If the antenna is far from resonance (or if antenna
Q is high), and especially if you are using a trans
ceiver that has a broadband solid state power
amplifier, the automatic final protection circuitry
in the transceiver may interfere with the auto-tune
system. This is because the output protection
circuitry automatically reduces the power of the
transmitter when the SWR is high, and such power
reduction may mislead the auto-tune sensors. In
this case, rotate the SENS control slightly counter
clockwise, and allow the auto-tune system to adjust
minimum SWR at the lower SENSitivity setting.
When the READY indicator lights, rotate the
SENS control to the fully clockwise position,
return to receive for a moment, then transmit
again. Any residual SWR will then be tuned out,
if necessary.
When the SENS control is set fully clockwise, any
time that the transmitter is activated and the SWR
is about 1.5: 1, the auto-tune system will seek to
retune the antenna. This will occur if the trans
mitter is retuned to a different frequency, or if
any changes in SWR appear in the antenna (due to
repositioning, weather changes, etc.). Therefore,
for mobile operation, the auto-tune system must be
desensitized once the antenna has been matched.
To do this, set the SENS control fully counter
clockwise.
Manual Matching
If you think you can match the antenna better
than the auto-tune system, if the auto-tune system
cannot find the match, or if you wish to use the
Tuner to match an antenna for receiving only,
manual impedance adjustment is possible.
TUNE and LOAD Adjustment Sequence
TUNE
LOAD
1st
2nd
3rd
4th
When using a transmitter on the amateur bands, set
the SENS control fully counterclockwise to disable
the auto-tune system, and set the METER SWR/PO
switch to the PO (undepressed) position. Now
transmit a carrier while pressing the four TUNE
and LOAD buttons (one at a time) while watching
the PO meter indication for maximum deflection.
When this is obtained, press the METER SWR/PO
switch to the SWR position, and make sure that
the SWR is below 3: 1. Return the transceiver to
receive. If the SWR indicated above 3: 1, it might
be possible to obtain a better match on another
band setting. To do this, set the MANUAL button
to the depressed position, press the DOWN or UP
button, key the transmitter and repeat the adjust
ment for maximum PO. If the SWR cannot be
brought below 3: 1 in any band setting or adjust
ment of the tuner, or if the WARNING indicator is
lit, do not use that particular antenna for trans
mitting at that frequency.
Manual adjustment settings will be stored in
memory when changing bands, just like automatic
settings.
Receive-Only Matching
The automatic and manual matching techniques
described above must not be used for operating
frequencies that are outside of the frequency
ranges in which the operator is authorized to
transmit, since it is necessary to apply power to
the antenna in both methods. However, the FC-
757 AT may be used manually to match antennas
for reception only, on frequencies outside of the
authorized bands. To do this, simply tune the
receiver to an AM station that is relatively free
from fading, set the receiver AGC to the fast posi
tion, and press the four TUNE and LOAD buttons
(
5th
6th
These are the six steps used by the auto-tune system.
They can be duplicated manually with the four
TUNE and
LOAD
buttons.
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