Our is a Lab gear one from TLC:
http://www.tlc-direct.co.uk/Products/MX27866R.html
It works pretty well most of the time in finding the best way to point the directional aerial before you initiate a "search" for digital channels.
It's used on the end of the coax cable that would plug into the TV [Status amplifier turned on]; find the best direction then unplug it and put the cable into the TV and initiate a channel "Search".
Sometimes there simply does not seem to be a "best" direction and your are back to looking at the local hoses.