I have a customer who is experiencing problems with a 1242 AP running 12.3(8) in a large metal factory building. I remember doing the survey on this one and placing the antenna's here was difficult due to the cranes they had running in the middle and sides of the plant which prevented any kind of omni-directional antenna mounts. My only option I felt at the time was to mount the AP and antenna to the I-Beams on the north and south walls of the plant (note, pic of one AP is attached) with a 2465 diversity patch. I know Cisco does not recommend this type of mounting but I think it was my only choice.
The customer is experiencing drops on it's mobile factory workstations running a mix of both internal WNICS and Cisco CB-21 cards using the version 3.5 ADU. The clients seem to be ok though. I have not had a chance to do a spectrum analysis, but I suspect multipath interference with all of the metal and the fact that it's mounted to an I-Beam.
I'm thinking of suggesting that the customer put a 1 inch (or more) thick piece of plywood behind the antenna with at least 4 inches of overlap around it. Does this sound like it would help?
Any other thoughts and suggestions?