At what distance are you losing video feed? What configuration do you have the antennas in on your ST16S?
If using the mushroom (omnidirectional) antenna it needs to be positioned so it looks like it just growing out of the ground.
View attachment 27621
If you are using the patch (flat directional) antenna you need to point the flat surface at the aircraft all of the time.
View attachment 27622
The wand antennas should be positioned as shown so the plane they are in is perpendicular to the ground and at 90 degrees to each other.