When I had my Yakima ski racks on the car one day, I noticed, via the car's shadow, that the stock antenna was flapping around, presumably due to the turbulence induced by the racks. The stock antenna is also tall enough that it can get in the way of things you might put on top of the car (in particular, a car-top box). Although there are short antennas available, when I did this there was none in black. So I decided to just make myself one. Note that...

Of course, you can always get Audi to sell you another full-sized antenna, and have both. I bet ClairParts has them, or can get them post-haste.

Anyway, it turns out that the (~15 1/2") antenna with the spiral up the outside on various Audi models is really a just fiberglass rod with a string wound around it (that's the spiral) for wind-noise reduction, I'm told, and a copper wire running along it. The picture below shows this via a view of the antenna with its weather protection removed. Note how the wire is wound tightly for about 1/4" and then runs straight up, then is wound tightly again, and finally runs straight to the top of the rod, where the wire just ends. I chose to do my surgery just above the first wrap. You can see this point without removing the weather coating as the place where the rod appears to get thinner and the single wire (with the string) sort of "appears."

For the surgery, I first cut the copper wire with the point of small side-cutters, and then I used a Dremel tool with one of those cutting disks on the rod. Then I dunked the exposed tip of the (much) shorter antenna into that black rubberized plastic goo made for coating handles of tools. Four dunk-and-set cycles later, it looked like the picture below. In terms of radio reception, it works much better than the 6mm machine screw I had been using and, as far as I can tell, nearly as well as the original — for the radio, at least. (I don't have the navigation unit or a car phone, so I don't know about how it affects those.)

If I were going to do it again, I'd just cut the thing off without removing the weather coating (I did this just to see what was in there, in case the wire ran back down the center or something). That way, I wouldn't have to deal with the fiberglass rod, which is that bad kind of 'glass that flakes off and gets in your skin. Also, it pays to practice on, say, a pencil with the goo, so you can make the tip shaped the way you want it.