Ionosphere
| Ionosphere | |
|---|---|
| What it is | The electrically charged region of the upper atmosphere |
| Height | About 50–400 km (30–250 miles) |
| Created by | Ultraviolet light and X-rays from the sun |
| Layers | D, E, F1 and F2 by day; mainly a single F layer at night |
| Key frequencies | |
| Critical frequency | Highest frequency returned from straight overhead |
| MUF | Maximum usable frequency on a given path |
| LUF | Lowest usable frequency on a given path |
| Measured by | Ionosondes |
The ionosphere is the electrically charged region of the upper atmosphere, roughly 50 to 400 km (30 to 250 miles) above the ground. Ultraviolet light and X-rays from the sun strip electrons from the thin gas there, leaving it ionized, and those free electrons bend HF radio waves back to Earth. That is what lets a modest station on the 20-meter band reach another continent. Because sunlight creates it, the ionosphere changes with the time of day, the season and the solar cycle.
How it returns signals
A radio wave climbing into an ionized layer is gradually refracted, or bent, as the density of free electrons increases. If the layer is dense enough for that frequency, the wave curves back down and lands far from the transmitter, a path called skywave. Higher frequencies need denser ionization, and above a certain point they pass through into space. The ground can reflect a returning signal up again for another hop.
The layers
| Layer | Approximate height | When present | Main effect on HF |
|---|---|---|---|
| D | 60–90 km | Daytime only | Absorbs signals, most strongly on the lower bands |
| E | 90–150 km | Daytime; very weak at night | Returns the lower HF bands on paths up to about 1,250 miles |
| F1 | 150–220 km | Daytime only | Minor; merges with F2 after dark |
| F2 | 220–400 km | Day and night | Main long-distance layer; hops of up to about 2,500 miles |
The D layer absorbs rather than returns HF signals, and the loss rises steeply as frequency falls. That is why 160 and 80 meters reach only short distances by day and 40 meters stays regional until evening. The layer fades soon after sunset.
The E layer peaks around midday and nearly disappears after dark. Dense, patchy clouds of E-layer ionization, called sporadic E, can return frequencies as high as 6 and even 2 meters.
The F layer does most of the long-distance work. By day it splits into F1 and F2, and the F2 layer, the densest of all, carries 20 meters and the higher bands around the world. At night the two merge into a single F layer that thins but survives until dawn.
Day, night and season
By day the F2 layer opens the higher bands while the D layer smothers the lower ones. After dark the picture reverses: 80 and 40 meters open for long distances, while the thinning F layer can no longer support 15 and 10 meters. Along the gray line, the band of twilight around sunrise and sunset, absorption is low while the F layer is still well ionized, and low-band signals can be unusually strong.
Summer brings more absorption and static but keeps the higher bands open later; winter's long, quiet nights favor the low bands. At mid-latitudes the daytime F2 layer is often denser in winter than in summer, so 15 and 10 meters can be excellent on winter afternoons.
Critical frequency, MUF and LUF
The critical frequency is the highest frequency a layer returns when a signal is sent straight up. For the F2 layer, written foF2, it ranges from a few megahertz on a winter night at solar minimum to more than 10 MHz at midday near solar maximum. It sets the upper limit for NVIS.
A signal meeting the layer at a slanting angle is returned more easily, so the maximum usable frequency (MUF) rises with path length; for the longest single F2 hop it is roughly three times the critical frequency. If foF2 is 7 MHz, a long hop may support about 21 MHz, and 15 meters is open. Signals are usually strongest a little below the MUF; a rule of thumb puts the best working frequency at about 85 percent of it.
The lowest usable frequency (LUF) is the bottom of the window, below which absorption and noise leave signals too weak to use. Unlike the MUF, it depends on the station as well as the sky: more power, a better antenna or a weak-signal mode such as FT8 pushes it down. When the LUF rises above the MUF, no HF frequency works on that path.
Absorption
Normal absorption is heaviest around local noon and in summer. A solar flare's X-rays strengthen the D layer within minutes across the sunlit half of Earth, causing a sudden ionospheric disturbance: HF signals fade out, lowest frequencies first, for minutes to hours. Energetic protons from the sun can cause polar cap absorption, closing paths over the poles for days.
Ionosondes
An ionosonde is a radar that looks straight up. It sweeps a transmitter across the HF spectrum and times the echoes, producing an ionogram that plots the apparent height of each layer against frequency. Critical frequencies and MUFs are worked out from it. Dozens of ionosondes report in near real time, feeding the foF2 and MUF maps many hams check before getting on the air.