User's Manual | ACOM 1010 | HF Linear Amplifier
July 2021
S e c t i o n OPERATION | Page 23 of 40
4.5.
Tuning
Tuning is possible only in the OPERATE mode.
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 2.5:1 SWR,
readings of 800 W and 150 W FORWARD POWER and REFLECTED POWER respectively, the real OUTPUT
POWER is 650 W. At 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 < 250 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 250 W. Failure to comply with these guidelines
will cause the protection circuits to trip.
For example, if the antenna SWR 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 SWR, an external antenna tuner may be deployed.
NOTICE
At elevated SWR levels, high voltages and high currents are distributed along the coaxial
cable to the antenna, risking internal arcing and heat generation, and likely damage to
the cable and any antenna switches that may be used. It is recommended that SWR
levels of more than 3:1 not be permitted with coaxial cable above 14 MHz.