Radar Indicators and Antennas

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3 RADAR INDICATORS AND ANTENNAS

Learning Objectives

After you finish this chapter, you should be able to do the following: 1. Describe the purpose of the A scope, the range-height indicator (rhi), and the plan position indicator (ppi). 2. State the relationship between range and sweep speed and length on a radar indicator. 3. Explain the purpose of timing triggers, video, and antenna position inputs to a radar indicator. 4. List the major units of a ppi and describe their functions. 5. Describe the basic operation of sweep deflection and sweep rotation in a ppi. 6. List and describe the operation of the three range measurement circuits.

7. Describe antenna directivity and power gain characteristics. 8. Describe the focusing action of a basic parabolic antenna. 9. Describe the basic radiation patterns of the most common parabolic reflectors. 10. Describe the basic characteristics of horn radiators. 3.1 INTRODUCTION Radar systems require an antenna to both transmit and receive radar energy and an indicator system to display the video information generated. This chapter will briefly describe some commonly used indicators and antenna systems. Antenna systems are described in more detail in NEETS, Module 10, Wave Generation, Transmission Lines, and Antennas, and Module 11, Microwave Principles.

3.2 RADAR INDICATORS The information available from a radar receiver may contain as many as several million separate data bits per second. From these and other data, such as the orientation of the antenna, the indicator should present to the observer a continuous, easily understandable, graphic picture of the relative position of radar targets. It should provide size, shape, and insofar as possible, indications of the type of targets. A cathode-ray tube (crt) fulfills these requirements to an astonishing degree. The cathode-ray tube's principal shortcoming is that it cannot present a true three-dimensional picture.

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The fundamental geometrical quantities involved in radar displays are the RANGE, AZIMUTH ANGLE (or BEARING), and ELEVATION ANGLE. These displays relate the position of a radar target to the origin at the antenna. Most radar displays include one or two of these quantities as coordinates of the crt face. The actual range of a target from the radar, whether on the ground, in the water, or in the air is known as SLANT RANGE. The majority of displays use as one coordinate the value of slant range, its horizontal projection (GROUND RANGE), or its vertical projection (ALTITUDE). Since slant range is involved in every radar situation, it inevitably appears in at least one display on every set. Slant range is the coordinate that is duplicated most often when more than one type of display is used. This is partly because displays presenting range have the highest signal-to-noise discrimination and partly for geometrical reasons.

Range is displayed by means of a linear time-base sweep, starting from a given point or line at a definite time in each pulse cycle. Thus, distances along this range sweep actually measure slant range. The sweep speed determines the scale factor, which relates the distance on the tube to actual range. The sweep length is the total distance represented. Distances are expressed in miles (statute or nautical) or yards. The origin of the range sweep may be on or off the tube face. The angle at which the antenna is pointing, either in azimuth or elevation, may provide two dimensional information in the display; that is, range and azimuth, or range and elevation.

A radar indicator, sometimes called a radar repeater, acts as the master timing device in analyzing the return of the video in a radar system. It also provides that capability to various other locations physically remote from the radar system. Each indicator should have the ability to select the outputs from any desired radar system aboard the ship. This is usually accomplished by the use of a RADAR DISTRIBUTION SWITCHBOARD. The switchboard contains a switching arrangement that has inputs from each radar system aboard ship and provides outputs to each repeater. The radar desired is selected by means of a selector switch on the repeater. For the repeater to present correct target position data, the indicator must have the following three inputs from the selected radar: 1. Trigger timing pulses. These pulses ensure that the sweep on the repeater starts from its point of origin each time the radar transmits. As discussed earlier, the repeater displays all targets at their actual range from the ship based on the time lapse between the instant of transmission and the instant the target's echo is received.

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2. The returning echo. The echo, in rf form, is detected (converted to a video signal) by the radar receiver and applied to the repeater. 3. Antenna information. The angular sweep position of a plan position indicator (ppi) repeater must be synchronized to the angular position of the radar antenna to display target bearing (azimuth) information. The three most common types of displays, called scopes, are the A-scope, the RANGE- HEIGHT INDICATOR (RHI) SCOPE, and the PLAN POSITION INDICATOR (PPI) SCOPE. The primary function of these displays will be discussed in this section.

However, detailed descriptions will be limited to the ppi scope, which is the most common display. 3.2.1 The A Scope The A-scope display, shown in figure 3-1, presents only the range to the target and the relative strength of the echo. Such a display is normally used in weapons control radar systems. The bearing and elevation angles are presented as dial or digital readouts that correspond to the actual physical position of the antenna. The A-scope normally uses an electrostatic-deflection crt. The sweep is produced by applying a sawtooth voltage to the horizontal deflection plates. The electrical length (time duration) of the sawtooth voltage determines the total amount of range displayed on the crt face.

Figure 3-1 A scope NEETS MODULE 18-Radar Principles

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The ranges of individual targets on an A-scope are usually determined by using a movable range gate or step that is superimposed on the sweep. Ranging circuits will be discussed in more detail later in this section. 3.2.2 Range-Height Indicator (RHI) The range-height indicator (rhi) scope, shown in figure 3-2, is used with height-finding search radars to obtain altitude information. The rhi is a two-dimensional presentation indicating target range and altitude. The sweep originates in the lower left side of the scope. It moves across the scope, to the right, at an angle that is the same as the angle of transmission of the height-finding radar. The line of sight to the horizon is indicated by the bottom horizontal line. The area directly overhead is straight up the left side of the scope. Target echoes are displayed on the scope as vertical PIPS or BLIPS (spots of increased intensity that indicate a target location). The operator determines altitude by adjusting a movable height line to the point where it bisects the center of the blip. Target height is then read directly from an altitude dial or digital readout. Vertical range markers are also provided to estimate target range.

Q1. What are the three fundamental quantities involved in radar displays? Q2. What are the required radar inputs for proper indicator operation? Figure 3-2 RHI scope NEETS MODULE 18-Radar Principles

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Q3. What coordinates are displayed on an rhi scope? 3.2.3 Plan Position Indicator (PPI) The ppi scope shown in figure 3-3, is by far the most used radar display. It is a polar coordinate display of the area surrounding the radar platform. Own ship is represented as the origin of the sweep, which is normally located in the center of the scope, but may be offset from the center on some sets. The ppi uses a radial sweep pivoting about the center of the presentation. This results in a map-like picture of the area covered by the radar beam. A long-persistence screen is used so that the display remains visible until the sweep passes again.

Figure 3-3 PPI scope NEETS MODULE 18-Radar Principles

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Bearing to the target is indicated by the target's angular position in relation to an imaginary line extending vertically from the sweep origin to the top of the scope. The top of the scope is either true north (when the indicator is operated in the true bearing mode) or ship's heading (when the indicator is operated in the relative bearing mode). 3.2.3.1 PPI Block Diagram The basic block diagram, figure 3-4, illustrates the major units of a plan position indicator. Synchronization of events is particularly important in the presentation system.

At the instant a radar transmitter fires (or at some predetermined time thereafter), circuits which control the presentation on the indicator must be activated. These events must be performed to a high degree of accuracy to ensure accurate range determination. The synchronization of these events is provided by the gate circuit. Figure 3-4 Basic ppi block diagram NEETS MODULE 18-Radar Principles

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GATE CIRCUIT.—The gate circuit develops pulses which synchronize the indicator with the transmitter. The gate circuit itself is synchronized by trigger pulses from the synchronizer. It then provides timing for the intensity gate generator, sweep generator circuit, and the sweep control circuit. SWEEP CONTROL CIRCUIT.—The sweep control circuit converts mechanical bearing information from the antenna into voltages which control sweep circuit azimuth. SWEEP GENERATOR CIRCUIT.—The sweep generator circuit produces currents which deflect an electron beam across the crt. Varying voltages from the sweep control circuit are applied to deflection coils. Gate voltages determine sweep rate, and therefore, the effective distance (range) covered by each sweep. Sweep potentials consist of separate north-south and east-west voltages; the amplitudes of these voltages determine sweep azimuth. The sweep generator is synchronized by an input from the gate circuit.

INTENSITY GATE GENERATOR.—The intensity gate generator provides a gate which unblanks the crt during sweep periods. The intensity of the trace appearing on the crt is determined by the dc level of this gate. This circuit is also synchronized by the gate circuit. VIDEO AMPLIFIER.—The video amplifier circuit amplifies the video signal from the receiver and applies it to the crt intensity-modulating element (control grid). POWER SUPPLY.—The power supply produces all voltages needed to operate the indicator. It also includes protective devices and metering circuits.

Although not shown in the basic block diagram, many indicators contain circuits which aid in range and bearing determination. These circuits are also synchronized by the gate circuit. 3.2.3.2 Sweep Deflection In modern indicator systems, electromagnetic deflection of the crt electron beam is preferred to electrostatic deflection. Reasons for this choice are (1) increased control of the beam, (2) improved deflection sensitivity, (3) better beam position accuracy, and (4) simpler construction of the crt. The primary difference between electromagnetic and electrostatic cathode-ray tubes lies in the method of controlling deflection and focusing of the electron beam. Both types employ electron guns and use electrostatic fields to accelerate and control the flow of electrons. The physical construction of a crt employing electromagnetic deflection is similar to an electrostatic type. The construction of a crt employing electromagnetic deflection is shown in figure 3-5.

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The electron gun in figure 3-5 is made up of a heater, cathode, control grid, second or screen grid, focus coil, and anode (composed of a special coating). Focusing the electron beam on the face of the screen is accomplished by the focus coil. A direct current through the windings sets up a strong magnetic field at the center of the coil. Electrons move precisely along the axis of the tube and pass through the focusing field with no deflection. This is because they move parallel to the magnetic field at all times. Any electron which enters the focusing field at an angle to the axis of the tube has a force exerted on it that is perpendicular to its direction of motion. A second force on this electron is perpendicular to the magnetic lines and is, therefore, constantly changing in direction. These forces cause the electron to move in a helical or corkscrew path shown in figure 3-6. With the proper velocity of the electron and strength of the magnetic field, the electron will be caused to move at an angle which allows it to converge with other electrons at some point on the crt screen. Focusing is accomplished by adjusting the current flow through the focusing coils.

Figure 3-5 Electromagnetic crt construction NEETS MODULE 18-Radar Principles

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The focused electron beam is deflected by a magnetic field that is generated by current flow through a set of deflection coils, as shown in figure 3-5. These coils are mounted around the outside surface of the neck of the crt. Normally, four deflection coils (N, S, E, and W) are used, as shown in figure 3-4. Two coils in series are positioned in a manner that causes the magnetic field produced to be in a vertical plane. The other two coils, also connected in series, are positioned so that their magnetic field is in a horizontal plane.

The coils (N-S) which produce a horizontal field are called the VERTICAL DEFLECTION COILS and the coils (E-W) which produce a vertical field are called the HORIZONTAL DEFLECTION COILS. This may be more clearly understood if you recall that an electron beam will be deflected at right angles to a deflecting field. The deflection coils are illustrated in view A of figure 3-7. View B shows the N-S windings in schematic form. Figure 3-6 Helical motion of electron through a uniform magnetic field Figure 3-7 Deflection yoke NEETS MODULE 18-Radar Principles

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Electron deflection in the electromagnetic crt is proportional to the strength of the magnetic fields. Magnetic field strength depends on current in the coils. The sweep circuits associated with electromagnetically deflected cathode-ray tubes must provide currents, rather than voltage, to produce the desired beam deflection. A sawtooth current is required to produce a linear trace. A deflection coil may be considered equivalent to the circuit shown in view A of figure 3-8. Because of the inductance of the coil, a trapezoidal voltage must be applied across the coil to produce a sawtooth of current through it. This is illustrated in view B. (Refer to NEETS, Module 9, Wave-Generation and Wave-Shaping Circuits, for a review of wave shaping.) 3.2.3.3 Sweep Rotation Azimuth indication of the ppi requires that the range trace rotate about the center of the screen. A very simple means of achieving sweep rotation is to cause the deflection coil to rotate about the neck of the crt in synchronization with the antenna motion. This method, however, has the disadvantages of inaccuracy and maintenance complications inherent to any mechanical gear-train assembly.

Figure 3-8 Deflection coil equivalent circuit and waveform NEETS MODULE 18-Radar Principles

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Most modem ppi systems employ fixed deflection coils and use special circuits to electronically rotate the magnetic field. Figure 3-9 illustrates a method of electronically producing a rotating sweep. In view A, a range sweep current, i, is applied to the vertical deflection coils only. The resulting magnetic field, represented by Φ , lies along the axis of these coils. The resulting range trace, shown by the short straight line, is vertical because the electron beam is deflected perpendicular to the magnetic field. In view B, range sweep currents are applied to both sets of coils, and the resultant magnetic field takes a position between the axis of the two sets of coils. Because of this shift of the magnetic field, the range trace is rotated 45 degrees clockwise from its previous position.

In view C, the sweep current is applied to the horizontal deflection coils only, and the range trace lies 90 degrees clockwise from its original position. Further rotation is obtained if the polarities of the deflection coil currents are varied in proper sequence, as illustrated in views D and E. To synchronize sweep rotation with antenna rotation, you must convert antenna azimuth (bearing) information into electrical signals. These signals, usually provided by synchros, control the amplitudes and polarities of the sawtooth sweep currents applied to the deflection coils.

Figure 3-9 Trace rotation NEETS MODULE 18-Radar Principles

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Figure 3-10 illustrates the waveforms of current required to produce a rotating range sweep. The amplitudes of the sawtooth sweep currents are varied sinusoidally (like a sine wave), corresponding to the rotation of the antenna. Notice that there is a 90 degree phase difference between the amplitude variations of the horizontal and vertical waveforms. Figure 3-10 Deflection coil currents NEETS MODULE 18-Radar Principles

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3.2.3.4 CRT Screen Persistence A ppi requires a crt in which the screen is coated with a long-persistence phosphor. This is necessary because each target reflects energy for only a short period of time during each rotation of the antenna. Therefore, the target indication on the face of the crt must be able to continue to glow during the portion of antenna rotation when the target is not reflecting energy. Q4. What coordinates are presented on a ppi scope? Q5. What type of deflection is preferred for a crt electron beam?

Q6. Which of the two types of deflection coils (fixed or rotating) is used most often? 3.2.4 Ranging Circuits The accuracy of target-range data provided by a radar varies with the use of the radar. For example, a weapons systems radar operating in a search mode is required to be accurate within a small percentage of its maximum range. However, an intercept radar, operating in a tracking mode, must supply range data that is even more accurate; it must be within a few yards of the actual range. In some applications of radar, the indicator sweep is calibrated by a transparent overlay with an engraved range scale. This overlay enables the operator to estimate the range of targets. In other applications, electronic range marks are supplied to the indicator. They usually appear as vertical pulses on A-scopes and as concentric circles on ppi scopes. The distance between range marks is generally determined by the type of equipment and its mode of operation.

In a weapons systems radar that requires extremely accurate target-range data, a movable range marker may be used. The range marker is obtained from a range-marker generator and may be a movable range gate or range step. When a ppi scope is used, a range circle of adjustable diameter may be used to measure range accurately. In some cases, movement of the range marker is accomplished by adjusting a calibrated control from which range readings are obtained. NEETS MODULE 18-Radar Principles

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The following discussion describes the operation of three types of range-marker generators: the RANGE-GATE GENERATOR, the RANGE-MARKER GENERATOR, and the RANGE-STEP GENERATOR. The range-gate generator, used in conjunction with a blocking oscillator, generates a movable range gate. The range-marker generator and the range-step generator, used in conjunction with an astable multivibrator, generate fixed range marks and a movable range step, respectively. 3.2.4.1 Range-Gate Generator Figure 3-11 shows a simplified block diagram of a typical range-gate generator. The pulse-repetition frequency is controlled by a master oscillator, or multivibrator, in which the output is coupled to a trigger thyratron (both in the synchronizer). The output of the trigger thyratron is used to trigger the radar modulator and the scope sweep circuits, thus starting the transmitter pulse and the range sweep at the same instant, referred to as time T0.

The PHANTASTRON in the sweep circuits is a variable timing circuit that supplies a sweep sawtooth to the sweep amplifier. The width of the gate and sawtooth is dependent upon the range selected by the radar operator. Figure 3-11 Range-gate generator NEETS MODULE 18-Radar Principles

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The range-gate circuit receives its input pulse from the trigger thyratron and generates a delayed range-gate pulse. The delay of this pulse from time T0 is dependent on either the range of the target when the radar is tracking, or the manual positioning of the range-volts potentiometer when the radar is not tracking (in the search mode). The range-gate triggers the range-strobe multivibrator, from which the output is amplified and sent to the blocking oscillator (which sharpens the pulses), as shown in figure 3-11. This range gate is used to select the target to be tracked. When in the track mode, the range gate brightens the trace or brackets the blip (depending on the system) to indicate what target is being tracked. Range-gate generators are used most often in weapons-control track radar A- scope presentations, but they can also be used with ppi presentations. When used with a ppi presentation, the range gate must be movable in both range and bearing.

The range-gate generator can easily be modified to produce a range strobe instead of a range gate. A range strobe is simply a single brightened spot that is movable both in range and bearing. In operation, the range strobe or range gate control also controls a dial or digital readout to provide a range readout to the operator. 3.2.4.2 Range-Marker Generator Several types of range-marker generators are in common use. Figure 3-12 shows a simplified version of a circuit that produces both range markers and the basic system timing triggers. The master oscillator in this case is a blocking oscillator that operates at a frequency of 80.86 kilohertz. By dividing 80.86 kilohertz into 1 (t = 1/frequency), we find the time required for one cycle of operation is 12.36 microseconds. Thus the blocking oscillator produces pulses 1 radar mile apart. These are fed to the 5:1 divider circuit. Five of the 1-mile marks are required to produce an output from the divider circuit. These five-mile marks are sent to the indicator for display and to the 10:1 divider circuit. In the latter case, ten of the five-mile marks are required to produce an output from the 10:1 divider. Thus the output triggers are 50 miles apart. These basic timing triggers are for a radar with a range of fifty miles. The period between triggers could be extended through the use of additional dividers for use with longer range systems.

Figure 3-12 Range-marker generator NEETS MODULE 18-Radar Principles

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Another version of a range-mark generator is shown in figure 3-13. This circuit provides range marks at 1,000-, 2,000-, or 3,000-yard intervals. Generation of the marks begins with the ringing oscillator, which is started by a delayed master trigger from the synchronizer. A ringing oscillator produces a sinusoidal output of a fixed duration and frequency when triggered. The output is synchronized to the input trigger. In this circuit, the trigger causes the oscillator to produce a 162-kilohertz signal that lasts for 4 1/2 cycles. The emitter follower isolates the ringing oscillator from the countdown multivibrator and clips the oscillator output signals. This action allows only the positive half of each sine wave to reach the multivibrator. The positive triggers from the ringing oscillator are at 1,000-yard intervals. This input signal results in an output from the countdown multivibrator of 1,000-, 2,000-,or 3,000-yard range marks, depending on the position of the RANGE MARK SELECT SWITCH.

Figure 3-13 Range-marker generator NEETS MODULE 18-Radar Principles

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3.2.4.3 Range-Step Generator The range step is often used to determine target range on an A-scope presentation. The appearance of a range step on an A-scope is illustrated in figure 3-14. View A of figure 3-15 is a block diagram of a simple range-step generator consisting of a sawtooth generator, a negative clipper, a range potentiometer, and a limiting amplifier. The position of the range step along the indicator's time base is controlled by the range potentiometer. When the range step coincides with the leading edge of a target's echo pulse, the range can be read directly from a calibrated readout associated with the potentiometer.

Figure 3-14 Range-step presentation NEETS MODULE 18-Radar Principles

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View B shows the time relationships of the voltage waveforms produced by the range- step generator. During the sweep gate, the sawtooth generator produces a sawtooth voltage that is sent to the clipper. The point at which the sawtooth is clipped is controlled by the range potentiometer. The clipped sawtooth is shaped in the limiting amplifier to produce the output voltage waveform. The portion of the output waveform from T1 to T3 is applied to the vertical-deflection plates of the indicator crt to produce the display shown in figure 3-14.

Figure 3-15 Range-step generation NEETS MODULE 18-Radar Principles

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Q7. What type of ranging circuit is most often used with a radar that requires extremely accurate range data? Q8. The range sweep in a range-gate generator is started at the same time as what other pulse? Q9. Range-marker generators produce pulses based on what radar constant? Q10. What radar scope uses a range step for range measurement? 3.3 RADAR ANTENNAS In this section, we will briefly review the requirements of radar antennas. Antenna characteristics are discussed in detail in NEETS, Module 10, Wave-Generation, Transmission Lines, and Antenna and in Module 11, Microwave Principles. A review of these modules would be helpful at this point to prepare you for the following radar antenna discussion.

Antennas fall into two general classes, OMNIDIRECTIONAL and DIRECTIONAL. Omnidirectional antennas radiate rf energy in all directions simultaneously. They are seldom used with modern radars, but are commonly used in radio equipment, in iff (identification friend or foe) equipment, and in countermeasures receivers for the detection of enemy radar signals. Directional antennas radiate rf energy in patterns of LOBES or BEAMS that extend outward from the antenna in one direction for a given antenna position. The radiation pattern also contains minor lobes, but these lobes are weak and normally have little effect on the main radiation pattern. The main lobe may vary in angular width from one or two degrees in some radars to 15 to 20 degrees in other radars. The width depends on the system's purpose and the degree of accuracy required.

Directional antennas have two important characteristics, DIRECTIVITY and POWER GAIN. The directivity of an antenna refers to the degree of sharpness of its beam. If the beam is narrow in either the horizontal or vertical plane, the antenna is said to have high directivity in that plane. Conversely, if the beam is broad in either plane, the directivity of the antenna in that plane is low. Thus, if an antenna has a narrow horizontal beam and a wide vertical beam, the horizontal directivity is high and the vertical directivity is low.

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When the directivity of an antenna is increased, that is, when the beam is narrowed, less power is required to cover the same range because the power is concentrated. Thus, the other characteristic of an antenna, power gain, is introduced. This characteristic is directly related to directivity. Power gain of an antenna is the ratio of its radiated power to that of a reference (basic) dipole. Both antennas must have been excited or fed in the same manner and each must have radiated from the same position. A single point of measurement for the power-gain ratio must lie within the radiation field of each antenna. An antenna with high directivity has a high power gain, and vice versa. The power gain of a single dipole with no reflector is unity. An array of several dipoles in the same position as the single dipole and fed from the same line would have a power gain of more than one; the exact figure would depend on the directivity of the array.

The measurement of the bearing of a target, as detected by the radar, is usually given as an angular position. The angle may be measured either from true north (true bearing), or with respect to the bow of a ship or nose of an aircraft containing the radar set (relative bearing). The angle at which the echo signal returns is measured by using the directional characteristics of the radar antenna system. Radar antennas consist of radiating elements, reflectors, and directors to produce a narrow, unidirectional beam of rf energy. A pattern produced in this manner permits the beaming of maximum energy in a desired direction.

The transmitting pattern of an antenna system is also its receiving pattern. An antenna can therefore be used to transmit energy, receive energy, or both. The simplest form of antenna for measuring azimuth (bearing) is a rotating antenna that produces a single-lobe pattern. The remaining coordinate necessary to locate a target in space may be expressed either as elevation angle or as altitude. If one is known, the other can be calculated from basic trigonometric functions. A method of determining the angle of elevation or the altitude is shown in figure 3-16. The slant range is obtained from the radar scope as the distance to the target. The angle of elevation is the angle between the axis of the radar beam and the earth's surface. The altitude in feet is equal to the slant range in feet multiplied by the sine of the angle of elevation. For example if the slant range in figure 3-16 is 2,000 feet and the angle of elevation is 45 degrees, the altitude is 1,414.2 feet (2,000 ⋅ .7071). In some radar equipments that use antennas that may be moved in elevation, altitude determination is automatically computed.

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3.3.1 Parabolic Reflectors A SPHERICAL WAVEFRONT spreads out as it travels and produces a pattern that is neither too sharp nor too directive. On the other hand, a PLANE wavefront does not spread out because all of the wavefront moves forward in the same direction. For a sharply defined radar beam, the need exists to change the spherical wavefront from the antenna into a plane wavefront. A parabolic reflector is one means of accomplishing this. Radio waves behave similarly to light waves. Microwaves travel in straight lines as do light rays. They may be focused and/or reflected just as light rays can. In figure 3-17, a point-radiation source is placed at the focal point F. The field leaves this antenna with a spherical wavefront. As each part of the wavefront reaches the reflecting surface, it is shifted 180 degrees in phase and sent outward at angles that cause all parts of the field to travel in parallel paths. Because of the shape of a parabolic surface, all paths from F to the reflector and back to line XY are the same length. Therefore, all parts of the field arrive at line XY the same time after reflection.

Figure 3-16 Radar determination of altitude NEETS MODULE 18-Radar Principles

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If a dipole is used as the source of radiation, there will be radiation from the antenna into space (dotted lines in figure 3-17) as well as toward the reflector. Energy that is not directed toward the paraboloid has a wide-beam characteristic that would destroy the narrow pattern from the parabolic reflector. This occurrence is prevented by the use of a hemispherical shield (not shown) that directs most radiation toward the parabolic surface. By this means, direct radiation is eliminated, the beam is made sharper, and power is concentrated in the beam. Without the shield, some of the radiated field would leave the radiator directly. Since it would not be reflected, it would not become a part of the main beam and thus could serve no useful purpose. The same end can be accomplished through the use of a PARASITIC array, which directs the radiated field back to the reflector, or through the use of a feed horn pointed at the paraboloid.

The radiation pattern of a parabola contains a major lobe, which is directed along the axis of revolution, and several minor lobes, as shown in figure 3-18. Very narrow beams are possible with this type of reflector. View A of figure 3-19 illustrates the parabolic reflector. Figure 3-17 Parabolic reflector radiation NEETS MODULE 18-Radar Principles

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3.3.1.1 Truncated Paraboloid View B of figure 3-19 shows a horizontally truncated paraboloid. Since the reflector is parabolic in the horizontal plane, the energy is focused into a narrow horizontal beam. With the reflector truncated, or cut, so that it is shortened vertically, the beam spreads out vertically instead of being focused. Since the beam is wide vertically, it will detect aircraft at different altitudes without changing the tilt of the antenna. It also works well for surface search radars to overcome the pitch and roll of the ship.

Figure 3-18 Parabolic radiation pattern Figure 3-19 Reflector shapes NEETS MODULE 18-Radar Principles

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The truncated paraboloid reflector may be used in height-finding systems if the reflector is rotated 90 degrees, as shown in view C. Because the reflector is now parabolic in the vertical plane, the energy is focused into a narrow beam vertically. With the reflector truncated, or cut, so that it is shortened horizontally, the beam spreads out horizontally instead of being focused. Such a fan-shaped beam is used to determine elevation very accurately. 3.3.1.2 Orange-Peel Paraboloid A section of a complete circular paraboloid, often called an ORANGE-PEEL REFLECTOR because of its shape, is shown in view D of figure 3-19. Since the reflector is narrow in the horizontal plane and wide in the vertical, it produces a beam that is wide in the horizontal plane and narrow in the vertical. In shape, the beam resembles a huge beaver tail. This type of antenna system is generally used in height finding equipment.

3.3.1.3 Cylindrical Paraboloid When a beam of radiated energy noticeably wider in one cross-sectional dimension than in the other is desired, a cylindrical paraboloidal section approximating a rectangle can be used. View E of figure 3-19 illustrates this antenna. A parabolic cross section is in one dimension only; therefore, the reflector is directive in one plane only. The cylindrical paraboloid reflector is either fed by a linear array of dipoles, a slit in the side of a waveguide, or by a thin waveguide radiator. Rather than a single focal point, this type of reflector has a series of focal points forming a straight line. Placing the radiator, or radiators, along this focal line produces a directed beam of energy. As the width of the parabolic section is changed, different beam shapes are obtained. This type of antenna system is used in search and in ground control approach (gca) systems.

Q11. Which of the two general classes of antennas is most often used with radar? Q12. The power gain of an antenna is directly related to what other antenna property? Q13. A parabolic reflector changes a spherical wavefront to what type of wavefront? NEETS MODULE 18-Radar Principles

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3.3.2 Corner Reflector The corner-reflector antenna consists of two flat conducting sheets that meet at an angle to form a corner, as shown in view F of figure 3-19. This reflector is normally driven by a half-wave radiator located on a line which bisects the angle formed by the sheet reflectors. 3.3.3 BROADSIDE ARRAY The desired beam widths are provided for some vhf radars by a broadside array, such as the one shown in figure 3-20. The broadside array consists of two or more half-wave dipole elements and a flat reflector. The elements are placed one-half wavelength apart and parallel to each other. Because they are excited in phase, most of the radiation is perpendicular or broadside to the plane of the elements. The flat reflector is located approximately one-eighth wavelength behind the dipole elements and makes possible the unidirectional characteristics of the antenna system.

Figure 3-20 Broadside array NEETS MODULE 18-Radar Principles

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3.3.4 Horn Radiators Horn radiators, like parabolic reflectors, may be used to obtain directive radiation at microwave frequencies. Because they do not involve resonant elements, horns have the advantage of being usable over a wide frequency band. The operation of a horn as an electromagnetic directing device is analogous to that of acoustic horns. However, the throat of an acoustic horn usually has dimensions much smaller than the sound wavelengths for which it is used, while the throat of the electromagnetic horn has dimensions that are comparable to the wavelength being used.

Horn radiators are readily adaptable for use with waveguides because they serve both as an impedance-matching device and as a directional radiator. Horn radiators may be fed by coaxial or other types of lines. Horns are constructed in a variety of shapes as illustrated in figure 3-21. The shape of the horn, along with the dimensions of the length and mouth, largely determines the field- pattern shape. The ratio of the horn length to mouth opening size determines the beam angle and thus the directivity. In general, the larger the opening of the horn, the more directive is the resulting field pattern.

Figure 3-21 Horn radiators NEETS MODULE 18-Radar Principles

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3.3.5 Feedhorns A waveguide horn, called a FEEDHORN, may be used to feed energy into a parabolic dish. The directivity of this feedhorn is added to that of the parabolic dish. The resulting pattern is a very narrow and concentrated beam. In most radars, the feedhorn is covered with a window of polystyrene fiberglass to prevent moisture and dirt from entering the open end of the waveguide. One problem associated with feedhorns is the SHADOW introduced by the feedhorn if it is in the path of the beam. (The shadow is a dead spot directly in front of the feedhorn.) To solve this problem the feedhorn can be offset from center. This location change takes the feedhorn out of the path of the rf beam and eliminates the shadow. An offset feedhorn is shown in figure 3-22.

Figure 3-22 Offset feedhorn NEETS MODULE 18-Radar Principles

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3.3.6 Airborne Radar Antennas Airborne radar equipment is used for several specific purposes. Some of these are bombing, navigation, and search. Radar antennas for this equipment are invariably housed inside nonconducting radomes, not only for protection but also to preserve aerodynamic design. Some of these radomes are carried outside the fuselage, while others are flush with the skin of the fuselage. In the latter case, the radar antenna itself is carried inside the fuselage, and a section of the metallic skin is replaced by the nonconducting radome. The radar antenna and its radome must operate under a wide variety of temperature, humidity, and pressure conditions. As a result, mechanical construction and design must minimize any possibility of failure. Transmission lines are usually hermetically sealed to prevent moisture accumulation inside them. Such accumulation would introduce losses. Because the low air pressures encountered at high elevations are very conducive to arcing, pressurization of equipment is widely used (the pressure is maintained by a small air pump). In some airborne radar equipments, practically all of the equipment is sealed in an airtight housing, along with the antenna and transmission line.

The antenna radome forms a portion of the housing. Airborne radar antennas are constructed to withstand large amounts of vibration and shock; the radar antennas are rigidly attached to the airframe. The weight of the radar antenna, including the rotating mechanism required for scanning, is kept to a minimum. In addition, the shape of the radome is constructed so as not to impair the operation of the aircraft. The airborne radar antenna must have an unobstructed view for most useful operation. Frequently, the antenna must be able to scan the ground directly under the aircraft and out toward the horizon. To meet this requirement, the antenna must be mounted below the fuselage. If scanning toward the rear is not required, the antenna is mounted behind and below the nose of the aircraft. If only forward scanning is needed, the antenna is mounted in the nose. When an external site is required, a location at the wing tip is common. A fire-control radar antenna is frequently located near the turret guns or in a special nacelle, where it can scan toward the rear or sides of the aircraft.

Q14. How many major lobes are produced by a paraboloid reflector? Q15. What type of radiator normally drives a corner reflector? NEETS MODULE 18-Radar Principles

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Q16. The broadside array consists of a flat reflector and what other elements? Q17. Horn radiators serve what purpose other than being directional radiators? 3.4 SUMMARY The following is a brief summary of the important points of this chapter. A radar INDICATOR presents the information (video) from the radar receiver in a usable manner. The display usually consists of one or more of the coordinates of range, bearing, and altitude. The CATHODE-RAY TUBE (crt) is the best available device for displaying the two- dimensional relationship produced by radar coordinates. The most commonly used crt displays are the A-SCOPE, the RHI, and the PPI. The A-scope presents range information only. The rhi displays range and height information. The ppi is the most widely used radar display indicator and presents range and bearing.

The range of a radar contact is determined by special RANGING CIRCUITS. The following three basic types of ranging circuits are used. RANGE-GATE GENERATORS produce a movable gate that measures range based on elapsed time and can be used on A-scope and ppi displays. RANGE-MARKER GENERATORS produce fixed interval range marks that can be used to estimate the range to a detected target. Range marks appear as an intensified series of vertical dots on an rhi and as concentric circles on a ppi. The RANGE-STEP GENERATOR produces a movable step that is displayed on an A- scope presentation.

RADAR ANTENNAS are usually directional antennas that radiate energy in a one directional lobe or beam. The two most important characteristics of directional antennas are directivity and power gain. Radar antennas often use parabolic reflectors in several different variations to focus the radiated energy into a desired beam pattern. Other types of antennas used with radar systems are the corner reflector, the broadside array, and horn radiators. NEETS MODULE 18-Radar Principles

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Answers To Questions Q1. Through Q17.

A1. Range, bearing, and elevation. A2. Triggers, video, and antenna information. A3. Range and elevation. A4. Range and bearing. A5. Electromagnetic. A6. Fixed. A7. Range gate or range step. A8. Transmitter. A9. The radar mile (12.36 microseconds). A10. The A scope. A11. Directional. A12. Directivity. A13. Plane. A14. One. A15. Half-wave. A16. Two or more half-wave dipoles. A17. Waveguide impedance matching devices.