What happens if you experiment by moving the antenna to under the stairs?
More details might help us help you.
I can install it on the down stairs, but how about interferences?
That suggestion was intended to diagnose the problem which you had not clearly described.
I am not sure that I understand. Why do you believe that your problem is the cable? Rather than, for instance, some noise that is present at your location?
More details might help us help you.
I believe that the cable is the problem because when a ts is in the vicinity i've a lot of noise, while when there isn't any ts i haven't any noise.
Oh, so there is not much noise when there is not a thunderstorm. Thanks for telling us that.
What is your noise sensitivity filter setting? Have you tried adjusting that (in particular during a thunderstorm)? What were those results?
What happens if you do the "spark test" but do the spark somewhere
not near the antenna, but near the cable? Or near the PC? If your system detects a spark only if it's done near the antenna, but not near the cable, then the cable is OK.
During a thunderstorm, try doing what I suggested: "What happens if you shield the antenna? For instance, put a metal can over it, or wrap aluminum foil over the pipe? " If the strikes go down, but the noise does not go down, then the problem is probably the cable.
What sort of building are you in? It's possible for buildings to reflect the lightning impulse, and sometimes that shows up as noise.