Make it an Impulse field, with a Min to Maximum boundary (Minimum 5 metres to Maximum 5.2 metres, so it makes a thin layer of impulse and doesn't throw whoever is in the shield out of it) then make a trigger that activates an Impulse field every 0.1 seconds.
Finally, use the same Impulse Min-Max values to make a Projectile Deletion feild that erases any projectile item that comes in contact with the shield boundary layer. Even better, a projectile detonation feild that makes the projectile Explode. You could also make anyone found inside the field invulnerable to impulse / radius damage.
Finally, no really, finally add a transparent spinning non-collision sphere with inner and outer skin as the basic explosion shape. to make it last longer simply add an animation sequence with 10 frames of still animation and give it an FPS of x/10 seconds as the time you want it to remain.
