Bushman,

I have a mast mounted radar, Raymarine 2kw, on a swivel to keep it
level while heeled. I don't experience any blank spots behind me. For
example, if there is land behind me, it is uninterrupted.

I do experience that many boats don't at all appear on my radar, from
any direction, (because they don't reflect enough energy) unless I
manually turn up my gain to 60%, which causes other things to appear.
If I had it to do again, I would get a 4kw radar so I can get a better
return from some of the smaller boats in our local waters.

I would also consider mounting the radar off my backstay. Then my jib
sail and spinnaker halyards would have one less thing to get tangled
around