Which effect allows radio signals to travel beyond obstructions between transmitting and receiving stations?

Knife-edge diffraction is the effect that allows radio signals to bend around obstacles, enabling them to travel beyond physical obstructions between transmitting and receiving stations. This phenomenon occurs when a radio wave encounters an edge or a sharp obstacle, causing the wave to diffract or bend around that edge. As a result, the signal can still reach the receiver even if it would otherwise be blocked by a structure like a building or a hill.

This capability is particularly relevant in the field of amateur radio, where operators often need to communicate over distances that involve various natural and man-made obstacles. Knowing how and when diffraction occurs can help operators determine effective communication strategies in such environments.

The other concepts mentioned do not relate directly to the bending of radio waves around obstacles. Faraday rotation deals with the rotation of the plane of polarized light in a magnetic field, quantum tunneling pertains to the behavior of subatomic particles, and Doppler shift refers to the change in frequency or wavelength of waves in relation to an observer moving relative to the wave source. Each of these concepts plays a role in different aspects of physics but is not applicable to the specific scenario of overcoming obstructions in radio signal propagation.