
38-6 Electromagnetic Variables
38.3.2 Aperture Antennas
Aperture antennas are commonly used for receiving and transmitting at microwave frequencies (above
1 GH
z)
. As re
ceiving
an
tennas,
th
ey
ar
e
co
nveniently
ch
aracterized
by th
eir
on
-axis
ga
in
g or e
ective
ar
ea
A
e
. Eective area is dened as the ratio of the received power P
r
to the incident power density S
inc
and can also be written in terms of the gain [9]:
A
P
S
g
eff
r
inc
= =
λ
π
2
4
(3
8
.9)
E
quation
38.9 ap
plies
to th
e
ca
se
wh
ere
th
e
in
cident
e
ld
is po
larization
ma
tched
to th
e
re
ceiving
an
tenna
. e in
cident
po
wer
de
nsity
in a pl
ane
wa
ve
is S
inc
= E
2
/η
0
, where E is the rms electric-eld
strengt ...