ARRL Technician Practice Exam

What effect might allow radio signals to be heard despite obstructions between the transmitting and receiving stations?

1. Ground reflection

2. Knife-edge diffraction

3. Skywave propagation

4. Groundwave propagation

The correct answer is: Knife-edge diffraction

Knife-edge diffraction is a phenomenon that occurs when radio waves encounter an obstruction that has a sharp edge, such as a hill or a tall building. As these waves travel, they can bend around the edge of the obstruction, allowing the signal to be received on the other side. This bending can enable reception of the signal even when there are physical barriers present between the transmitter and receiver. In contrast, ground reflection refers to radio waves bouncing off the ground, which impacts signal propagation but does not specifically address obstructions. Skywave propagation involves signals bending back to the Earth's surface after bouncing off the ionosphere, which allows for long-distance communication but does not directly relate to overcoming physical barriers. Groundwave propagation pertains to signals that travel along the surface of the Earth, which can be affected by obstacles but does not exploit the edge diffraction phenomenon that specifically enables reception behind obstacles. Thus, knife-edge diffraction effectively allows signals to bypass obstructions entirely, making it the correct response to the question.