I've got a book on my shelf somewhere (not Trek related) that explains what kind of forcefields (or shields) you could have with sufficiently advanced science and technology. From what I remember some of it was pretty wild and rather effective. I'll try to find it.
Now if you can manipulate gravity (which Starfleet seems able to do on some levels) than a sufficiently powerful antigravity field could be a quite effective shield since you could repel physical objects (such as missiles) and even deflect energy beams. If the antigrav field weakens then things start too get through.
But there is some suggestion in TOS that the deflector shields seem to absorb/dissipate energy and successive hits can eventually weaken the field. That doesn't really sound like a "deflector" shield or screen whereas an antigrav field would indeed be a "deflector."