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The reader is referred to Fig. 1, which contains the approximately
correct positions of the BAND, LOAD,
and TUNE controls for a
50 ohm load impedance, such as that presented by a dummy load.
A dummy load is extremely useful for tuning up a transmitter in
preparation for adjustment of matching of an antenna. If the control
positions shown in Fig. 1 are utilized in conjunction with a 50 ohm
dummy load, only very minor adjustments of the TUNE and LOAD
controls will be necessary for a perfect match.
Of course, a 50-ohm
dummy load should require no matching for a perfect match to a 50-chm
transmitter output, but this information is presented because the
control positions in Fig.
1 represent useful starting points for
matching unknown impedances.
'The following, then, is a step-by-step procedure to follow for the
matching of an antenna feedline that has not been matched previously.
1) Tune up the transmitter in the normal way into a 50-ohm dummy
load (if 50 ohm coax is used between the transmitter and the FC-902)
or other resistive 50 ohm termination.
Reduce the gain control on
the transmitter
to a minimum,
and place the transmitter in "standby"
for the moment,
2) Place the BAND, LOAD, and TUNE controls in the positions
indicated in Fig. 1 for the frequency at which the line is to be
matched.
Place the ANT SELECT switch in the position which
corresponds to the rear panel connector of the antenna to be
matched.
Place the POWER & SWR switch in the SWR SET
position, and place the SWR SET knob in the 12:00 position.
3) It is desirable to make preliminary matching adjustments
with as little transmitter output power as possible.
Place the
transmitter in the "transmit" condition, and slowly advance
the output level until deflection of the FWD POWER meter is
observed. Adjust the SWR SET control to line up the meter
needle of the FWD POWER meter with the SWR SET position
at the far right end of the FWD POWER meter, using the minimum
power necessary to accomplish this. With the needle in the SWR
SET mark
o f the FWD POWER meter, the SWR meter will accurately
read the SWR.
4) Adjust the TUNE control for minimum reading on the SWR
meter.
If necessary, adjust the SWR SET control and/or trans-
mitter
power to ensure proper calibration.
Once the "dip" has
been found using the TUNE control, move the LOAD control either
to the left or right, by a small amount.
Adjust the TUNE control
for a "dip" again, and if the SWR improves (again, make sure that
you are reasonably well
calibrated) move the LOAD control slight-
ly more in the same direction.
Again "dip" the TUNE control, and
continue this procedure until no further improvement is noted. If
the initial direction of adjustment of the LOAD control worsened the
SWR, move it an equal direction from the starting point IN THE
OPPOSITE DIRECTION and follow the above procedure with
sive adjustment of the TUNE and LOAD controls.
suce
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