Question T3C05
From subelement T3 - T3C
Which of the following effects may allow radio signals to travel beyond obstructions between the transmitting and receiving stations?
Why is this correct?
Knife-edge diffraction allows radio signals to bend around sharp edges of obstacles like mountains or buildings, enabling communication beyond direct line-of-sight obstructions. The other options don't help signals travel around obstacles: Faraday rotation affects signal polarization in space, quantum tunneling is a physics phenomenon unrelated to radio propagation, and Doppler shift only changes frequency due to relative motion between transmitter and receiver.
Memory tip
Remember the pattern: when you see 'beyond obstructions' in questions, look for propagation mechanisms that actually bend or redirect signals around physical barriers. Knife-edge diffraction is the key VHF/UHF phenomenon for this, while the other choices relate to completely different signal effects.
Learn more
In practical amateur radio operation, knife-edge diffraction is especially valuable for VHF and UHF communications where line-of-sight is typically required. This phenomenon allows repeater access from locations with intervening terrain or buildings. Understanding diffraction helps explain why moving your antenna position slightly can sometimes restore a blocked signal path. The effect works best with sharp-edged obstructions and becomes more pronounced at higher frequencies within the VHF/UHF spectrum.
Think about it
Why do you think knife-edge diffraction works better around sharp obstacles like mountain ridges than around smooth, rounded hills?