Matching Circuit; Change Of Antenna Impedance - Furuno FS-1503 Service Manual

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.
RF IN
Z
C
(50 Ω )
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
L
RF IN
(Tank coil)
Z
∑ = 4788 pF
(50 Ω)
L = Lt + Ll
Lt Ll
Ra
(Tank coil) (Loading coil)
∑ = 102 µ H
C = Ct + Cc
Ra
Ct
Cc
(Coupling C) (Annten C)
(+jXL)
Inductive
Ra: Resistance
30 to 40 Ω
λ/4
Capacitive Reactance
(-jXC)
Figure 2-25 Change of Antenna Impedance
Z > Ra
ANT
1
= Z
ω C
Cs
ω L = Z
Z < Ra
ANT
1
=
ωC
(Ra - Z)
Cs
ωL =
Z (Ra - Z)
Antenna Length
λ/2
3λ/4
2-30
ANT equivalent circuit
Ra
Ca
(Z - Ra)
Ra
R (Z - Ra)
Antenna equivalent circui
Z
Ca La
Ra Ra