How Radar Operates
October 1945 Radio-Craft

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

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 to function.

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 next burst.

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 provided.

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 detection apparatus.

"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 batteries.

Display: Methods of showing the radio echo on the cathode ray tube.

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 layers exist.

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 transmitter.

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 "Elsie."

T.W.S.: Tail Warning Sets - used to give warning of approach of other aircraft from the rear.

Posted August 12, 2021