[Elecraft] KPA500 - SWR into K3

[Don Wilhelm]

Fred,

You logic is correct *except* for 2 things.
First, the KAT3 BYPASS switches in elements to make the output 50 ohms.  
That compensates for residual reactance in the tuner itself (relay paths 
etc.) - so it is not a direct path from in to out.

Secondly, the K3 SWR indication is at the input to the KAT3 - so if the 
KAT3 gets "mixed up" and somehow loses its memory for the inductors and 
capacitors to use for the BYPASS condition. the SWR indication will go 
up even though there is a good 50 ohm load on the output.

73,
Don W3FPR

On 12/9/2012 4:48 PM, Fred Jensen wrote:
> On 12/8/2012 7:57 PM, Ron D'Eau Claire wrote:
>> Well, putting the KAT3 in operation will "correct' whatever is 
>> causing the
>> high VSWR in the KPA500 by providing a conjugate match, but when the 
>> KPA500
>> is in Standby it passes the K3 output through to the KAT500. Now the 
>> input
>> to the KAT500 is no longer the same value it was with the KAT3 
>> bypassed so
>> it needs to retune too.
>
> Too many K's, P's, T's and A's. :-)  Let's start over.
>
> 1.  Original config on *all* bands was:  KAT3=BYPASS, KPA500=OPER, 
> KAT500=tuned [all segments, all bands, all applicable antennae for 
> each band].
>
> 2.  In original config, SWR seen by K3=1.1:1 max, often 1:1.  SWR seen 
> by KPA500=whatever the KAT500 could produce, always below 1.5:1.  All 
> this is as expected.
>
> 3.  Still in original config, I started ARRL 10 CW and after awhile 
> noticed K3 SWR=1.5:1 and wondered if something had gone south in the 
> KPA500 input circuit.  Note:  KAT3 is still in bypass, SWR seen by 
> KPA500 is still 1.1:1 where it was when I started on 10M.
>
> 4.  Posted a question to the list, Gary offered: "Put KAT3 in AUTO, do 
> ATU TUNE on the K3, put KAT3 back in BYPASS.  When I did this, three 
> things happened:
>
> a)  I heard the KAT3 tune, and I heard a quick relay noise from the 
> KAT500 [not the amp], as if it *too* was adjusting.
>
> b)  The SWR seen by the K3 went to 1:1
>
> c)  The SWR seen by the KPA500 was now off scale, and it faulted.
>
> 5.  I then did a TUNE cycle on the KAT500, and SWR seen by KPA500 went 
> back to 1.1:1 where it had been.  SWR seen by the K3 [i.e. looking 
> into the KPA500] remained 1:1.
>
> <digression>When I did all of the above, I was somewhere between 28000 
> and 28050.  It was late afternoon Sat, contest activity was waning, 
> and I ended the day without going above 28050.  This AM, there was 
> contest activity up to about 28075 or so.  The first time I tried 
> transmitting in that segment, I had to do a TUNE cycle on the KAT500, 
> which is even more puzzling since I had already done it in that 
> segment and the KAT500 had memorized it.</digression>
>
> I understand your reply, that's not what puzzles me:
>
> -- With KAT3 in BYPASS, is the K3 not looking directly into the input 
> impedance of the KPA500?  SWR seen by K3 should be 1:1 [or very 
> close].  It wasn't, that's what started this puzzle in the first place.
>
> -- Obviously, I can use the KAT3 to minimize the SWR seen by the K3 
> ... shouldn't have to but I can.  And yes, that has side effects 
> should I go to low power with KPA500 in STBY.  Remember though, after 
> doing the ATU TUNE on the K3, *the KAT3 went back to BYPASS*.
>
> -- If "BYPASS" on the KAT3 really means what it says, the sequence: 
> "KAT3 AUTO, ATU TUNE [on K3], KAT3 BYPASS" should do nothing to the 
> original issue ... 1.5:1 seen by the K3 ... but it did, just like Gary 
> said it would.
>
> -- Regardless however, messing with the load seen by the K3 should not 
> affect the load provided to the KPA500 by the KAT500, but it seems it 
> did.
>
> -- Why would doing ATU TUNE [on K3 feeding the amp] cause the KAT500 
> to do anything?
>
> I've checked all the K's, P's, T's, and A's in the above and I'm 
> convinced I got them all right.  You have to read carefully. :-))
>
> I haven't figured out how to repeat the issue since I don't know how 
> to get the KAT3 back to whatever it was doing in BYPASS when this all 
> started.
>
> 73,
>
> Fred K6DGW
> - Northern California Contest Club
> - CU in the 2013 Cal QSO Party 5-6 Oct 2013
> - www.cqp.org
>
>
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