Question T3A08
From subelement T3 - T3A
What is a likely cause of irregular fading of signals propagated by the ionosphere?
Why is this correct?
Irregular fading occurs because ionospheric signals take multiple paths to reach the receiver - some signals travel via different ionospheric layers or reflection points. Since these paths have different lengths, signals arrive at slightly different times and can combine either in phase (strengthening) or out of phase (canceling). The ionosphere constantly changes, making this combining effect random and unpredictable, causing the characteristic fading. Faraday rotation causes polarization changes, not fading. Thunderstorms and intermodulation are different interference sources.
Memory tip
Look for 'multipath' concepts across propagation questions - whether VHF mobile 'picket fencing' or HF ionospheric fading, the pattern is the same: multiple signal paths creating random reinforcement or cancellation. The key insight is that different path lengths create timing differences that cause signal strength variations.
Learn more
In practical HF operation, this fading is why you might hear a distant station's signal slowly rise and fall over seconds or minutes, especially during changing ionospheric conditions. The multiple reflection paths through different ionospheric layers create a constantly shifting interference pattern. Understanding this helps explain why antenna polarization is less critical on HF bands - the random path combinations scramble polarization anyway, making the signal elliptically polarized rather than purely horizontal or vertical.
Think about it
Why do you think ionospheric fading tends to be slower and more gradual compared to the rapid 'picket fencing' effect experienced with VHF mobile signals?