...
The sonic is mounted 73" or 1.85 m from the Rohn towers. The sonic boom is at an angle of 30 degrees with respect to the vertical plane of containing the closest guy wire (PGWVPGW).
Thus the sonic is 1.85 m * x sin(30 deg) = 0.93 m from the PGW VPGW and the horizontal distance of the sonic from the tower in the PGW VPGW is 1.85m * x cos(60 deg) = 1.6 m.
The ball is placed 10 m below the top of the tower and the upper guy is attached 5' = 1.5 m below the top of the tower. Thus the ball is 8.5 m below the top of the outer guy and displaced 0.8*8.5 m = 6.8 m horizontally from the tower. It is displaced 6.8 -1.6 = 5.2 m horizontally from the sonic in the PGWVPGW.
Since the sonics are at 5 m height intervals, the best case is a 2.5 m vertical separation between the ball and a sonic. I could refine this with the exact heights of the sonics on each tower, but the spatial separation will be principally defined by the 5.2 m horizontal separation. Thus the best case (maximum) separation is sqrt(0.93^2 + 2.5^2 + 5.2^2) = 5.8 m. The worst case is for a sonic at the same height as the ball or sqrt(0.93^2 + 5.2^2) = 5.3 m. These only differ by 10%.
Wyngaard (JAM, 1981) calculated the flow distortion caused by a sphere with a potential flow (laminar) model. Note that for a turbulent flow, this model is adequate only upwind of the sphere. Downwind of the sphere, the turbulent wake complicates the flow around the sphere (not to mention periodic vortex shedding of the wakes). Upwind of the sphere, the distortion of the mean flow is on the order of 3/2 (a/r)^3 where a is the radius of the sphere and r is the distance from the center of the sphere. Thus for a 36" diameter sphere, the flow distortion is estimated to be 1e-3 in the worst case and 7e-4 in the best case. These are certainly acceptable levels of flow distortion, but these estimates are only valid upwind of the sphere. For an 24", or 600mm, diameter ball the flow distortion is decreased by a factor of (1/2)^3 = 0.3.
The most desirable compromise would be to place these balls only on the two guy wires 'opposite' to the sonic, i.e. those at 90 degrees and 150 degrees from the sonic boom. The next best would be to use three 24" diameter balls.