It depends on what you want to hear. A CB antenna would probably work acceptably on the VHF lo band, but get progressively worse as the frequency rises.
You can get a good idea of what other folks are using for scanner antennas by posting in the antennas forum
. Our Scanner antenna wiki
article lists numerous models, distributors and reviews as well as technical stuff.
Note: all links are in blue
A little duckie that comes with most scanners is at best a compromise, and in fact may even exhibit some loss (like in the 30 mhz range, where it's much too short to be efficient). How much the antenna will improve reception is a combination of numerous factors, like base or mobile, frequencies you wish to hear, power, height of both the receiving and transmitting antenna, obstacles and many other factors. It is also important to keep in mind that some scanners overload very easily in urban areas, so sometimes in this case, less antenna really ends up being more. It's hard to make any recommendations without knowing where you are (county/state is fine), what you want to hear and your situation (handheld used in car, at home, base mobile in the car, etc). When you go to the antennas forum be sure to be as specific as you can. The more specifics, the better the quality of the answer.