Sig strength is good but there's a good chance that radio interference is causing your loss of supervision.
As gts said relocate the sounder or I'd bring the sounder indoors and place it near the panel with rear tamper closed then run it to see if you still get the supervision fault.
Least then you will know if it's interference or hardware fault.
Just an idea but you defo better off with an hardwired sounder if possible.
e manno
March 2014