I bought some no name RG-59/U coax (RG-59/U-SP-95 Made in the USA). I
went to cut and tune a 1/4WL matching section for 3.8Mhz. I cut the
coax a little longer because I intended to tune it with my MFJ259B
or.LP100 exactly to the design frequency. I have a 50 Ohm dummyload
that actually is about 54 Ohms @ 3.8Mhz. Plugging the numbers into
ON4UN's program for calculating impedance along a coax line: Given the
coax is 75 Ohms and the load is 54 Ohms, the program reported that if
the line was a 1/4WL long the impedance at the source end would be
about 105+j0 Ohms.

How long is the coax (in feet)? Are you sure you didn't actually cut
a half wavelength?

Tor
N4OGW