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The coaxial connection from your arial needs to be looped through the DVD recorder, i.e. the main aerial cable connects to its aerial input and a second cable connects from its aerial output to the TV’s aerial input. In this setup, it should pick up the same set of digital TV channels as what your TV tunes in directly. Like watching a Comcast digital channel on the TV, the comcast digital receiver needs to be tuned on the channel you intend recording.
However, if the DVD recorder still will not pick up anything (or just that channel 3) with the aerial connected directly to it, then your recorder likely has a faulty tuner.