Stryped wrote:
"What do you mean it is a "short-circuit for low frequencies?"

A folded dipole, you might agree, is essentially a short-circuit for
d-c, also for 50-60- Hz a-c. At some high frequency, the folded dipole
is no longer a short-circuit, but offers considerable opposition to
current at its input terminals.. Its impedance is very high at its first
resonance where its overall length is 1/4-wave, and its impedance is
often about 300 ohms near its second resonance where its overall length
is about 1/2-wave.

At frequencies much lower than the resonant frequencies of the dipole
(most of the energy contained in a lightning stroke) the opposition of
the folded dipole is very low, or almost a short-circuit. This puts the
voltage at both terminals of the dipole to the same value and makes the
lightning applied to the transmission line a common-mode phenomenon.

Best regards, Richard Harrison, KB5WZI