Digital Signal Processing Reference
In-Depth Information
The proportionality factor has the dimension of a length and is therefore called the
effective length l
0
(also
effective height h
) (Meinke and Gundlach, 1992). The following
is true:
S
·
Z
F
U
0
=
l
0
·
E
=
l
0
·
(
4
.
88
)
For the case of the matched antenna (i.e.
R
r
=
R
T
) the effective length can be
calculated from the effective aperture
A
e
(Kraus, 1988):
2
A
e
·
R
r
Z
F
l
0
=
(
4
.
89
)
If we substitute the expression in equation (4.86) for
A
e
, then the effective length
of a matched antenna can be calculated from the gain
G
, which is normally known (or
easy to find by measuring):
l
0
=
λ
0
G
·
R
r
π
·
Z
F
(
4
.
90
)
4.2.5.6 Dipoleantennas
In its simplest form the
dipole antenna
consists solely of a straight piece of line (e.g.
a copper wire) of a defined length (Figure 4.66). By suitable shaping the characteristic
properties, in particular the
radiation resistance
and bandwidth, can be influenced.
A simple, extended
half-wave dipole
(
λ/
2 dipole) consists of a piece of line of
length
l
=
λ/
2, which is interrupted half way along. The dipole is supplied at this
break-point (Figure 4.67).
The parallel connection of two
λ/
2 pieces of line a small distance apart (
d<
0
.
05
λ
)
creates the
2-wire folded dipole
. This has around four times the radiation resistance of
Figure 4.66
915 MHz transponder with a simple, extended dipole antenna. The transponder
can be seen half way along (reproduced by permission of Trolleyscan, South Africa)
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