Where is it fed? [center, end, off center, etc.]

If center fed, the impedance looking into the feedline exceeds the
tuning range of your tuner. I would start by adding a little less than
1/4 wavelength at 28.3 MHz of additional feedline [something around 6 feet].

I use a CF doublet with 600 Ohm tuned feeders and an OLD [OLD] Murch
UT2000A tuner with a very good balun.

+ + +

Larry Gagnon wrote:
I have a 135' long doublet, 45' in the air fed with 450 ohm ladderline. It
tunes fine and works well through an MFJ tuner on all bands up to 21MHz.
Above 21MHz I have a very difficult time tuning the antenna to get any SWR
below 3:1. I cut 6' off the feedline when it was finally dressed into my
shack and that made it worse. I have now reinstalled the extra 6' and that
is better for tuning, but not yet perfect. I guess I'll just have to keep
adding or subtracting small amounts of feedline to get it right?

If so, are we talking inches or what? Any ideas appreciated.

Larry VE7EA