I built a 10' camera jib arm that works really well, but has a bit of bob and wiggle when moved too fast.

the arm is 1.5" aluminum tube. I'm considering ways to stiffen it. My thought is to add an 18" vertical upright in the middle of the span, and then run a metal cable from one end of the jib to the other over the upright. by tightening the cable, I'm hoping to reduce the up and down bobbing of the arm.

Just read that steel would have been a better, stiffer choice for the arm - and the same weight as an aluminum tube of equal stiffness. and cheaper!

any thoughts?

How about two spars at 45 degrees to the left and right as well as two cables. That will reduce springiness in both the vertical and horizontal directions.