Why is everyone guessing the same? The poster said:

also connected the antenna to
each of splitter’s outputs (which go to TVs) – exactly the same
outcome -

which discards the splitter as a possible culprit. It must be these
cables going downstairs.

On 6 Des, 05:20, Shawn Hirn wrote:
My guess is the splitter is defective. Splitters are very inexpensive,
so try replacing it.- Amaga el text entre cometes -