Been there, done that Mike😉 Led to my rule with baluns/transformers:Â
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
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