Module 18—Radar Principles
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Index-1 to 3
Figure 3-10.—Deflection coil currents.
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?
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.
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.
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.
Figure 3-11.—Range-gate generator.
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.
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.
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.
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.
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.
Figure 3-14.—Range-step presentation.
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 3-15.—Range-step generation.
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.
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?
In this section, we will briefly review the requirements of radar antennas. Antenna characteristics are discussed in detail in NEETS, Module 10, Introduction to 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 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.
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.
Figure 3-16.—Radar determination of altitude.
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.
FiFigure 3-17.—Parabolic reflector radiation.
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 parraboloid.
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 3-18.—Parabolic radiation pattern.
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 3-19.—Reflector shapes.
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.
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.
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?
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.
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.
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.
Introduction to Matter, Energy, and Direct Current,
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Introduction to Amplifiers, Introduction to
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Introduction to Test Equipment
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The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
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