Very Important Note: If the tower is against the house, as in my installation, be sure the tower hinges away from the house and not toward it. The time to discover this truth is before, not after the concrete has cured!!!
Although they are not easily visible in the black hole, you can see the pairs of nuts holding the tops of two of the three-foot long J-bolts which will support the tower. The top of the concrete will be just below the 2 x 4's supporting the hinged tower base. The rebar cage is seated on three-inch tall cement piers in the bottom of the hole and the top of the rebar is at least three inches below what will be the top surface of the concrete. Additionally, the UBC requires that the cage is at least three inches from the sides of the hole at all points. This insures that the rebar cage is completely imbedded in the concrete.
The reason for the rebar is to provide tensional strength. Concrete is very strong when compressed, but is surprisingly weak when under tension (pulled). Steel, on the other hand, deforms when compressed, but is very strong under tension. The two materials work together to provide the necessary strength to respond to the forces exerted by the tower which is mechanically a very long lever. In conditions of high wind or earthquakes the tower can exert thousands of pounds of stress upon the foundation.