Question T3C10
From subelement T3 - T3C
Which of the following bands may provide long-distance communications via the ionosphere’s F region during the peak of the sunspot cycle?
Why is this correct?
Only 6 and 10 meters can provide long-distance F region propagation during sunspot peaks. Higher sunspot activity increases ionization, allowing the F layer to reflect these higher HF frequencies. The other choices (23 cm, 70 cm, 1.25m) are UHF/microwave frequencies that pass through the ionosphere rather than being reflected by it, making ionospheric propagation impossible on these bands.
Memory tip
Remember the frequency pattern: higher sunspot activity enhances propagation on higher HF bands. The 10-meter band especially benefits from solar maximum conditions, while VHF/UHF frequencies remain line-of-sight regardless of solar activity. Think 'sunspots boost HF, but can't help VHF.'
Learn more
During solar maximum, the enhanced F region can support skip propagation on frequencies up to about 50 MHz, but this upper limit rarely extends into the VHF spectrum. The 6-meter band (50-54 MHz) sits right at this boundary, occasionally experiencing F layer skip, while 10 meters (28-29.7 MHz) reliably benefits. Understanding this frequency threshold helps explain why VHF and higher frequencies rely on line-of-sight propagation or specialized modes like meteor scatter and tropospheric ducting.
Think about it
Why do you think the ionosphere acts like a mirror for some frequencies but is transparent to others, and what determines this cutoff point?