If your digital TV reception seems to be strong but perhaps you want to see if you can improve it a bit, you can probably get away with a simple VHF, UHF or combo indoor antenna. A VHF antenna is the old "rabbit ears" type of set-top antennas. A UHF antenna looks quite similar to a VHF antenna, except the antenna is circular rather than the two long telescoping arms a VHF antenna boasts. A combo indoor antenna is simply both a VHF and UHF antenna combined. Most digital broadcasts are on UHF channels. However, certain stations will move their broadcasts back to VHF channels in the future. Opting for a combination of both may be your safest bet. There are many VHF/UHF set top antennas on the market.

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If you have only fair reception and often experience station outages, you may need a stronger antenna. A higher quality or amplified indoor TV antenna may work, but you might want to invest in a simple outdoor antenna. If you have trouble getting any reception on your digital TV, a good outdoor antenna may be the only answer (aside from from paying for cable or satellite). The installation and choice of an outdoor antenna usually requires input from a contractor or installer, but you can get some ideas about your options from the antennaweb link above.
Generally, if you are inside a town or city, where broadcast towers are closer, but could be in several directions, a multi directional antenna is best. If you are in a more spread out area, and most of the towers are in the general direction of the nearest town, but further away, you probably want to go with a directional antenna.
Any time you install or adjust an antenna, perform a channel scan with your digital converter to see if you have improved reception and perhaps more channels available to you. It's also a good idea to scan periodically, as new digital television stations will crop up from time to time.
Confused about DTV, HDTV and all the other acronyms being thrown around? Read DTV, HDTV and HDTV Ready Television: What is the Difference? to learn more.