Pi-Matching - Furuno FS-1575 Service Manual

7.9 Antenna Coupler

2. Pi-matching

The pi-matching circuit is usually used on the HF band. To calculate the constant of a
matching circuit, "L" is divided into two channels and a virtual resistor "R" is mounted to
each channel. Two-stage matching circuit is designed centering on these resistors. The
circuit on the input side is a matching circuit of Rg and R, while the circuit on the load side
is that of Ra and antenna impedance.
With a practical antenna, antenna impedance switches between "capacitive" and "inductive"
depending on feed frequency. "Cc" consists of reactance that negates the antenna impedance
and matching constant.
L=L1+L2
L1 L2
Rg
Ct
Cc
R
Ra
Q1 Q2
Fig. 7.9.8 Calculation of Pi-matching
L
Rg
Ra
Ct Cc
Fig. 7.9.9 Pi-matching Measurement Constant for 50-ohm Loads
7-94
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