"Frank" wrote:
What is the best type of "wire" for making a homemade
car antenna to pick up FM radio. Is it copper? Aluminium?

Traditional is steel (i.e. coat-hanger wire :-)

In reality, it makes no difference except mechanically.

What thickness or size.

It is a mechanical issue. What makes it self-supporting?
Electrically it is of no significance.

Lenght?

Well, the middle of the FM broadcast band is ~98MHz
and 1/4 wavelength is ~28.5 inches.
http://www.csgnetwork.com/antennagen...eqlencalc.html