Why the Bands Can Stay Open After Sunset

Sunset reaches different heights at different times. A playful view of the same ground location in darkness while the F region above it is still sunlit.

The Sun has slipped below the horizon, the yard is getting dark, and a station on 10 or 15 meters is still coming through. What keeps the contact going? The answer starts well above the horizon you can see.

Sunset on the ground and in the ionosphere

From your station, sunset is the moment the Sun’s upper edge disappears below an unobstructed horizon. Civil twilight follows, ending when the Sun’s center is 6 degrees below it. These familiar milestones describe the light outside. Radio propagation follows a different timetable.

The upper atmosphere has a longer view of the Sun. Because Earth curves away beneath it, a point in the F region can still be sunlit after sunset reaches the ground. In this guide, “ionospheric sunset” means the estimated loss of direct geometric sunlight at 300 kilometers altitude. It is a useful landmark, rather than an exact cutoff for the ultraviolet radiation that creates ionization.

Why the F region matters

For long-distance contacts on 15 meters, around 21 MHz, and 10 meters, around 28 MHz, the F region—especially F2—does much of the work. It stays illuminated later than the ground, and its ionization fades gradually afterward. As the path’s maximum usable frequency falls, 10 meters generally fades first, with 15 meters following later.

The E region sits lower, roughly 90 to 140 kilometers above Earth, and has its own sunlight horizon. Its ordinary daytime ionization fades much faster after sunset. Sporadic E is a separate mechanism that can occur by day or night; it does not follow a dependable F-region sunset schedule.

What about 20 meters

Twenty meters often rewards one more turn of the tuning dial after the higher bands fade. At around 14 MHz, it can use a path that no longer supports 21 or 28 MHz. The F region must have enough electron density, with the right profile and path geometry, to bend the signal back to Earth. That path’s maximum usable frequency, or MUF, must reach your operating frequency, and the arriving signal must be strong enough to copy.

Sunset does not empty the F region of electrons. Ionization declines gradually, and enough may remain for 20 meters to work for hours or even all night. Reduced D-region absorption after dark can also help signals. As electron density falls, the path’s MUF may drop below your operating frequency and end ordinary F-region support. Absorption, interference, noise or changing geometry can make a contact fade sooner.

Morning sunlight can rebuild ionization and lift the MUF back above 14 MHz; changing geometry can also create an opening. Solar activity, season and geomagnetic disturbances all influence the result. That is why 20 meters gets an explanation here rather than a fixed closing time in the tables.

The morning counterpart

Dawn brings the same geometry in reverse: sunlight reaches the upper atmosphere before it reaches the ground. Ionizing radiation helps rebuild electron density, allowing higher frequencies to return. Fifteen meters will often become usable before 10 meters as a path improves. Ionospheric sunrise starts that process; it does not ring an opening bell for either band.

Using the monthly tables

Use the monthly tables to decide when to listen more closely as evening moves across a path. They show ground sunset and estimated F-region sunset alongside simple operating guidelines: sunset to one hour later for 10 meters, and one to three hours later for 15 meters. These are broad guides, not measured monthly averages or deadlines. The F-region time is shown separately; do not add the band offsets to it.

Further reading: Australian Bureau of Meteorology, Introduction to HF Radio Propagation (sws.bom.gov.au/Educational/5/2/2); ARRL, Radio Waves and the Ionosphere (arrl.org/files/file/Technology/pdf/119962.pdf).

Will 20 Meters Stay Open Tonight

Wondering whether to stay at the radio for another hour or two? Start with an hourly path prediction, then check it against what the ionosphere and other stations are doing now. These five checks give you a useful outlook without turning the evening into a research project.

1 Check the next few hours for your actual path

In VOACAP Online, set your station, destination, month, mode, power and antennas. Check 20-meter reliability (REL) and signal-to-noise ratio (SNR) for now and the next two or three UTC hours. Favorable values across several hours suggest a worthwhile window; falling values suggest fading. VOACAP describes monthly statistical expectations, so use the current-day checks below alongside it.

2 Look for frequency margin and the MUF trend

Compare a path-specific MUF with your actual frequency—for example, 14.2 MHz. A MUF well above it gives more room than one barely above it. A trend down toward 14.2 MHz warns that the path may be nearing its limit. Use the same comparison at roughly 21 MHz for 15 meters and 28 MHz for 10 meters.

On KC2G, choose MUF(3000 km) and check its timestamp. It estimates support for a 3,000 km hop; look near the relevant hop midpoints along your route. Compare successive updates for a trend. This is a current estimate, not your exact path or a forecast. The separate foF2 map shows vertical critical frequency, which cannot be compared directly with your operating frequency.

3 Confirm what is happening now

On PSK Reporter, select 20 meters and recent reports, such as the last 15–30 minutes. Repeated receptions between your region and the destination are encouraging; listen on your intended mode too. FT8 reports do not guarantee an SSB contact, and a blank map may simply mean nobody is transmitting or reporting.

4 Check NOAA for a disruption

Check NOAA’s Kp index, alerts and D-RAP absorption map. Kp 0–3 generally means relatively quiet geomagnetic conditions; 5 or higher marks a geomagnetic storm. D-RAP highlights flare- or particle-related HF absorption. These checks flag trouble; neither Kp nor solar flux alone supplies a band-closing time.

5 Read the clock and the date

Keep every tool on the same clock: UTC, also called Z time. For Eastern Time, subtract 5 hours in EST or 4 in EDT; for Atlantic Time, subtract 4 in AST or 3 in ADT. UK time is UTC in GMT or one hour ahead in BST. Watch the date: 02:00 UTC is 9 p.m. EST or 10 p.m. EDT on the previous day in Chattanooga.

A favorable hourly outlook, MUF above your frequency, continuing reception reports and no major disruption give you good reasons to keep listening. Recheck as the evening develops; none of these tools supplies an exact closing minute.

Tools and further reading

VOACAP Online | KC2G MUF map | PSK Reporter | NOAA Kp | NOAA D-RAP

Monthly Sunset and Band Closing Guide

Chattanooga Tennessee

2026 • The 15th of each month • Times rounded to the nearest 5 minutes

Eastern Time: EST in winter and EDT in summer.

Read local time first and Chattanooga Eastern Time second where they differ. Seasonal clock changes are included. Use the band columns as sunset planning guides for that location, not a prediction of the entire radio path.

How to read the calculations

The ground sunset calculation includes standard refraction and solar-disk corrections. F-region sunset uses a spherical Earth and a nominal 300 km altitude, where direct geometric sunlight ends with the Sun’s center about 17.2° below the ground horizon. It is not an exact cutoff for ionizing radiation or usable propagation.

The shorthand is simple: sunset to +1 hour for 10 meters, and +1 to +3 hours for 15 meters. These are operating guidelines, not calculated monthly averages. Apply them to ground sunset, not to the F-region sunset column.

Great Britain represented by London

2026 • The 15th of each month • Times rounded to the nearest 5 minutes

UK time: GMT in winter and BST in summer.

Read local time first and Chattanooga Eastern Time second where they differ. Seasonal clock changes are included. Use the band columns as sunset planning guides for that location, not a prediction of the entire radio path.

Summer sunlight at F region height

London provides an interesting summer example: the point 300 km overhead stays geometrically sunlit all night on the June and July sample dates. That does not guarantee an all-night band opening. The band columns still use the ground sunset rules. London represents southern Great Britain; northern locations have different times.

New Brunswick represented by Fredericton

2026 • The 15th of each month • Times rounded to the nearest 5 minutes

Atlantic Time: AST in winter and ADT in summer.

Read local time first and Chattanooga Eastern Time second where they differ. Seasonal clock changes are included. Use the band columns as sunset planning guides for that location, not a prediction of the entire radio path.

Using the three location tables

For Great Britain or New Brunswick, compare the destination with Chattanooga using the EST or EDT entries. Watch the end of the path moving into darkness, while remembering that the ionosphere between stations matters too. Local clock times help you coordinate; they are not separate closing deadlines for your station.

Calculation and propagation references

Solar position: NOAA solar equations based on Jean Meeus, with solar declination and equation of time iterated at each event. Coordinates: Chattanooga 35.0456° N 85.3097° W; London 51.5074° N 0.1278° W; Fredericton 45.9636° N 66.6431° W. Earth radius 6,371 km; nominal F region altitude 300 km.

NOAA Solar Calculation Details: https://gml.noaa.gov/grad/solcalc/calcdetails.html

ARRL Here Comes the Sun: https://www.arrl.org/files/file/On%20the%20Air%20Email/Here%20Comes%20Sun.pdf

Equatorial Sunset Reference

A geographic definition for this guide

For this guide, the equatorial belt means approximately 10° north to 10° south of the geographic equator—a working definition rather than a universal boundary. The wider tropics reach about 23.4° north and south. This table uses exactly 0° latitude as a clear baseline, not an average of the whole belt.

Reading the equatorial table

This table uses LOCAL APPARENT SOLAR TIME, with solar noon at 12 p.m. It does not use a time-zone clock. Converting it to Eastern Time requires a longitude and date; a city’s clock also reflects time-zone boundaries, daylight saving and the equation of time. Entries are for the 15th of each month in 2026, rounded to five minutes.

Sunlight geometry and radio behavior

At the geographic equator, daylight changes little through the year. A point 300 km overhead keeps its direct geometric sunlight roughly 65–75 minutes beyond ground sunset. The 10- and 15-meter columns only illustrate the arithmetic used earlier: sunset to +1 hour, and +1 to +3 hours. They are not validated equatorial closing windows.

For radio, geography is only part of the picture. The magnetic equator matters too: F-region plasma transport and ionization enhancements roughly 15°–20° to either side can change evening openings. Use the sunlight times as geometric landmarks, rather than expecting one closing schedule to fit every equatorial longitude and path.

Source: Australian Bureau of Meteorology, Introduction to HF Radio Propagation, sections 1.4.3 and 1.4.4 — https://sws.bom.gov.au/Educational/5/2/2. Solar geometry uses the method described in the earlier tables.