UNITS AND FREQUENCY RANGES 



11 



performance characteristics of particular radar and 

 communication sets. 



In Chapter 3 vdW be found a discussion of the 

 radiating properties of a wide variety of antennas. 

 Particular attention is devoted to a consideration of 

 the shape of the radiation patterns, methods for 

 improving directivity of antennas, and computation 

 of the gains. 



A general discussion of the factors which modify 

 the manner in which radio waves are transmitted 

 through the atmosphere is given in Chapter 4. Here 

 also is given the reflecting properties of sea water 

 and various types of soil. 



An important practical problem is that of diffrac- 

 tion of waves around obstacles such as hills and trees. 

 A simplified treatment of this problem is given in 

 Chapter 8. 



Chapter 9 is devoted to a presentation of the 

 reflecting problems of targets and their bearing on 

 the operation of radar sets. Successful operation of 

 these sets is dependent, in no small measure, on the 

 proper choice of siting. Factors bearing on the siting 

 problem are evaluated in Chapter 10. 



» 5 UNITS AND FREQUENCY RANGES 

 >5i Units 



In this book the units used are those of the mks 

 rationalized system, in which distances are ex- 

 pressed in meters, masses in kilograms, and time in 

 seconds; and the formulas have been rationalised 

 so that the factor iw appears in equations involving 

 point sources, 27r in equations involving line sources, 

 and is generally absent from equations for uniform 

 or unidirectional fields. 



The Coulomb formula, for illustration, for point 

 sources in classical electrostatic units, 



/ = 



9i92 



(13) 



TOth / in dynes, gi and (/2 in statcoulombs, r in centi- 

 meters and £ referred to unity in free space, is 

 transformed to 



/ = 



glg2 



4:irtotrr^ ' 



(14) 



Here force / is given in newtons (1 newton = 10^ 

 dynes), gi and q^ in coulombs, r in meters, «, is the 

 dielectric constant relative to that of free space co. 



/ = 



(15) 



Similarly, the Coulomb formula for magnetic poles, 



WW2, 



ixr- 



with nil and m« in unit poles in the electromagnetic 

 system and ti equal to the permeability, transforms to 



f = 



(16) 



4:irfiQH,r~ 



where tih and vh are now given in webers, Hr is the 

 permeability relative to that of free space hq. 



In the mks rationalized system, the free-space 

 values of eo and ixo must carry the burden of the 

 change of units and the inclusion of Aw, and thus 

 take on the values 



eo = 8.854 • 10"'- ^ — 10^^ farads per meter, (17) 

 SGtt 



Mo = 4ir. 10"^ ^ 1.257 . 10"® henries per meter. (18) 



With these values, c, the velocity of light in free 

 space, is equal to 

 1 



= 2.998 . 10** ^ 3 • 10** 



«oMo 



meters per second 

 (19) 



and the impedance of free space is 



^l 



— = 376.7 ohms. 



(20) 



This system of units has been chosen because it is 

 unified, free from numerical factors required in 

 equations using arbitrary choices of units, and has 

 been adopted by the International Electrotechnical 

 Commission. Since the various Armed Services 

 use differing sets of units for their operational 

 instructions, it would have been impossible to choose 

 any one that would have been satisfactory to all; 

 hence the choice of using the only system which is 

 generally recognized and scientifically sound. 



1.5.2 Symijolg for Frequency Ranges 



The following symbolism has been adopted for 

 various ranges of frequency. 



Table 2. Symbols for frequency ranges. 



