To help explain, the modulation type used by a station (AM, FM, the various digital modes) is important to the radio circutry but not to the antenna. The antenna generally only cares about what frequency range is used.
For example, if your local police use the 400 MHz band (generally called UHF), you need an antenna that will cover the 400 MHz band. If the police changes from regular analog to a trunked system, the same antenna will still work. When they migrate this trunked system to a digital system, the antenna will still work just as well. Where you may need to change antennas is if they move from a 400 MHz system to an 800 MHz system.
Based on what DPD1 indicated in his post, you'll need a dual band antenna that covers UHF (400 MHz) and VHF-Hi (150 MHz) for best results. The antenna he mentioned should work well for you. Since he does antennas for a living, his advice should be given quite a bit of weight and is normally spot on.