A radio wave and a beam of light are the same thing: a ripple sent out by shaking electrons. Shake them slowly and it is radio; shake them a million times faster and your eye can see it. Shortwave is the radio that the sky throws back down, so a 100 kW transmitter in Cuba can land in a kitchen in Minnesota. Every picture below is live: drag the sliders and watch what changes.
Start with one electron sitting still. It pushes on every other charge in the universe: hard up close, weakly far away. That push, mapped at every point in space, is its electric field. Now move the electron. The map has to change, but the news can only travel at the speed of light, so the field far away lags behind. Shake the electron back and forth and the lag becomes a ripple racing outward. That ripple is a radio wave. Click through the six steps.
The lines are not wires or paths; they show which way a second charge would be shoved at each spot. The “meter bands” on the dial are wavelengths: 9580 kHz is a 31-metre wave, so it lives in the 31 m band. An antenna works best when it is about half a wavelength long, which is why a shortwave antenna is a wire and a phone antenna is a sliver.
Every wave has four numbers. Amplitude is how tall it is: the strength of the signal. Wavelength is crest to crest. Frequency is how many crests pass per second, in hertz. Phase is where in its cycle a wave is at a given moment, and it only matters when two waves meet, which on shortwave they constantly do.
Slide the phase to 180° and the two waves cancel: that is destructive interference, and on shortwave it is heard as fading. Slide it back to 0° and they add up to twice the height.
A steady 9580 kHz wave is a carrier. It says nothing. To send sound, the transmitter changes the carrier in step with the voice. AM (amplitude modulation) makes the carrier louder and softer with the sound, so the voice is the outline of the wave. Nearly all shortwave broadcasting is AM because a receiver can decode it with a single diode. SSB (single sideband) is AM with the carrier and one mirror-image sideband removed: all the power goes into the voice, which is why hams, aircraft and ships use it. FM wiggles the frequency instead and is used on VHF, not shortwave.
Top: the voice. Middle: the carrier with the voice on it. Bottom: the spectrum, what the signal occupies on the dial. AM takes the carrier plus a sideband on each side (about 10 kHz for broadcast audio); SSB keeps one sideband, so an SSB signal tuned in AM sounds like Donald Duck until the receiver puts the missing carrier back.
From about 60 km up to 400 km, sunlight is strong enough to knock electrons off air molecules. That thin electrified gas is the ionosphere, and it bends radio waves. It comes in layers. The low D layer only exists in daylight and mostly absorbs; it eats the low bands during the day. E and F1 are daytime layers. The high F2 layer is the one that matters: it survives the night, because at 300 km the air is so thin that freed electrons take hours to find a molecule again.
Drag the sun across the sky. At night the D layer vanishes, which is why the low bands (120 m to 41 m) open up after dark: nothing is left to absorb them, and F2 is still there to send them back. More solar flux means a denser F2 layer, which can turn back higher frequencies; that is the eleven-year sunspot cycle, felt on the dial.
The transmitter sprays waves at every angle. Those that hit the F2 layer at a shallow angle bend back down and land a long way off; those that go up steeply punch through into space. Whether a given angle comes back depends on the frequency and on how dense the layer is: the highest frequency that still returns from a given distance is the MUF, the maximum usable frequency. Waves also skim along the ground, but at shortwave frequencies the ground wave dies within a few tens of kilometres. Between where the ground wave dies and where the first skywave lands is the skip zone: a station can be booming 2,000 km away and silent 200 km away.
Bright rays return to Earth; faint ones escape. In daylight, rays on the low bands dim as they cross the D layer: absorption. Heights are drawn to scale with the distances, which is why the ionosphere looks so thin: 300 km up against a 3,000 km hop. The bounce is really a gradual bend, like light in a hot road mirage, but a mirror at the turning height gives the same answer.
Because the D layer absorbs by day and the F2 layer weakens by night, each band has its hours. The low bands want an all-dark path. The high bands (19 m and up) want daylight, when F2 is dense enough to turn them. The sweet spot is the gray line, the band of twilight sweeping round the globe: the D layer has collapsed on the dark side while F2 is still charged from the day, and signals along the terminator go unusually far.
A signal rarely arrives by one path. A one-hop and a two-hop copy of the same broadcast land together, having travelled different distances, so they arrive in different phases. As the ionosphere drifts, the phases slide past each other and the two copies add, then cancel, then add. That slow swell is QSB, fading, and it is the sound of shortwave. The S-meter reads the result on a scale where each S-unit is 6 dB, a doubling of voltage; S9 is a strong signal (−73 dBm), and above S9 the meter counts decibels.
Left: the two arriving copies and their sum. Right: what the tuning meter does with it. Set the second path to zero and the fading stops; that is what a single clean path sounds like, which on shortwave is rare.
Now go turn the dial. The set works out all of this for wherever you are.
Open the receiver →