Dr. Slick wrote:
How do they measure a "surge impedance"?

For a transmission line, it is the same as the characteristic
impedance. For a traveling wave antenna, it is the same as
the feedpoint impedance. So for a center-fed standing wave
antenna, like a dipole, you could terminate the ends of the antenna
to eliminate reflections and then measure the feedpoint impedance.

Standing wave antennas are like transmission lines with standing
waves. The impedance at any point is the (forward voltage wave
plus the reflected voltage wave) divided by the (forward current
wave plus the reflected current wave). The feedpoint impedance
of traveling wave antennas is usually about 600-800 ohms according
to The ARRL Antenna Book.
--
73, Cecil, W5DXP