Learn / Guide 5
Propagation without mystery
Ionospheric layers, day and night, solar cycle and SFI and Kp indices, with Brazil and the southern hemisphere as a reference.
intermediate 7 min read
The mirror up above
Between about 60 and 1,000 km of altitude, solar radiation strips electrons from the atoms of the air. This electrified region is the ionosphere. For radio waves below about 30 MHz, it acts as an imperfect mirror: the signal goes up, is curved back and comes down hundreds or thousands of kilometers away. Each trip up and down is a hop, and a signal can make several.
They are layers, and each one handles the radio differently.
| Layer | Altitude | When there is | What it does |
|---|---|---|---|
| D | 60 to 90 km | daytime only | absorbs low frequencies. It is because of it that medium waves and 80 meters do not go far with a high Sun |
| And | 90 to 150 km | by day; at night it almost vanishes | reflects the low bands in short hops. In the summer it forms dense clouds (sporadic E) that open up 10 and 6 meters |
| F | 150 to over 400 km | day and night | is the one that makes DX possible. By day it splits into F1 and F2; at night the two merge |
Day and night
During the day, the Sun feeds all layers. The F2 layer becomes dense and can bounce back high frequencies, which is why 20, 15 and 10 meters open up. The D layer is also there, and swallows anything below about 7 MHz.
When the Sun sets, the D layer disappears in minutes. Suddenly 40, 80 and 160 meters, tropical bands and medium wave start going far. The F layer remains there, only thinner, and stops reflecting high frequencies back. The upper bands close one after another.
MUF, the maximum usable frequency
For every path between two points there exists, at any given moment, a maximum usable frequency, the MUF. Above it, the signal punches through the ionosphere and goes into space. Just below it lies the best band, because the higher the frequency, the less the D layer absorbs it.
By day with an active Sun, the MUF exceeds 28 MHz. At dawn, it may drop below 7 MHz.
Gray line
The Gray Line is the boundary between day and night that circles the planet. Along it the D layer has already vanished, or has not yet formed, and the F layer remains strong. For half an hour or so, low band signals traveling along this boundary arrive with a strength they do not have the rest of the day.
The solar cycle
Sun activity rises and falls in a cycle of about 11 years. More sunspots mean more ultraviolet radiation, a denser F layer and a higher MUF. At the peak, 10 meters opens to the world with a few watts. At the trough, it spends weeks in silence.
We are in cycle 25, which began in December 2019. In October 2024, NASA and NOAA announced that the Sun had entered its maximum period, and the official forecast placed the peak between late 2024 and March 2026. From now on it is a descent: the high bands open on fewer days and the low bands gain importance.
How to read the indices
| Index | What it measures | How to interpret |
|---|---|---|
| SFI (solar flux at 10.7 cm) | the Sun's radio noise at 2,800 MHz, which tracks ionization | rule of thumb, not an exact limit: below 80, weak high bands, from 100 to 150, good, above 150, 10 meters opens frequently |
| Kp | disturbance of the Earth's magnetic field, from 0 to 9, every 3 hours | 0 to 2, calm; 3 and 4, unstable; 5 or more, geomagnetic storm |
| X-rays (classes C, M, X) | solar flares | a strong flare blacks out shortwave on the illuminated side of Earth for minutes or even an hour |
A good day has high flux and a calm magnetic field. A geomagnetic storm drops the MUF and hits the paths passing close to the poles first.
Which band to try
| Status | Where to look |
|---|---|
| Day, high SFI, low Kp | 10, 12 and 15 meters |
| Day, low SFI | 20 and 17 meters |
| Night | 40, 80 and 160 meters, tropical bands, 49 and 31 meters broadcasting |
| Sunrise and sunset | 40 and 80 meters, via Gray Line |
| Kp 5 or higher | lower bands and north-south paths, away from the poles |
What changes in Brazil
Almost all propagation text was written for Europe or North America.
- Stations are swapped. Winter, when the lower bands are quieter, runs from June to August. Sporadic E season falls in our summer, peaking near the December solstice.
- We are close to the magnetic equator. Ionization concentrates in two crests, one on each side of it, and one passes over Brazil. The MUF here tends to be higher than in middle latitudes.
- Transequatorial propagation. In late afternoons and early evenings, especially near the equinoxes, 10 and 6 meters can open between southern South America and the Caribbean region, crossing the equator in a single hop.
- Bubbles after sunset. The equatorial ionosphere becomes irregular at night. Signals acquire a fast flutter. INPE tracks these effects in the EMBRACE program.
- Static. In the summer, thunderstorms fill 160, 80 and 40 meters with static crackles.
See instead of predict
The index is a forecast. What actually propagated through the band appears on the maps of who heard whom, and that says more than any number. Without transmitting, you can check these maps in two ways:
- Open PSKReporter (the guide FT8 and WSPR shows how), pick a band and see which regions currently have signals passing through.
- Open KiwiSDR receivers on different continents on the same frequency and compare.