11
4-3. RTTY Mode
Select the
RTTY
mode to operate continuous-duty modes such as RTTY, SSTV, or other data modes. The
LED indicator above the
RTTY
button illuminates and the amplifier operating parameters are changed
to reduce tubes dissipation. In the
RTTY
mode, the amplifier output power is reduced to a maximum
of 500 W. There is no need to adjust tuning when changing between
RTTY
and normal modes.
CAUTION
To avoid damage not covered under warranty, do not change modes during
transmission. That is, do not change to or from
RTTY
or any other mode when
transmitting.
4-4. Antenna change
By pressing the
A1-A2
button, the amplifier output is switched between the two corresponding antenna
outputs,
ANT1
and
ANT2
. The lights above the button indicate the current antenna selection.
CAUTION
To avoid damage (not covered under warranty) do not change the antenna during
transmission.
4-5. Tuning
Tuning is possible only in the
OPER
mode.
a) Preliminary information.
Tuning the amplifier involves a procedure of matching the impedance of the antenna and transmission
line to the amplifier tube characteristic load resistance. This will ensure maximum plate efficiency and
RF gain at nominal output power, with minimal distortion and spurious output. Note that
REFLECTED
POWER
readings depend on the antenna and transmission line impedances only, and not on amplifier
tuning. If the load impedance is not a nominally resistive 50-Ohms, the
REFLECTED POWER
reading will
always show a reading, no matter what the tuning settings. Proper tuning is always necessary, however,
and will allow you to operate at a high power level, without distortion or any danger to the amplifier.
Note also that the real OUTPUT POWER presented to the load (the antenna and transmission line)
is equal to the difference between the
FORWARD
and
REFLECTED
power readings. For instance,
with a 2.5:1 VSWR, readings of 800 W and 150 W
FORWARD POWER
and
REFLECTED POWER
respectively, the real OUTPUT POWER is 650 W. At very high VSWR levels, such as when no antenna
is connected or a badly mismatched antenna is used, the
FORWARD
and
REFLECTED
readings will
be almost equal, while the real OUTPUT POWER (the difference between them) will be nearly zero.
The amplifier can operate safely as long as the
REFLECTED POWER
is LESS THAN 250 W. Matching
is assured for loads presenting a VSWR of up to 3:1. Nevertheless, for some loads and bands, matching is
possible at even higher VSWR levels, but the drive power must be reduced to prevent the
REFLECTED
POWER
from exceeding 250W. Failure to comply with these guidelines will cause the protection circuits
to trip. For example, if the antenna VSWR were 5:1, the maximum attainable forward power would be
540 W, 240 W of reflected power and real output to the antenna and transmission line of only 300 W.
In the event your antenna cannot be adjusted to produce a lower VSWR, an external antenna tuner may
be deployed.
