Question 6A540

Direction finders (DFs) determine the bearing of a radio signal by sensing the direction of arrival of radio waves. **A) A received wave which has been bent by night effect:** "Night effect" refers to the propagation of radio waves via the ionosphere (skywave), particularly on MF and lower HF frequencies. At night, the ionosphere's D-layer dissipates, allowing signals to reach the F-layer, which refracts (bends) them back to Earth. If a DF receives a skywave signal that has been bent, its apparent direction of arrival will differ from the true great-circle bearing to the transmitting station, introducing significant error. **B) By any changing of the position of nearby metallic objects:** A DF antenna's performance and calibration are highly sensitive to its immediate electromagnetic environment. Nearby metallic objects, such as cranes, rigging, or other shipboard equipment, can reflect, absorb, or reradiate radio waves, distorting the incoming wavefront and causing deviation errors. If these objects change position *after* the DF has been properly calibrated, the local field around the antenna changes, rendering the original calibration inaccurate and introducing new bearing errors. Since both factors independently degrade DF accuracy, **C) Both A & B** is the correct answer.