In HF radio communications, what percentage of the maximum usable frequency (MUF) is optimal for transmission?

The optimal frequency for HF radio communications is generally considered to be around 85% of the maximum usable frequency (MUF). This percentage is based on the principle that while the MUF represents the maximum frequency at which radio waves can successfully propagate between two points, the optimal operational frequency is slightly lower to ensure reliable communication.

Using a frequency at 85% of the MUF provides a buffer that accounts for fluctuations in ionospheric conditions and improves the chances of maintaining a strong, stable signal. Operating at this percentage also mitigates risks associated with potential fades that can occur at frequencies closer to the MUF, which may be subject to sudden changes in the ionosphere that can disrupt communication.

In contrast, the other percentages given do not optimize for reliability in communications. While lower percentages like 75% and 60% may be used in some contexts, they may not take full advantage of the ionosphere's reflective properties for longer ranges. Operating right at the MUF (100%) can lead to unstable signal quality due to the rapidly changing conditions, which is why it's not recommended. Thus, 85% strikes the right balance for effective HF communication.