P.S. - one area I forgot to comment on - consider your coax choices as carefully as you do your antenna choice. At higher frequencies, coax has higher loss. As you move to higher ranges, you'll definitely want better coax and better connectors for lower loss. I use LMR-400 for my connection to my DA3200 discone. I also use "N" connectors at each end and a converter to BNC right at the entrance to the shack (I have one of those window panels where all my coax comes into the house). A great antenna with crappy coax will provide crappy results. You can lose a LOT of your signal in the coax run, particularly if you have a long coax run like I do (150', because my discone is hanging in a very high tree). Here's a great page with cable loss charts:
http://www.w4rp.com/ref/coax.html. Look at the 2nd chart, specifically at the 150MHz and 220MHz lines and compare RG-58/U to LMR-400. That's a LOT of signal to lose between the antenna and your scanner. When I can afford it, I plan to upgrade from LMR-400 to LMR-600 for my discone for even less loss (at 150', it matters). Be careful of what connectors you choose for your coax run. Again, it depends on what the highest frequency is that you plan to monitor. UHF connectors (PL-259) are very poor above 300MHz (look at this chart:
Frequency Range Chart for RF Connectors | Amphenol RF). BNC and N connectors are both good choices. If you go for higher end coax (e.g. LMR-400), you'll want N connectors on each end with a N to BNC converter at the radio (I am assuming your radio has a BNC connector).
Best (again) - David