Way too general a question, my friend. There are just too many variables.
First, find out how far you are from the various transmitters - and which directions they are from you.
What might be in the way - buildings, hills, etc.? Are they all pretty much in the same direction from you? Are they more than a few miles away?
If they're close, a very simple cheap UHF antenna might be plenty. Grab one from the junk pile and see what happens. Note that you might hang your 811 when scanning OTA (no matter what antenna you might have) - don't worry, a reboot will fix it.