Reception of the US stations calls for a high gain UHF antenna such as the Antennas Direct 91XG or Winegard HD9095P. The signals are weak, so a preamp needs to be mounted at the antenna. The Antennas Direct CPA-19 would be my choice.
I would treat this as a separate antenna system. You can use an A/B switch or an HD STB/tuner.
http://forum.tvfool.com/showthread.php?t=2882