Which frequency will have the least attenuation for long-distance skip propagation?

For long-distance skip propagation, radio waves interact heavily with the ionosphere. The Maximum Usable Frequency (MUF) is the highest frequency that will be refracted back to Earth for a given path. Frequencies just below the MUF experience the least amount of absorption in the lower D layer of the ionosphere. As you drop lower in frequency, attenuation increases because the signals interact more heavily with the denser D region. Therefore, operating just below the MUF provides the strongest signals and least attenuation for long-distance communication. Frequencies above the MUF will escape into space rather than refracting back down, and those near the Lowest Usable Frequency (LUF) suffer maximum absorption.