When using an HF radio and considering propagation, how must HF frequencies be selected?

• A. Lower for longer distances, higher for shorter distances
• B. Higher for longer distances, lower for shorter distances
• C. Lower for day, higher for night
• D. Higher for day, lower for night

Answer: (D)

HF propagation relies on how the ionosphere refracts radio waves. For a given path, the usable frequency is limited by the MUF (maximum usable frequency) and the LUF (lowest usable frequency). During daylight, solar radiation increases ionization, especially in the F2 layer, which raises the MUF. That means you can use higher frequencies to reach long distances without them being reflected too low or not returning. The D layer, present in daylight, also absorbs lower frequencies, so relying on very low frequencies for long-range daytime paths is discouraged.

At night, the D layer largely disappears and overall ionization decreases, so the MUF falls. Lower frequencies become usable for the same long-distance paths, while very high frequencies tend to be less reliable at night.

So, frequencies are chosen higher during the day and lower at night to stay within the MUF for the intended path.