Hi,
Don't know about the TV switching itself off when losing signal but regarding loss of signal in general check the following:
1. Ensure that you have selected the correct antenna input source in the TV. See p.38 of the User Guide.
2. If you are in a weak signal area try enabling the Low Noise Amplifier in the TV. See p.42 of the User Manual.
Here is a link to the User Manual for your TV.
http://downloadcenter.samsung.com/content/UM/201104/20110426134742093/BN68-01178A-00L02-110426.pdf