The claim you are testing
The propagation lessons assert that the D region absorbs the lower HF bands during daylight and vanishes at night, and that the MUF falls after sunset. Both are testable in one day, with one receiver, using signals that transmit continuously.
The observation is worth more than the reading, because propagation is the one part of the syllabus where the textbook answer and your own experience will eventually disagree — and you need to know which to trust.
The instrument: WWV and WWVH
15 CFR 200.107 is the authority for what NIST actually transmits, and it is worth being precise because the details make the experiment work:
- WWV, Fort Collins, Colorado — 2.5, 5, 10, 15, and 20 MHz.
- WWVH, Kauai, Hawaii — 2.5, 5, 10, and 15 MHz.
Continuous, at known powers, from known locations. That is a purpose-built propagation experiment: five frequencies, two well-separated paths, always on.
You will see 25 MHz mentioned. It is experimental — not listed in the regulation, switched off and on over the years — so treat it as a bonus rather than part of the protocol.
Distinguishing the two stations matters, because on 5, 10, and 15 MHz you may be hearing either or both. §200.107(b) gives you the discriminator for free: WWVH announces the time at 15 seconds before the minute with a female voice; WWV at 7.5 seconds before, with a male voice. Two different paths, resolvable by ear, on one frequency.
Tune each frequency in turn and note the signal-to-noise ratio and which station you are hearing. You are measuring path versus frequency, and if you are in the continental US the Colorado and Hawaii paths will not behave alike.
The protocol
Do this on one day. It takes five minutes at each of four times.
| When | What to expect |
|---|---|
| Local noon | 15 and 20 MHz strong; 2.5 and 5 MHz weak or absent |
| Sunset | the low bands rising as the D region decays |
| Two hours after dark | 5 MHz strong, 20 MHz weak or gone |
| Local midnight | 2.5 and 5 MHz best; the high bands closed |
Record the SNR for each frequency at each time. Four columns, five rows.
What the table shows you is the D region turning on and off. It absorbs in proportion to , so it attacks 2.5 MHz forty times harder than 15 MHz. During the day the low bands are unusable over distance; within an hour of sunset they open.
And the high bands do the opposite, for a different reason: with no sunlight the F region recombines, ionisation falls, and the MUF drops until 20 MHz will no longer refract back.
The grey line
Around sunrise and sunset there is a window — often twenty minutes, sometimes an hour — when the low bands are dramatically better than either the day or night figure.
The D region decays much faster than the F region. For a while you have F region ionisation still supporting long paths with no D region left to absorb them. That is the grey-line enhancement, and it is the strongest single-day propagation effect you can observe.
Watch for it deliberately. Tune 5 MHz twenty minutes before your local sunrise and listen for ten minutes past it.
Scaling up with WSPR
One receiver sees one path. The WSPR network sees thousands.
- Open
wsprnet.organd query the last hour on 20 metres. - Repeat for 40 and 80 metres.
- Look at where the reported paths are, relative to the terminator.
You will see the same physics at continental scale: 80 metre paths clustered in darkness, 20 metre paths in daylight, and a dense band of reports along the grey line. Every one of those spots is a real decode by a real receiver, which makes this the largest continuously running propagation experiment in existence — and it is free to query.
Then run one yourself. A receive-only WSPR station is an SDR, an antenna, and
WSJT-X in WSPR mode with “upload spots” enabled. Your station joins the map,
and your own data appears in the same query.
Comparing indices to observation
Once you have a day of your own data, check it against the space-weather numbers for that day — solar flux, A and K indices, and Bz from the Extra propagation lesson.
The correlations are real but loose, and finding out how loose is the point. A high solar flux figure does not guarantee an open band; a K index of 5 does not close every path. Learning where the indices predict well and where they do not is what separates reading the numbers from using them.
Check yourself
- Why is 5 MHz weak at noon and strong at midnight, while 20 MHz does the reverse?
- What causes the grey-line enhancement?
- You want to know whether 20 metres is open to Japan right now, without transmitting. What do you do?
- You are hearing a time station on 10 MHz and want to know whether it is the Colorado path or the Hawaii path. How do you tell, without moving the dial?
Answers
- 5 MHz is absorbed by the D region, which exists only in daylight. 20 MHz passes through the D region but needs a high enough MUF, which requires the F region ionisation that daylight produces.
- The D region decays faster than the F region after sunset, leaving F region refraction with no absorption beneath it.
- Query WSPRnet or the Reverse Beacon Network for recent spots on that path — or run a receive-only WSPR station and watch what decodes.
- Wait for the minute. WWVH (Hawaii) announces at 15 seconds before the minute in a female voice; WWV (Colorado) at 7.5 seconds in a male voice. Hearing both means both paths are open at once.