Moisture meters

Anyone know how pinless radio “search” moisture meters work? I’m not sure of the name. It’s easy to find out how surface meters work but not the penetrating kind.

reflected radio frequencies. Higher density material (moisture) reflects more signal. Which is why most won’t work on concrete or metal as it’s already to dense.

Thanks Donald. If anyone knows anything else about how they work, I’d like to learn.

Are these two main types of moisture meters are the only types? Do most meters have both types or just one?