Learn · a beginner’s guide
Six short chapters, each with something to play with. No maths and no prior knowledge needed. The whole guide takes about half an hour, or you can dip into whichever chapter you’re curious about. Underlined words have a plain-English definition: hover or tap them.
A radio signal is a wave of energy travelling at the speed of light. The number that describes it is its frequency: how many times it vibrates every second. HF, short for high frequency, covers 3 to 30 million vibrations a second, written 3–30 MHz.
Every frequency also has a wavelength, the distance from one wave crest to the next. The two are locked together: divide 300 by the frequency in MHz and you get the wavelength in metres. That’s why radio people name bands by length. 7 MHz waves are about 40 m long, so that’s the “40 metre band”.
Drag the slider or tap a band, and watch the wave stretch and squeeze.
Why care about wavelength? Because antennas are sized to it. The most basic antenna is half a wavelength long, so one for the 10 m band fits on a car roof, while one for 160 m needs a very large garden.
Radio waves travel in straight lines, but the Earth is round. On its own, an HF signal would sail off into space past the horizon. What brings it back is the ionosphere: layers of thin, electrically charged gas 60–400 km up, made by sunlight.
The layers act like a mirror, but a fussy one. Whether a signal comes back depends on its frequency, how steeply it hits the layer, and how strongly the sun has charged the layer that day. If the frequency is too high or the angle too steep, the signal goes straight through into space.
Long-distance signals travel in hops: up to the ionosphere, down to the ground, and up again. One hop covers at most 3,000–4,000 km. Every trip through the D layer costs some signal, which is why paths with many hops eventually fade out.
Put those ideas together and you get the most useful rule in HF: every path has a usable window of frequencies. The top of the window is the MUF. Go higher and the signal escapes or skips right over the other station. The bottom is the LUF. Go lower and the D layer soaks the signal up.
The window moves through the day. Midday sun raises the MUF, which is good for the high bands, but it also thickens the D layer, which is bad for the low ones. At night the D layer vanishes and the low bands open up. The MUF sinks too, though, so the high bands close.
Pick how far away the other station is, then slide through the day, or run a finger along the chart. Bands inside the shaded window are worth trying.
✓ open · ~ marginal · × closed (skips over or absorbed). Local time at the middle of the path.
A rule of thumb falls straight out of this: the higher the sun, the higher the band. Use 20 m and up by day, 40 m around dawn and dusk, and 80 m and 160 m after dark. For nearby stations, stay low: 40 m or 60 m by day, 80 m at night.
The strongest signals are often just under the MUF. That’s the FOT, the optimum frequency. Live right at the edge, though, and the path drops out whenever the ionosphere wobbles.
An antenna doesn’t just “send the signal”. It decides where the signal goes. On HF the direction that matters most is up: the take-off angle, or how steeply the energy leaves.
Low angles, under about 15°, reach the ionosphere far away and come down thousands of kilometres off. That’s long distance, or DX. Steep angles go nearly straight up and come back down all around you. That’s NVIS, perfect for regional nets within a few hundred kilometres.
For the simple dipole, the angle is set by its height above the ground, measured in wavelengths. The ground acts like a mirror, and the reflected signal combines with the direct one to shape where the energy goes.
Slide the height and watch the shape change. The dashed line is the angle your chosen distance needs, so try to get the shape to reach it.
Rules of thumb: under a quarter of a wavelength high, a dipole is an NVIS antenna. At half a wavelength it’s the classic all-rounder. At a wavelength or more it favours low angles and long distance, with extra lobes appearing above. So the same 10 m-high dipole is an NVIS antenna on 80 m and a DX antenna on 10 m.
It’s tempting to think more power is the answer. It helps less than you’d expect, because receivers, and our ears, work on a logarithmic scale. Radio people measure it in decibels (dB). Doubling the power adds 3 dB, and ten times the power adds 10 dB.
A receiver’s S-meter is marked from S1 to S9, and each S-unit is 6 dB, which is four times the power. So going from 100 W to 400 W moves the other station’s meter by just one S-unit.
Whether you’re heard depends less on how strong your signal is than on how far it stands above the noise, the hiss and buzz of electrical interference. That gap is the SNR. Cities are noisy and the countryside is quiet.
You’re calling a friend about 1,500 km away on 40 m. Change your station and where they live, and watch their meter.
Notice how the mode moves the goalposts. Digital modes like FT8 can be decoded 20 dB below the noise, from signals you’d never hear by ear. That’s why low-power stations love them.
Everything in this guide runs on sunlight. The sun’s activity rises and falls over a roughly 11-year solar cycle, tracked by counting sunspots (the SSN) or measuring the sun’s radio brightness (the solar flux, SFI). More activity charges the ionosphere harder, so the MUF climbs and the higher bands open.
The sun can also cause trouble. Bursts of particles shake the Earth’s magnetic field. That’s a geomagnetic storm, measured by the Kp index from 0 (quiet) to 9 (extreme). The Earth’s magnetic field funnels storms toward the poles, stirring up the aurora, so paths across high latitudes suffer first and worst. Paths near the equator often carry on. A big solar flare can also black out HF on the sunlit side of the Earth for minutes to hours; that’s an SID.
You now know enough to read every HFKit tool. They use the ITU’s own prediction model, which runs right here in your browser, instead of the simple teaching model above.