Newbie general antenna question

I just purchased a Home Patrol 1 scanner and am in the process of picking a mobile antenna out for it. My question is, is there a specification to compare when trying to find out which has a better distance to pick up signals? Sorry for my lack of better wording, I couldn't think of a better way to ask the question. I understand there are different bands and frequencies that some are better designed to monitor. But if I have two tri-band antennas I'm looking at, is there a way to tell if one will pick up distant agencies better than the other? Does that have to do with gain or watts power? Again, newbie here. I apologize for my ignorance.



I live in Wright City, MO and will be monitoring agencies on a Interoperability - 700 MHz / P25 system. Thanks for your help.

Newbie general antenna question