PROPAGATION OF RADIO WAVES 335 
antenna height is increased, the lobes are narrower 
and depressed toward the horizon. The range is 
improved. However, there are broad nulls for the 
higher antenna in‘ which detection will fail. Below 
the horizon, the corresponding decibel contours are 
pushed to the right so that point-to-point com- 
munication is improved. It should be observed that 
the effect of height upon the lobe structure is similar 
to that of frequency. 
Which contour actually represents the limit of 
detection for a given radar depends on the power 
output of the transmitter, the minimum power 
detectable by the receiver, the antenna gains, and 
the radar cross section of the target. For com- 
munication sets, the same quantities, except the 
target cross section, apply. 
UNITS AND FREQUENCY RANGES 
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 rationalized 
so that the factor 47 appears in equations involving 
point sources, 27 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, 
= ah 
i s (13) 
with f in dynes, gi: and q in statcoulombs, r in centi- 
meters and e referred to unity in free space, is 
transformed to 
ye EE (14) 
Amene,r? 
Here force f is given in newtons (1 newton = 10° 
dynes), g: and q2 in coulombs, r in meters, e¢, is the 
dielectric constant relative to that of free space ¢. 
Similarly, the Coulomb formula for magnetic poles, 
— Mmm, 
Tee (15) 
with m, and min unit poles in the electromagnetic 
system and u equal to the permeability, transforms to 
= (16) 
Amr pop,T* 
where m, and m are now given in webers, yp, is the 
permeability relative to that of free space po. 
In the mks rationalized system, the free-space 
values of ¢9 and po must carry the burden of the 
change of units and the inclusion of 47, and thus 
take on the values 
8.854 - 10°? = = 10° farads per meter, (17) 
Tv 
wo = 4-107 & 1.257 - 10° henries per meter. (18) 
With these values, c, the velocity of light in free 
space, is equal to 
ll 
€ 
c= me = 2.998 - 10° = 8 - 10° meters per second 
€oHo (19) 
and the impedance of free space is 
\# = 376.7 ohms. (20) 
£9 
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. 
Symbols for Frequency Ranges 
The following symbolism has been adopted for 
various ranges of frequency. 
TaBLE 2. Symbols for frequency ranges. 
Frequency | Frequency Wavelength 
Symbol name mec meters 
LE | Low 0.03-0.3 10,000—1,000 
MF | Medium 0.3-3 1,000-100 
HF | High 3-30 100-10 short waves 
VHF | Very high 30-300 10-1) ultra-short 
UHF | Ultra-high | 300-3,000 1-0.1 { waves 
SHE | Super-high >3,000 <.1 microwaves 
