In article ,
micky wrote:

Actually, I don't believe that is the purpose. I think I've read that on automotive antennas, the winding is simply a means to
reduce wind induced oscillation and noise.

Terry

Well, either one is a good reason and I'm glad to hear that there is a
reason.

It's triply important to me to reduce wind noise becausethe car is a
convertible, and I'd hear any noise it made, but I should mention that
this is my 8th convertible over 50 years and all the other ones had
standard antennas, usually in the front** but sometimes in the rear, and
I never heard any noise from them.

My fuzzy recollection is there's a bit of a tradeoff.

Without the helical spoiler, the antenna tends to generate a fairly
uniform "wake vortex" - an alternating flow of air behind the antenna,
which switches directions repeatedly. It'll tend to be "in sync" from
the top of the antenna to the bottom. If the frequency at which the
vortex is alternating happens to match the resonant frequency of the
antenna, the antenna can vibrate - a bit like a reed in an oboe. So,
at certain speeds, the antenna can buzz or hum.

The spoiler breaks up the airflow, making it more turbulent... and due
to the helical winding it makes the turbulence different at each point
up the antenna. As a result, you can get more "broadband" wind noise,
overall, but the turbulence won't excite a physical resonance in the
antenna anywhere near as well as a "regular" vortex would, and so the
antenna doesn't sound off at specific driving speeds.