Try a separate antenna. If you have UHF terrestrial transmissions, use a UHF TV-antenna which should be available at a very cheap price from any TV supplier.
Don't forget to aim it towards the TV transmitter site and be prepared to experiment a bit with the pointing of the antenna to get the best image.
If you can disable the Automatic Gain Control (AGC) on the card it will assist you to peak the pointing, but even if you can't over-ride the AGC you should still be able to get a decent picture if it's pointed properly and the cable is correctly terminated...and make sure it is because I have seen new installations where the installers managed to put a short-circuit on the cable by a poor connector installed in haste. Snowy pictures and the transmitter was only 8 miles away!
Communal coaxial systems rely on distribution amplifiers and these are built to a price, not to a standard. Their performance is usually marginal, they get zero maintenance and pick up any local RF fields because their shielding is pathetic. Built by putting in enough parts to make sure it worked, then removing parts one by one until it stopped working, then putting that last part back in - keeps the costs down, likewise the quality. Masthead amplifiers, communal distribution amplifiers...more trouble than a bag of cats.