Explanation: ADF systems receive signals from Non-Directional Beacons (NDBs) which transmit in the Low Frequency (LF) and Medium Frequency (MF) bands (typically 190-1700 kHz). A key characteristic of these frequencies is ground wave propagation, where radio waves follow the curvature of the Earth. This allows NDB signals to be received far beyond the optical line-of-sight, often hundreds of miles, enabling over-the-horizon navigation. Skywave propagation, especially at night, can extend this range even further as signals reflect off the ionosphere. Therefore, statement D is correct. Option A is incorrect because ADF is a receiver, not a transmitter. Furthermore, NDB transmissions are specifically chosen for their ground wave properties, which are not primarily line-of-sight. Option B is incorrect as a single omnidirectional antenna cannot provide directional information; ADF typically uses a combination of directional (loop) and omnidirectional (sense) antennas to determine bearing. Option C is incorrect because aircraft ADF systems are designed to tune into and utilize frequencies within the standard commercial AM broadcast band (550 to 1700 kHz) for navigation, in addition to dedicated NDB frequencies.