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KXAT100 "Hunting"

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Edward A. Dauer

KXAT100 "Hunting"

My KX-line tuner (the KXAT100 installed in the KXPA100 amp) recently
started an odd behavior. I wonder if anyone can help explain it. When
tuning into an indoor Buddipole set at resonance, the ATU finds a
near-perfect solution when the power applied during tuning stays below
about 40 watts. With the ATU set in “auto” however, if the power out is
higher than 40 watts the tuner hunts without stopping. I have watched the
SWR readings change, often going briefly to 1.0 or 1.1:1, but the tuner
just keeps hunting, endlessly, even when the frequency is unchanged. Yet
when it is first tuned at a lower power, either in Auto or Manual-tune,
and the ATU is set to Manual, it stays at the solution perfectly well no
matter how high the power goes. I can live with this, just by tuning at
low power and then switching to Manual, but I wonder why the Auto function
works well up to a certain point but goes berserk at any power higher than
that? Is the higher ERP sneaking back into the control circuitry somehow?

Ted, KN1CBR
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Vic Rosenthal

Re: KXAT100 "Hunting"

My guess is the same as yours -- common-mode current on the outside of
the coax upsetting the tuner circuitry. If you have a large ferrite
toroid (ferrite #31 should work on all bands), try winding several turns
of the coax around it. It might work better near the rig or near the
antenna.

Also, changing the length of the feedline might help. Odd multiples of
1/4 wavelength are bad.

73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/

On 30 Jan 2016 19:54, Dauer, Edward wrote:

> My KX-line tuner (the KXAT100 installed in the KXPA100 amp) recently
> started an odd behavior. I wonder if anyone can help explain it. When
> tuning into an indoor Buddipole set at resonance, the ATU finds a
> near-perfect solution when the power applied during tuning stays below
> about 40 watts. With the ATU set in “auto” however, if the power out is
> higher than 40 watts the tuner hunts without stopping. I have watched the
> SWR readings change, often going briefly to 1.0 or 1.1:1, but the tuner
> just keeps hunting, endlessly, even when the frequency is unchanged. Yet
> when it is first tuned at a lower power, either in Auto or Manual-tune,
> and the ATU is set to Manual, it stays at the solution perfectly well no
> matter how high the power goes. I can live with this, just by tuning at
> low power and then switching to Manual, but I wonder why the Auto function
> works well up to a certain point but goes berserk at any power higher than
> that? Is the higher ERP sneaking back into the control circuitry somehow?
>
> Ted, KN1CBR

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Don Wilhelm-4

Re: KXAT100 "Hunting"

In reply to this post by Edward A. Dauer

Ted,

You may need additional current mode chokes on the feedline in addition
to the small ones provided by Buddipole.

I would suggest "training" the tuner on each band segment that you want
to use and then put the tuner into MAN mode.
I realize that is more difficult with the Buddipole because after
changing bands with its taps and element lengths, you might not end up
with exactly the same thing each time.
So as an alternative, set the band on the Buddipole, then with the
KXPA100 in AUTO, do a TUNE at low power - when that tune completes,
switch to MAN mode and operate. The KXPA100 should not automatically
tune in MAN mode.

If you are operating SSB (and less often CW), there can be a false
sensing of a high SWR condition that will cause retuning if in AUTO mode.
That is because of a combination of two things - first the syllabic
nature of SSB speech and secondly because the KXAT100 (and other tuners)
do not sample the forward and reverse voltage at the same instant. So
if the forward voltage is sampled at a time when the RF output (do to
the variations in SSB speech) is low, and the subsequent sampling of the
reverse voltage happens to be at a time when the RF output is high, a
false computation of the SWR will result (it is all a matter of the math).

In other words, once the antenna is tuned, switch to MAN mode on the tuner.

73,
Don W3FPR

On 1/30/2016 12:54 PM, Dauer, Edward wrote:

Edward A. Dauer

KXAT100 "Hunting"

In reply to this post by Edward A. Dauer

Thanks, Don. I will try adding feed line chokes (after rereading K9YC’s
tutorials on the subject.) Meanwhile, tuning at low power and then
switching to manual does work, though it’s inconvenient with the Buddipole
which in some configurations has a very sharp SWR null. The Auto function
is useful when prowling the entire band quickly from 7005 to 7055 or the
like. Tuning at low power then switching to manual with any significant
frequency change adds a couple of steps . . .

However, I have to confess that this is not one of life’s major issues. I
remember some years ago hearing someone complain that the driver’s side
power window in his car did not have the function that allows the window
to travel its full length with one flip of the switch, as most other cars
do today. His required that he actually hold the switch until the window
was all the way up. What an awful burden to bear, I thought. My first
car (a ’57 MGA) didn’t even have crank-up windows. What MG then called
windows were solid pieces of fabric and plastic in a steel frame that
mounted into the doors on posts. They could be in either of only two
positions - on the door, or in the trunk.

Useful to remember when complaining about the limits of an auto-tune
function.

Cheers,

Ted

On 1/30/16, 3:15 PM, "Don Wilhelm" <[hidden email]> wrote:

>Ted,
>
>You may need additional current mode chokes on the feedline in addition
>to the small ones provided by Buddipole.
>
>I would suggest "training" the tuner on each band segment that you want
>to use and then put the tuner into MAN mode.
>I realize that is more difficult with the Buddipole because after
>changing bands with its taps and element lengths, you might not end up
>with exactly the same thing each time.
>So as an alternative, set the band on the Buddipole, then with the
>KXPA100 in AUTO, do a TUNE at low power - when that tune completes,
>switch to MAN mode and operate. The KXPA100 should not automatically
>tune in MAN mode.
>
>If you are operating SSB (and less often CW), there can be a false
>sensing of a high SWR condition that will cause retuning if in AUTO mode.
>That is because of a combination of two things - first the syllabic
>nature of SSB speech and secondly because the KXAT100 (and other tuners)
>do not sample the forward and reverse voltage at the same instant. So
>if the forward voltage is sampled at a time when the RF output (do to
>the variations in SSB speech) is low, and the subsequent sampling of the
>reverse voltage happens to be at a time when the RF output is high, a
>false computation of the SWR will result (it is all a matter of the math).
>
>In other words, once the antenna is tuned, switch to MAN mode on the
>tuner.
>
>73,
>Don W3FPR
>
>On 1/30/2016 12:54 PM, Dauer, Edward wrote:
>> My KX-line tuner (the KXAT100 installed in the KXPA100 amp) recently
>> started an odd behavior. I wonder if anyone can help explain it. When
>> tuning into an indoor Buddipole set at resonance, the ATU finds a
>> near-perfect solution when the power applied during tuning stays below
>> about 40 watts. With the ATU set in “auto” however, if the power out is
>> higher than 40 watts the tuner hunts without stopping. I have watched
>>the
>> SWR readings change, often going briefly to 1.0 or 1.1:1, but the tuner
>> just keeps hunting, endlessly, even when the frequency is unchanged.
>>Yet
>> when it is first tuned at a lower power, either in Auto or Manual-tune,
>> and the ATU is set to Manual, it stays at the solution perfectly well no
>> matter how high the power goes. I can live with this, just by tuning at
>> low power and then switching to Manual, but I wonder why the Auto
>>function
>> works well up to a certain point but goes berserk at any power higher
>>than
>> that? Is the higher ERP sneaking back into the control circuitry
>>somehow?
>>
>> Ted, KN1CBR
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>> Message delivered to [hidden email]
>

Bob McGraw - K4TAX

Re: KXAT100 "Hunting"

Candidly, I would be more concerned about exceeding the RF Exposure
limits with this arrangement. Perhaps the "Tuner issue" is an
indication of such. From your description, at 100 watts you will likely
be way outside of the FCC limits of safe RF Exposure.

Now with that said, the amount of RF at the TUNE level is not such to
confuse the tuner. However at 40+ watts the RF back into the system is
more than the system can accurately handle. A bunch of #31 beads or #73
beads, where "bunch" = 50 pcs or more, on the feed line at the antenna
feed point and again a bunch at the radio. This may help the radio and
antenna problem associated with the auto tune routine. You may find the
same treatment required on the DC power cable at the radio.

73
Bob, K4TAX

>>
>>
>> On 1/30/2016 12:54 PM, Dauer, Edward wrote:
>>> My KX-line tuner (the KXAT100 installed in the KXPA100 amp) recently
>>> started an odd behavior. I wonder if anyone can help explain it. When
>>> tuning into an indoor Buddipole set at resonance, the ATU finds a
>>> near-perfect solution when the power applied during tuning stays below
>>> about 40 watts. With the ATU set in “auto” however, if the power out is
>>> higher than 40 watts the tuner hunts without stopping. I have watched
>>> the
>>> SWR readings change, often going briefly to 1.0 or 1.1:1, but the tuner
>>> just keeps hunting, endlessly, even when the frequency is unchanged.
>>> Yet
>>> when it is first tuned at a lower power, either in Auto or Manual-tune,
>>> and the ATU is set to Manual, it stays at the solution perfectly well no
>>> matter how high the power goes. I can live with this, just by tuning at
>>> low power and then switching to Manual, but I wonder why the Auto
>>> function
>>> works well up to a certain point but goes berserk at any power higher
>>> than
>>> that? Is the higher ERP sneaking back into the control circuitry
>>> somehow?
>>>
>>> Ted, KN1CBR
>>> _