What type of signals are typically constrained above 30 MHz?

The chosen answer indicates that atmospheric refracted signals are typically constrained above 30 MHz. This is correct because signals in the VHF (Very High Frequency) and above frequency ranges are more likely to be subject to line-of-sight propagation, limiting their ability to refract through the atmosphere. As frequencies increase, the ability for signals to bend around the curvature of the Earth diminishes, leading to challenges in long-distance communication using these higher frequencies.

In contrast, ground wave signals, which operate effectively below 30 MHz, can follow the contours of the Earth and travel further distances due to their propagation characteristics in the lower frequency spectrum. Satellite signals, while they may operate at various frequencies, are not inherently constrained above this threshold in the same way that refracted atmospheric signals are. Underwater acoustic signals typically operate at much lower frequencies than 30 MHz, as sound propagates better in water at lower frequencies, making them irrelevant to the discussion of constraints at this frequency range.