That is, the cable from the back of the TV to the wall plate.

I need to watch the TV in the annexe which has no wall plate, so I
could run an extension from, say, the kitchen's wall plate. But what
is the limit to the length and would I need some sort of booster?

The aerial is currently driving a 14"analogue (CRT) Sony portable, but
will soon be connected to an LCD TV.

MM