Automobile antennas-radial ribs?

What is the purpose of the spiral ribs seen on modern car radio antennas?

 

The rings and the antenna itself are an integral metal piece...e.g. the rings are not electrically isolated so it couldn't be to increase antenna length. It took awhile but I did some further research on the subject and apparently the rings are there to cut noise and keep the antenna from vibrating at certain speeds. The rings set up little ebbs in the air current which dampen movement at the natural frequencies of the antenna. The speeds and frequencies at which this damping occurs is determined by the spacing of the rings. I was quite suprised to read that much thought went into a car antenna.

 

From http://www.theautochannel.com/mania/industry/Transcripts/2000/118.htm
[Why'd They Do That? - Car Antennas]
John McElroy, American Driver
"And, finally tonight, in a segment we like to call “Why’d they do that, I’m going to attempt to explain why all car antennas are not alike. While many antennas are smooth, have you ever noticed how some others look like they have a wire wrapped around them, or have a spiral groove in them? You know what that’s for? It’s to cut down on wind noise and vibration. That spiral shape actually helps the antenna slice through the wind more easily, so it’s quieter. When an antenna is mounted on the front fender, as the wind comes off it, that noise is actually directed right at the driver. These spiral-type antennas direct the noise off to the side of the car. And you know how automotive engineers test these antennas to see how quiet they are? They go outside and swish them around like swords!"

From http://www.topspeed.com/cars/ford/2007-ford-edge-ar32661.html
"Even the radio antenna was scrutinized. By modifying the pitch and height of the spiral by tenths of a millimeter, wind noise was reduced by as much as two decibels."

From http://www.autoworld.com/cutlass.htm
"The radio antenna has a spiral-ground groove to prevent wind whistles."