Skip Signal Fade (T3A08)
The 2022-2026 Technician License question pool notes that signals propagated through the ionosphere, or "skip" propagation, often fade in and out in strength at the receiving station. Why is this?
T3A08: What is a likely cause of irregular fading of signals propagated by the ionosphere?
A. Frequency shift due to Faraday rotation
B. Interference from thunderstorms
C. Intermodulation distortion
D. Random combining of signals arriving via different paths
With long distance propagation using ionospheric skip received signals will often fade in and out, being stronger and weaker over the course of a few seconds. What’s the cause of this irregular fading?
A signal arriving at your station antenna may take more than one path to get from its originating station. Signals taking off at slightly different angles from the transmitting station may encounter different densities of ions in the ionosphere and be bent back to the earth along slightly different paths, but each still arriving at the receiving antenna. Signals may also be reflected from the earth’s irregular surface at different angles, both vertically and laterally, resulting again in significant path differences taken between transmitting station and receiving station. But, how does that cause signal fading?
Let’s consider the simplest case of two signal waveforms arriving at a receiving antenna. In the figures 1-4 one signal is indicated in red and the other blue.
Signal Combination #1: In the first case, the two signals took slightly different skip paths to the receiving antenna, but they arrive very nearly in phase with one another. That is, the waveforms’ electric fields are aligned, or in step, with the positive voltage halves of the cycles and the negative voltage halves of the cycles reinforcing one another. The electric field voltages are summed by the receiving antenna, so the well-aligned waves produce a combined signal on the antenna that is about twice as strong as either signal alone. This is depicted with the upper purple waveform in which the signal amplitude is shown to be about twice that of the individual waves’ amplitudes. This high amplitude produces a relatively strong signal at the receiving antenna.
Signal Combination #2: In the second graphic the waves have again taken different paths to the antenna, but the path lengths worked out so that the two waveforms are almost exactly out of phase with one another. When the red signal has a peak positive voltage the blue signal has a peak negative voltage. When these two signals are summed at the antenna the positive voltages and negative voltages sum to zero volts, canceling one another out! (A very low amplitude signal is depicted as the summation in purple, assuming the alignment of the two is not quite exactly opposed.)
Signal Combination #3: Of course, other relative relationships of the received signals are possible between the two extremes of perfect alignment and perfect misalignment. The third scenario depicts the two waves somewhat out of phase, but not perfectly opposed. Again, the electric field voltages depicted as amplitude will sum for each position on the waveforms, in this case producing an intermediate amplitude summation signal. You can imagine that there are an infinite number of combination possibilities resulting in variable signal strengths when summed as the receiving antenna’s induced voltages.
Signal Combination #4: Another factor that comes into play is signal polarization, as illustrated in the fourth graphic. Not only will signals travel by different path lengths and have variable phase relationships, but the orientation of the electric field oscillations gets scrambled during skip propagation, too. Signals arriving at the receiving antenna with a polarization identical to the antenna’s polarization will produce relatively strong signals (red waveform) as compared to signals arriving with unaligned polarization (blue waveform).
So, consider that a receiving antenna may combine two, three, dozens, or thousands of signals arriving via different path lengths, and the phase relationships of all those different waves will be combined into some signal strength at the antenna. Consider further that the polarization of those signals will all be somewhat different, contributing yet more variability into the antenna’s summation function. And finally, consider that as ionospheric conditions of density and ion cloud location shift over time, and even as items on the earth from which signals may reflect move along the surface (ground vehicles, for instance), both the arrival phase relationships and the polarizations will change and shift from moment to moment! The result at the receiving antenna is a moment-to-moment variation in the summation signal strength that produces an irregular fading of signals.
The answer to Technician Class question T3A08, “What is a likely cause of irregular fading of signals propagated by the ionosphere?” is “D. Random combining of signals arriving via different paths.”
-- Stu WØSTU