Radar System Engineering

SEC. 37] USE OF ABSORBENT MATERIALS 71
The minimum reflection clearly occurs when sin @ = Oand cos @ = – 1.
Hence
@ = (2p – l)7r p=o, 1,2, . . . (19)
d=(2p– 1):.
Thus for cancellation the phase change must be r and the layer an odd
multiple of a quarter wavelength thick. The minimum reflection will
actually be O if
g=lnl~~,
a
Mid since
~.—.
gn gn
—
(20)
or
2/l
~=—.
T
‘l’his condition determines the necessary attenuation in the layer in order
that the emergent wave ~rill produce complete cancellation. It is qualitatively
obvious that the attenuation for each passage must become
smaller as the reflection at the front surface becomes greater.
When Eq. (20) is satisfied the power reflection coefficient is given by
the relation
+!azcoS2~
R2 = -. (21)
(a’
– 1)’ + 4a2 Cos’;
If the bandwidth AX of the absorber be defined as the range of wavelengths
in which more than half the incident power is absorbed, it can be shown
from Eo. (21) that
Therefore the bandlvidth of an absorber is proportional to p and inversely
proportional to n. The qualitative nature of this behayior follows at
once from the way in which a depends upon n and p. For a given refractive
index, a continuous transition from absorbers of the first kind to
those of the second kind may be effected by allowing the value of p to
ringe from 1 to c,
Ahsmbent materials have been pI educed in Germany for the
(22)