October 1945 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Here is a brief synopsis of radar
(radio detection and ranging). Today, most people
who would be reading a magazine like Radio-Craft would have at least a
layman's level of knowledge of what radar is and how it works. However, in late
1945 when the transition from a wartime society to a "normal" existence was solidly
underway, many new terms and types of inventions previously withheld for defense
security reasons were being released into the public domain. I have mentioned previously
that some people were vehemently against making a lot of the stuff known, but government
agencies wanted to get the information out in order to promote innovation for improvement,
to provide new technology for manufacturers, and to reward citizens for the lifestyle
and personal safety sacrifices made in order to help secure victory.
How Radar Operates - Résumé of All Technical Information Released to Date
A typical mobile radar installation. Functions of the more important
parts are indicated.
Practically every radar set is made up of the following major parts or components:
1 - A modulator; 2 - A radio-frequency oscillator; 3 - An -antenna with suitable
scanning mechanism ; 4 - A receiver; and 5 - An indicator.
While the physical form of each of these components may vary widely from one
kind of radar set to another, each radar must have this complement of parts in order
1 - The modulator is a device for taking power from the primary power source
(which may be the commercial power line a special engine or motor-driven generator,
or storage batteries) and forming suitable voltage pulses to drive the R.F. oscillator
in its bursts of radio frequency oscillations. In other words it is the modulator
which turns on the radio frequency oscillator to oscillate violently for a millionth
of a second or so, turns it off sharply and keeps it in repose until time for the
2 - The radio-frequency oscillator is a vacuum-tube of suitable design, or a
group of such tubes, which will oscillate at the desired radio frequency and give
the desired bursts of radio frequency power when connected to the modulator. The
development of suitable oscillator tubes has been one of the major achievements
of the radar art. It is a relatively simple job to produce a radio frequency oscillator
which will give oscillations of any desired frequency provided one is satisfied
with a power of only a few thousandths of a watt. In the receiving part of a radar
circuit this amount of power is adequate. A practical radar transmitter, however,
must generate during its momentary bursts of oscillation a power which may run into
hundreds of kilowatts. Since the oscillator is turned on a small fraction of the
time, the average power is usually hundreds of times less than the peak power, but
even the average power may run up to the order of one kilowatt. Thus, practical
radar equipment requires extremely high frequency oscillators running at powers
thousands of times greater than was thought possible a few years ago.
3 - The problem of antenna design is also
one of the major problems in radar, incomprehensible as this may seem to the operator
of a home radio receiver, who finds a few yards of wire strung up on his roof adequate
for his purpose. A suitable radar antenna must have the following characteristics:
a. It must be directional; that is, it must concentrate the radio energy into
a definitely defined beam, since this is the method by which the direction to the
objects detected is determined;
b. It must be highly efficient. All of the generated power must go into the beam
and none must leak off into "side lobes" in other directions, since such side lobes
may often be fatally confusing; and,
c. The radar antenna must be capable of being directed or scanned from one point
in space to another, and on shipboard and in aircraft it must frequently be stabilized
to take out the motions of the ship or airplane itself.
An antenna may be made directional either by building it up of an array of small
antennas or dipoles, suitably spaced and phased to concentrate the energy in one
direction, or it may be built on the searchlight principle of spraying the energy
into a large parabolic "mirror," which focuses the energy into a beam. In either
case, the larger the antenna, the sharper the beam for any given wave-length. Sometimes
antennas may be longer in one direction than the other, giving a beam which is sharper
in the first direction and thus fan shaped.
The scanning of the portion of space which the radar set is intended to cover
must usually be done by mechanical movement of the antenna structure itself. This
means that the structure, whatever its size, must swing around or up and down to
direct the beam in the necessary direction. In certain cases where one needs to
scan only a small sector, techniques have been worked out for rapid electrical scanning
not requiring the motion of the whole antenna structure itself. So far, however,
there has been no method for extending this rapid electrical scanning to cover more
than a relatively small sector. Radars for directing guns which need ac curate and
fast data in a small sector are making use, however, of this valuable technique.
To carry the radio-frequency energy from the oscillator to the antenna, and the
echo from the antenna to the receiver, wires and coaxial cables are used at ordinary
wave-lengths. For microwaves, however, it is more efficient to use wave guides,
which essentially are carefully proportioned hollow pipes-and the transmission system
hence is often called "plumbing."
4 - The problem of the receiver for radar is also a complex one. In practically
all radars the superheterodyne principle is employed, which involves generating
at low power a radio frequency fairly close to that received, and "beating" this
against the received signals, forming an intermediate frequency, which is then amplified
many times. Curiously enough the crystal, used as a detector and mixer, has again
come into its own in microwave receivers. The peculiar characteristics of pulse
signals require that receivers be built with extremely fast response, much faster
even than that required in television. The final stages must prepare the signals
for suitable presentation in the indicator. The receiver normally occupies a relatively
small box in the complete radar set, and yet this box represents a marvel of engineering
ingenuity. A particularly difficult piece of development is concerned with a part
closely connected with the receiver. This is a method of disconnecting the receiver
from the antenna during intervals when the transmitter is operating so that the
receiver will not be paralyzed or burned out by the stupendous bursts of radio frequency
energy generated by the transmitter. Within a millionth of a second after the transmitter
has completed its pulse, however, the receiver must be open to receive the relatively
weak echo signals; but now the transmitter part of the circuit must be closed off
so it will not absorb any of this energy.
5 - It is the indicator of a radar that presents the information collected in
a form best adapted to efficient use of the set. early (but not quite) all
radar indicators consist of one or more cathode-ray tubes. In the simplest or "A"
type of presentation the electron beam is given a deflection proportional to time
in one direction - say, horizontally - and proportional to the strength of the echo
pulse in the other - say, vertically. If no signals are visible, then one sees a
bright horizontal line (the "time base") across the tube face, the distance along
this line representing time elapsed after the outgoing pulse. A returning echo then
gives a V-shaped break in the line at the point corresponding to the time it took
the echo to come back. The position of the "pip" along this line measures the distance
to the reflecting object. There are many variations of this type of indicator for
special purposes, but most radars have an A-scope, even when other types are also
Many types of radar whose antennas "scan" various directions employ the PPI tube.
Here the time base starts from the center of the tube and moves radially outward
in a direction corresponding to that in which the antenna is pointing. This time
base rotates in synchronism with the antenna. The returning signal, instead of causing
a break in the time base, simply intensifies its brilliance for an instant. Hence
each signal appears as a bright spot of light at a position corresponding to the
range and bearing of the target. Thus a maplike picture of all reflecting objects
appears in the cathode-ray tube face.
Since the antenna can usually be rotated only slowly (e.g., from 1 to 20 r.p.m.)
and since the light from an ordinary cathode-ray tube fades away almost instantly,
one might expect not to see a "map" at all, but only bright flashes at various spots
as the antenna revolves. Some way had to be found to make the brightness of these
flashes persist for many seconds after they were produced. Special screens were
developed which continue to glow for some time after being lighted by a signal.
Thus the whole map is displayed at once.
The above article is directly reprinted from Radar, a Report on Science at War,
released by the Joint Board on Scientific Information Policy for the Office of Scientific
Research and Development, the War Department and the Navy Department. The Glossary
follows that in the bulletin Radar, issued by the British Information Services.
A Glossary of Radar Terms
A.I.: Air Interception - sets used in aircraft, especially night-fighters,
to detect and attack enemy aircraft.
Asdic: Apparatus for detecting submarines (not radar).
A.S.V.: Air to Surface Vessel - sets used in aircraft to detect
and attack ships and surfaced submarines.
A.W.S.: Aircraft Warning Set of general application to long-range
"Blip" or "Echo": The visual indication on the "scope" given
by the reflected impulses from the object located. Also referred to as the "Break."
The use of the second named term led to the adoption of the tune "Little Sir Echo"
as the theme song of the radar operators.
B.T.O.: Bombing Through Overcast.
Cathode Ray Tube or "Scope": The Cathode Ray Tube or Oscillo-scope
used in radar sets as an indicator unit.
Centimeter Waves: Radio waves of very short wave length (only
a few centimeters) and of extremely high frequency.
C.D.U.: Coast Defence Units for the protection of coastal and
harbor installations for sea attack and for assistance of fire control of shore
Display: Methods of showing the radio echo on the cathode ray
Electron: An electrical particle.
"Floodlighting": Covering a wide area with radar waves in a
wide fixed beam.
G.C.I.: Ground Control Interception - an apparatus specially
designed for the control of fighters in the interception of enemy aircraft.
"Gee": Radar navigational aid, by which bombers are guided from
the home field.
"Gen Box": See "Mickey."
G.L.: Gun-laying sets designed specifically for gun control.
"Headlight Sets": Radar sets operating in a similar manner to a searchlight.
Heaviside Layer: The lowest reflecting layer in the ionosphere.
I.F.F.: Identification, Friend or Foe - sets carried in friendly
aircraft to give a special radar response which will identify them as friendly aircraft.
Ionosphere: Is that portion of the earth's upper atmosphere in which reflecting
M/C: Megacycle - a measure of frequency representing one million
cycles or oscillations per second.
"Mickey": Special type of Plan Position Indicator used by planes
to survey the terrain below.
"Oboe": Improved form of "Gee."
"Pip": Same as "blip." "Pip" is the American term.
P.P.I.: Plan Position Indicator.
"Pulse": A transient burst of radio energy sent out by the radar
Racons: Radar beacons used to assist friendly aircraft in navigation
and homing and for blind landing.
Radar: Term coined in the U. S. A. to signify Radio Direction
Finding And Ranging, now in common use.
Radar Altimeter: A device giving a constant indication of the
height of the aircraft above the surface of the ground.
Radiolocation: Term first generally used in the British Services
to describe this development.
RCAF: Royal Canadian Air Force.
R.D.F.: Reflection Direction Finding - the initials used in
Britain in the early days to describe radar.
S.L.C.: Searchlight Control Radar Set - familiarly known as
T.W.S.: Tail Warning Sets - used to give warning of approach
of other aircraft from the rear.
Posted August 12, 2021