Spiral antenna on cars

[bobilgner]

Some cars have a whip antenna with a spiral of wire attached to the stalk of the antenna, as per the attached picture.

I have read (on various groups) that the spiral is there to reduce wind resistance and noise, alternatively that it covers AM reception as opposed to the content of the stalk covering FM reception.

Does anyone have a good reference to the correct answer. What is this spiral around the whip antenna?

 

[chuckey]

The spiral will aid wind load shedding and hence flexing of the main element. When you drive along, the aerial gets a high pressure on its front and low pressure on its rear, because the main aerial is round this is not a stable state and the pressure areas move from side to side, imparting a sideways force on the aerial, causing it to oscillate sideways. The external spiral causes the air flow to be directed and hence stable.
The wind load shedding phenomena caused a 1000 feet tall Emley Moor TV mast to fall down in 1968. The mast went into oscillations.
Frank

 

[bobilgner]

The spiral will aid wind load shedding and hence flexing of the main element. When you drive along, the aerial gets a high pressure on its front and low pressure on its rear, because the main aerial is round this is not a stable state and the pressure areas move from side to side, imparting a sideways force on the aerial, causing it to oscillate sideways. The external spiral causes the air flow to be directed and hence stable.
The wind load shedding phenomena caused a 1000 feet tall Emley Moor TV mast to fall down in 1968. The mast went into oscillations.
Frank

That does make sense. I have seen the wild oscillations on the stubby antenna used by the Le Mans Porsches. The rapid oscillations resulted in a blur where the antenna was positioned. I was wondering whether this rapid oscillation would have an effect on the tx or rx of the telemetry. Probably not.