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The three letters ?EDX? is a registered trademark of EDX Wireless, LLC. This name is a combination of the letter ?E? which represents the word ?equivalent? and ?DX? which is the radio Morse code abbreviation for ?distance?. Harry Anderson, the founder of EDX began writing radio design software programs for personal computers in the 1980?s. One of his first programs was named ?EDX?. The program predicted groundwave field strength at a distance for medium wave (550-1600KHz) transmitters. He liked the combination of the letters and decided to also name his company EDX. The EDX program integrated the ground conductivity tables found in the rules of the Federal Communications Commission. These tables contain families of curves for a variety of ground conductivities. When a radio signal travels across the earth it can encounter multiple areas (zones) of varied ground conductivity. In order to account for these transitions from one conductivity zone to another, the Equivalent Distance Method simply assumes that the signal at the end of a preceding zone determines the amplitude of the signal (from an imaginary transmitter) at the beginning of the new zone. This is described in the FCC rules (47CFR73.183(d)).