What is the purpose of the spiral ribs seen on modern car radio antennas?
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.
my understanding was that the antenna is mechanically supported by a flexible internal fiberglass rod wound with the antenna wire.
if the wire starts in tight turns which spread apart the further along the antenna, then it is to cover a wider frequency range.
with the typical spacing on antenna wire turns, i'd suggest that the mechanical benefits are minimal.
you are right about the spiral structure of cords/pipes though, which are made to be quite flexible. the key is that it provides no real resistance to bending, but it does provide resistance to crimping and prevents axial splitting in pipes. If you were to get an aerial to the crimping stage the fiberglass rod would have shattered already.
The spiral shape increases the bandwidth of the antenna, at the expense of gain. It also mechanically shortens the antenna, versus if the same length of antenna were kept straight.
Would the spiral give the antenna not just an omni directional pattern in the horizontal plane but a vertical component? Thinking of transmission here.
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.
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.
I have yet to see one built to this purpose.
All I have seen are as berkeman describes.
Do you have a specific example?
[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!"
"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."
"The radio antenna has a spiral-ground groove to prevent wind whistles."
However, as noted previously it does decrease the efficiency somewhat.
Separate names with a comma.