Which characteristic is true for Line of Sight radio systems?

Line of Sight (LOS) radio systems primarily rely on a direct line between the transmitting and receiving antennas. This direct connection is crucial for effective communication, which makes atmospheric conditions a significant factor in their performance. However, while options that mention extensive equipment or multiple obstructions suggest complexities in set-up, the most relevant characteristic of LOS systems is that they are indeed affected by atmospheric conditions such as fog, rain, or even snow, which can scatter or absorb radio signals.

The correct answer highlights the characteristic that LOS systems, being highly dependent on straight paths for signal propagation, can face significant challenges when atmospheric conditions are less than ideal. This means that while they operate optimally in clear weather, adverse atmospheric factors can greatly hinder their effectiveness. Other options address the power needs for longer distances, equipment requirements, and the ability to navigate through obstructions, which do not align as closely with the established knowledge regarding LOS systems.