When using frequencies above 30 megahertz, you are normally limited to using which range?

When using frequencies above 30 megahertz, the communication is primarily limited to line of sight. This phenomenon occurs because higher frequencies, especially those in the VHF (very high frequency) and UHF (ultra high frequency) ranges, tend to travel in straight lines and are significantly affected by obstacles such as buildings, hills, and other terrain features.

In line of sight communication, the transmitting and receiving antennas need to be within each other's visual range to maintain a strong and clear signal. As frequencies increase, the propagation characteristics change, making it crucial for the antennas to be positioned without obstructions. This contrasts with lower frequencies, which can diffract around obstacles or reflect off surfaces, allowing for greater coverage areas and non-line-of-sight communication.

The other options present different methods or characteristics of communication that are less applicable at frequencies above 30 megahertz. For example, on-ground communications typically refer to lower frequency operations that can utilize ground wave propagation. Tropospheric scattering and reflective communications are also not as effective as line of sight for these higher frequencies, since they rely on different propagation principles that are more suitable for specific frequency bands or situations.