What is the purpose of the spiral ribs seen on modern car radio antennas?
my understanding was that the antenna is mechanically supported by a flexible internal fiberglass rod wound with the antenna wire.I think its purpose is mechanical (rather than electronic). It provides strength to the antenna while allowing it to remain flexible, spreading any shear over the length of the antenna.
You see the same spiral structure on steam iron power cords, commercial kitchen sink hoses, etc., anywhere where you need flexibility spread evenly over a length so that you do not get a crimp.
That sounds plausible. Certainly the horizontal gain falls, and if the radiation resistance remains real, then the energy has to be going some other direction.Would the spiral give the antenna not just an omni directional pattern in the horizontal plane but a vertical component? Thinking of transmission here.
I have yet to see one built to this purpose.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.htmI have yet to see one built to this purpose.
All I have seen are as berkeman describes.
Do you have a specific example?