Bob Bob wrote:

If I use a 4:1 coaxial balun cut for 20M, does the 4:1 effect (now a
full wavelength) also occur on 10M?

It works on the odd harmonics, not the even--I published a computer
model of a 2M/70cm balun in the January 2000 QEX RF column. However,
as the coax multiple goes up, the phase change also goes up, compared
to a shorter balun, making long baluns impractical.

73--Zack W1VT

Reprints of the article are available for a nominal charge from the
ARRL.

(Thinking a 3/4 wavelength Q section is the same as a 1/4 wavelength
one
when it comes to translating the Z)

I am not and will not be using a tuner... (in case anyone wants to
suggest it)

TIA Bob VK2YQA