So let's see ... 100' LMR-400 at 10 MHz with source end VSWR of 3
And Owens calculator says the line loss less .5 dB? That's less then the rated attenuation.
What am I missing? Should be much much higher.
I think you may be missing this line in the returned results * Note that estimating line loss from VSWR is prone to error.
If you have an antenna analyzer and use Zin (Z you get from your analyzer) you will get better results.