Just got my new FT-857D which I love, but I haven't found a menu option for
reducing the RF power while tuning antennas. Currently I only have a G5RV
HF antenna, and an MFJ-901 Tuner for it. Will transmitting at full power
(100 watts) while using the antenna tuner, for a few seconds harm this radio
any, or am I too worried about nothing?

The radio is going to go into high-SWR foldback, almost certainly, if
it sees an SWR over 1.5:1 or so. The manual doesn't seem to say
whether this situation puts an undue strain on the finals.

There are a couple of ways to reduce power when tuning. To do it
manually, just adjust menu #75 before tuning and then turn it back up
afterwards.

The Yaesu FC-30 autotuner supports this radio directly (it plug into
the CAT port), Similarly, the Yaesu ATAS antenna can be connected to
the radio. For both of these, once you set up the radio, you can turn
to multi-function row K, press TUNE once to enable the tuner, then
press and hold TUNE for a second - the radio will then transmit a
low-power carrier and activate the FC-30 autotune sequence or tune the
ATAS position. When done tuning it returns to whatever power level
you've set in menu 75.

There are aftermarket products which permit automatic low-power tuning
of one sort or another with non-Yaesu antennas. BetterRF sells a
"Yaesu Tune Control" which plugs into the CAT port... it makes the
radio "think" that it's connected to an FC-30 autotuner, and enables
the low-power TUNE feature of row K. You can use this with a
manually-controlled screwdriver antenna and adjust the antenna
position via a separate toggle switch (that's the approach I use). The
tune-controller also has a 4-pin Molex jack into which one can plug an
Icom AH-3 or AH-4 or compatible autotuner... the tune-controller in
effect translates the Yaesu autotuner control protocol into an
Icom-compatible protocol.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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