Hi Fred, I did just as you did - first got lowest SWR settings on the Matchbox using my AA-55 impedance analyzer, recorded those settings on each band of interest in the middle of the CW or phone sub-bands, and then used the internal ATU to compensate for higher SWR away from the fixed tune points. I didn't attempt to twiddle the Matchbox caps for lower SWR at any new frequency away from the mid-band points, but let the ATU do its thing.
Thanks for your comments! 73, Mike, K8CN ________________________________ From: Fred Jensen <k6dg...@gmail.com> Sent: Tuesday, March 11, 2025 6:38 PM To: Michael Carter <mike.car...@unh.edu>; Elecraft Reflector Reflector <Elecraft@mailman.qth.net> Subject: Re: [Elecraft] Elecraft Digest, Vol 251, Issue 9 [was K3 power foldback] CAUTION: This email originated from outside of the University System. Do not click links or open attachments unless you recognize the sender and know the content is safe. Been there, done that Mike😉 Led to my rule with baluns/transformers: If it gets hot, get a bigger toroid. Lacking a larger toroid, you can rewind it using two stacked cores, but one bigger one is better. A problem with my experiment at the radio-->int tuner-->ext tuner was that the ext tuner is really just a passive 2-port impedance transformation network and it can easily transform your feedpoint impedance to something wildly different than 50+j0 ohms which the internal tuner then tries to match. My result was that even small frequency excursions caused widely varying impedances at the ATU terminal. Until beefing up the transformer [balun], I solved it by tuning the ext tuner first using an antenna analyzer and then connecting to the radio and letting the ATU do it's thing. Worked but I was still glad to simplify the arrangement. 73, Fred ["Skip"] K6DGW Sparks NV DM09dn Washoe County Michael Carter via Elecraft<mailto:elecraft@mailman.qth.net> Tuesday, March 11, 2025 3:04 PM Hi Bob, Thanks for your comment. As I noted in our private email exchange, the Johnson Matchbox was a last-minute substitution for an Elecraft BL-2 balun that was overheating on some bands at 100 watts. I had not been using that configuration previously, but faced with the choice of going QRP or QRT in the ARRL DX CW contest at its commencement, I opted to put the Matchbox in place of the balun. I used that same configuration again for my brief foray in the ARRL DX SSB weekend, and that led to my original post here about K3 TX power foldback with only modest rises in SWR presented to the internal ATU. For my particular antenna and feedline length, the indications are that common-mode currents are high on several bands, which is not surprising to me. The Matchbox can reduce the SWR presented to the rig on some bands, but not all of the ones I needed for good contest outcome. That motivated use of the internal ATU rather than just putting the ATU in 'BYPASS' mode and relying on the Matchbox alone. I agree that using a good, high common-mode impedance choke is preferable to the Matchbox, and the internal ATU of my K3 is quite capable of handling the native SWR presented by the antenna/feedline using the balanced-to-unbalanced transformer in its 1:1 configuration. It appears that the balun's common-mode impedance is not sufficient to reduce the common-mode currents presented by my particular antenna/feedline system. That's not the balun's fault, and one day soon I'll measure its common-mode impedance to better understand its power-handling limitations. With thanks as always for your insight and comments, 73, Mike, K8CN ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net<http://www.qsl.net/> Please help support this email list: http://www.qsl.net/donate.html Message delivered to k6dg...@gmail.com<mailto:k6dg...@gmail.com> [https://s-install.avcdn.net/ipm/preview/icons/icon-envelope-tick-green-avg-v1.png]<http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> Virus-free.www.avg.com<http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
