Antenna lengths

[Ghoste]

How are antenna lengths determined for automotive applications?  We went from the telescoping masts on our cars, to fixed masts shortly after and now they have the stubby little fin type ones, I thought there was a specific length needed?

[John_Kunkel]

The perfect FM antenna length depends on the actual frequency of the station, I believe they chose 29-30" as a good mid-band length to cover the entire frequency band.

I imagine the current "stubby" antennas incorporate some sort of device to simulate a longer length. 

[Ghoste]

Are AM stations affected the same way?  And I will guess not here since fixed mast coincides with the rise of FM as the preferred way.

[ODZKing]

If you are really that curious:
http://www.ehow.com/how_5990599_calculate-fm-antenna-length.html

[b5blue]

  AM or amplitude modulated could benefit from a varying mast length as the actual length of the antenna could help receive (or tune out unwanted) reception. FM or frequency modulated is much less sensitive to tweaking mast length. Your tuner in the radio is actually tweaking the mast length in AM. As music is frequency based FM naturally works much better.
  Think of AM as transmitting bigger and smaller waves and FM as transmitting more and less waves. So by farting around with the telescoping mast you can find a length that give you the best reception for your favorite AM station. As FM became 1st choice a solid mast became a cheaper more practical alternative.   
   Stubby antennas have windings that simulate a full length like a pole mast would be.   
       

[Ghoste]

Okay, that makes sense, thanks.