information obtained about
the Venusian surface and atmosphere.
The telescope will also be used to study very faint meteors and
the aurora borealis by the radar technique. As a receiver it will
be applied to many other problems such as the radio emissions from
the sun and the planets. In all these programs the great power gain
coupled with the adaptability and ease of steering of the telescope
is confidently expected to give results of outstanding interest
as well as importance.
* Director of the Jodrell Bank Experimental
Station, University of Manchester.
** A device for accelerating electrons or protons
in a circular orbit in an increasing magnet field by applying an
alternating electric field in synchronism with the orbital motion.
At sufficiently high speeds polarized light and, it is believed,
polarized radiation, some in the radio spectrum, is produced. The
polarization is in the direction of motion.
*** Radio Astronomy. Lovell and Clegg. Radio
Astronomy, Pawsey and Bracewell. The Changing Universe, Pfeiffer.
Posted July 30, 2014