As far as I know, all UHF antennas are capable of receiving both Analog (NTSC) and Digital (ATSC) TV signals. HDTV signals are transmitted in the same UHF/VHF frequency range as analog.
Omni-Directional antennas are generally fairly low gain*, but if you are in a city and surrounded by unobstructed stations, they might be adequate.
Omnií antennas can generally be used at small to medium distances from the broadcaster, provided that there are no big reflective "ghost" producing structures near your location. In more remote
areas directional tv antennas should be used.
* Gain is a measure (in dB or dBi) of how much signal the antenna
You should be able to get specific advice from the manufacturer:
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