INTRODUCTION	1-4
Visual detection methods and limitations	1-4
Radar overcomes visual limitations	1-5
Derivation of word "radar"	1-5
Information given by radar	1-5
Importance of radar	1-6
Battle of Britain
Battle of Midway
Navy types of radar	1-6
Search
Fire-control
Special
Navy letter system	1-8
Radars on board ship	1-8
Importance of radarman	1-8
Security	1-9
BASIC PRINCIPLES OF RADAR	1-9
Definitions	1-9
Frequency code	1-11
Equivalent measurements	1-11
Principle of pulse reflection	1-11
Bearing determination	1-12
Range determination by sound	1-12
Range determination by radar	1-13
Summary	1-13
MAIN PARTS OF A RADAR SYSTEM	1-13
The transmitter	1-14
Wave length or frequency
Duration and power of pulse
Pulse rate
The antenna system	1-16
Transmission line
Antenna: an emitter of radio waves
Non-directional antenna
Directional antenna

Closely related to this primary purpose of the Navy is the secondary purpose: self preservation. The ship that fights the most effectively has the best chance of coming safely into home port again. A poorly fought ship may never have another chance to fire its guns. Sunken ships do not shoot.

You, as a radar operator, may be wondering how radar helps the ship fire the guns and come safely home again. The purpose of this hook is to clear up that question. Radar performs the old task of finding the enemy but uses new methods. If you understand how this task was performed before the development of radar, it will help you to realize the superiority of radar over any device formerly employed.

Visual detection methods and limitations. Ships have used visual lookouts since the early days of sail. A lookout, however, cannot see through fog

Navy men of the past realized that when an enemy vessel appeared over the horizon or out of a fog bank too little time remained to prepare for battle. So when in search of the enemy, they sent out pickets, a line of the faster, smaller ships in the direction from which the enemy was expected. Then, when the ship farthest ahead saw the enemy rise above his horizon, it signaled the next vessel, and thus the word was passed.

But the picket system did not help much in bad weather or at night. The black of night or a curtain of fog still could hide an enemy's approach. After radio came into general use, pickets or patrol vessels could relay the information beyond the horizon at night or through fog. But even so, the visual lookout of the patrol vessel was handicapped by his limited field of view.

Even if the presence of an enemy were known, battles could not be waged very successfully on dark

Figure 1-2. Indiscriminate firing betrays your position.

Radar overcomes visual limitations.

Radar, generally speaking, can reach out beyond the visual horizon. It can detect through darkness, fog, and smoke as well as through sunshine. You no longer have to wait until the enemy appears over the horizon before you know several facts about him-his presence, number, size, course, and speed. You can be preparing a plan of action before the enemy even knows where you are. Thus, radar enables your ship to "shoot, the guns," even in the dark, and to make hits with the first or second salvo.

Figure 1-3

Derivation of word "RADAR".

Let us digress for a minute to study the derivation of the word radar. We know that radar uses radio techniques, hence the first two letters RA. We know that it is used in detection and this gives us the letter D. In addition to detection, radar is useful in giving the range of an object. This then gives the last two letters AR, A for and, and R for ranging. If we combine them we have:

RA	Radio equipment for
D	Detection
A	And
R	Ranging

Information given by radar.

Up to this point, we have discussed the function of radar without mentioning just how it operates, other than remarking that it uses radio methods and techniques. Now let us consider what information it furnishes, and how it functions.

Radar gives the following information:

1. Presence of an object 2. Bearing. 3. Range. 4. Position angle (angle of elevation) or altitude. 5. Composition.

Radar operation consists of sending out a series of radio frequency (R.F.) pulses from a high power ultra-high-frequency radio transmitter. These pulses are directed into a beam by a directional antenna. When this beam strikes an object in its path, most the R.F. energy will go around the obstruction, and a small amount, depending on the size of the object, will be reflected toward the sending antenna, the transmitter position there is a highly sensitive receiver which will receive or detect the small amount returning R.F. energy. From the receiver the

In radar the reflected R.F. energy is called an echo. The presence of an object is indicated by the echo appearing on the indicator.

Since the echo returns to the antenna when it is pointed at the object, it can be said that the object must have the same direction, or bearing, as the antenna.

The range to an object can be determined by measuring the time it takes for the pulse to go out from the transmitter to the object and return. To avoid confusion, enough time is provided between pulses to allow an echo to return from the greatest distance at which radar can be expected to function.

Position angle is the angle above the horizontal at which a plane may be seen. Since protection against enemy aircraft is of great importance, some radars have been adapted to give this position angle as well as range and bearing. This has been accomplished in several Navy sets with sufficient dependability to permit full radar control of AA batteries.

As an operator you can learn to get the information previously mentioned with relatively few hours practice. A superior operator, however, can gain far more information than this. He can determine the composition of the target, including the number and type of units involved. With experience and a reasonable amount of practice you will soon be able to recognize the difference in appearance of the blip (radar indication) representing a surface vessel and that representing an aircraft. Presently, you will be noting differences in blips caused by large ships as compared with those caused by small ones, and can estimate the size of the ship from the size of the blip and the way it behaves. You will also be able to estimate the number of planes or ships producing blips on your screen.

Remember that you will learn how to do these things only by keeping your eyes open and actually trying to learn. The extra information gained can be of vital importance to your ship. You should not be disappointed because you are unable to establish such data the first time that you stand a radar watch, since this ability comes only from skill and familiarity with your set. However, you cannot use inexperience as an excuse for laxity. Excuses will not save your ship; your ability and experience can.

Importance of Radar.

It is imperative that you as a U. S. Navy radar

Battle of Britain. The Nazi Luftwaffe, intent on bombing England, was itself defeated in part through the use of radar. With it, the British beamed directly on Germany and occupied Europe and saw the enemy planes shortly after they rose from the ground. As the huge armadas approached, the RAF, at that time vastly outnumbered, was always at the right place at the right time to intercept them.

On a Sunday evening, January 17, 1943, the then mighty Luftwaffe appeared in force over London in reprisal for RAE raids on Berlin. There was a bright moon and everything seemed to be in their favor. Much to their surprise, however, the searchlights, which previously on similar occasions had scanned the skies in futile search action, now followed the planes with unerring precision. Radar was not only directing the few planes of the RAF to the right spot at the right time, but aiming the searchlights and guns as well.

Without radar, the Air Ministry has said, the Battle of Britain, one of the greatest decisive battles of all history, would have been lost. Radar helped turn the tide of the war. In a sense, it probably saved our entire civilization.

Battle of Midway. Another illustration of radar's effectiveness occurred in the Battle of Midway. A large Japanese force was approaching the island, presumably to attempt occupation. At the island there were several squadrons of land-based bombers. Near by was the carrier Yorktown. Without radar a continuous patrol depending on visible detection would have been necessary. With radar, the enemy was detected while still about a hundred miles distant, and his course plotted. The Japanese were first allowed to close in (thus saving fuel); then the carrier planes and the land-based bombers were sent to the attack. Directed by the large radar stations on the carrier and on Midway, our pilots went straight for their targets. Thus our force, though considerably outnumbered, was able to disperse and defeat the larger Japanese force.

Navy types of radar.

The fundamental principles of all radar sets are alike. However, radar lends itself to many different uses. Each use requires a different application of these principles. In this section, the different types of

Search. Due to the great speeds with which enemy aircraft make their approach, need for detecting them in ample time has become only too evident. A large portion of shipboard radar has been designed for just this purpose. It picks up targets far at sea, giving our own ships sufficient time to prepare for immediate action. Reports from this radar are given at regular intervals to the combat information center (covered later in this book). This makes possible the rapid and accurate plotting of the enemy's course and speed. It is a most valuable aid in sending our own planes aloft to intercept the enemy.

There is also a need for detecting surface targets and securing knowledge of their movements. For this purpose every ship in the U. S. Navy has some type of surface search radar aboard. This equipment is not only invaluable for the location of the enemy task force, but is equally important in the location of surfaced submarines (it can detect even their periscopes), or for obtaining positions of ships in convoy.

Both surface and aircraft search radar are fundamentally alike. However, since each has a different task to perform, special consideration was given to the design of their respective antennas. In the days following the birth of this revolutionary weapon, a group of technical experts devoted a considerable amount of time to the development of the antenna, realizing that therein lay the means of obtaining more accurate target information. For instance, it was found that surface search radar, in order to do its job efficiently and effectively, requires an antenna

Engineers and designers were also agreed that aircraft search equipment should have a greater range capability. This was necessary because of the rapid approach of the enemy.

Fire control. During the first year of the war, radar was widely employed as a searching device. Even at that time there were those who thought it could be used equally well in controlling the fire of the ship's guns. Before long, a fire control radar was produced: radar that not only gave the direction and distance of the enemy, but also aimed the guns. This particular type of radar has been used extensively and effectively in the vast Pacific in night operations against the Japanese fleet, and it did an equally fine job in silencing the shore batteries at Casablanca.

Shipborne fire control radar, like search gear, is divided into two general classes, namely, anti-ship and anti-aircraft. Here again, the main difference lies in the antenna and beam it emits. An anti-ship fire control antenna must provide a beam which is extremely narrow in the horizontal plane in order to obtain sharp bearing accuracy. This beam is made somewhat wider in the vertical to allow for the roll and pitch of own ship. Otherwise, the beam might go over or fall short of the target. An anti-aircraft

Special. As you progress in your study of radar, you will encounter equipment that is entirely portable and can easily be moved about from one location to another. This apparatus is especially valuable when a beachhead has been established, and it is used to warn against both aircraft and surface targets.

IFF (Identification, Friend or Foe) equipment is a part of the radar in use today. This equipment, rather than being an actual radar, is an aid to radar. It has its own transmitter and receiver and answers the all important question as to whether the target is enemy or friendly.

Navy letter system.

In conclusion, it is important that you learn a little about your Navy's way of naming the gear with which you will soon work. There are many radars in the Fleet, each doing its own particular job, each having its own particular name.

First, there are two large divisions of radar, those used for searching action, and those used for fire control. To distinguish them, their first letter is always "S" if the instrument is for search, and "F" if for fire control.

The second letter in the designation of search radars is usually an indication of the model of one particular type. For instance, a model SC radar is a search radar and is older than an SK. An SO model, on the other hand, is more recent than an SK, all, however, are used for search. When a modification is developed, the Navy uses numbers to designate the new model; i.e., an SC-2 is a modification of an SC.

All models and types of lire-control radar are now named by the Mark system. This system is employed by the Bureau of Ordnance and is used in connection with all gunnery equipment. Some of the earlier models of fire-control equipment were also known by the letter system used on search gear. For example, the Mark-3 radar was also referred to as FC; the Mark-4 as the FD; the Mark-8 as the FH, etc. Later fire-control models are known only by their Mark number such as the Mark-9, Mark-12, Mark-19, etc.

Usually, all types of recognition (IFF radar) equipment used with radar, both airborne and shipborne, have the letter B in their designation. Examples of these are the BL or BK models. Following this same system, the airborne model becomes the ABK. The combinations of IFF units are designated Mark IFF radars; i.e., Mark 3 IFF, Mark 4 IFF, etc.

Radars on board ship.

It should be mentioned that there are certain natural combinations of search gear from the standpoint of the functions the sets serve and the ships which carry them. The SG (surface search) and SC or 5K (air search) always go together. These sets are designed for combatant ships of DD size or larger. The SL (surface search) and SA (air search) also go together. These sets are designed for ships of the DE class. Another group of sets consists of the SO and SF (the SL can also be included). These sets are all for surface search and one is used on small ships and auxiliaries, such as PT's, PC's, SC's, AK's. and AT's, which do not carry an air search radar. You will be expected to become an expert operator of one of these combinations.

Importance of the radarman.

Since radar does let you "look" through fog and smoke and darkness, you find that it is a great help not only for detecting the enemy, but also for locating your own vessels and for warning you of nearby rocks, islands, icebergs, and similar objects. It is most important for the safety of your ship that you have this information. Of course, if your radar is operated carelessly, you cannot expect it to give good results. If you fail to do your task well, fail to notice instantly the indication of an enemy ship or of a rock, you may well be responsible for the sinking of your own ship. Radar is capable of performing its task well, but only if there is an efficient, alert operator at the controls. Yours is indeed a big responsibility, and the safety of your ship and shipmates depends on how well you do your job. Keep this fact before you every minute you are on duty.

You as a radar operator, are the first aboard to know of the enemy's presence, his strength, and his precise

The information you receive is of no use whatsoever if you fail to relay to the proper place the

Figure 1-5

Security.

The more familiar you become with your equipment, the more you will realize the importance of keeping what you know to yourself. You are being entrusted with a vital military secret when you learn about radar. It is imperative that you keep it a secret. Stop and consider what radar does for you in guarding your ship and in helping to protect your life and the lives of your shipmates. This miracle weapon is largely on our side. That is where it will remain if security functions. Keep what you learn to yourself!

BASIC PRINCIPLES OF RADAR

Before beginning with the basic principles of radar there are a few terms, symbols, and abbreviations that you should learn in order to help you understand the material that follows.

Definitions.

The word cycle is familiar to most of us, occurring in such expressions as "vicious cycle," "cycle of prosperity," "cycle of life," etc.

The cycle is one complete series of events at the end of which conditions are back at the starting point. Beginning with any particular phase or condition, one cycle is completed as soon as it starts repeating itself.

Figure 1-6.

Since these cycles occur regularly, there must be a definite time required for each cycle. This time is known as the period. Remember that it is a measure of time required for one cycle to occur. Hence it is reasonable to expect the time for each cycle to decrease when the number of cycles per second increases.

If one second is divided into one hundred equal parts, each part will be one one-hundredth of a second long. The period is 1/100-second when the frequency is 100 c.p.s. (cycles per second). Now, note this carefully: The period is the same as one divided by the frequency.

Another term that will help to make our discussion simpler is wave length. This is the actual distance traveled by the energy while it is completing one cycle.

You are familiar with the fact that as long as you travel at a definite speed, you can cover a distance proportional to the time. In other words, if you travel at a speed of 30 miles an hour for one hour you go 30 miles; if you travel for one-half hour, you go only half as far; if you travel fur two hours, you go twice as far, etc. The distance increased with the time or,

wave length (distance) = velocity (speed) x period (time).

Usually, you will not know the period, but this information is not necessary for finding the frequency or wave length. Therefore, a formula which gives the wave length when the period is not known will be better for our purpose. Remember that the period equals 1 divided by the frequency: P = 1/f. Substituting 1/f for its equal P the new formula will read:

wave length = velocity x 1/frequency, or WL = V/F

this is the usual formula used in calculating the wave length.

Radar energy travels at a velocity (or speed), of 162,000 nautical miles a second. If frequency is 2,000 cycles per second, what is the period, and what, then, is the wave length?

Frequency (f) = 2,000

Therefore the period 1/f = 1/2,000 second = 1,000,000/2,000 microseconds = 500 microseconds.

Thus you see that the energy travels 81 miles while it goes through one cycle.

You have learned that radar may be used to determine the range and bearing of a target. Range is the distance of the target from you. Bearing is the direction of the target.

The table at the bottom of page 1-10 gives the most common symbols and abbreviations which you may encounter in reading the various radar publications,

Frequency code.

Radar equipment operates on many frequencies. some of which are just being explored today. To safeguard this system, operating frequencies or wavelengths are classified and described only by the following code:

Frequency in megacycles	Code
0 to 300	P
300 to 1,500	L
1,500 to 5,000	S
5,000 to 10,000	X
10,000 and above	K

Equivalent measurements.

1 inch	= 2.54 centimeters
1 yard	= 36 inches
1 meter	= 100 centimeters
1 meter	= 39.37 inches

Principle of pulse reflection.

Now that you know something of the importance of radar, its various types and their uses, and have built up a vocabulary of terms frequently used, let us consider next just how this equipment functions in securing information.

If you understand the principle of sound echoes you have mastered one of the basic understandings of radar. Suppose that you are in a canyon and that some distance from you is one of the walls of this canyon. You shout loudly-then wait. What happens? The shout comes back in the form of an echo. Why? It is simply that the sound wave from your voice travel through the air, hit the wall of the canyon, and bounce back. You hear the reflected sound wave. If you want to hear it distinctly you do not shout continuously, but utter one brief sound and then maintain silence until the echo returns.

By shouting for a short interval and then waiting, it is possible for you to shout loudly again the next time. In other words, you are sending out pulses of energy of short time duration, thereby making it possible for you to shout at maximum strength without straining your voice.

This is the basic principle of echo ranging: sending out brief pulses of energy and measuring the time it takes them to return.

SENDING ENERGY ....... TIME TO RETURN

In radar we deal not with sound waves, but with radio waves. In shouting, the sound waves are produced by your vocal cords. In radar the radio waves are produced by a unit called the transmitter. This radar transmitter is turned on for a very short period, so short that the time is measured in microseconds (millionths of a second). This short time during which the radar is sending out radio energy is called the pulse width, or pulse duration. During this short time the transmitter produces a maximum amount of energy.

After the transmitter is turned off, there is a definite amount of time (measured in microseconds) before it is turned on again. This is called the rest period. It is during this rest period that the echo returns, if there is a target in the path of the radio waves. This rest period must be long enough to allow time for an echo to return before another pulse of energy is sent out. Accordingly, we have the pulse width during which the strong radio signal is sent out into space, and the rest period during which time the echo returns. The transmitter is also turned on a certain number of times a second, thus setting up a pulse repetition rate.

Bearing determination.

Suppose that there are several large cliffs or walls in different directions from you in a canyon in which you are shouting, and that you desire to receive an echo from one certain cliff. By shouting and sending the sound waves in all directions, you have no way of telling which wall is sending the echo back to you; perhaps they are all sending back a small amount of the energy. By placing a megaphone to your mouth, or by cupping your hands around your mouth, it is possible to direct most of the sound waves in any desired direction. If you desire to receive an echo from one certain wall or cliff, you point the megaphone in the direction of that cliff and shout into it. The sound waves are concentrated into a narrow beam, go out, hit the cliff, and are reflected back from the same direction. Since you know the direction

In radar the radio waves are concentrated into a narrow beam by a special antenna which will be described later. This narrow beam is much the same as the concentrated beam of light a searchlight sends out. Just as you can point a megaphone or searchlight, you are also able to point the antenna and direct the narrow beam of radio waves in any desired direction. If now you receive an echo while the antenna is pointing in a certain direction, you know the target is in that direction. Radar works the same at night or during bad weather as it does in daytime or during good weather. Accordingly, it is possible to detect a target with radar when it is impossible to see it with optical equipment. By turning the antenna through a complete circle or 360 degrees around you, it is possible to detect any target. You will know the direction or bearing of this target by knowing the direction the antenna is facing.

Range determination by sound.

Detecting the target and knowing its bearing are not enough. You must also know how far it is from you, or the range of the target. By knowing both the bearing and range of a target, you locate the target exactly.

The following analogy will help to make the concept of range clear. If a stone is dropped into a pool, a small wave will start out from where the stone hit. This wave spreads out in a circle in all directions. If there is a pole or piling in the pool a short distance from where the stone is dropped, the wave going out will hit the piling and a reflected wave will start back. Assume for the sake of explanation that the wave is traveling at a speed of one foot per second through the water. If you start a stop watch when the stone hits the water and note how many seconds elapse before the reflected wave returns to the starting point, you can easily tell how far away the piling is. For example, if it takes eight seconds for the wave to go out and return, then the distance traveled is eight feet. The range, which is the distance out to the piling, will be one-half of the total distance, or four feet.

A special stop watch could he devised with the face marked off in feet instead of seconds. The dial would read distance instead of time. For example, in place of one, two, three seconds, and so on, the face would read one, two, three feet, etc. Better yet, since we are interested only in the distance to the target, the face

What you actually do is measure the elapsed time from the instant the stone hits the water until the reflected wave returns to the starting place. Knowing the speed of the moving wave, you multiply the time by the speed. in order to compute the distance. As range is only half the distance traveled, you divide the distance by two and obtain the range. Put in the form of a formula, R = (s x t)/2 where R is range, s is speed, and t is time.

Let us go back to the example of sound echoing in a canyon. Suppose you want to know how far away that cliff (i.e., its range) which sent back the echo is. You need to know that the speed of sound is approximately 1,100 feet per second, and you also need a watch, or better still, a stopwatch. It is now an easy matter to obtain the range. If, for instance, four seconds pass from the time you shout in this canyon until you receive the echo, you know that the distance traveled by the sound waves out to the cliff and back is 4 x 1,100 feet or 4,400 feet. You know that range is one-half the distance out and back: therefore the range is (4 x 1,000)/2 or 2,200 feet. Since you divided by two, why not divide by two at the start, and make a statement that for sound, range equals 550 feet multiplied by time? Thus, it would he 550 x or 2,200 feet.

On your stop watch, you could make a special face to measure range in terms of echoes. Since for sound one second is equal to 550 feet range, in place of one second on the watch have 550 feet, in place of two seconds have 1,100 feet, etc. Thus, whenever the echo returns, it is possible to note where the second hand is at that instant on the face of the watch and to read directly the range of the target.

Range determination by radar.

How does all this fit in with radar? Let us next see how see are able to tell how far away the target is, or how to obtain the range with radar.

Light travels so fast that it is almost instantaneous. Radio waves, electricity, and light, all travel at about the same speed, which is 186,000 land miles per second, or almost seven and one-half times around the earth in one second. Expressing this in nautical miles

For example, suppose that the transmitter sent out a short pulse of radio energy and that the reflected wave was received after 1,000 microseconds. The distance traveled out to the target and back is 1,000/*1,000,000 x 162,000 = 162 nautical miles. Referring to the sound analogy, remember that in order to compute the range, the total distance out and back must he divided by two. So also with radar; the total distance must be divided by two. In this case range is 162/2 or 81 nautical miles.

However, the speed of radio waves is much greater than that of sound. Therefore, an ordinary stop watch cannot be used. A special timing device is needed to measure such small time intervals as microseconds. This special timing device is called the cathode-ray tube, and on it there is a special time base which takes the place of a second hand on the stop watch. Instead of being marked off in divisions of one, two, three microseconds, etc., this time base can be marked off directly in miles or yards of range.

Summary.

By sending out a very short pulse of energy from a high powered transmitter, and receiving the echo which is called the pip, you have detected a target. By knowing the direction the antenna is facing, you know the direction or bearing of the target. By measuring the time it takes the wave to go out to a target and return, you have a means of obtaining the range of the target. If you know the bearing and range of a target, you then know the exact position of this target at any instant. In good weather or bad, in daytime or at night, whether surface craft or aircraft, you are able to establish the desired data. But these are only some of the many things you are able to tell about the target. In the pages which follow you will learn about other information which a good operator can get through the medium of radar.

MAIN PARTS OF A RADAR SYSTEM

In the preceding section you have learned about the principles of radar. This section will deal with

* The 1,000 microseconds must be divided by 1,000,000 to give seconds.

The transmitter.

Wave length or frequency. To make radio detection and ranging possible, it is necessary at the outset to send out a pulse of radio waves. It is the function of the transmitter to generate the pulse. Since the transmitter creates the pulse, it is the source of the radio energy. There are well defined differences between the radar transmitter and the radio transmitter that operates in the radio station. One of the main differences is in wave bands on which the two types of transmitters operate. Radio stations operate on either the broadcast or the short wave bands, but radar uses the ultra (very) short waves that were previously used only for experimental purposes. Another difference is that radio broadcast transmission is continuous while radar transmission is intermittent.

Duration and power of pulse. An important point to remember about the compact radar transmitter is its ability to send out pulses of radio energy as powerful as, and in many cases more powerful, than the transmissions from the biggest broadcast transmitters. High power is necessary in radio transmissions to carry the broadcasts to the listeners at distant points. In radar very strong pulses must be emitted in order to get back even a small echo or reflection from the waves striking the target while the rest of the waves continue into space until they die out. Unfortunately, only a minute amount of the energy of the pulse sent out is bounced back. In generating the strong pulses that are needed, high voltages, dangerous to life, are required. Everything possible, however, has been done to make the equipment safe for the operator, so long as certain safety precautions are observed.

One advantage that results from the transmitter's working only in brief periods is the long rest or idle period during which sufficient electrical energy can be stored up to provide the extremely high power necessary for the next pulse. Thus the overall or average power output of the transmitter is low, and within the transmitter's capacity. During the brief moment of transmission, considerable heat is generated by the tubes. During the rest period the tubes cool. Should the transmitter be allowed to operate continuously at such high power the unit would be badly damaged or even destroyed by the intense heat. Motor-driven fans or blowers circulate cool air inside the cabinet of the radar set to aid in keeping the temperatures at safe operating levels. The length of the rest period (expressed in microseconds) is dependent upon the pulse interval (the time between the beginning of one pulse and the start of the next one) and the pulse width (the working time).

Pulse rate. The keyer unit performs the task of pulsing the transmitter. The keyer, which will be described later in this section, does exactly what its name implies, it keys the transmitter, turning it on for an instantaneous surge of radio energy for a few microseconds (or even a fraction of a microsecond); then, after the pulse, it turns it off for a comparatively long time until the next pulse. It is during this period while the transmitter is resting that the echoes return from any object that was in the path of the outgoing pulse. The rate at which the keyer pulses the transmitter is the pulse rate or pulse repetition rate, which simply means the number of times the transmitter is sending out a pulse of radio waves each second. The keyer operates at a constant rate, spacing the pulses so that the interval between any two is always the same. The length of time of

The antenna system.

The antenna system is one of the most important parts of the radar equipment since it radiates the radio frequency energy into space and receives the reflected energy or echo. The antenna system is made up of two parts: (1) the transmission line, and (2) the antenna.

Transmission line. The purpose of the transmission line is to carry the high frequency energy from the transmitter to the antenna, and to carry the reflected energy from the antenna to the receiver. This transfer of energy must he done with a minimum of loss.

Two types of transmission lines are in general use in radar equipment: the concentric or coaxial line, and the hollow wave guide. The coaxial line consist of one conductor inside another. Both conductors may be tubular, or the outside one may be a hollow tube and the inside a solid conductor. Since the inner conductor must be exactly in the center of the outer conductor, a great number of insulating disks are required. Moisture on these insulators may cause a flash-over or breakdown between the two conductors so it is necessary to keep the line filled with nitrogen or dry air at a pressure of about five pounds per square inch. The wave guide is a hollow metal pipe, the cross section may be rectangular or circular. The rectangular wave guide is most commonly used today.

When comparing the coaxial line with the wave guide, the following advantages seem to favor the wave guide: (1) construction is simpler. (2) losses are lower, and (3) power capacity is greater.

The wave guide's principal disadvantage is that unless the frequency is very high, the size of the pipe must be unreasonably large.

Antenna: an emitter of radio waves. In order to utilize the radio energy created by the transmitter pulse, it must be converted into radio waves, which can be shot out to strike any object in the outgoing path. The antenna functions as the converter of the radio energy into radio waves which can be radiated in any desired direction into the atmosphere. To better illustrate this fact, consider the situation resulting when a flashlight is supplied with new batteries but lacks a bulb. Even though the necessary power is available, the means of using it are lacking. As soon as you insert a bulb in the socket and close the switch, the battery energy is put to work and its conversion to light waves results in a beam of light. The transmitter can be likened to the battery-filled flashlight, for it, too, is a source of stored energy-radio energy. The transmitter is inoperative without an antenna for the same reason that the flashlight is inoperative without a bulb. The antenna converts the pulse of radio energy from the transmitter into radio waves, just as the bulb converts the battery energy to light waves.

The fundamental element from which most antennas are built is the half-wave antenna, or dipole. This is a metal rod or wire one-half wave length long. Since the velocity of radio waves is constant, 300,000,000 meters per second, the length of a

Non-directional antenna. A single dipole will send its energy out in all directions around itself. The greatest amount of power goes directly outward at right angles to the length of the rod with decreasing amounts of energy out in other directions except in the direction of the rod, where no energy is sent out. Consequently, the side view of the lobe looks like a pair of circles touching the dipole, as shown in figure 1-11.

A target at right angles to this dipole would give a strong echo, while one in the actual direction of the dipole would give a very weak echo (theoretically no echo). One of these dipoles mounted vertically would send a strong signal toward anything around it and on its level. It would give a weaker signal to anything above it. Some of our sets use antennas like this so that any target around them, regardless of its bearing, can be detected. Because the radio waves are going out in all directions, bearing cannot be indicated and only range can be determined.

Directional antenna. A reflector can be used with a radar antenna to make it unidirectional, that is, to cause all the waves to leave the antenna in one direction rather than in two directions. This reflector serves the same purpose, and functions in the same manner as the reflector used in a flashlight. The four most popular types of reflectors are: (1) the flat or "bedspring" type, (2) paraboloidal or "dish pan", (3) parabolic or "barrel stave", and (4) semi-parabolic.

The type of radar antenna using a number of dipoles, called a curtain array, or bedspring array, has a metal screen reflector. The dipoles are a definite distance forward of the screen. The more dipoles used, the sharper the beam. However, the energy is strongest in a direction approximately at right angles to the screen. The metal reflector is perforated to reduce

Figure 1-11. Dipole and radiation pattern.

If the antenna is wide, you can expect a narrow horizontal lobe because there will be several dipoles horizontally. If there are many dipoles vertically, the

Figure 1-12. Wave lengths.

vertical lobe will be sharp. Of course, the physical size (in feet and inches) of any particular lobe width will depend on the wave length. When comparing antenna sizes, be sure to measure them in square wave lengths.

As an example, if you have an antenna like the one shown in figure 1-12 you can measure the area easily by noting that each rod is one-half wave length long, and that each row of rods is one-half wave length

Figure 1-13. Wave lengths.

away from the next. This antenna, then, is three wave lengths broad, one wave length high, and has an area of three square wave lengths.

The antenna in figure 1-13, though physically smaller, is four wave lengths wide, two wave lengths high, and has an area of eight square wave lengths-almost three times as great as the previous example.

As a general rule, a sharp vertical lobe is not desirable for search sets because the lobe might shoot over the target and miss it as the ship rolls. If this happened, an echo would he reflected from the target only occasionally, instead of almost every time the

Fire-control sets must give accurate bearings, and (if used to control anti-aircraft fire) accurate position angles. By reducing the wave length (increasing the frequency), the antenna necessary for this bearing accuracy can be reduced until its size is physically practical. If, also, the reflector is bent into a semi-parabola, the sharpness of the vertical lobe can he increased. From the side, this antenna has the appearance of a "V" with a rounded bottom, the "V" being on its side with the wide opening toward the target.

This antenna is not entirely satisfactory for anti-aircraft fire control. Being a wide antenna (in wave lengths), it gives satisfactory bearing accuracy, but lacks sufficient position angle accuracy for AA gun laying. To increase this accuracy, a modification has been made. In effect, two of these semi-parabolic antennas have been fastened together, one atop the other. In this way, the sharpness of the vertical lobe

As you go to higher frequencies, dipoles and curtains are of less concern. Reflectors of the type used with searchlights can be employed. This is due to the fact that radio waves at the higher radar frequencies behave much like light.

Paraboloidal reflectors, bowl-shaped, are usually called "dishpan" reflectors, or "spinners". They are used for surface-search and some air-search and fire-control sets. Their hearing accuracy depends on the diameter of the spinner measured in wave lengths (the unit of measure used for other types of antennas).

Since this type of antenna produces a "pencil" beam, the beam is as narrow in the vertical plane as it is in the horizontal, which for some types of radar necessitates a helical search (a spiral search, changing the position angle as well as the bearing angle).

On some sets, a sharp, vertical lobe is a definite disadvantage. If a large spinner is used (for good bearing accuracy), the vertical lobe may be too narrow. To increase the vertical beam width, the rep and the bottom parts of the spinner are cut off, which reduces the vertical size of the spinner without reducing the

Another type of antenna involves an entirely different principle. Tapered plastic rods about three feet long are used, and the energy comes out along the full length of a given rod. A rod by itself will produce a beam about 30 degrees wide. By placing 14 of them side by side, the beam is narrowed to 2 degrees. Three vertical rows are used to narrow the beam to 6 degrees in the vertical plane. When these rods are energized at different times, the lobe goes out to one side-toward the side which received the energy last. Constantly changing the amount of delay causes the lobe to move steadily from 15 degrees to the left of the center line to 15 degrees to the right in 1/10 of a second. The great advantage of this system is that 30 degrees can be seen at once, with the accuracy of a 2 degrees beam (similar to television scanning). This permits accurate spotting in both range and deflection. Such an antenna, however, is extremely heavy and complicated.

Figure 1-15. Parabolic or barrel stave antenna.

The foregoing discussion of antennas has avoided technical treatment of the subject because for purposes of this handbook only general ideas are needed. It will he helpful to keep the following in mind:

1. The antenna size, in wave lengths, determines the size and shape of the lobe. 2. A wide antenna gives a narrow, sharp horizontal lobe, while a narrow antenna produces a broad horizontal lobe and poor bearing accuracy. 3. Use of high frequencies permits use of parabolic

How does radar determine bearing?

You have already learned how radar detects the presence of a target and determines range. In this section you will discover just how it establishes the direction, or bearing and position angle.

When you shout in the direction of a cliff or big building you hear an echo, but when there is no cliff or building to reflect the energy there is no echo. If the radar energy, like sound, is sent out in one general direction, you can tell approximately the direction of an object by simply observing the direction from which the echo returns. By knowing this direction, you know the target's direction since it is the same.

When you shout, the sound as you know, does not go only straight out, but can also he heard to either side of the direction you are facing. The degree to which the energy is scattered will determine how accurately you can judge from the echo whether or not you are facing the cliff. If the sound energy is scattered over a wide angle, perhaps you can receive an echo when you are facing, and shouting, in a direction far off to one side of the target. A small amount of the energy goes off in the direction of the target, and will he reflected as an echo, but it will be a weak echo.

You are probably wondering now how you can tell whether you are getting the echo back from the direction you are facing, rather than from a direction off to one side? The answer to this is that you get the largest echo when you are facing the target directly.

Concept of lobe. Your radar set sends out its energy like sound in a general direction, but with some scattering. You receive radar echoes from a target even if you are not pointing the antenna in the exact direction thereof. But, as in the case of sound, you get the strongest echo when you are directly on the target.

Since you get the strongest echo from an object when the antenna is pointed directly at it, you can reasonably expect to locate a small target in that direction more easily than in any other direction. Likewise, you can detect an echo from a ship at a greater range in the direction the antenna is pointing than in any other direction. It is logical to expect that a ship will reflect a stronger echo when it is only slightly off the antenna hearing than when it is considerably

In fact, if a ship sails around your antenna hearing in such a way as to always produce the same strength echo, it will follow the path shown in figure 1-16. This shape is called a lobe, and is actually a representation of the range at which you can get an echo of any particular size in any direction from the antenna. It represents approximately the amount of power sent out in any direction. Figure 1-19 shows how great a change in echo height results simply from turning the antenna around, sweeping the lobe past the target.

"E" units. Referring to the actual height of the echo in inches is of little value because you can increase or decrease the height of the echo at will merely by varying the gain control. Even a weak signal can easily be "blown up" in this manner until it approaches saturation, but this increase may not help because of the corresponding increase in grass height.

Grass is a disturbance on the cathode, ray tube (C.R.T.) screen, caused by noise in the tubes, static, etc. It shows up as a fuzzy, jumpy fur along the time base on "A" and "R" scopes, and as many small, bright spots (sometimes called snow) on the B and PPI scopes. It is always present to some degree, and pips smaller than the grass are very difficult to find. The grass seems to wave just as actual grass does, and may cover the pip. The height of this grass can be controlled by the receiver sensitivity (or gain) control low sensitivity means low grass height and high sensitivity means a large amount of grass. You know that the pip height can he made larger by increasing the gain, and smaller by reducing the gain. Since both the pip height and the grass height vary together,

The "E" unit system of discussing echo strength is standard in the Navy. It is a convenient system to use; by providing common terms in which to discuss the strength of echoes confusion and misunderstanding are reduced. A small pip about the same height as the grass is an E-1 echo. It will always he difficult to detect, and only a wide-awake operator will notice it. An echo of this strength will probably he overlooked on a PPI scope by even the very best operator. That is the main reason that an operator should not devote all his attention to the PPI scope.

The complete E system includes five values: E-1, E-2, E-3, E-4, and E-5. E-1, as is indicated above, includes those signals whose ratio of signal to noise, or of pip height to grass height is one to one (i.e., the pip and grass are the same height). An E-2 echo is an echo whose pip is twice as high as the grass. An E-3 echo is one producing a pip four times as high as the grass. If the pip is eight times as tall as the grass, we say we have an E-4 echo. An exceptionally great echo reaching saturation, or 16 times, or more the height of the grass, is spoken of as an E-5 echo.

An E-1 echo is very weak, an E-2 echo is weak, and F-3 echo is good, and E-4 echo is strong, and an E-5 echo is very strong. The F-5 echoes are the echoes we get from large, nearby targets.

The E-number system enables you to report echo strength in a definite way. You can also use the E-number system for labeling lobes which, as you know, are representations of the range at which you can get an echo of any particular size in any direction from the antenna. If lobes were drawn to represent all echoes for a particular size target from E-1 (very weak-) to E-5 (very strong) they would appear as

The closer the ship is to you, the larger the blip produced, provided that it stays in any one direction from the lobe center. Also, for any range, the size of the returned echo (and thus the blip) is smaller as the direction of the target gets farther and farther from

Maximum echo method. Knowing the D.L., you have a way of telling just where the ship is. Simply turning the antenna until you get the largest blip or the brightest spot, will give you the direction of the target.

Now, let us repeat the procedure in a brief, summarized form. As the target comes steaming in, keeping always at a certain angle with the D.L., the signal

You cannot affect the range of the target, and hence are unable to do much to increase the height of the blip in this way, while keeping the target on a certain angle with the DL., but if you swing the antenna around a larger echo might come hack to you. lithe target steams along, keeping at the same range, but moving nearer and nearer the D.L., you know that an increasing amount of the energy is hitting it, resulting in a bigger and bigger echo. Conversely, as soon as

This method of setting the antenna (which determines the direction of the lobe), to find the bearing of a target is called the maximum echo method, since you read the direction from which you get the maximum echo. It is the easiest, and consequently the most frequently used method.

Minimum echo method. Look at the lobe again. Where can you get a bigger change in blip height than near the D.L.? The biggest change you can get will be right at the edge of the lobe. For this sketch, a ship at "A" would be on the barely pick-up, or the minimum echo line of the lobe. It would give you an E-1 echo. If you swing the lobe around toward the direction of the target (the ship), the

However, you find one difficulty: you can read the hearing of the lobe (or the antenna), but not the bearing of the target. Since you want the direction of the target, you are interested in the antenna direction only if it can give you this information. In your present situation, it is obvious that you do not know the difference in direction of the antenna and the target. Consequently when the target is in one edge of the lobe, you cannot find the direction of the target simply by knowing the direction of the antenna. You need to know something more.

If you can find the angle between the antenna direction and the direction of the target when it is in the edge, you can either add or subtract this angle from the bearing of the antenna and arrive at the desired answer.

You can set your antenna so that the target is accurately in one edge of the lobe and then in the other edge, getting two bearings: one larger than the bearing of the target by a certain (unknown) amount, and the other smaller than the bearing of

Lobe switching. So far, you have found two ways of setting your antenna (to direct the lobe), to find the bearing of the target: the maximum echo setting and the minimum echo settings. The minimum echo setting gives you the bigger change in echo size for each degree change in antenna train; hence it is the more precise. However, there is an even more accurate way of setting your antenna, and for two reasons: first, because the side of the lobe is used instead of the blunt end, consequently the size of the echo is extremely sensitive to any small change in antenna train, and second, because an improved method of indication is used, based on the comparison of two pips heights (it is easier to judge when two pips are the same size than to judge when a single pip is at maximum size). This is the procedure called lobe switching.

If you have an object that increases in height while another object decreases, the difference in their comparative heights will change twice as fast as the height of either one. If two echoes work together in this way to show when you are on the target (one going up and the other down when you get off the target), you can get the antenna set in the target direction just twice as easily as you could by looking at the change in only one echo.

1. Direct the energy out to the port side of the antenna direction and get an echo hack. The size of this echo will depend, of course, on the part of the lobe in which the target appears (see fig. 1-22). With this sketch, the echo would be about an E-2 echo.

Figure 1-22. Lobe to port.

2. Then direct the energy out to the starboard side of the antenna direction to see how big that echo is. Of course, the echo size depends on its position in this lobe (see fig. 1-23).

Figure 1-23. Lobe to starboard.

In comparing the size of these echoes you found that they were not the same. Why? Simply because the target is nearer to the center of one lobe than it is to the other. The target is nearer the edge of

Several things must be kept in mind in lobe switching. One point is that you send the energy out on only one direction at one time. You send a few pulses out to one side of the antenna direction, and then an equal number of pulses out to the other side of the antenna bearing. You are sending the energy out one way or the other; not both at the same time. Another is that you need to separate the echoes so that you can compare them easily. You can make the blips from the starboard lobe show up a little to one side of the blips from the port side (fig. 1-24), so that the blips will appear side by side. It is vitally important that all the echoes returning from pulses sent out to port appear at the same definite position, and that all those returning from starboard pulses appear at another precise position.

Notice that you do not change the direction of the antenna between these pulses: you simply switch the lobes from one side to the other. When you change the antenna hearing, you change the direction of the lobes as well, maintaining their position a certain number of degrees from the antenna bearing. When you get the blips matched you know that the hearing of the antenna is very close to the desired bearing of the target, but you realize that the target is not in the center of either lobe. Since this is true the echo height is smaller than it would be if the target happened to be centered in either lobe:

The lobe switching method is used in some of our fire-control sets, where bearing accuracy is absolutely vital. When bearing accuracy is as important as in the case of fire-control gear, you usually have a separate scope on which to match pip heights. You can find details of the actual method employed in Part 4 of this book, and in the instruction books furnished by the manufacturers of the various sets. On radars such as the SJ or SA, in which cases the precise bearing is not as important as it is in a fire-control set, we use

You have found that you cannot read the bearing of the target directly, but only the bearing of the antenna. But if you know when your antenna is bearing directly on the target, you can read the bearing of the antenna as the bearing of the target. If the antenna is off the target slightly when you read, you naturally get a bearing that is incorrect. But, if you can tell when you are off the target, you can tell when you are on the target. The simpler the method for finding when you are off, the mote accurately you can read the bearing of the target. The bigger the change produced in whatever you are looking at (the blip height, for instance), with a small change in antenna bearing, the greater the accuracy will be. The maximum echo method is the least accurate of the three methods you have studied here because the change in echo strength is small per degree of antenna train. The minimum echo method is more accurate than the former, but it takes more time, and so it is not used very often. The lobe switching method is the most accurate method in general use. It gives a big change in height difference for a small change in antenna bearing.

Minor lobes. So far, in our discussion of lobes, antennas and bearings, we have assumed that most of the

When enough energy goes our to form a distinct lobe like this, we say that we have minor lobes, or side lobes (see fig. 1-25).

What does this mean to you as an operator? The primary thing is that you can pick up a target in a side lobe and report it as being in the main (or major) direction of the antenna assuming that the antenna is trained in the direction of the target. If you have a target in a side lobe, however, and are unaware that it is there, you might think that your antenna is bearing directly on the target, when such is not the case at all, and any bearing reading that you may take will be incorrect. If you have side lobes, the assumption that you can pick up targets only when your antenna bears on them is wrong.

For example, let us suppose that you have side lobes 60 degrees to each side of the main lobe. If your target actually bears 135 degrees, you might read a bearing of 195 degrees or of 075 degrees if you have it in one of the side lobes. That is evident, for your antenna is actually directed 60 degrees to one side or the other of the target.

These side lobes occur to some extent with all types of antennas. They are generally most noticeable and troublesome with a curtain array. They occur because the dimensions of our antennas are only a small number of wave lengths. We cannot escape them entirely with our present radar techniques; we must recognize their presence, and be careful to avoid errors resulting from them.

Although these minor lobes may cause errors in establishing bearing, they will not result in incorrect range readings. How to recognize pips from side lobes is discussed in Part 3 under Composition."

The receiver.

To all intents and purposes the radar receiver bears a close resemblance to an ordinary radio receiver. Of course, the two sets differ in frequencies of operation, for the receiver must be tuned to the same spot in the wave band as its associated transmitter, and as earlier emphasized, the bands used by radio and radar are widely separated.

Signal amplification. The receiver used in radar must be very sensitive so as to operate on weak echoes. The power represented in the echo would be of little value if it were not built up in some way. This reinforcing or strengthening action takes place in the receiver, and is called amplification. It is the amplifier that builds up the weak radio signals into energy that is finally converted into sound issuing from the loud speaker of a radio set.

In the radar receiver the echo is amplified or increased by similar amplifiers. A manually operated gain control enables you to control the amount that the

The new signal, having been made much stronger and lowered in frequency, is now one that you can use. So far, however, you have nothing to indicate your receiver output. For this purpose, you must connect the receiver to a cathode-ray tube, or INDICATOR, which will indicate the return of the echo. The speaker on your home radio set corresponds to the indicator.

The indicator.

In radar, extremely small divisions of time are measured. The unit is the microsecond, a millionth of a second. Obviously, no ordinary time-measuring device will serve this purpose. However, it has been found that a device used for many years by television people fits radar's demands well. This is called a cathode-ray tube, or the scope. Learning about some construction features of the C.R.T. (cathode-ray tube) will help you to understand how it is used to measure time. Basic electron theory. Before discussing the functions of the various parts of the C.R.T. you should know some fundamentals of electron theory with particular reference to how an electron beam is used to measure time.

Scientists tell us that everything is made up of atoms. which are extremely small particles of matter, Each

Figure 1-26. Structure of on atom showing electrons in their orbits.

Scientists discovered that by heating material such as metal it is possible to make the atoms within the metal collide with sufficient force and such rapidity that the metal will emit or give off electrons in large numbers. They also discovered that some metals give off more electrons than others, indicating that some materials release their electrons more easily than others.

How well these electrons flow in materials is quite important when selecting materials for conductors and insulators. Conductors are materials used for electrical wires because the electrons are held together loosely and can move through the material with relative freedom. Insulators, on the other hand, are materials which hold their electrons very tightly around the center of the atom. Electrons have great difficulty in moving through insulators.

Electrons have a small negative charge. Usually, this negative charge is cancelled by the positive charge of the center part of our atom. This is true, of course, only if the atom has exactly the right number of electrons. If an atom has too many electrons, a negative charge exists, if it has lost some electrons, it is said to have a positive charge.

In conducting materials a large number of extra

If it is possible for them to do so, the electrons will return to atoms which do not have enough electrons. As soon as the electrons are paired off with atoms lacking electrons, everything is back to normal, or a state known as zero potential.

Whenever you collect electrons in one place to produce a negative charge, you must get these electrons front somewhere. When electrons are taken from atoms, those atoms do not have enough electrons to be neutral. Since there is a lack of electrons the atoms in question have a positive charge.

There are certain facts about negative charges and positive charges that may be stated in the following general law: electrical charges of like kind repel each other, and charges of unlike kind attract each other. In terms of negative charge and positive charge the law is: a negative charge will repel a negative charge; a positive charge will repel a positive charge; a positive charge will attract a negative charge. This is exactly like the principle of magnetism: like poles repel and unlike poles attract each other.

The following is a summary of the main ideas we have discussed on the fundamentals of electron theory.

1. Electrons are small particles of negative electricity. 2. Each atom has a certain typical number of electrons. So long as it has this number of electrons, it has no net charge, and thus has zero potential. 3. Conducting materials have some electrons attached very loosely. The atoms can gain or lose electrons easily. 4. Non-conducting materials have the electrons firmly bound to the atoms. Any free electron finds great resistance to its movement. 5. When electrons are caused to assemble on, or in, something, that object is said to have a negative charge. 6. When electrons are taken away (so that there is an insufficient number to satisfy all the atoms), the object has a positive charge. 7. Electrons will go from a negative charge to a

In order to measure the time it takes for the energy to go out, and echo hack, we must have some instrument to indicate its return. Mechanical means, such as an ordinary pencil with gear and lever arrangements Could not operate rapidly enough for this: their weight prevents them from being moved quickly enough to give an indication of the presence of the reflecting object. Since electrons are so extremely light, a beam of electrons may be made to move when the echo returns, and move in just a fraction of a microsecond. This electron beam is used as a convenient pencil to draw the picture of what is happening.

Structure of the electrostatic cathode-ray tube. In a cathode-ray tube the hot metal from which the electrons are boiled is called a cathode. This is usually only a small piece of fine wire (something like the filament of an ordinary light bulb), which is heated by an electric current. In order to make the electrons boil off more easily, this wire is usually given a chalky covering of a special material, which is merely a substance from which electrons can easily be boiled. The cathode, then, just furnishes the electrons.

You could control the number of electrons by varying the cathode temperature, but this is a very slow process. Some other method of controlling the number of electrons must be used. A man by the name of De Forest found that a negatively charged piece of metal in the path of these electrons could stop them altogether, while a wire with a smaller negative charge would permit some of them to go by, the number depending on how negative this wire was. He also found that by weaving wire into a grid he could do this more easily than by using just a single wire. We still call this controlling part of the C.R.T. the grid, but its actual physical shape is more like a miniature tomato can fitted around (but not touching the cathode). There is a small hole in one end to let the electrons out in a stream, or beam. The potential of

The electrons coming out of the grid are moving relatively slowly, and more or less at random. If they are to he used, they must be speeded up (accelerated) greatly so that they will get from the cathode to the screen in a short time. To do this two more parts are placed in the C.R.T.: the first anode and the second anode. Remembering the properties of electrons, you know that a positive voltage attracts the electrons and causes them to rush toward it. This force of attraction depends on the magnitude of the voltage, so to get a strong force, the anodes are put at a high positive voltage. This causes the electrons to move very rapidly toward the anodes, and to shoot through them toward the center of the screen.

The anodes are cylindrical (see fig. 1-28) and are mounted so that the electrons may shoot through the hole in the grid, the hole in the first anode, and the hole in the second anode. Since the electrons

Figure 1-28.

emerge from the second anode at extremely high speed, the complete arrangement of the cathode, grid, first anode, and second anode is sometimes called the electron gun.

You know that you can attract the electron with a positive charge and repel it with a negative charge, and that the attraction or repulsion is proportional to the charge (or potential). You can move the beam upward by placing a positive charge near the top, or a negative charge near the bottom of the tube, or by both. Likewise you can move the beam to the side by putting positive or negative charges to the side. It is convenient to put these charges in place by using two pairs of metal plates, one pair horizontal and the other pair vertical. These are known as the deflection plates, since they deflect the electron beam.

One of the vertical plates is above and the other below the beam. If there is a positive charge on the upper one and a negative charge on the lower, the beam will move upward, since the positive charge attracts the electrons and the negative charge repels them. These charges can be placed on the plates this way by connecting the + terminal of a battery to the upper plate and the - terminal of the same battery to the lower plate. Since these plates move the beam up or down, they are called the vertical deflecting plates (V.D.P.). Remember that they lie horizontally in the tube, but the direction in which they move the beam is important. They are called the

There is another pair of plates which are exactly like the vertical plates, except that they are turned half-way around so that they are at right angles to the vertical plates. They are able to move the beam to the right or the left, depending on the charges on these plates. Since they control the back-and-forth position of the dot, they are called the horizontal deflection plates.

It is obvious that by merely varying the vertical position and the horizontal position of the dot, you can make the dot take any position you want, anywhere on the screen. Consequently, by adjusting the voltages on the two sets of plates properly, you can make the dot appear at any place on the screen. You can make it move in any definite manner by changing the voltage on one or both sets of the plates properly.

Concept of sweep and time base. If the dot moves from left to right at a certain speed, you can use this spot movement as a yardstick with which to measure time. Suppose the distance the dot moves is three inches, and that it covers this distance in exactly 1,200

The spot on the screen of the radar cathode-ray tube moves across the face of the tube in the same way. It starts at one place and moves toward some

Since the time and the range to a target always have a fixed relationship, let distance on the scope be marked in range instead of time. Do not forget, though, that you are actually measuring time. In all radar sets, the path of the sweep is marked in units of range such as miles or yards (and not time) for convenience in reading. This sweep-line is sometimes called the time basis.

So far, you have studied the electrostatic cathode-ray tube. You have found that you can deflect the beam by changing the charges (or voltages) on the deflection plates. You have learned that the sweep is produced by varying the voltage on these plates in a very definite way.

Electromagnetic cathode-ray tube. However, getting the sweep on a PPI scope is more difficult because the sweep must change direction but not speed. It is troublesome to get the voltages to cooperate and vary properly. Fortunately, another type of cathode-ray tube is available; using this tube you can get the sweep to move in the desired way with little difficulty. This type is known as the magnetic cathode-ray tube.

The stronger the magnetism, the harder it tends to get out, and consequently the farther from the center of the screen it will appear. If you vary the magnetism between a pair of poles mounted vertically across the neck of the tube, the spot on the tube face will move sideways, farther from the center if you increase the magnetism, and closer to the center if

If you have ever experimented with electromagnets you know that it is possible to vary the amount of magnetism simply by changing the current through the coils around them. That is just what is done in radar. To get the spot to move and form the sweep, you increase the current through the magnetic coils. The magnet (or held) current determines the position of the spot on the screen. The current in the control coils above and below the neck, controls the horizontal position of the spot, and that in the coils to the right and the left controls the vertical position.

By varying these currents (and thus the magnetism) you can change the spot position in exactly the same way as was done by changing the charges on the deflection plates. By varying the magnetism properly, you can make the spot draw any desired picture, duplicating any picture that could he drawn on the screen of an electrostatic tube. However, it is easier to produce certain voltage changes than it is to change the currents correspondingly. Likewise, current may sometimes be changed more easily than voltage. Making the spot on the screen move outward first in one direction and then in another (to form the PPI sweep) without changing the sweep speed is relatively easy in the magnetic tube. This action is difficult to achieve in an electrostatic tube. You know that the spot must be made to move in this manner, if you are to have a true picture of the PPI (Plan Position Indicator) scope.

The deflection coils are mounted around the neck of the tube within easy reach. Since the sweep is perpendicular to the direction of the magnetism, you can turn the coils around the neck of the tube to change the direction of the sweep. To produce a PPI scope, then, you merely need to rotate the coils as the antenna turns, change their magnetism properly, and intensify the beam when the echo comes back. It would be troublesome trying to use rotating deflection plates on the outside of the neck of the tube, otherwise we could use an electrostatic C.R.T. as well as we can use the magnetic C.R.T. Even if the coils are not rotated, it is easy to make the sweep behave properly for PPI purposes by employing simple electrical devices which keep the currents varying correctly.

Echo indication by deflection and by intensity method. You were told that the reason for using an electron beam is to have a "pencil" that can be moved very rapidly. Why is that necessary? Before you

By connecting the receiver so that either a negative voltage is applied to the lower vertical plate or a positive voltage to the upper vertical plate of the C.R.T., it is possible to move the electron beam up and then down again in a very short time. It may jump up and back down again within a fraction of a microsecond producing the pip or blip. What h