Reading Propagation Forecasts

Propagation forecasts can look like a wall of numbers and colored maps. They make much more sense once you know there are really three different things on offer: forecasts of what the Sun and Earth’s magnetic field will do, predictions of which frequencies should work on a given path, and live reports of what is being heard right now.

Each one answers a different question. Used together, they help you pick the right band at the right hour instead of calling CQ into silence.

What a Forecast Can and Cannot Tell You

A space weather forecast tells you how likely a flare, a radiation storm or a geomagnetic storm is over the next few days. Flares are given as odds, not times. Storms from coronal mass ejections give more warning, because the cloud takes many hours or days to reach Earth.

A path prediction tells you which frequencies should work between two places at each hour, on a typical day for the month. It does not know about today’s flare or this afternoon’s storm. If the basic ideas of solar flux and the K index are new to you, start with our article on space weather and the solar cycle.

Products from NOAA SWPC

The NOAA Space Weather Prediction Center publishes a range of free products. These are the ones hams tend to find most useful:

• 3-Day Forecast: A plain-language, single-page forecast issued every 12 hours, broken down by NOAA’s R (radio blackout), S (radiation storm) and G (geomagnetic storm) scales.
• 27-Day Outlook: A weekly table forecasting the solar flux, the planetary A index and the largest daily K value for each day ahead. It is good for spotting quiet stretches for a contest or a trip.
• Alerts, Watches and Warnings: Notices of events as they happen or are expected. You can subscribe to receive them by email.
• D-Region Absorption Prediction (D-RAP): A world map showing the highest frequency affected by absorption from flares and proton events, with an estimate of when conditions will recover.
• Dashboards: SWPC’s radio dashboard gathers current scale levels, solar wind, solar flux and the D-RAP map on one page.

MUF and LUF

Two terms appear on almost every prediction. SWPC defines the maximum usable frequency (MUF) as the highest frequency that allows reliable HF communication over a given distance by bending off the ionosphere. Above the MUF, your signal passes through and heads into space.

The lowest usable frequency (LUF) is the lowest frequency that allows reliable long-range HF communication. Below it, the signal is soaked up by absorption, mostly in the D region during daylight.

Your usable window lies between the two. Pick a band a little below the MUF for the strongest signals. When a flare hits, D region absorption climbs and pushes the LUF up, so the lower bands often vanish first while the higher bands hang on. The D-RAP map shows you how high that absorption reaches.

Path Prediction Tools

VOACAP is free, professional HF prediction software from the Institute for Telecommunication Sciences, part of NTIA, and was first developed for the Voice of America. The voacap.com site offers free online versions.

You give the program your location, the other end of the path, the month and a sunspot number, and often your power, antennas and mode. It returns a chart of frequencies by hour, with an estimate of how often each should work.

Read the results with care. The VOACAP documentation explains that it uses a monthly smoothed sunspot number, that it is not accurate for a single day within the month, and that its timing can be off by 30 minutes to an hour. Think of it as a climate guide, not today’s weather report.

Real-Time Evidence

Live reports show what is really happening. These free networks turn receiving stations all over the world into your own propagation meter:

• WSPR: The Weak Signal Propagation Reporter mode, part of the free WSJT-X program, sends low-power beacon transmissions in two-minute cycles. Stations that decode you report it to the WSPRnet database and map. See our WSPR page.
• PSK Reporter: Volunteer receiving stations automatically report the digital mode signals they decode, mostly FT8 today. Call CQ, and within a few minutes you can see on a map where you were heard.
• Reverse Beacon Network: A network of listening stations that report which stations they hear, when, and how strong, on CW and on FT8 and FT4. Call CQ and look up your own callsign to see who heard you.
• NCDXF/IARU Beacons: Eighteen beacons around the world take turns on 14.100, 18.110, 21.150, 24.930 and 28.200 MHz. Each sends its callsign in CW and then four dashes stepping down from 100 watts to 10 watts, 1 watt and 0.1 watt, and each beacon is heard on each frequency once every three minutes.

The beacons are a lovely tool for a beginner. If you can hear the 1 watt dash from another continent, the band is in good shape on that path. Our gray line tool shows you which parts of the world are in daylight and darkness at the same moment.

When to Trust a Forecast

A forecast is most useful for planning days or weeks ahead, such as choosing a weekend for a portable outing. It is least useful in the moment, because the ionosphere changes from hour to hour.

Trust the live evidence when the two disagree. If the prediction says 10 meters is closed but the beacons and spotting networks show signals, the band is open. If a storm alert is out but the K index has stayed low, conditions may be better than feared. Predictions also cannot see brief, patchy openings such as sporadic E.

A Simple Routine

This takes five minutes before an operating session:

1. Check the current R, S and G levels and the Kp on the SWPC site.
2. Glance at a path prediction for the region you want to work, to choose two or three likely bands.
3. Tune to the NCDXF/IARU beacon frequency on your first choice and listen for a few minutes.
4. Call CQ or send a few WSPR cycles, then look yourself up on PSK Reporter, WSPRnet or the Reverse Beacon Network.
5. Pick your band from what you actually hear, and keep notes so you learn how your station behaves.