A 800 mhz antenna is way too short to be even close to resonant on any of the milair frequencies (but I'm sure you already know that). Yes, you would do well to consider a widebanded antenna like a discone or something from
DPD Productions. And I'd be careful about using that preamp when you do get a better antenna - it wouldn't surprise me to hear that you would start hearing intermod or other junk. You're undoubtedly adding unnecessary noise to the amplified signal, and that simply isn't helping your results. The further you get from the design freq of the antenna, the less effective that whole setup is going to be, plain and simple. And remember your coax choice is also just as critical.
Take some time to do a little research before you make a purchase; you might find some additional filtering is needed, you might not. It's all dependent on your RF environment, but one thing is sure - doing it right will bring in the results...HTH...Mike