[altis]

TBH, I doubt that 5m of even the grottiest coax will make an enormous difference to the signal levels. My guess is that there will be a problem with the terminations (ends). What, exactly, have you got at each end of this mysterious cable and do you know that the two ends are connected directly with nothing else in between?

Anyway, if all else fails, I would recommend installing some nylon tubing (such as used in pneumatics) instead of an actual cable. That way you can replace the cable whenever you like.