Wood does not hold heat Thus you must create a sufficient thermal mass below the wood floor. Picture a rock on the beach in the Sun all day. Well after dark that rock stil is warm, a smaller rock will drop the heat quicker a log will go cold very quick. Someone said here that you only need 1/8 of a inch thinset. Might work under tile at that thick but under wood you will need a half inch as wood doesn't store heat. It will transfer heat.  The Cable is the way to go but must be four inches apart or it might warp the wood.

Is this going to be nail-down? If so, WarmlyYours recommends using the system encased in thinset or SLC. Use a system of 3/8" high "sleepers" laid down 19" apart across the room, perpendicular to the direction of the wood. This supplies the installer with "troughs" that the mats or cables are laid in. This trough is then filled with SLC or thinset up to and even with the tops of the sleepers. This supplies you with a "heat bank" and gives the wood full support across its span. The installer then nails the wood into the sleepers.

We have a diagram showing this installation method. I would attach it here but this is a discussion concerning a competitor and I do not want to be seen as foisting spam on this list. If anyone has any questions concerning this method of installation, let me know.