[Elecraft] KPA500 - SWR into K3

[Fred Jensen]

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