Topband: radials

[Lee K7TJR]

Anybody know what method the broadcast industry uses for determining the
physical length of the radials used for vertical bc band antennas?  I have
been doing some most interesting research on the surge impedance of ground
radials and am not familiar with their processes... One other question for
the masses is what is a quarter wave ground radial... Is that measured
physically or electrically? Thanks   Lee K7TJR   Sherwood OR.