Externmal ATU

[Joe Subich, W4TV-4]

 > 3) Leave the tuner in manual mode and press TUNE on the tuner when you
 > change bands or antennas.  This interrupts the amp-key line before
 > tuning starts.  But this requires a TUNE REQUEST input to the K3
 > which is not available.

Since the manual mode tune interrupts the amp-key line, you can
use the "Key" jack in the K3 for your tune request.  The K3 output
will be limited to your "PWR" setting which with most amplifiers
(other than a pair of 3-500Z) will (should) be less than full
power - typically 60-70 W and as low as 25-30 W for 8877 and
similar high gain tubes.  That level should be plenty safe for
both the "full power" tuner and the K3.

In addition, one could always add a relay switched 3-6dB attenuator
enabled by the Tune Request signal to the input of the tuner.  That
would both decrease the tune power to the antenna and isolate the K3
from any transient mismatches due to relay switching in the tuner.

73,

    ... Joe, W4TV

On 8/28/2010 7:07 AM, Phil & Debbie Salas wrote:

> "...MFJ released a new Firmware for the MFJ-998 dated 08/02/2010. ..."
>
> The MFJ-998 wasn't interrupting the amp-key line when it tuned from memory.
> I guess it was assumed that this happend very quickly so this would not
> cause a problem.  However, as the relays operate you can have a momentary
> high VSWR condition.  So this will help.  HOWEVER - it takes a finite period
> of time for the tuner to "decide" that the VSWR is high and to measure the
> incoming frequency.  So during this "decision" process (when you change
> bands or antennas), you could have a high VSWR presented to the amplifier
> before the amp-enable line is interrupted and the new tuner values are
> snapped in place.   There are four ways around this:
>
> 1) With the tuner in full auto, flip the amp to standby and hit TUNE on the
> K3 when you change bands/antennas.  If the VSWR is greater than the MFJ-998
> programmed "start tuning" value, the tuner will tune.  Then flip the amp to
> operate.  This is what I do now (K3 tune output set for 15 watts).
> 2) Provide band data to the tuner so it tracks band changes before you even
> transmit.  Not supported by the current MFJ-998.  And this requires more
> untelligence if you have more than one antenna/band.
> 3) Leave the tuner in manual mode and press TUNE on the tuner when you
> change bands or antennas.  This interrupts the amp-key line before tuning
> starts.  But this requires a TUNE REQUEST input to the K3 which is not
> available.
> 4) Use resonant antennas on all bands and forget the tuner!
>
> Phil - AD5X
>
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