🔍 Key Concepts

• ADF (Automatic Direction Finder) always points to the apparent direction of the NDB (Non Directional Beacon) signal, not necessarily the true geographic location if the signal is distorted • How radio waves in the low/medium frequency band behave differently in daytime versus nighttime, especially with the ionosphere changing after sunset • The difference between ground waves (following Earth’s surface) and sky waves (bouncing off the ionosphere) and how that can affect bearing accuracy at night

💭 Think About

• Think about what physically changes in the atmosphere after sunset that could affect how radio signals travel from the NDB to the aircraft • Consider whether the ADF system itself starts transmitting anything, or whether it only receives signals from the NDB • Ask yourself: which option describes an effect that could cause the ADF pointer to give an unreliable or wandering bearing to the station at night?

✅ Before You Answer

• Verify that NDBs normally operate 24 hours a day, not just in daylight • Confirm that an ADF in the aircraft is a receiver, not a transmitter, so any explanation involving ADF ‘transmissions’ should be treated carefully • Check which option correctly involves signal propagation and reflections at night, and could plausibly cause the ADF to point in a misleading direction