FWIW
I have a CM15A with "F" connector mod and an antenna in the attic. Over past three or so weeks I am suddenly having problems receiving RF signals from the two most remote motion sensors. One of the MS is using the 18" passive "repeater" antenna (it's not a true "passive repeater" but ya gotta call it something). Replacing batteries had no effect. Check of the attic antenna showed everything normal.
So what has caused the sudden decrease in CM15A sensitivity?
I, at this point only have three theories:
1. sun spots. (yeah, seriously)
2. moisture content of shingles which changes the pickup pattern of the antenna. (yeah, still seriously)
3. Crappy CM15A receiver gain is unstable.
My point here is: from an RF standpoint, my system has been working well since 2006, now it suddenly gets crappy. I can't explain it. Nor have I been able to fix it. Someone on the forum has a signature that paraphrased says: " One day it works, next day it does not, that's the way X10 is."