46 OCEAN ELECTRONIC NAVIGATIONAL AIDS 



scope move in the opposite direction causing a trail to be left on the scope 

 due to its persistence. In addition, the PPI must be closely observed during 

 the turn or later confusion will result in the new placement of objects about 

 the scope. In the second method of bearing indication which has won 

 favor with the Navy, the PPI is stabilized in azimuth so that the top of the 

 PPI is always north. A marker is then flashed when the antenna is pointed 

 toward the bow to indicate true heading. This method enables both true and 

 relative bearings to be determined readily, preserves the resolution of the 

 equipment and does not have the undesirable feature of the relative bearing 

 presentation. The picture on the scope is then similar to a chart with the 

 addition of the movable objects. For this latter method, however, the ship 

 must be gyrocompass-equipped. 



INSTALLATION. — To meet the varying requirements for installation, commer- 

 cial Radars have been broken into 2, 3, or 4 packages. One package, for 

 example may be the indicator alone, or the indicator and receiver. The 

 transmitter-modulator unit may be a separate package to be installed in 

 any convenient location; or the indicator, receiver, modulator, and trans- 

 mitter may be combined in a single package. The antenna is generally a 

 separate unit, but in some cases may have the transmitter and modulator 

 units mounted with or near it in a watertight cabinet. On direct-current 

 ships a converter or motor generator is required which can be located where 

 it may receive proper attention. 



Locating the antenna is a special problem. While in the case of naval 

 installations antennas were mounted on the masts, this may present diffi- 

 culties on a merchant vessel due to the use of the masts to support rigging 

 for cargo handling. It is desirable however, from the standpoint of cover- 

 age that the antenna be located as high as practicable. It is also desirable 

 that the antenna be located so that 360° azimuth coverage is provided. A 

 satisfactory solution in some cases has been to place the antenna on a short 

 tower or platform mounted above the pilot house. In any event it is poor 

 economy to pay a relatively large amount for a Radar and then limit its 

 capabilities by failing to install the antennna in a good position. 



BRIEF DESCRIPTION OF RADAR COMPONENTS 



Basically Radar employs very short electromagnetic waves and utilizes the 

 principle, that these waves can be beamed, that they travel at a definite 

 speed in a straight line, and that they will be reflected from any discontinuity 

 in the medium through which they are transmitted. 



The typical surface Radar consists of five components: the transmitter, 

 modulator, antenna, receiver, and indicator. In addition to these com- 

 ponents, the power supply is an important factor to be considered in determin- 

 ing the actual characteristics of any Radar set. While the physical form 

 of each of these parts may vary widely from one type to another, all Radars 

 contain them. Figure 4-5 illustrates the arrangement of the fundamental 

 components in a block diagram. 



The transmitter consists of the radiofrequency oscillator which produces 

 the electromagnetic waves of energy. Because of the necessity for beaming 

 this energy, while at the same time being able to receive suitable echoes, the 

 oscillator generates very high-frequency energy. The development of a suit- 

 able oscillator with sufficient power has been one of the major accomplish- 

 ments of the Radar technicians. 



In order that range may be determined accurately, electromagnetic waves 

 are emitted in the form of pulses and each pulse is transmitted for a very 

 short period of time, one-millionth of a second (1 microsecond) or less. 

 After each pulse the transmitter is silent while echoes from that emitted pulse 

 are being received. The procedure is then repeated about 1,000 times a sec- 

 ond. The modulator, or keyer, is the unit which turns the transmitter on 

 and off and forms these pulses. 



The antenna assembly is so designed as to beam the energy at the target, 

 normally being accomplished by the use of an antenna and reflector in much 

 the same manner that the headlights of an automobile are directed. The 

 echo is received back through the same antenna and directed to the receiver. 

 The antenna must be directional and concentrate the radio energy into a 

 well-defined beam, since this is the method by which the direction of the 

 detected objects is determined. It must also be capable of being rotated or 

 trained in order that the surrounding area can be properly scanned. 



