[Elecraft] Elecraft Digest, Vol 251, Issue 9

[Bob McGraw]

You are using the matchbox to convert the balanced line to unbalanced 
and then using the internal ATU to resolve a match.

Use the matchbox connected direct to your radio, bypass the internal 
ATU, and tune the matchbox as needed.  That will work great.

If you insist on using the radio internal ATU, get a proper balun to 
feed the balanced line, remove the matchbox, and be done with it.  That 
will also work great.

73

Bob, K4TAX


On 3/10/2025 11:00 AM, elecraft-request at mailman.qth.net wrote:
> Message: 2
> Date: Sun, 9 Mar 2025 17:52:52 +0000
> From: Michael Carter<Mike.Carter at unh.edu>
> To: Elecraft Reflector Reflector<Elecraft at mailman.qth.net>
> Subject: Re: [Elecraft] K3 TX power foldback with SWR
> Message-ID:
> 	<BL3P223MB03855E806499171CB94BF013E4D72 at BL3P223MB0385.NAMP223.PROD.OUTLOOK.COM>
> 	
> Content-Type: text/plain; charset="iso-8859-1"
>
> Hi Jim,
>
> Thanks for your observation.  I think that duplicates what I found by fooling around during the SSB weekend.  I tried retuning the ATU, of course, after the 'Hi SWR' message flashed, but that didn't restore full power.  I don't have CW in SSB enabled (because I so rarely operate SSB), but I do think I switched modes to CW and hit the key, then back to SSB.  That might have done the trick, but I wasn't taking notes : ) .
>
> I just verified that by tuning my Matchbox for lowest SWR (using an AA-55 impedance analyzer) at the midpoint of each U.S. phone segment (40, 20, 15, and 10 meter segments - no WARCs), the SWR is always less than 3.0 across the entire segment, and for a couple bands it is less than 2.0 across the band, without further adjustment of the Matchbox controls.  That SWR would seem to be well within the capability of the internal KAT3 ATU.  While I didn't train the KAT3 by first moving up the band in 20 kHz increments and re-tuning the ATU, it is still surprising that a 2.0 SWR would cause the 'HI SWR' warning and TX power foldback.  I will look at the actual R+ JX impedance values for a better picture than SWR alone can provide.
>
> 73,
> Mike, K8CN