Tile on 3/4" plywood on hearth

I am redoing my fireplace and build a new hearth and topped it with 3/4" plywood. I would like to put 18" tile on top of the hearth. Do I need to add a backer board on top of the plywood or can I tile directly to the plywood? There will be very little weight ever put on the hearth so I am not really worried about how much weight the tile can hold.

You need a second layer of something. You can't tile over a single layer. If you want to keep it thin as possible, might as well make it either Ditra, which will only add another 1/8", or 1/4" cement board. Either one would be acceptable.