I have always heard that "13 feet, 6 inches" is a standard for the maximum
height of an antenna mounted on a vehicle.

However, I cannot find a credible source on the web verifying this "fact".
NHTSA, DOT, don't seem to have such an antenna regulation on their
respective web sites. I am not the most skillful user of Google, but my
searches have sure not turned up a reference to this magic number on any
authoritative site. Perhaps there is no national standard, only 50 separate
state standards, and the state with the lowest height limit would then
become the 'national standard'??