Posted by
Wes Stewart-2 on
Jun 22, 2018; 6:11pm
URL: http://elecraft.85.s1.nabble.com/KPA-1500-ATU-not-working-well-tp7642239p7642259.html
I do not, and probably never will, have a KPA1500. I do have a KPA500 and
KAT500 so I think I can comment. In my opinion, the function of these tuners is
to provide a match into which the amp can deliver full power. If 1.5:1 does
this, then that's good enough in my book. Furthermore, a better match might
entail higher tuner losses. So why worry about it?
Another thing; a 1.0:1 SWR infers infinite return loss. To measure infinite
return loss, you need a directional coupler with infinite directivity or some
mathematical correction derived from very well known calibration standards and
no other errors, So maybe you say, 1.0:1 can't be measured (you would be
correct), let's shoot for 1.22:1. That's a return loss of ~20 dB. Now I have
no idea of what the directivity of the coupler is in the KAT500 or KPA1500 but
considering it has to work from 1.8 to 54 MHz and to keep the numbers easy, 20
dB wouldn't be unreasonable. (Note: I'm only considering the directivity error,
there are several others, internal mismatches, frequency tracking, detector
non-linearities, etc.)
If we measure a load with 20 dB RL using a directional coupler with 20 dB
directivity the answer can be anywhere between infinity to 14 dB. (Full
cancellation of the two reflection coefficients to the sum of the reflection
coefficients) In SWR terms, a 1.22:1 load can measure anywhere between 1.0:1
and 1.5:1. A "perfect" load will measure 1.22:1.
Folks, these things aren't laboratory instruments and until Elecraft builds in
vector network analyzers with full error correction this is what we get.
Wes N7WS
On 6/21/2018 11:17 PM, Paul Baldock wrote:
> First let me say that other than some issues with the ATU, I am after a week
> of use, very happy with the KPA1500.
>
> So here's the ATU issues:
>
> I find that the ATU very rarely will learn a setting to get the SWR below
> 1.2:1 on any band. I can usually tweak it down to 1.0:1 using the Utility
> Program. And yes, before anybody asks, I have ATU STOP TUNE and the ATU
> BYPASS set at 1.0:1.
>
> On 6M into a perfect load (laboratory quality to 8GHz) with the ATU bypassed,
> the internal SWR meter reads 1.4:1 I would have expected it to be 1.0:1, but
> Elecraft tell me it is normal. So the ATU on 6M will have to tune out what is
> left when the 1.4:1 internal mismatch sees your external antenna mismatch.
> Maybe this is contributing to your problem. Try adding a quarter wave of coax,
> maybe it will achieve a better match. Fortunately my 6M antenna is flat enough
> that I can work CW, SSB and FT-8 in BYPASS.
>
> For your interest, on 10M in to perfect load with ATU bypassed the internal
> SWR meter reads 1.2:1. Not 1.0:1.
>
> Another issue I have with the ATU is that the internal frequency counter has
> 8KHz resolution which according to one of the Elecraft techs can lead to a
> 16KHz error in measurement. It appears that during "learning" that the ATU is
> based on the internal frequency counter measurement, not the frequency you
> transceiver sends to the KPA1500.This means that on the bands that have 10KHz
> or 20KHz segments, you cannot be sure which segment you are using, and when
> you use the tuner it could be in a different segment. Being off by a segment
> could be a problem with a high Q antenna.
>
> Apparently the counter originally had 1KHz resolution but was changed to fix
> some other problem. An Elecraft tech told me they currently have no plan to
> change it back to 1KHz. I am not suggesting that the tuner should change
> segments exactly on their edge. Clearly there has to be some hysteresis to
> stop hunting. 25% of a segment width might width be a good number to choose,
> but this would require the current frequency counter to have improved resolution.
>
>
> - Paul KW7Y
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