Why is 40m shown as "fair" while 20m is "poor" during the day. Shouldn't 7 MHz be getting absorbed by the D-layer more than 14 MHz?

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    $\begingroup$ All the things on there say "SAMPLE", so maybe they're not meant to be accurate? $\endgroup$ Commented Jul 16, 2020 at 3:18
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    $\begingroup$ @PhilFrost-W8II the banners in the center of the page say "SAMPLE" because they're examples, but the one on the left margin is from live data, and it does indeed say Fair for 80m-40m and Poor for 30m-20m. $\endgroup$
    – rclocher3
    Commented Jul 16, 2020 at 13:21
  • $\begingroup$ it might be good to include a screenshot in the question so it's clear exactly what's being asked $\endgroup$ Commented Jul 20, 2020 at 15:00
  • $\begingroup$ I wrote to the author of the table, asking him to join this discussion. $\endgroup$
    – Brian K1LI
    Commented Jul 20, 2020 at 20:29

2 Answers 2


The "HF Conditions" indicate the general, worldwide, relative propagation to be expected from the bands during your station's local day or night. I wrote to Paul, N0NBH, for his response to your question, which he agreed could be posted here:

Thanks for the email and question. I’ll do my best to answer it, but have to remind all using these that its not an exact science. It’s a prediction based on available data and a most likely scenario.

For HF predictions I use what’s available to predict the band conditions, given the obvious night vs day differences (given SFI does not differentiate, its 3 measurements in Canada). Also note I combined 80-40, 30-20, 17-15 and 12-10 to simplify, but that adds a level of error in the calculations. And of course the dayline, short and long hops, etc all are impacted but not in the predictions.

For HF predictions I use K-Index, Solar Wind, X-Ray, and Solar Flux reading for the predictions. In your example of 20 and 40, you are right the D-Layer impacts 80-40 meters the most, but the primary factor for D-Layer Ionization is X-Ray (Hard). And lately its been <A1.0 or non existent. And at Night the D-Layer goes away, hence the difference between day and night. That leaves only the F Layer at Night to impact 20m, but during the day both E and F2 impact that band. That’s where the SFI reading most impacts the band. I created the following slide to try and put all together. Hope it helps.

Again, I am open and honest about the predictions that are based on available data and a likely scenario BASED ON DATA ALONE, but I always recommend the only way to know for sure is to turn on the rig and call CQ!

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  • $\begingroup$ So, this is essentially a less concise more long winded version of the answer I gave. Again, the reason is primarily due to low SFI which usually affects >10 MHz. Lower ionization indicated by SFI will result in 20m passing through the E/F layers into space since it is below the MUF, but 80-40m will continue to refract. $\endgroup$
    – Macuser
    Commented Jul 23, 2020 at 13:09

The reason is because of SFI (solar flux index). Solar flux (10.7 cm) indicates the level of ionization, which affects HF propagation above 10 MHz. The solar flux does not affect 40m and below, since the MUF rarely drops below 10 MHz. This is why the lower bands are always open.


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