Furuno FS-1503 - 3. Matching Circuit

2-30
3. Matching Circuit
AT-1503 uses two L-type matching circuits with a nominal input impedance Z of 50
Ω
as
shown below. Refer to Figure 2-25 for the variation of the input impedance with fre-
quency.
The matching circuit in the MF band is shown below, where the antenna length is less
than one quarter of the wavelength. The antenna impedance includes not only a resistance
but a capacitive reactance (-jXCa). In addition to matching Z to Ra, an inductance L1
(loading coil) is needed to cancel an antenna capacitance Ca.
At the output side of the matching circuit, a shunt capacitor Cs is included, which
equivalently increases Ca to make it possible to match to an antenna with small Ca.
Reference)
When a 6 m whip antenna is hoisted, the Ca is about 80 pF in 1.6 MHz. In
this case LI is 120
µ
H that is too high. Adding Cs (50 pF) increases the equivalent value
of Ca to about 130 pF and leads to the necessary value of LI 75
µ
H that is within a
matching range.
Z Ra
RF IN
ANT equivalent circui
t
Z > Ra
1
ω
C
= Z
Ra
(Z - Ra)
ω
L= Z
R (Z - Ra)
C
L = Lt + Ll
(50
Ω
)
Lt Ll
(Tank coil) (Loading coil)
Ca
Ra
Cs
∑
= 102
µ
H
ANT
The matching circuit for the antenna length of longer than a quarter of the wavelength is
shown below. The actual antenna has not only a resistance Ra but also a capacitance, or
an inductive reactance in an antenna length range. In addition to matching Z to Ra, a
coupling capacitor Cc is needed to cancel the antenna reactance
Z
Ra
RF IN
Z < Ra
1
ω
C
=
Z
(Ra - Z)
ω
L=
Z (Ra - Z)
Ct
L
(50
Ω
)
Antenna equivalent circui
Ca
Ra Ra
La
Cs
(Tank coil)
Cc
C = Ct + Cc
(Coupling C) (Annten C)
∑
= 4788 pF
ANT
Inductive
Capacitive
Antenna Lengt
h
λ
/4
λ
/2
Reactance
Ra: Resistance
30 to 40
Ω
(+jXL)
(-jXC)
3
λ
/4
Figure 2-25 Change of Antenna Impedance