Before you choose a replacement antenna, make sure that you actually need one in the first place. To that end, you might want to start off by checking the cable that connects your antenna to your head unit. If it isn’t firmly seated in the head unit, or it is corroded or damaged in any other way, then you should address that issue first.
Another easy test is to tune into a radio station and then attempt to wiggle your antenna mast. If you find that the mast wiggles around a lot, and that your radio reception is affected, then you might be able to simply tighten the mast or the assembly.
If the mast is broken or you find rust, corrosion, or other damage, then you’ll have to decide what kind of replacement antenna to purchase. If, on the other hand, there isn’t actually anything wrong with the antenna, you can check out these other ways to improve your car radio reception.