I lost the antenna on mine sometime over the last couple of weeks. I went to Advance Auto and got one of thier universal replacement antennas. I went cheap and got a 13" rubber coated, flexible antenna for $8. It comes with various screw adapters and I found the one that worked. I trimmed about an inch off of the plastic base cover that came with the replacement antenna just to make it look good.
The results are great and signal reception is only a little worse than the original antenna. More importantly, the antenna does not stick out much from the back and won't interfere with a cover. Advance Auto did have a longer antenna for about $14 that would probably get better reception.
H-D Dyna Convertible