VOR and Doppler VOR
VOR operates in VHF (108-117.95 MHz) and provides magnetic bearing from the station (radial). The reference signal is frequency-modulated on a 30 Hz subcarrier, while the variable signal is amplitude-modulated by a rotating pattern (or electronically rotated in DVOR). The phase difference between reference and variable signals corresponds to the radial.
Candidates must understand radials, the CDI and OBS, TO/FROM indication, and the cone of confusion overhead the station. Know that VOR gives bearing information only, not distance (that is DME). Understand line-of-sight limitations and terrain/height effects. DVOR uses a wide-aperture antenna array and reverse rotation to reduce site errors, especially in mountainous terrain.
Traps: confusing radial with heading; forgetting that the CDI shows the selected course relative to the radial, not the aircraft heading; misinterpreting TO/FROM when overhead the station. Another trap is assuming VOR is immune to line-of-sight; it is not.
Worked example: OBS set to 270, CDI centred with TO indication means the aircraft is on the 090 radial, tracking outbound to the station on course 270.