Yeah - looking up "what kind of system" is important. Luckly, here in NYC, the major services are NOT trunked (although DoITT has been putting in a 900Mhz trunked system for the city)

I have 2 scanners, and rarely use them - in fact, what I did was program what I want/need into my ham radios, which can all receive "out of band"

As for antennas - it's an "interesting" problem

_IF_ what you are listening to is in the "VHF High" and 470 Mhz bands (common), a good "dual band" ham Radio antenna will work really well, and give you some gain - the problem then comes in if you want to listen to stuff "away" from those bands - if you want to JUST add VHF low (aka 45-60Mhz or so), going to a "tri Band" 6m-2m-70cm ham antenna works well

Now - if you want full flexibility, you go to an antenna called a "discone" - which covers all bands - but equally badly Discones work at basically any frequency above their desigen frequency (well, they can have an upper limit based on design), but they have NO gain, and don't tend to eliminate frequencies you don't want