Most of the stations, in cities like Chicago, Philadelphia, New York and Dallas, moved their digital broadcasts from the UHF band to the VHF band on June 12, when they turned off their analog broadcasts.
The VHF band was previously used only for analog broadcasts, and was largely untried as a carrier for digital broadcasts. While UHF can be received well with small indoor antennas, the best VHF antennas are large rooftop units. Many antennas sold as "digital" ones in the last few years receive UHF only.
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Source: Physorg