avoid the old style 'rabbit ears' aerial or the fm rod type, as found on radios- they're for VHF not UHF and are utter cr@p.
Try investing in the type which look more like a roof aerial, or the ones that have an amplifier built in and look like a mini satellite dish. Then put it as close to the window as possible.if your TV is not near the window, that shouldn't be a problem if the aerial has a bult in amplifier - then you can buy an extra couple of metres of RF cable to extend the connection lead to run to the TV whilst keeping the aerial near the window.
as others have said, if the rom is on a low floor of a block and is surrounded by othe rbuildings or hills, forget it! Also, if analogue reception is bad then digital almost certainly won't work - with analogue you get a partial , snowy pic but with digtial it's usually all perfect or nothing.