The frequencies that you plan to listen to will determine the type of antenna you decide on.
Personally, I prefer a discone style. It covers all frequencies between 30mhz up thru 1.3 ghz.
And of course you'll want to mount it outside, the higher, the better.
One antenna covers it all and it is what I use here. I have 1 Diamond and 1 Radio Shack.
They are both mounted about 35 feet up, feed all of my scanners, and they perform identical to each other.
By the way, the Diamond cost twice as much as the Radio Shack and I defy anyone to tell which one gets better reception.
Just my opinion.
It's all Mind over Matter....If you don't Mind....It don't Matter.
10-BC350A, 4-BCT8, 4-BC560XLT, 3-Pro77A, 2-Pro7A, 2-Pro48