Definition: | the ratio, generally expressed in decibels, of the electromagnetic field at a given point, to the field at another point on the same vertical line at a reference height
NOTE 1 – The height gain when expressed in decibels may have a negative value.
NOTE 2 – In the case of spherical diffraction by the Earth, the reference height generally used depends on the wavelength and on the ratio of the effective radius of the Earth to the actual Earth radius.
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