[Elecraft] KPA1500 ATU

[N4ZR]

I'm seeing a similar phenomenon, Scott, but figured I'm too much of a 
newbie with the amp to question it.

73, Pete N4ZR
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On 2/6/2019 10:49 AM, K9MA wrote:
> Ever since I received my KPA1500 (SN 00178), I've observed that the 
> ATU sometimes won't achieve a match better than about 1.4:1, even 
> though the antenna SWR is well within range. (ATU STOP TUN SWR = 
> 1.0:1, tune power 40 W) Now, 1.4:1 doesn't sound that high, but the 
> amplifier will often not operate at full output at that SWR, either 
> due to excessive current or drive power. This is particularly 
> troublesome when tuning across a band. Clearly, the KPA1500 is 
> designed to deliver 1500 W to a 50 Ohm load, with little margin for a 
> mismatch. More margin would have increased cost, size, and weight, of 
> course.
>
> Just to be sure there isn't something unique wrong with my KPA1500, 
> have others observed similar behavior?
>
> A little ice on the antenna this morning provided an opportunity for a 
> test. On 40 meters, the antenna SWR was 2.2:1. The best match the 
> KPA1500 ATU could attain automatically was 1.4:1, in multiple 
> attempts. I was able to manually get it down to 1.1:1. (SWR as 
> indicated by the KPA1500 display.) It was a little tricky, requiring 
> one value to be increased beyond the SWR minimum before the other was 
> adjusted, as often happens with manual tuners. With the KPA1500 ATU 
> bypassed, the K3 ATU automatically got it down to 1:1 on the first try.
>
> My conclusion is that there's something about the KPA1500 ATU 
> algorithm that prevents it from achieving a better match, in some 
> cases. (I'm assuming that, aside from their power ratings, the 
> components in the KPA1500 ATU and the K3 ATU are similar.)  I'm 
> wondering if the KPA1500 ATU algorithm could be improved to perform 
> more like that of the K3 ATU. That would allow operating the KPA1500 
> closer to full power across a band without having to ride the power 
> control.
>
> Further details of ATU component values, etc. are available.
>
> 73,
>
> Scott K9MA
>
>