you will need a very wideband amplifier. not all cable channels correspond to the standard off-air channel numbering scheme, and many cable systems run the gamut from 6Mhz to over 700Mhz you would be limited most likely to whatever range of frequencies are near the resonant frequency of any antenna you would have connected. also, your local/national agency in charge of radio spectrum usage might not take kindly to your retransmission of wideband video signals. public officials have no sense of humor, it's part of their job description.