Power Supply In figure I-1 the power supply is represented as a single block. Functionally, this block is representative. However, it is unlikely that any one supply source could meet all the power requirements of a radar set. The distribution of the physical components of a system may be such as to make it impractical to group the power-supply circuits into a single physical unit. Different supplies are needed to meet the varying requirements of a system and must be designed accordingly. The power-supply function is performed by various types of supplies distributed among the circuit components of a radar set. In figure 1-3 the modulator, transmitter and receiver are contained in the same chassis. In this arrangement, the group of components is called a TRANSCEIVER. (The term transceiver is an acronym composed from the words TRANSmitter and reCEIVER). Modulator The function of the modulator is to insure that all circuits connected with the radar system operate in a definite time relationship with each other and that the time interval between pulses is of the proper length. The modulator simultaneously sends a synchronizing signal to trigger the transmitter and the indicator sweep. This establishes a control for the pulse repetition rate (PRR) and provides a reference for the timing of the travel of a transmitted pulse to a target and its return as an echo. Transmitter The transmitter is basically an oscillator which generates radio-frequency (r-f) energy in the form of short powerful signals from the modulator. Because of the frequencies and power outputs required, the transmitter oscillator is a special type known as a MAGNETRON. Transmitting and Receiving Antenna System The function of the antenna system is to take the r-f energy from the transmitter, radiate this energy in a highly directional beam, receive any echoes or reflections of transmitted pulses from targets, and pass these echoes to the receiver. In carrying out this function the r-f pulses generated in the transmitter are conducted to a FEEDHORN at the focal point of a directional reflector, from which the energy is radiated in a highly directional pattern. The transmitted and reflected energy (returned by the same dual purpose reflector) are conducted by a common path. This common path is an electrical conductor known as a WAVEGUIDE. A waveguide is hollow copper tubing, usually rectangular in cross section, having dimensions according to the wavelength or the carrier frequency, i.e., the frequency of the oscillations within the transmitted pulse or echo. Because of this use of a common waveguide, an electronic switch, a TRANSMIT-RECEIVE (TR) TUBE capable of rapidly switching from transmit to receive functions, and vice versa, must be utilized to protect the receiver from damage by the potent energy generated by the transmitter. The TR tube, as shown in figure 1-3, blocks the transmitted pulses from the receiver. During the relatively long periods when the transmitter is inactive, the TR tube permits the returning echoes to pass to the receiver. To prevent some or all of the very weak echoes from being absorbed by the transmitter, another device known as an ANTI-TR (A-TR) TUBE is used to block the passage of these echoes to the transmitter. Since the r-f energy is transmitted in a narrow beam, particularly narrow in its horizontal dimension, provision must be made for directing this beam towards a target so that its range and bearing may be measured. Normally, this is accomplished through continuous rotation of the radar beam at a rate of about 10 to 20 revolutions per minute so that it will impinge upon any targets which might be in its path. Therefore, in this basic radar system the upper portion of the waveguide, including the feedhorn, and the reflector are constructed so that they can be rotated in the horizontal plane by a drive motor, as illustrated in figure 1-5. This rotatable antenna and reflector assembly is called the SCANNER. Figure 1-5 illustrates one of the more common types of reflectors used, a TILTED PARABOLIC REFLECTOR. The feedhorn is positioned so that it will not obstruct the reflected beam. Feedhorn Figure 1-5.-Scanner. Gear box From The Use of Radar at Sea, 4th Ed. Copyright 1968, The Institute of Navigation, London. Used by permission. Figure 1-6.-Slotted waveguide antenna. Figure 1-6 illustrates another type of antenna being used. In this SLOTTED WAVEGUIDE ANTENNA, no reflector or feedhorn is used. The last few feet of the waveguide is constructed so that it can be rotated in the horizontal plane. The forward and narrower face of the rotatable waveguide section contains a series of slots from which the r-f energy is emitted to form a narrow radar beam. Returning echoes also pass through these slots and then pass through the waveguide to the receiver. Receiver The function of the receiver is to amplify or increase the strength of the very weak r-f echoes and reproduce them as video signals to be passed to the indicator. The receiver contains a crystal mixer and intermediate frequency amplification stages required for producing video signals used by the indicator. The primary function of the indicator is to provide a visual display of the ranges and bearings of radar targets from which echoes are received. In this basic radar system, the type of display used is the PLAN POSITION INDICATOR (PPI), which is essentially a polar diagram, with the transmitting ship's position at the center. Images of target echoes are received and displayed at either their relative or true bearings, and at their distances from the PPI center. With a continuous display of the images of the targets, the motion of the target relative to the motion of the transmitting ship is also displayed. The secondary function of the indicator is to provide the means for operating various controls of the radar system. The CATHODE-RAY TUBE (CRT), illustrated in figure 1-7, is the heart of the indicator. The CRT face or screen, which is coated with a film of phosphorescent material, is the PPI. The ELECTRON GUN at the opposite end of the tube (see figure 1-8) emits a very narrow beam of electrons which impinges upon the center of the PPI unless deflected by electrostatic or electromagnetic means. Since the inside face of the PPI is coated with phosphorescent material, a small bright spot is formed at the center of the PPI. If the electron beam is rapidly and repeatedly deflected radially from the center, a bright line called a TRACE is formed on the PPI. Should the flow of electrons be stopped, this trace will continue to glow for a short period following the stoppage of the electron beam because of the phosphorescent coating. The slow decay of the brightness is known as PERSISTENCE, the slower the decay the higher the persistence. At the instant the modulator triggers the transmitter, it sends a TIMING TRIGGER signal to the indicator. The latter signal acts to deflect the electron beam radially from the center of the CRT screen (PPI) to form a trace of the radial movement of the electron beam. This radial movement of the electron beam is called the SWEEP or TIME BASE. While the terms trace and sweep are frequently used interchangeably, the term trace is descriptive only of the visible evidence of the sweep movement. Since the electron beam is deflected from the center of the CRT screen with each pulse of the transmitter, the sweep must be repeated very rapidly even when the lower pulse repetition rates are used. With a pulse repetition rate of 750 pulses per second, the sweep must be repeated 750 times per second. Thus, it should be quite obvious why the sweep appears as a solid luminous line on the PPI. The speed of movement of the point of impingement of the electron beam is far in excess of the capability of detection by the human eye. While the sweep must be repeated in accordance with the PRR, the actual rate of radial movement of the electron beam is governed by the size of the CRT screen and the distance represented by the radius of this screen according to the range scale being used. If the 20-mile range scale is selected, the electron beam must be deflected radially from the center |