Chapter 6 
SCATTERING AND ABSORPTION OF MICROWAVES 
Re oBJect of the present chapter is to summarize 
the status of absorption and scattering of micro- 
waves by different solid obstacles, by liquid water 
or ice particles floating or falling in the atmosphere 
like those present in clouds, fog, rain, hail, and snow. 
The absorption of microwaves by the atmospheric 
gases as well as the aforementioned meteorological 
elements will also be summarized here. 
The following grouping of tke material included 
suggests itself naturally: absorption and radar cross 
section; targets (planes, ships); absorption and scat- 
tering by rain, hail, snow, clouds, and fog; and 
absorption by the atmospheric gases, oxygen, and 
water vapor. 
ABSORPTION AND RADAR 
CROSS SECTION 
Any object irradiated by electromagnetic waves 
will in general remove energy from the incident 
beam both by absorption and by scattering. The 
absorbed energy is transformed into heat in the . 
body, while the scattered energy appears in the form 
of radiation propagated generally in every direction 
aroud the scatterer as the source. 
Let us call P. the power removed from the beam 
tnrough the internal absorption of the object. Its 
absorption cross section is defined by 
Pa 
A= W ’ (1) 
where W, is the power density in the incident beam, 
that is, the power passing a unit cross-sectional area. 
Similarly, if P, is the total power removed from 
the beam through scattering in every direction, then 
the scattering cross section associated with this 
object is 
Ps 
S Sims 
(2) 
The value of S gives information about the total 
scattered energy, but this is not directly useful in 
radar work because one is interested only in that 
fraction of the total scattered power which travels 
in the direction of the receiver. One wants then a 
parameter involving the scattered power per unit 
area W, at the radar receiver instead of the total. 
If the target is an isotropic scatterer, 
P, 
We Tears 
(3) 
d being the distance from the target to the receiver. 
45 
The scattering cross section can thus be written as 
SiS ara? ; (4) 
1 
For targets other than isotropic scatterers, however, 
this procedure fails. since one cannot say that the 
power per unit area at the radar is P;/4rd?. Never- 
theless, it is useful to define a parameter, 
W, 
o = 4rd? W,? 
(5) 
which is called the radar cross section in analogy 
with the scattering cross section S of an isotropic 
scatterer. This cross section « may be thought of 
as the scattering cross section which the target in 
question would have if it scattered as much energy 
in all directions as it actually does scatter in the 
direction of the radar receiver. For an isotropic 
scatterer o = S, but in general it does not. 
Tasie 1. Radar cross sections. 
Radar 
Targets Condition cross section 
Conducting A<<a ma? 
sphere, 
radius @ A>>a 14475a8 
é 
Metallic plate, All dimen- 47S? 
area S sions >> 2 
Cylinder, Axis of cylinder dl? 
diameter = d__ parallel to Xr 
length = . electric field, X 
<<dr»<<l 
Matched load Oriented parallel 9n? 
dipole to the incident 167 
electric field 
Shorted dipole Oriented parallel 9n2 
to the incident ar 
electric field 
Corner reflector 4S? 
r2 
S = cross section 
of triply 
reflected beam 
Triangular L = length of AnL’ . 
corner reflector’s edge. 3r2 (1— 0.00766") 
reflector @ = angle between 
direction of incidence 
and axis of symmetry 
of reflector 
Square corner 127L4 (1—0.02748) 
reflector 2 
