The 2020-2024 Extra License question pool asks a deep question about DX techniques requiring some good knowledge of propagation science:
E3A06: What might help to restore contact when DX signals become too weak to copy across an entire HF band a few hours after sunset?
A. Switch to a higher frequency HF band
B. Switch to a lower frequency HF band
C. Wait 90 minutes or so for the signal degradation to pass
D. Wait 24 hours before attempting another communication on the band
This question about DX operations picks at multiple domains of ham radio knowledge, requiring some savvy conceptual integration to select the correct response. It blends general knowledge of the HF bands, matters of RF propagation characteristics among the several HF bands, matters of ionospheric dynamics, and matters of practical operational practices. Let’s see if we can efficiently wrap all this together in succinct consideration.
We want to restore a contact with a station in another country, probably quite far away. Barring that IRLP or Echolink cheating, this means the continued use of an HF band since VHF and UHF rarely travel such distances. However, this does not narrow the response options for us.
The signal became weak across an entire HF band with the loss of sunlight. Aha! Here is a good hint. Our understanding of RF propagation in the HF bands reminds us that the higher HF bands, especially those above the 20-meter band (10m – 17m), often require a densely ionized ionosphere to be bent sufficiently back to earth for communication over-the-horizon. When the sun is blaring and charging up the ionosphere these bands often work amazingly well, but at night time they tend to close, or provide insufficient bending of signals for reliable communications. So, the assumption in this question is that we were initially using one of the higher HF bands that closed with the setting sun. Further, the sun isn’t going to shine on us again in 90 minutes, and in 24 hours it’s going to be dark yet again. THAT I can tell you! Nix responses C and D.
Drawing on our knowledge of the ionosphere, we recall that during the daylight hours that pesky D-layer resides relatively low in the sky and it readily absorbs HF of the lower frequencies, such as those comprising the bands 40-meters, 60-meters, and 80-meters. Signals in these low HF bands cannot easily penetrate the D-layer and skip from higher ionospheric layers during the daylight hours. However, the D-layer dissipates at night and liberates those lower HF bands, allowing their signals to fly high into the sky and be bent by the E-layer and combined F-layers. In other words, the lower HF bands open at night for long distance, over-the-horizon communication. Now we’re getting to the solution. .
Using the combined knowledge above you can see that it will not be fruitful to switch to a higher frequency band after dark, as the bending power of the ionosphere reduces with higher frequency. If your band crapped out you’re not going to have any better luck with skip propagation at a higher frequency. Nix response A.
We are left with response B, and it makes perfect sense based on our replete knowledge of these various factors. We can move to a lower frequency HF band – perhaps one that has opened with the night as the higher bands have closed – and try to re-establish our DX contact. Additionally, the 20-meter band often has adequate performance around the clock, day or night, so it is a viable “lower HF band” candidate if you were initially operating on, say, 15-meters.
Lastly we will mention gray-line propagation. Along the twilight boundary of the earth, where day is transitioning to night and vice-versa, the lower HF bands (below 20-meters) can be very effective along the line of the boundary – along the gray line. Along the gray line the lower altitude D-layer is rapidly disappearing with diminishing sunlight, and its lower altitude causes darkness to consume it before the high altitude F-layers are affected. The F-layers remain in sunlight for a greater duration near the twilight boundary and they remain very effective in bending signals over the horizon. The result is enhanced lower HF band communication along the gray line, generally north-south and varying in specific angle with the seasonal tilt of the earth.
The answer to 2020-2024 Extra Class question E2C12, “What might help to restore contact when DX signals become too weak to copy across an entire HF band a few hours after sunset?” is “B. Switch to a lower frequency HF band.”
-- Stu WØSTU