Mike,
You probably need a LNA if you are comparing relatively weak TV signals. It would be a good idea to use a 75 ohm input LNA, then the mismatch at the output to the 50 ohm tinysa would be negligible. You also need to use the pilot signal (if you can isolate it) to compare as the rest of the signal bandwidth varies with time.
www.arrl.org/files/file/Technology/TV_Channels/8_Bit_VSB.pdf
Gary
W9TD
Gary W9TD