Question T3A02
From subelement T3 - T3A
What is the effect of vegetation on UHF and microwave signals?
Why is this correct?
Vegetation absorbs UHF and microwave signals, reducing their strength and range. Trees, leaves, and heavy vegetation contain water, which absorbs these higher frequency signals. This is why antennas should be positioned above trees rather than within them. The other options are incorrect: knife-edge diffraction occurs around sharp obstacles, amplification would increase signal strength (which vegetation doesn't do), and polarization rotation changes signal orientation rather than absorbing energy.
Memory tip
Higher frequencies are more susceptible to absorption by materials containing water. Remember the frequency-dependent pattern: lower frequencies (HF) penetrate better, while higher frequencies (UHF/microwave) are more easily absorbed by obstacles like vegetation, rain, and buildings.
Learn more
This absorption effect is why cellular towers and Wi-Fi access points operating at UHF and microwave frequencies require careful site planning around trees and foliage. In amateur radio practice, VHF/UHF repeaters are typically installed on tall towers or mountaintops to clear vegetation. The same principle applies to microwave dish antennas, which need clear line-of-sight paths free from tree branches that would attenuate the signal path.
Think about it
Why do you think rain affects microwave signals more than HF signals, and how does this relate to vegetation's effect on UHF signals?