A radio wave is a rapid wave of force, the same as light. Shortwaves travel far by bouncing off the atmosphere. Start with the scope: see a real, live shortwave signal and learn what it is.
Flip POWER, press FIND. A public receiver somewhere on Earth tunes a station on the air right now, and the tube draws what it hears. SOUND is what came off the wave; RADIO is the wave itself.
Where is the radio wave? A station like WRMI drives an alternating electric current through its antenna, which oscillates (switches direction) 9,395,000 times (cycles) a second. That movement in the force, in the field, creates a wave, like shaking water: a radio wave, whose 9.395 MHz rhythm is the carrier. The sound is the shape imposed on it: in AM, the strength of the carrier rises and falls with the voice, from a few dozen to several thousand times a second. Imposing a shape means nothing grander than shaking harder, then softer — the rhythm stays the same, the size of it carries the voice. Then how does anything reach you at all? Below, a carrier is already running: fast, steady, and saying nothing. Take the lever and set how tall each cycle is as it leaves the aerial — that height, and nothing else, is the message. Watch the change set off down the path: the wave already in flight keeps the height it was born with, because a transmitter writes on the wave as it makes it, not afterwards.
The public receiver strips the carrier off and hands over the sound; the sound is what crosses the internet to you. SOUND draws that. RADIO draws the same sound riding its carrier, with the carrier thinned thousands-fold so a screen can hold it. SPECTRUM shows which pitches are in the sound. MIC puts your own voice on the tube, and on a carrier, as if you were the station. RECEIVER chooses whose antenna you borrow: any of about 850 public sets, anywhere on Earth; the stations FIND offers are the ones on the air within reach of that antenna. The receiver ↗ tunes properly and shows where the signal comes from.
An electron pushes on every other charge, through empty space. Shake it and that push ripples outward at the speed of light. That ripple is a radio wave: a wave of the force itself — in the textbooks, an oscillation of the electric and magnetic field, which is that force written down for every spot in space. Start with step 5 and press SHAKE IT. The tube at the top ↗ is drawing exactly this, arriving from a real station.
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. Wavelength is the speed of light divided by the frequency, which in the units on the dial is 300,000 ÷ kHz in metres. An antenna works best when it is about half a wavelength long — a half-wave dipole, the length in the readout above — 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 — the radius of the circle under the scope ↗. Wavelength is crest to crest, written λ. 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: constructive interference.
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, its envelope. 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, the signal’s bandwidth); SSB keeps one sideband, so an SSB signal tuned in AM sounds like Donald Duck until the receiver puts the missing carrier back. See it for real: put a live broadcast on the scope at the top of this page and watch the outline move with the voice.
From about 60 km up to 400 km, sunlight is strong enough to knock electrons off air molecules, to ionise them, which is where the ionosphere gets its name. 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, to recombine.
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. The skip zone is the gap between where the ground wave dies and where the first skywave lands: 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 stretched about three times here so the bounce is visible at all; distances along the ground are honest. To scale the ionosphere would be a hairline — 300 km up against a 3,000 km hop. The bounce is really a gradual bend — refraction, like light in a hot road mirage — but a mirror at the turning height gives the same answer, and the textbooks call that height the virtual height.
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. On the dark side the D layer has collapsed while F2 is still charged from the day, so signals along the terminator go unusually far.
A signal rarely arrives by one path: multipath. 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 →