Remember that an antenna is pretty much all about frequency. Mode doesn't matter, analog, digital, whatever if the frequency is right the antenna will work just fine. Polarity is important on all frequencies. For nearly all scanner activity, you'll want a vertically polarized antenna. On a hand-held, that indicates that you'll want the antenna pointing straight up and down for best reception. Be aware, however, that in some circumstances, you may find that pointing the antenna in other directions may give you better reception due to the signal being bounced around on buildings or other objects. For transmitting, keep the antenna pointed up and down though.
There are some great 800 MHz antennas on the market so if you only listen to the many trunking (or standard) systems that transmit on 700, 800, or 900 MHz bands (often called the 800 MHz band), they will do you well. What they won't do well on is the VHF-Hi (120 - 170 MHz) or UHF (420 - 480 MHz) bands. Similarly, there are good antennas for VHF-Hi band that won't work well on the 800MHz band. Some may work well on the UHF band, some won't. There are also dual-band VHF-Hi/UHF antennas that work well on both bands but still not so well on 800 MHz. If you listen to the VHF-Low band (30 - 50 MHz) there are a few that will work well on that band but they'll be quite long and not work as well on the other bands.
Be aware that many sellers will have an antenna that is designed for one thing but they'll say it will work on everything on the planet (and some from outer space!). If they list where the antenna will transmit then remember that that's where it will receive properly as well and ignore the rest of the hype. If they say that their antenna will work from 30 MHz through 1.5 GHz just know that they're lying and move on. You don't want to be stuck with a bad antenna just because they lied about the specs.
