Chapter 27: Propagation

15, 12, 10 and 6 m

The high bands are the top of HF and the bottom of VHF: about 15, 12, 10 and 6 m. The mid-bands run from about 40 m to 17 m, and the low bands are 160 to 60 m.

160, 80 and 60 m

The low bands are the bottom of HF and the top of MF: 160, 80, 60 and perhaps 40 m. The mid-bands run from about 40 m to 17 m, and the high bands are 15 to 6 m.

VHF

6 m is about 300 ÷ 6 = 50 MHz, inside VHF (30 to 300 MHz). The manual’s “high bands” put 6 m beside 15 to 10 m, but those three are HF.

1 m down to 100 mm

λ = 300 ÷ F, with F in MHz: 300 ÷ 300 = 1 m and 300 ÷ 3 000 = 0,1 m = 100 mm. 10 m down to 1 m is VHF, 30 to 300 MHz.

Long contacts, judged by what is usual on the band

DX means long distance, and what counts depends on the band: a contact that is routine on the high bands can be great DX on the low bands or VHF. There is no fixed distance.

6,25 %

Power falls with the square of distance: 20 ÷ 5 = 4 times as far leaves 1 ÷ 4² = 1/16 = 6,25 %. 25 % treats the power as falling simply in proportion to distance.

20 dB

Ten times the distance leaves 1/10² = 1/100 of the power, and a ratio of 100 is 20 dB. 10 dB would mean the power falling only in proportion to distance, not with its square.

10 dB

The power ratio is (40 ÷ 12)² = 3,33² = 11,1, and 10 log₁₀ 11,1 = 10,5 dB, so 10 dB is nearest. 5 dB comes from the distance ratio without squaring it.

6 dB

Twice the distance leaves a quarter of the power (1 ÷ 2²), and a quarter is 6 dB down. 3 dB is half the power, as if it fell only in proportion to distance.

Its power is spread over an ever larger sphere

The power spreads out over the surface of a growing sphere, so each square metre receives less the farther it is from the antenna. Free space absorbs nothing; the loss is all spreading.

Direct wave propagation

By the manual, a wave that goes straight from one antenna to the other is the direct wave, also called line of sight. A groundwave instead bends to follow the earth’s surface, mostly at LF and MF.

Direct wave

There is no groundwave at VHF, and 8 km over flat ground is well within direct-wave range (about 20 km between 10 m masts). Tropo scatter and sporadic E are modes for hundreds of kilometres or more.

A high site can be seen from far off, and VHF needs a clear path

VHF repeaters depend on direct waves, so a high site can be “seen” from far off, giving coverage out to perhaps 100 km. VHF has no groundwave, so no site makes it hug the ground.

Either kind works, but both must use the same

Horizontally and vertically polarised waves travel equally well over a line-of-sight path, but the wave keeps its polarisation, so both antennas must match. Crossed polarisations lose much of the signal.

At any frequency at all

The Summary (p260) says signals of any frequency can travel directly from transmitter to receiver; on p252 a word is missing. It only seems a VHF mode because VHF has no groundwave or regular skip.

Radio waves pass through some materials, like wood, that block light

Some materials that stop light, such as wooden structures, let radio waves through, so a path can work without a clear view. Fog and smoke do the reverse: they dim light but hardly touch radio.

The ground slows the wave near it, bending it downwards

The nearby ground slows the lower part of the wave, so it bends towards the ground (refraction) and follows the earth’s curve. Bouncing between ground and ionosphere is multi-hop skywave, not groundwave.

Losses in the ground weaken it as it travels

The same ground that bends the groundwave round the earth also attenuates it, so its range depends on power and reaches a few hundred kilometres. Following the curve is exactly what a groundwave does.

LF and MF

Bending by the ground is strongest at low frequencies, so groundwave matters most in the LF and MF ranges. It is weaker at HF and absent at VHF and above.

VHF has no groundwave, so FM needs a clear path

Medium waves (530 kHz to 1,6 MHz) follow the earth as a groundwave, but at 88 to 108 MHz there is no groundwave, so FM reaches only as far as the direct wave. Power is not the reason.

80 m works far better, since groundwave favours low frequencies

Groundwave is strongest at low frequencies (best at LF and MF, weaker at HF), so 3,5 MHz beats 28 MHz. In practice groundwave also needs vertical, not horizontal, polarisation.

X-rays, ultraviolet and particles from the sun

The sun’s X-rays, ultraviolet and solar-wind particles knock electrons off gas molecules, leaving a plasma. Visible light and heat carry too little energy to ionise, and meteors leave only brief trails.

About 70 km

The D layer lies about 50 to 90 km up, so about 70 km. 120 km is in the E layer (90 to 150 km), 250 km in the F2 layer and 10 km at the top of the troposphere.

About 180 to 300 km

By day the F2 layer is the highest, from about 180 km to 300 km and above. 150 to 180 km is the F1 layer, 90 to 150 km the E layer and 50 to 90 km the D layer.

The E layer

The E layer lies from about 90 to 150 km, between the D layer below (50 to 90 km) and the F1 layer above (150 to 180 km).

It absorbs them, the lower frequencies most

The D layer mainly absorbs, and its absorption shrinks with the square of the frequency, which hits the low bands hardest. Bending signals back to earth is the work of the E, F1 and F2 layers.

4 dB

Absorption falls with frequency squared: 32 × (3,7 ÷ 10,1)² = 32 × 0,134 = 4,3 dB, nearest 4 dB. 12 dB scales by the frequency ratio without squaring it.

Only by day; it fades quickly after sunset

The D layer exists only while the sun ionises it; after sunset its ions and electrons recombine almost at once. It is the F layers that lose their ionisation slowly after dark.

They merge into one F layer, less strongly ionised

At night the F1 and F2 layers merge into a single F layer, more weakly ionised than by day, so it returns only lower frequencies. It does not vanish: it carries the low bands at night.

The Kennelly-Heaviside layer

The E layer is also called the Kennelly-Heaviside layer; the Appleton (Appleton-Barnett) layer is the F layer. The ozone layer lies in the stratosphere, and the Van Allen belts far above the ionosphere.

Lower frequencies are bent more

Refraction in the ionosphere is stronger at lower frequencies, so a high enough frequency is not bent back enough to return and goes on into space. That is why every path has a highest usable frequency.

The F2 layer

Refraction by the F2 layer (the single F layer at night) carries most long-distance HF. The F1 and E layers are lower, so their hops are shorter, and the D layer mainly absorbs.

It passes through the ionosphere and is lost in space

Above the MUF the layers cannot bend the wave enough to return it, so it carries on into space. The E layer returns only lower frequencies than the F layer does, so it cannot help.

The LUF, as a stronger signal is heard above the noise

More EIRP lets a signal weakened by D-layer absorption still beat the noise, so the LUF falls. The MUF depends on the ionisation: above it, extra power simply goes into space.

The highest frequency that works on a given path at a given time

The MUF is the highest frequency that works on a particular path at a particular time. The highest frequency that returns when sent straight up is the critical frequency, usually lower than a path’s MUF.

The highest frequency sent straight upwards that still comes back

Signals sent straight up come back only below the critical frequency. A path’s MUF is usually higher, because waves meeting the layer at a shallow angle return more readily.

More readily, so a path’s MUF can exceed the critical frequency

Like a flat pebble skipping on a pond, a wave arriving at a shallow angle can be returned when one going straight up passes through. So a long path’s MUF can be much higher than the critical frequency.

Just below the MUF

Both D-layer absorption and atmospheric noise fall as the frequency rises, so the best frequency (the OTF or FOT) is usually just below the MUF. Right on the MUF the path is marginal and drops out.

The gap between groundwave range and the first hop

In the skip zone a receiver is beyond the reach of the direct wave and groundwave but short of where the first hop comes down, so little or nothing is heard. One hop’s length is the skip distance.

Roughly 4 000 to 5 000 km

The manual gives about 5 000 km for one F-layer hop (in practice nearer 4 000 km) against about 2 500 km for the lower E layer. Longer paths need several hops, with ground reflections between them.

Signals arriving by several paths, adding and cancelling

Signals arriving by two or more paths have different delays, so they add or cancel as the ionosphere shifts, giving fades tens of dB deep. Interference adds unwanted signals; it does not make the wanted one fade.

The F layers lose ionisation slowly and stay sunlit longer

The F layers lose their ionisation slowly (a plasma conducts poorly) and, being high, stay sunlit after local sunset; on east-west paths the turning point lies west, in sunshine. The D layer vanishes quickly.

About 11 years

Solar activity rises to a maximum about every 11 years, with a minimum in between. Seven or eight years is the manual’s figure for the fall from a maximum to the next minimum, not the whole cycle.

They can rise well above 50 MHz, opening 10 m and 6 m

High solar activity means intense ionisation, so daytime MUFs can pass 50 MHz and 10 m and 6 m open for long-distance contacts. MUFs below 20 MHz are typical of the solar minimum.

The mid-bands

Near a minimum MUFs are low, so daytime DX moves down to the mid-bands, about 40 to 17 m, with the low bands used for DX at night. The high bands carry daylight DX near the maximum.

The solar flux index (SFI)

The solar flux index and the sunspot number correlate very well, so flux measurements can be compared with cycles counted by eye. The A and K indices show how stormy conditions are instead.

How stormy or disturbed conditions are

A and K show how stormy conditions are: the manual says on the solar surface, and strictly they measure disturbance of the earth’s magnetic field. The level of activity is given by the sunspot number and solar flux.

Tokyo, as the whole path is in daylight

Propagation depends on conditions along the whole path: at 09:00 here it is afternoon in Tokyo, so that path is all daylight, while New York is in darkness. A mixed day-night path is difficult, whatever its length.

South-east, over Australia

Any two places are joined by two great-circle paths. To the USA the short path heads north-west, over the Atlantic; the long path leaves the opposite way, south-east, crossing Australia and the Pacific.

21,1 m

For DX the manual wants a horizontal antenna at least one wavelength up: λ = 300 ÷ 14,2 = 21,1 m. Half a wavelength (10,6 m) gives a higher main lobe, about 27° in chapter 26.

Around sunrise and sunset

At dawn and dusk the change between day and night tilts the ionospheric layers along the twilight zone, giving brief openings over huge distances that low-band DXers wait for. Noon and midnight are not grey-line times.

Sporadic E

Strong, steady VHF signals over 1 800 km for hours on a summer day point to sporadic E. Tropo scatter reaches only about 500 km, and auroral scatter serves only stations near the poles.

In summer, in daylight hours

Sporadic E appears unpredictably, but most often in summer daylight, and an opening may last several hours. It is neither a winter-night mode nor tied to the sunspot minimum.

Small, intensely ionised patches in the E layer

Sporadic E is VHF refraction by small, intensely ionised patches in the E layer, which come and go unpredictably. Signals trapped under an inversion are tropospheric ducting.

Distant ground scatters some energy, which the ionosphere returns

Where the first hop lands, the ground scatters some energy towards the sender, and the ionosphere returns it into the skip zone, weak but readable. Round-the-world signals are far rarer, and aircraft scatter is a VHF effect.

They rely on bursts of propagation lasting seconds

A meteor’s ionised trail lasts only a few seconds, perhaps tens of seconds, before recombining, so contacts are pieced together from short bursts, with fast Morse or digital modes such as FSK441 (today MSK144).

6 m and 2 m

Meteor trails refract VHF signals, and the manual names 6 m and 2 m as the typical meteor-scatter bands. 15 m and 10 m are HF bands, carried by the F layer.

It absorbs HF but can reflect VHF and UHF

Aurora absorbs HF, yet strong aurora can reflect VHF and UHF; the moving reflector adds Doppler shift, so CW sounds rough and raspy. Only stations near the poles can use it.

It extends their range a little beyond the visual horizon

Bending in the troposphere curves VHF and UHF waves slightly back towards the earth, so the radio horizon is about 15 % beyond the visual horizon. Normal bending adds range rather than reducing it.

69 km

Tropospheric bending puts the radio horizon about 15 % beyond the straight-line (visual) one: 60 × 1,15 = 69 km. 80 km scales the distance by four-thirds, but that factor enlarges the earth’s radius, not the horizon distance.

To allow for bending in the troposphere

A larger earth curves less, which mimics the slight downward bending of VHF and UHF waves in the troposphere, so the radio horizon is about 15 % farther. Scatter is a separate, weaker mode, and antenna height a separate term.

FM needs more power than CW or SSB to be understood

Tropo-scatter signals are weak, and FM needs a stronger signal than CW or SSB to be intelligible. Since tropo scatter lasts, ordinary CW and SSB work; no special fast modes are needed.

About 100 to 500 km

Irregularities of temperature and humidity in the lowest 10 km scatter VHF and UHF signals weakly over about 100 to 500 km. Thousands of kilometres need ducting or sporadic E.

VHF signals are caught beneath an inversion layer and carried far

VHF signals get trapped under an inversion layer, with the ground or a second inversion below, and can cover thousands of kilometres with little loss, mostly along coasts. Clouds do not reflect VHF.

The ionosphere very rarely returns such signals

On VHF and above, ionospheric propagation is exceedingly rare, so bouncing signals off the moon is the main way to span continents. The moon is no better a reflector at VHF; the ionosphere makes the difference.

Big stations, and modest ones using weak-signal digital modes

EME was once only for high-power stations with large steerable arrays, but weak-signal digital modes such as JT65 (now often Q65) let modest stations make contacts too, especially with a big station at the other end.

10²⁵ times

A power ratio in dB is 10 log₁₀ of the ratio, so 250 dB is 10^(250 ÷ 10) = 10²⁵; the percentage printed on p258 has one zero too many. 3,2 × 10¹² divides by 20, the voltage rule.

2,5 s

The echo goes to the moon and back: 2 × 384 000 km ÷ 300 000 km/s = 2,56 s, so 2,5 s is nearest. 1,3 s counts the one-way trip only.

A sunspot maximum at the time

The manual lists construction, antenna work, high-power transmitters, low-noise receivers, tracking and astute operating. EME goes through the ionosphere to the moon, so sunspots do not help it.

Transmit on 70 cm, listen on 2 m

The first letter names the uplink and the second the downlink; U is UHF (70 cm) and V is VHF (2 m). So you transmit on 70 cm and listen on 2 m, the reverse of the manual’s Mode V/U.

SSB, CW, FM and digital

Amateur satellites relay signals in various modes, FM, SSB, CW and digital among them. They work much like repeaters, but usually listen on one band and transmit on another.

Doppler shift, as it comes closer, then recedes

A satellite moves at high speed, so its signal arrives Doppler-shifted, higher as it approaches and lower as it recedes. Operators retune the downlink and adjust the uplink to allow for it.

Tracking satellites across the sky

Azimuth is the compass bearing and elevation the angle above the horizon; gain antennas for satellites must follow both as a satellite crosses the sky. An HF beam needs only azimuth.

Just enough for reliable contacts

A satellite’s power is limited, and a too-strong uplink makes its on-board AGC cut the receiver’s sensitivity, drowning other users. Use only the power needed for reliable contacts.

Low-power access, but a small footprint and short passes

Low satellites are close, so low power and simple antennas reach them, but each covers a small footprint and a pass may last only minutes. High-orbit satellites give larger footprints and longer passes.

It is geostationary, so a fixed dish sees it at any hour

QO-100 is geostationary, staying put in the sky, so a fixed dish of about 800 mm works it at any hour. It needs microwave equipment, not 2 m FM.

The path loss to expect, in dB

Forecasting tools, many based on VOACAP, use solar data to work out a path loss in dB. Knowing the power sent and the power needed, you can then judge whether the path will work.

102 dB

At 300 MHz λ = 1 m, so LP = 20 log₁₀(4π × 10 000 ÷ 1) = 102 dB. 80 dB leaves out the 4π, 51 dB uses 10 log, and 42 dB mixes km with m.

About 9,5 dB more on 432 MHz

LP = 20 log₁₀(4πd ÷ λ), and the 432 MHz wavelength is a third of the 144 MHz one, so the loss is 20 log₁₀ 3 = 9,5 dB more. Higher frequency, shorter wavelength: more path loss.

190 dB

λ = 300 ÷ 2 400 = 0,125 m, so LP = 20 log₁₀(4π × 36 000 000 ÷ 0,125) = 20 log₁₀(3,6 × 10⁹) = 191 dB: 190 dB is nearest. 170 dB leaves out the 4π.

−77 dBm

PRx = PTx + GTx − LTx − LP − LM + GRx − LRx = 40 + 10 − 2 − 130 − 3 + 10 − 2 = −77 dBm. −87 dBm counts only one antenna gain.

14 dB

The link margin is the predicted signal-to-noise ratio minus the minimum needed: 23 − 9 = 14 dB of unexpected loss can be taken before the link fails. Adding the two (32 dB) means nothing.

A tally of power, gains and losses, to judge if the path will work

A link budget adds up transmitter power, antenna gains, feeder and other losses and the path loss to predict the received signal, and so whether a link is feasible. Path loss alone is just one entry.

+20 dBm

PTx = PRx − GTx + LTx + LP − GRx + LRx = −100 − 12 + 2 + 140 − 12 + 2 = +20 dBm (100 mW). +68 dBm adds the antenna gains instead of subtracting them.