Question T3B10
From subelement T3 - T3B
What frequency range is referred to as HF?
Why is this correct?
HF (High Frequency) is defined as 3 to 30 MHz. Option A (300-3000 MHz) is UHF, option B (30-300 MHz) is VHF, and option D (300-3000 kHz) equals 0.3-3 MHz, which is below HF. The frequency spectrum is systematically divided: HF enables long-distance ionospheric propagation, VHF provides regional communications, and UHF supports local high-bandwidth operations.
Memory tip
Remember the ascending pattern: each range increases by a factor of 10. HF starts at 3 MHz, VHF at 30 MHz, UHF at 300 MHz. The 'H' in HF doesn't mean highest—it's historically 'high' compared to earlier radio frequencies. This systematic organization helps you quickly identify propagation characteristics.
Learn more
HF's 3-30 MHz range corresponds to wavelengths of 100-10 meters, explaining why HF bands are called '80 meters,' '40 meters,' etc. These frequencies interact optimally with the ionosphere's F layer, located 90-400 miles above Earth. During high sunspot activity, increased ionization enhances HF propagation on higher frequency HF bands like 10 meters, enabling worldwide communication without repeaters or satellites.
Think about it
Why do you think HF frequencies (3-30 MHz) are particularly effective for long-distance communication compared to the higher VHF and UHF ranges?