What does a spiral do on a car radio antenna?

Answer 1

Wind noise reduction and stability.

When engineers first designed tall circular towers (such as industrial boiler flues for factories), they ran into a problem - wind currents whipping round the towers would create strange forces that shook the tower from side to side, undermining its structural integrity. They discovered that they could solve the problem by adding a spiral round the top of the tower. It breaks up the wind currents and balances them out.

The same problem and solution has been found in some steel suspension wires on bridges.

So, the spiral on a car antenna is there solely to reduce wind noise and stop the antenna vibrating. It has absolutely no effect on the antenna's ability to receive radio frequency signals.