The correct answer is: A. Wired telephone systems
Free space propagation is the propagation of electromagnetic waves through a vacuum or through air. It is a common mode of propagation for radio waves, microwaves, and light.
Wired telephone systems do not undergo free space propagation because they are not using electromagnetic waves to transmit information. Instead, they use electrical signals that are carried through wires.
Wireless line of sight radio links, microwave line of sight radio links, and satellite communication systems all use electromagnetic waves to transmit information. Therefore, they all undergo free space propagation.
Here is a more detailed explanation of each option:
- A. Wired telephone systems use electrical signals that are carried through wires. These signals do not propagate through free space.
- B. Wireless line of sight radio links use radio waves to transmit information. Radio waves are a type of electromagnetic wave that can propagate through free space.
- C. Microwave line of sight radio links use microwaves to transmit information. Microwaves are a type of electromagnetic wave that can propagate through free space.
- D. Satellite communication systems use satellites to transmit information. Satellites are located in space, which is a vacuum. Therefore, the signals from satellite communication systems propagate through free space.