[Elecraft] Elecraft Digest, Vol 251, Issue 9 [was K3 power foldback]

[Fred Jensen]

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 at 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
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