Question T3C02
From subelement T3 - T3C
What is a characteristic of HF communication compared with communications on VHF and higher frequencies?
Why is this correct?
HF signals (3-30 MHz) can refract off the ionosphere and return to Earth hundreds or thousands of miles away, enabling worldwide communications without infrastructure. VHF and UHF signals are primarily line-of-sight and rarely propagate via the ionosphere, limiting them to local/regional communications. The other options are incorrect: HF antennas are actually larger due to longer wavelengths, HF doesn't inherently accommodate wider bandwidths, and HF typically has more atmospheric noise than VHF/UHF.
Memory tip
Remember the frequency hierarchy: as frequency increases, propagation distance decreases but line-of-sight reliability increases. HF bounces globally, VHF works regionally, UHF excels locally. This trade-off between distance and predictability drives band selection for different communication needs.
Learn more
The ionosphere acts like a selective mirror—it reflects HF frequencies back to Earth but allows VHF and UHF signals to pass through into space. This is why international broadcasters use HF bands, while repeater systems use VHF/UHF for reliable local coverage. Solar activity affects this ionospheric 'mirror,' making HF propagation variable but enabling amateur contacts across continents during favorable conditions.
Think about it
Why do you think emergency communications networks often combine both HF and VHF capabilities rather than relying on just one frequency range?