Ground Wave, Sky Wave and Space Wave

This is the lesson that turns a licence into a hobby. Everything else in Section A explains how the signal is made; this explains where it goes. It answers the questions that puzzle every new operator: why 40 metres is dead at three in the afternoon and full of Japan at ten at night, why a station 200 km away cannot hear you while one 3000 km away can, and why 5 watts on 2 metres from a hilltop beats 50 watts from a valley. It is also the largest single block of marks in the propagation module.

Three ways a signal can travel

Energy leaving your antenna reaches the far station by one of three routes, and which one dominates depends almost entirely on frequency.

Strictly, the ground wave is the whole of the signal travelling near the earth, and it divides into the surface wave, which is dragged along in contact with the ground, and the space wave, made of the direct ray and the ray reflected off the earth. In amateur usage “ground wave” almost always means the surface wave, and that is how the exam uses it too.

The ground wave

A vertically polarised wave travelling along the surface induces currents in the ground beneath it, and those currents dissipate energy as heat. The wave is therefore being attenuated continuously by the very surface that guides it. Two things decide how far it gets:

• Frequency. Losses climb steeply with frequency. At 200 kHz a ground wave travels a thousand kilometres; at 1.8 MHz a few hundred; at 30 MHz a few tens; at 145 MHz it is gone within sight of the antenna.
• Ground conductivity. The better the ground conducts, the lower the loss. Sea water is the best surface there is, wet farmland is good, and dry rock, sand or the laterite of the Deccan plateau is close to the worst.

Put those together and you have the exam answer: the ground wave reaches furthest at the lowest frequency over sea water. It is why 160 m gives reliable within-state contacts at any hour whatever the ionosphere is doing, and why nobody has ever heard of ground-wave working on 2 metres — at 145 MHz the surface wave is absorbed within a kilometre or two, and everything you hear on VHF is space wave.

The ionosphere

Between roughly 60 and 400 kilometres up, the air is so thin that the ultraviolet and X-radiation from the sun can strip electrons from gas molecules faster than they can recombine. The result is a region of free electrons and ions — the ionosphere. A radio wave entering a region of free electrons is slowed and bent; if the electron density is high enough and the wave is not too high in frequency, it is bent far enough to come back down. Papers usually say the layer “reflects” the signal. It is really refraction, a gradual bending, and it is worth knowing the difference because it explains why the effect depends so sharply on the angle at which the wave arrives.

Different gases predominate at different heights and different wavelengths of sunlight penetrate to different depths, so the ionisation is not uniform. It forms recognisable layers.

The two facts to fix in your memory are that D absorbs and F2 does the DX, and that after sunset D vanishes and the two F layers become one. Almost every day-and-night question on the paper is answered by those two sentences.

Critical frequency, MUF and LUF

Point an antenna straight up and sweep the frequency upwards. Below a certain frequency, everything comes back down. Above it, the signal punches through the layer and carries on into space. That dividing frequency, measured at vertical incidence, is the critical frequency of the layer.

Real contacts do not use vertical paths. A wave arriving at the layer at a shallow angle needs to be bent much less to be turned back, so it can be returned at a much higher frequency than the critical frequency. The highest frequency that a given path will support at a given moment is the maximum usable frequency, the MUF. The MUF is always higher than the critical frequency, and the shallower the angle the higher it goes.

Work right at the MUF and the path is on a knife edge: a small drop in ionisation and it fails completely. Work below the LUF and absorption kills you. The usable window is between the two, and the sweet spot is the OWF. Note the corollary that satellite work depends on: to get out of the ionosphere you must be above the critical frequency, which is why amateur satellites use VHF and UHF and never HF.

Angle of radiation, skip distance and the skip zone

Because refraction depends on the angle at which the wave meets the layer, the take-off angle of your antenna decides where the signal lands. A steep ray is turned back quickly and comes down close to home. A shallow ray travels a long way before it reaches the layer, is bent through a smaller angle, and comes down a very long way off.

Lowering the angle of radiation lengthens the skip distance, which is what you want for DX. The usual way to lower the angle of a horizontal dipole is to get it higher above ground — half a wavelength up is the point at which a dipole stops firing mostly upwards and starts firing usefully towards the horizon. On 20 m that is 10 metres of mast. This is the entire reason experienced operators talk about antenna height before they talk about power.

See it move

Drag the take-off angle slider below and watch the first hop run outwards as the angle falls; then push the frequency past the MUF and watch the ray leave through the top of the picture altogether. Those two behaviours are this whole section in one diagram.

Two definitions the exam separates carefully:

• Skip distance — the distance from the transmitter to the point where the sky wave first returns to earth. It is a property of the frequency, the antenna angle and the ionosphere. It is not the distance between transmitter and receiver.
• Skip zone — the ring of silence between the point where the ground wave dies out and the point where the sky wave first comes down. Inside it, nobody hears you at all.

Worked example — the skip zone

A station's ground wave is usable out to 120 km. Its sky wave first returns to earth at 300 km. What is the skip zone?

skip zone = skip distance − ground wave range = 300 − 120 = 180 km

Anyone between 120 and 300 km from that station hears nothing. The skip distance is the 300 km figure; the skip zone is the 180 km gap.

Hops

A signal returned to earth can be reflected back up by the ground and refracted down again, and so on. Each up-and-down is a hop.

Those are the working figures the syllabus material uses, and they are the ones an arithmetic question will expect. Beyond about 4000 km the losses at each ground reflection start to mount, so a long path is always weaker than its length alone suggests. If a question states a hop distance, use the number it gives you and divide.

Worked example — counting hops

A single hop covers at most 2000 km. What is the minimum number of hops for a 3600 km path?

3600 ÷ 2000 = 1.8, so one hop is not enough and you round up: two hops, each about 1800 km.

Every ground reflection costs signal, so a five-hop path is much weaker than the distance alone suggests. This is one reason a contact into Europe from India on 20 m can be strong while one into South America, which needs more hops, is marginal.

Fading — four different things with one name

“Fading” on the paper means several distinct mechanisms, and telling them apart on the air is a genuinely useful skill.

The distinction the examiner wants is between general fading, where the whole signal fades equally and AGC copes, and selective fading, where parts of one signal fade differently and no amount of AGC will fix it.

The sun sets the conditions

The ionosphere exists because of the sun, so solar behaviour governs everything above.

• The sunspot cycle. Sunspot numbers rise and fall over roughly 11 years. At the peak, F2 ionisation is high enough to keep 15 m and 10 m open for hours a day and to make worldwide contacts possible with modest power. At the bottom of the cycle those bands can be closed for months and the action moves to 20 m and below.
• Solar flux. The 10.7 cm solar flux index, measured daily, tracks ionising output more directly than sunspot counts do; a flux above about 150 usually means good high-band conditions. The sun's 27-day rotation brings the same active region back round, so conditions often repeat at that interval.
• Sudden ionospheric disturbance. A solar flare floods the sunlit side of the earth with X-rays, the D layer ionises enormously, and HF absorption rises so much that the bands go abruptly silent — sometimes within a minute — for anything from a few minutes to a couple of hours. Only the daylight side is affected. If every band goes quiet at once and the receiver noise floor drops, suspect an SID rather than your antenna.

Day and night, from India

Put the D layer and the F layers together and the daily rhythm of the bands follows inevitably.

The reason is the D layer: it exists only in daylight and it absorbs most fiercely at the lowest frequencies. In the daytime it removes 80 m and 40 m from DX use entirely. At sunset it goes, leaving the F layer to refract, and those bands open. The high bands, by contrast, need strong F2 ionisation to be returned at all, and that only exists in daylight — so they close when the sun goes down. Watch also for the grey line, the twenty minutes or so around your sunrise and sunset when the D layer has gone but the F layer is still ionised: the low bands can be remarkably good along the terminator.

Propagation without the ionosphere

Tropospheric propagation and ducting

The troposphere is the lowest 10–14 km of the atmosphere, and its refractive index falls gradually with height. That gradient bends a VHF or UHF ray slightly downwards, which is what extends the radio horizon past the optical one. When a temperature inversion forms — warm air lying over cool, very common over the Arabian Sea and the Bay of Bengal, and over the plains on still nights after a hot day — the gradient becomes strong enough to trap signals in a duct, and 2 m contacts of several hundred kilometres become possible with ordinary equipment. This is tropospheric, not ionospheric: it works on VHF and UHF, where the ionosphere does nothing.

Sporadic E

Patches of intensely ionised cloud sometimes form at E-layer height, lasting minutes to hours and covering a few hundred kilometres. They will return frequencies far above the normal E-layer critical frequency, giving sudden strong openings on 10 m and 6 m and occasionally on 2 m, over roughly 1000–2000 km. Sporadic E is commonest in the northern summer and has nothing to do with sunspot number, so it is the one route to a 6 m opening even at solar minimum.

VHF and UHF: line of sight and the radio horizon

Above about 30 MHz the ionosphere no longer bends signals back down under normal conditions; they pass straight through. What remains is the space wave, and range is set by geometry — how far you can see, plus a little. Because the troposphere bends the ray gently downwards, the radio horizon is about 15% further than the optical horizon.

The optical version of the same formula uses 3.57 rather than 4.12, and 4.12 ÷ 3.57 is 1.15 — the 15% is built into the constant. For a path between two stations, work out each station's horizon and add them.

Worked example — how far will 2 m reach?

Your antenna is 30 m above ground on a Bengaluru rooftop. The station you are working has hers at 10 m.

d1 = 4.12 × √30 = 4.12 × 5.48 = 22.6 km

d2 = 4.12 × √10 = 4.12 × 3.16 = 13.0 km

total = 22.6 + 13.0 = 35.6 km

Notice what does not appear in that calculation: power. Doubling your transmit power moves that figure by almost nothing, while doubling your antenna height moves it by 41%. Height, then a clear path, then power — in that order, always, on VHF.