At what frequencies is line-of-sight transmission the only method of propagation?

Line-of-sight transmission is typically the only method of propagation for frequencies above 30 MHz. This includes VHF (Very High Frequency), UHF (Ultra High Frequency), and SHF (Super High Frequency) bands. These frequencies are used for FM radio, television broadcasts, and cellular data, among other applications.

Here are the steps to understand why:

1. Understanding Propagation: Radio waves can reach the receiver through several paths. They can travel in a straight line (line-of-sight), reflect off the ionosphere (skywave propagation), or diffract around obstacles or the Earth's surface (groundwave propagation).

2. Frequency and Propagation: Lower frequencies (below 30 MHz) can use skywave and groundwave propagation because they can easily bend around obstacles and reflect off the ionosphere. However, higher frequencies do not have these properties.

3. Line-of-Sight Transmission: For frequencies above 30 MHz, the waves tend to travel in straight lines. They don't reflect off the ionosphere or diffract around obstacles well. Therefore, the transmitter and receiver generally need to be within 'line of sight' of each other.

4. Applications: This is why these frequencies are used for purposes where the transmitter and receiver are not too far apart and have a clear line of sight, such as television broadcasts and cellular data.