4.3. Tuning
Tuning is possible only in the OPER mode.
Press the OPER button in order to illuminate its LED (unless AUTO-OPERATE is active).
a) Preliminary information
Tuning the amplifier involves a procedure of matching the impedance of the currently used antenna and
transmission line to the amplifier tube's optimum characteristic load resistance. This will ensure maximum
plate efficiency and optimum 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
indicator 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 2.4:1 SWR
load, readings of 1200 W and 200 W FORWARD POWER and REFLECTED POWER respectively, the real
OUTPUT POWER is 1000 W. At a very high SWR 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 if the following rule is obeyed:
"REFLECTED POWER < 300 W".
Also, impedance matching capability is assured for loads presenting a SWR of up to 3:1. Nevertheless, for
some loads and bands, matching is possible at even higher SWR levels, but the drive power must be
reduced to prevent the REFLECTED POWER from exceeding 300 W. Failure to comply with these guidelines
will cause the **REFLECTED POWER** SOFT-FAULT protection to trip.
For example, if the antenna SWR were 4.5:1, the maximum attainable forward power would be 750 W,
300 W of reflected power and real output to the antenna and transmission line of only 450 W. In the event
your antenna cannot be adjusted to produce a lower SWR, an external antenna tuner may be deployed.
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