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A theoretical investigation has been made of the behavior of low-frequency automatic direction finder systems in aircraft on courses near and over the radio station. This behavior is explained in the light of electromagnetic field theory and the equipment characteristics. An equation is derived for the locus of all points where the ADF indicator will start to reverse due to sense-antenna signal phase shift with respect to the loop-antenna signal as the station axis is approached and passed. The equation contains six geometrical and electrical variables. The locus thus defined is similar to a cone or paraboloid, apex down, with the apex located at the radio station in most cases. The equation predicts that for a phase advance of the loop signal, relative to the sense-antenna signal, of slightly more than the 90Â° value applied by the basic ideal receiver, the apex of the cone-like space figure locus rises off the ground, and that it would be possible to fly under this apex, producing but one reversal of ADF indicator needle with this single reversal starting exactly over the radio station. The altitude of this apex is a relatively simple function of the variables. The results of the flight tests verifying the theoretical findings are given together with some requisites for the use of the findings to eliminate the effects due to improper sense antenna location, provided the location error is not too extreme.