Overview Of Matching Procedure - Furuno FS-1575 Service Manual

3. Overview of matching procedure

To define a direction for whether to vary first the coil or the capacitor of the matching circuit
in initiating matching operation, it has to be determined from the VSWR whether the
antenna reactance is inductive or capacitive. The method is as follows:
1) If the dip point of VSWR can be detected by varying "L" (inductance) in series to the
antenna, the antenna reactance will be capacitive. In this case, matching operation will
be initiated from the gamma matching circuit.
2) If no dip-point is detected even by varying the "L," find the dip-point by adding 50 pF,
and 100 pF capacitors on the antenna side and varying the "L" again. Then, if the
dip-point can be detected, the antenna reactance will be inductive. In this case,
matching operation will be initiated from the pi-matching circuit.
Gamma-matching
In case of capacitive antenna reactance, matching operation will be initiated in the gamma-
matching circuit.
Set the Cin-side capacitor to "0," and then vary "L." Set the Cin-side capacitor to a
calculated value, and then vary "L" near the calculated value. Depending on the calculated
value, set "L," and then vary the Cin-side capacitor. Follow the said procedure to find the
minimum VSWR. If successful matching is not achieved, add the Cout-side capacitor to try
matching following the same procedure as that aforementioned.
In other words, vary "L" and "Cin" based on a locus that minimizes VSWR in terms of the
Smith chart.
Pi-matching
Basically, this method initiates matching operation by setting the Cout-side capacitor to "0"
to conduct it following the same procedure as that for gamma-matching.
Burnout prevention
On low transmitted frequency bands, antenna impedance comes to capacitive reactance
(1/jωCa) with antenna resistance of several ohms and reactance of −j500 ohms or more.
The shorter the antenna length, the higher the capacitive reactance becomes.
In this case, in order to negate capacitive reactance components, add coils (inductive
reactance (jωL) in series.
1/jωCa = jωL
In this case, each end of a larger coil located near the antenna side generates a high voltage,
possibly causing the matching circuit to burn out.
To prevent burnout, the matching circuit is designed to detect voltage at the antenna
connection and choose a matching point within the specified threshold when the voltage
exceeds the threshold even if the matching point shows acceptable VSWR.
This operation is conducted in the low transmitted frequency range, i.e., in the capacitive
reactance range of the antenna.
The voltage is given as:
CONFIDENTIAL (internal use only)
This confidential document is used only by FURUNO authorized persons.
It is strictly prohibited to reproduce the document in whole or in part without prior written permission of FURUNO.
7-75
7.9 Antenna Coupler