Roy Lewallen, W7EL wrote:
"The attenuation of the ground wave is the same as the free space
attenuation?"

No, it is greater. The attenuation of the ground wave is more rapid
because the soil has resistance and imposes a loss on the passing wave.

In free space, there is no ground loss and the signal loss is from the
spreading of the signal which reduces its density. It declines 6 dB (its
power is quartered) every time the distance from the transmitter
doubles, that is its volts per meter or signal strength is cut in half.

Ground waves decrease more rapidly with distance from the transmitter
due to imperfect conductivity in the earth`s surface. Loss goes up with
the operating frequency and with higher resistivity in the soil.

The FCC`s ground-wave field intensity charts cover limited frequency
ranges and each curve is for a specific soil conductivity.

Best regards, Richard Harrison, KB5WZI