"Ed G" Huckleberry_ large SWR when operating below about 3825 KHz.

We recently replaced the entire antenna assy with a 122' half wave
dipole, fed with 600 ohm ladder line to an SGC antenna coupler at the
base of the center mast. The SGC 237 coupler tunes the antenna
well above 3825, and all other bands, too. However, we still exhibit a
very high SWR when going below about 3825 and the tuner fails to tune...
in fact the Radio's power drops
way down, possibly preventing the tuner from working properly. A
different radio shows the same problem.

Ed, sounds like your radios are into VSWR turndown from the inability of the
tuner to match at the low end. It could be that the Z at 3825 is going high
which a lot of tuners can't handle--or could be very low, same problem, no
match. Try adding some more length to the dipole if you can and see if it
tunes better at the low end. Even if you only drape pieces down at the ends
would help if you can't extend more horizontally.
73,Bill W0BVR