Digital Signal Processing Reference
In-Depth Information
(
G
i
=
1
.
64) in relation to an isotropic emitter is known, it is easy to convert between
the two figures:
P
EIRP
=
P
ERP
·
1
.
64
(
4
.
72
)
4.2.5.3 Input impedance
A particularly important property of the antenna is the complex
input impedance Z
A
.
This is made up of a complex resistance
X
A
, a loss resistance
R
V
and the so-called
radiation resistance R
r
:
Z
A
=
R
r
+
R
V
+
jX
A
(
4
.
73
)
The loss resistance
R
V
is an effective resistance and describes all losses result-
ing from the ohmic resistance of all current-carrying line sections of the antenna
(Figure 4.63). The power converted by this resistance is converted into heat.
The radiation resistance
R
r
also takes the units of an effective resistance but the
power converted within it corresponds with the power emitted from the antenna into
space in the form of electromagnetic waves.
At the operating frequency (i.e. the resonant frequency of the antenna) the complex
resistance
X
A
of the antenna tends towards zero. For a loss-free antenna (i.e.
R
V
=
0):
Z
A
(f
RES
)
=
R
r
(
4
.
74
)
The input impedance of an ideal antenna in the resonant case is thus a real resistance
with the value of the radiation resistance
R
r
.Fora
λ/
2 dipole the radiation resistance
R
r
=
73
.
4.2.5.4 Effectiveapertureand scatter aperture
The maximum received power that can be drawn from an antenna, given optimal align-
ment and correct polarisation, is proportional to the power density
S
of an incoming
Dipole
U
0
R
T
R
r
R
v
U
T
X
a
Z
T
X
T
Transponder
Antenna
Figure 4.63
Equivalent circuit of an antenna with a connected transponder
Search WWH ::
Custom Search