346 PROPAGATION THROUGH THE STANDARD ATMOSPHERE 
antenna pattern factors F, and F2 in computing the 
interference pattern above the line of sight. This 
subject. is dealt with on pages 384-385. 
Standing-Wave Antennas 
An important class of antennas is that in which 
standing waves of the currents and the voltages are 
set up. In a transmitting antenna of this type, for 
instance, a progressive or traveling wave is supplied 
from the connected source of power. This is re- 
flected from the end of the antenna and the inter- 
action of the two sets of waves moving in opposite 
directions results in a standing-wave system. 
In this event the current amplitude is zero at the 
ends of the antenna and assumes differing vaiues 
at the other positions on the antenna. The dis- 
tribution of current amplitudes is usually assumed 
to vary sinusoidally with the distance from the end 
of the antenna. This is a good approximation where 
the diameter of the antenna wire is small compared 
with the length, but may be seriously in error for 
thick-dbire antennas. 
The simplest, and one of the most commonly 
used, standing-wave antennas is the half-wave 
dipole antenna, discussedon pages. 34!7-'349 
Resonant Antennas 
Many antennas are operated at or near resonance, 
which means that the reactive component of their 
impedance vanishes or is very small. 
Two types of resonant antenna may be dis- 
tinguished: either (1) the radiating element as a 
whole is resonant, as in the case of the half-wave 
dipole, shortened the right amount; or (2) the an- 
tenna system is made resonant by adding suitable 
reactive components to the radiative elements. To 
illustrate, the center-fed half-wave dipole of exactly 
half-wavelength, assuming sine distribution of cur- 
rent, has an inductive reactance;-it- may be made 
resonant by the addition in series of a capacitive 
reactance. This is known as antenna loading and is 
common at the longer wavelengths where half-wave 
dipoles would be too cumbersome. Another example 
is that of a dipole radiator shorter than the half- 
wave dipole and having the form.of a metallic tube; 
this is combined with a tunable cavity resonator 
inside the tube that acts as a shunt impedance, the 
whole system being tuned to resonance (Figure 3). 
JeLESS THAN RESONANT LENGTH + 
INNER COAXIAL LINE 
TUBE INPUT” |<-METAL SUPPORT 
Ficure 3. Antenna tuned to resonance by a shunt 
impedance. 
Although the actual antenna impedance is made 
up in a complicated way of distributed capacitances 
and inductances, the input impedance of the simpler 
types of antennas for a limited frequency band 
containing the resonance frequency is essentially 
that of an ordinary series resonant circuit [the resist- 
ance at resonance being essentially the radiation 
resistance of the antenna (see text below )]. The 
input impedance of certain other antennas is essen- 
tially that of parallel-resonant circuits with very 
large shunt resistances at resonance (see text on 
p- 347).For illustration, see Figure 6. 
Traveling-Wave Antennas 
In this type of antenna there is no standing-wave 
system set up since the progressive or traveling 
wave of current fed into the antenna is absorbed, 
without reflection, by a terminal resistance placed 
at the end of the antenna, which is equal to the 
characteristic impedance of the antenna regarded 
as a transmission line. Such antennas are necessarily 
nonresonant. 
The traveling-wave antenna radiates most strongly 
in the general direction of the wave motion. The 
major lobe makes an angle a < 90 degrees with this 
direction as indicated in Figure 20. Here we have a 
long-wire antenna with the input at the left and the 
characteristic impedance (resistance) at the right. 
A traveling-wave V antenna uses two of these 
elements (see text on p 353) and a rhombic is com- 
posed of four elements (see text on p 354), with the 
elements arranged at angles which produce maximum 
directivity of the combinations. 
Antennas of the nonresonant or traveling-wave 
types are used both for longer and for very short 
waves. (However, there is an intermediate fre- 
quency region extending from about 100 to 3,000 me 
where the half-wave dipole is of such convenient 
size that standing-wave dipoles or dipole arrays are 
most frequently employed.) 
In the microwave band where transmission is 
effected by wave guides it is possible to terminate a 
wave guide with a horn which “matches the imped- 
ance of the wave guide to that of free space” and 
acts as a directive antenna (seepage 363). A slot 
or a series of slots in the side of a wave guide may 
also act as an antenna at these frequencies. 
Radiation Resistance 
The radiation resistance R, of an antenna is the 
ratio P, of the total power radiated in all directions 
to the square of the current at the point of measure- 
ment. The power may be computed by integrating 
the radial component of the Poynting vector over a 
spherical surface surrounding the antenna. Then 
