Antenna Change; Tuning - Acom 1500 Manual

When Auto-Operate is enabled, the amplifier will be maintained in operate mode by default, unless you
use the OPER button manually to go to standby (in which case the Auto-Operate is suppressed
temporarily). Pressing OPER again will restore the Auto-Operate feature.
When the Auto Operate function is disabled, the amplifier will shift and stay in STBY mode after each
protection trip. To restore operation, the OPER button must be depressed.
When the Auto Operate function is enabled (see S 5-2), the amplifier will shift to the STBY mode on a
protection trip. It will return to the OPER mode after about four seconds automatically.

4-3. Antenna change

By pressing the ANT button, the amplifier output can be switched between three antenna outputs. The
three lights above the button indicate the current antenna selection. Selection of any of the antenna
outputs can be disabled if not used - see S. 5-3 - Antenna outputs assignment. This prevents
inadvertent selection of antenna outputs, preventing no-load transmission and possible amplifier
damage.
To avoid damage (not covered under warranty) do not change antennas while
transmitting. Do not transmit into an antenna output (1, 2, or 3) if it is not connected
to an antenna or a dummy load.

4-4. Tuning

Tuning is possible only in operate mode, so you may need to press the OPER button in order to
illuminate the LED above it (unless Auto-Operate is active).
a) Preliminary information.
Tuning the amplifier is a procedure of matching the impedance of the currently used antenna to the
optimum tube load resistance. This will ensure optimum plate efficiency and RF gain at nominal output
power, with minimum resulting IMD.
Please note, that the REFLECTED POWER readings and the measured VSWR depend on the
antenna impedance only, and not on the amplifier tuning. If the antenna impedance is different from
50-Ohm pure resistive (nominal), the REFLECTED POWER reading will always indicate some
reflected power (even with optimal tuning). Proper tuning will assure operation at maximum power
without distortion or danger to the amplifier.
Note also that the real OUTPUT POWER in the load is equal to the difference between the
FORWARD- and REFLECTED- readings. For instance, at a reading of FORWARD 1500W and
REFLECTED 250W, the real OUTPUT POWER will be equal to their difference - 1250W (into a 2.4:1
VSWR load). At a very high VSWR (no antenna or badly mismatched antenna), the FORWARD and
REFLECTED readings will be almost equal, while the real OUTPUT POWER (the difference between
them) will be practically zero.
The amplifier can operate safely if the following rule is obeyed: "REFLECTED POWER < 300W."
Matching is assured for loads with VSWR up to 3:1. Nevertheless, for some loads and bands,
matching is possible at even higher VSWR figures. The maximum usable forward power at a VSWR of
3:1 is 1200W with 300W reflected. For higher than 300W reflected power a ** REFLECTED POWER
** soft-fault protection trip will occur. This can happen at full-scale 1500W forward and 375W reflected
power, into an antenna having a VSWR of 3:1 or worse.
Using a coaxial cable with VSWR > 3:1 on HF, and particularly on the 10 and 6
meters bands, is not recommended. At such high values of VSWR, the high
voltages, high currents, and heat associated with line losses risk permanently
damage to your coaxial cable or antenna switch.
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