[Elecraft] KPA-1500 ATU not working well

[Paul Baldock]

I have found that if the KPA1500 is feeding  a 
largish SWR (like 1.5:1 as you suggest) then the 
drive power required to maintain a constant 
output will vary significantly as you tune across 
a band. This means you have to keep adjusting the 
power control within a single band. This appears 
not to occur if the SWR is 1.1:1 or better.

I don't know, but I would guess a solid state 
untuned amplifier efficiency is better into a 1:1 
SWR  than a 1:5:1. This could lead to the dreaded more fan noise.

By the way by suggesting 1.0:1. I did not mean 
exactly 1.0000000000000000000:1. I tired to infer 1 digit resolution/accuracy.

- Paul  KW7Y


At 11:11 AM 6/22/2018, Wes Stewart wrote:
>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)Â  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 
>1.4:1Â  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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