Radio waves and propagation

Navigation · Chapter 36 · DGCA CPL and ATPL

What a radio wave is, the band table and what lives in each band, how waves travel, the ionosphere and HF, modulation and aerials.

Remember

What is a radio wave?

An electric and a magnetic field

  • Where it comes from • Push a current up and down a wire many times a second • Its magnetic and electric fields change with it • Fast enough, the fields cannot sink back into the wire: they break away and travel. That is a radio wave
  • Two fields • An electric field and a magnetic field, at right angles to each other: an electromagnetic wave • Each keeps making the other as they go, so the wave needs no air to travel • Both fields are also at right angles to the way the wave travels
  • Polarisation • polarisation = the plane of the electric field • It copies the aerial: a vertical aerial → vertical polarisation • Receive it best with an aerial in the same plane
  • Circular polarisation • The field can also turn as the wave travels • Radar: rain sends it back turned the other way, so rain echoes can be filtered out • Any aerial angle works: handy for satellites and phones
  • Speed • 300 000 km/s, the speed of light (light is the same kind of wave) • About 162 000 NM every second

How long is a radio wave?

Frequency and wavelength

  • One cycle in time • One full wave takes a time: its period (τ) • Frequency = how many cycles each second, measured in hertz (Hz) • So f = 1 ÷ τ
  • Short names for big numbers • 1 kHz = 1 000 Hz • 1 MHz = 1 000 000 Hz • 1 GHz = 1 000 000 000 Hz
  • One cycle in space • In one period the wave moves one wavelength (λ) • The speed never changes, so more cycles a second = shorter cycles • λ (m) = 300 ÷ f (MHz)
  • Examples • 150 MHz → 2 m · 30 MHz → 10 m • 300 kHz = 0.3 MHz → 1 km

Solve it: wavelength and band from a frequency

Solve it: wavelength and band

  • Solve it: wavelength and band from a frequency

Which band is it?

The band table

  • Why bands matter • The frequency decides how a wave travels (along the ground, off the sky, or in a straight line) • and how big the aerial must be • So every radio aid lives in the band that suits its job
  • Eight bands, ×10 each • Radio runs from 3 kHz to 300 GHz, cut into 8 bands • Each band up: ×10 the frequency, ÷10 the wavelength
  • Read one row • VHF: 30–300 MHz • Wavelengths 10 m down to 1 m • The stripe says how it travels: gold = straight lines
  • Easy anchors • 1 m = 300 MHz (VHF | UHF) · 10 cm = 3 GHz (UHF | SHF) • 100 m = 3 MHz (MF | HF) · 1 km = 300 kHz (LF | MF)
  • Wavelength names • Each name says the wave’s size: LF kilometric (1–10 km), MF hectometric (100s of m) • HF decametric (10s of m), VHF metric, UHF decimetric, SHF centimetric
  • How each band travels • Teal: LF, MF follow the ground (surface wave) • Pink: HF bends back off the ionosphere (sky wave) • Gold: VHF and up go in straight lines (line of sight)

Which aid uses which band, and why there?

What lives in each band

  • LF and MF: beacons • NDB 190–1 750 kHz: the ADF needle points at it • Why here: long waves follow the ground, so a cheap beacon reaches you low down and far away • The price: the most static (lightning noise is loudest at low frequencies)
  • HF: across the oceans • HF R/T, 2–30 MHz • Why here: HF is the band the ionosphere bends back, so it hops thousands of miles • The only long-range voice without a satellite
  • VHF: the busy band • VOR 108–117.95 · ILS localizer 108.1–111.95 · VHF R/T 118–137 · markers 75 · VDF • Why here: straight lines, little static, short aerials: clean and accurate, but only as far as you can “see”
  • UHF: pulses and satellites • Glide path 329–335 · DME 962–1 213 · SSR 1 030 / 1 090 · GNSS 1 575 · SATCOM • Why here: short waves make narrow beams and sharp pulses, and they pass straight through the ionosphere to satellites
  • SHF: radars • Radio altimeter 4.2–4.4 GHz · MLS ≈ 5 GHz · weather radar 9 375 MHz • Why here: centimetre waves give pencil beams and fine detail; 3 cm waves bounce best off big raindrops
  • Your cheat sheet • Tap every chip and watch it work • For each aid learn: principle, band, errors, use

How do you compare two waves?

Phase comparison

  • Out of step • Two waves on the same frequency can be out of step, like two clocks set to different times • The gap, in degrees of one cycle (360°), is the phase difference
  • Why it matters • A VOR sends two such waves; how far out of step they are is your radial • So measuring phase = measuring direction
  • Measure it • Start where the reference is at zero (A) • Go forward until the variable is at zero (B) • A → B is the phase difference
  • Or subtract • Reference − variable • Negative? Add 360° (a full cycle changes nothing)

Why does a signal get weaker with distance?

Why signals fade

  • Attenuation • Signals weaken as they travel: attenuation • Absorption: air, water drops, ground soak energy up as heat • Shorter waves are soaked up more: big above 1 GHz
  • Spreading out • The energy spreads over a growing sphere, like paint over a bigger balloon • Twice as far: the same energy covers 4 squares, so each gets ¼ • This is the inverse square law: strength falls with the square of the distance
  • Three times as far • Nine squares: ÷ 9 • So to double the range you need 4 × the power
  • Static and fading • Static: lightning and machines make radio noise, loudest at low frequencies; tiny at VHF and up • Fading: the same signal arrives by two paths, out of step, and partly cancels
  • What adds range • More power (but only as the square root) • A more sensitive receiver • A narrow beam: the energy goes one way instead of all round

Which way does each kind of radio wave travel?

Surface, space, sky

  • Surface wave • Follows the curve of the Earth: the ground drags the bottom of the wave, so it leans forward and bends round • Works for long waves: about 20 kHz to 2 MHz (LF, MF)
  • Space wave • A straight line, like light: line of sight • VHF and up (above about 50 MHz): too short to bend round the Earth, too fast for the ionosphere to turn back
  • Sky wave • HF (about 2–30 MHz) goes up and is bent back down by the ionosphere • It lands far beyond the horizon
  • Satellite • UHF and SHF go straight through the ionosphere • So satellites can talk to the ground: SATCOM, GNSS
  • All together • LF, MF: surface (sky wave too at night) • HF: sky (a little surface) · VHF and up: space wave only

How far does a surface wave go?

The surface wave

  • It bends with the ground • The wave is not a line on the ground: each front stands up off it, strongest low down and weaker higher up • The ground slows its feet, so it leans forward and follows the curve (diffraction) • Each front gives some energy to the ground as it goes: the wave slowly dies
  • Why lower frequency goes further • A longer wave bends round the curve more easily and loses less to the ground • So LF and MF go far; by HF the surface wave dies within tens of miles
  • Sea beats land • Sea water conducts electricity well: it takes less energy from the wave • Dry land soaks up more: shorter range
  • Why the aerial is vertical • Near the ground a horizontal electric field is shorted out by the earth and dies at once • A vertical one survives: surface-wave aerials stand upright
  • A rule of thumb • At 300 kHz: sea 3 √P, land 2 √P (NM, P in watts)

How far can a VHF signal reach?

Line-of-sight range

  • Two paths • VHF reaches you by a direct wave • and by one bounced off the ground (together: the space wave)
  • The radio horizon • The Earth bulges between you: beyond the horizon there is nothing, however strong the transmitter • Watch the aircraft fly out: above the station’s gold line it hears; below it, in the radio shadow, the bulge is in the way • Higher aerial or higher aircraft → the horizon moves out
  • Why a square root • From a tower you see further, but each extra foot adds less and less: horizon distance grows with √height • Radio bends a little with the air, so its horizon is a bit beyond what the eye sees: hence 1.23
  • Worked • Station 400 ft, aircraft 10 000 ft

Solve it: line-of-sight range

Solve it: VHF range

  • Solve it: line-of-sight range

Can VHF ever reach beyond the horizon?

Super- and sub-refraction

  • Why VHF bends at all • Air gets thinner and drier with height, and radio travels a touch faster in thinner air • So the top of a wave runs ahead and the wave curves gently down, round the Earth a little • That is why radio reaches about 15 % beyond the straight-line (geometric) horizon from the same tower
  • Super-refraction • If the air thins out faster than usual (dry air above moist, temperature falling slowly), the bend is bigger: super-refraction • Up to 40 % more range: fine weather, high pressure, warm air over a cool surface
  • Ducting • A strong inversion (warm, dry air over cool, moist air) bends VHF right back down, again and again: a duct • Hundreds of NM: distant stations interfere
  • Sub-refraction • The opposite is sub-refraction: moist air above, temperature falling fast, low pressure, cold air over a warm sea • The wave bends less: range down by up to 20 %

What is the ionosphere, and why does it change?

The ionosphere

  • Sunlight makes it • From about 60 km up, the Sun’s ultraviolet knocks electrons off air atoms • The freed electrons are what bend radio waves • No sunlight, no new electrons
  • A tug of war • All the time, freed electrons find atoms and rejoin them • So the electrons at any moment = what the Sun makes minus what rejoins
  • Layers, not one sheet • The electrons gather in layers, each densest in its middle • D ≈ 75 km · E ≈ 125 km · F ≈ 225 km
  • Why D vanishes at night • D is low, where the air is thick: a freed electron meets an atom within moments • At sunset nothing new is made, so D is gone almost at once • At sunrise it is rebuilt just as fast
  • Why F lasts the night • At 250 km the air is so thin an electron can go hours without meeting an atom • So F fades slowly and survives the night; F1 and F2 join into one F • E, in between, weakens but stays
  • Why the peak is at 1400 • Like the warmest time of day: the Sun keeps making more than is lost until early afternoon (about 1400) • All night the loss wins: lowest just before sunrise
  • More sun, more electrons • Summer and low latitudes: the Sun is higher, its ultraviolet stronger • Solar flares: sudden bursts, can upset radio badly • Dawn and dusk: the layers are building or fading, so HF is unsettled

How does the ionosphere send a wave back, and where does it land?

Critical angle, skip, dead space

  • Why a wave bends • In a layer, radio runs a little faster where there are more electrons • A slanting wave front has its top in more electrons: the top runs ahead, the front tips over and the wave turns down
  • The searchlight • Now imagine the transmitter sweeping its beam from straight up down to the horizon • Watch which rays come back and where they land
  • Too steep escapes • A steep ray must turn almost all the way round to come back: the layer cannot bend it that much • It punches through and is lost to space
  • The critical angle • Coming down from the vertical, the first ray that does come back sets the critical angle (measured from the vertical) • Every ray lower than that comes back too
  • Skip distance • That first returning ray lands closest to the transmitter • Transmitter → that point = the skip distance: no sky wave lands nearer
  • Dead space • The HF surface wave dies within a few tens of miles • Between where it dies and where the sky wave lands: dead space, nothing heard
  • Which layer bends it • Up to about 2 MHz: the E layer is enough • 2–50 MHz: it takes the denser F layer • Above 50 MHz: no sky wave (only in rare solar storms)

What makes the skip distance change?

What moves the skip

  • Raise the frequency • A faster-wiggling wave hardly moves the electrons, so it is bent less • Only shallower rays come back: critical angle, skip and dead space all grow
  • Higher still • 20 MHz: the first sky wave lands much further out • Drag the frequency and watch the pink zone move
  • More ionisation • More electrons bend more: steeper rays come back • Critical angle, skip, dead space all shrink • Move the clock: noon versus night
  • Night • Fewer electrons at night: the wave goes higher before turning, so the skip grows
  • A higher layer • Bent back by a higher layer (F instead of E), the wave comes down further away • Like throwing a ball higher: it lands further

Why do NDBs misbehave at night?

LF and MF at night

  • Day • In the D layer the air is thick: electrons shaken by the wave keep bumping into air, turning its energy into heat • Low frequencies shake them most: below about 2 MHz the sky wave is swallowed • Only the clean surface wave reaches you
  • Night • After sunset D is gone, so nothing swallows it: the E layer bends LF/MF back down • Sky wave and surface wave arrive together, out of step: the ADF needle wanders. This is night effect

How far can a sky wave go?

One hop, many hops

  • The longest single hop • Longest when the wave leaves and lands skimming the ground: any lower and the Earth’s bulge blocks it • E layer: about 1 350 NM · F layer, higher up: about 2 200 NM
  • Many hops • The ground (best of all the sea) reflects it up again: multi-hop • Like a stone skipping across a pond: five F-layer hops reach about halfway round the world

Which HF frequency should you use, by day and by night?

Choosing an HF frequency

  • MUF • For a station at a given range, the muf is the frequency whose first sky wave lands right there • It is the best: it spends the least time inside the ionosphere, so it picks up the least loss and noise
  • OWF • But the ionisation wobbles: if it dips, the skip grows and jumps past you • So use a little less: OWF ≈ 0.85 × MUF
  • Day, night, range • Night: fewer electrons → about half the day frequency • Longer range → higher frequency; shorter → lower
  • Where the wave bends • What counts is day or night at the refraction point, about halfway • Mid-ocean at sunrise: east station (day) 13 MHz, west station (night) about 6.5 MHz

How is a voice or a code put on a radio wave?

Keyed and AM

  • Why a carrier • Your voice is sound, a few kHz: far too slow to leave an aerial (it would need one kilometres long) • So a fast radio wave, the carrier, takes it along. Putting the message on it is modulation
  • Keyed • Simplest: switch the carrier on and off in Morse • NDBs still send their ident this way
  • Amplitude modulation • AM: the audio’s height changes the carrier’s height • The receiver follows the outline (dashed) to get the voice back • VHF voice uses AM

How wide is an AM signal, and what is SSB?

Sidebands and SSB

  • When you talk • Nobody talking: the radio sends one frequency, a thin line on the dial (the carrier) • You talk: two extra bands appear, one each side. These are the sidebands • Like a truck: a crate alone fits a thin lane. Tie a parcel to each side and it needs a wider lane
  • How wide • Each sideband is as wide as the highest note in your voice • There are two, so width = 2 × audio • Drag the note and watch the lane change
  • Who carries the message • The crate (carrier) is empty: it says nothing, yet takes about ⅔ of the power • The parcels (sidebands) both hold the same words • So only about ⅓ of the power carries your voice
  • Single sideband • One parcel is enough. Remove the crate and the second parcel: that is SSB • Half the width, so twice as many radios fit the same stretch of the band • Also less power and less noise. Used for HF voice

What are FM, phase and pulse modulation used for?

FM, phase and pulse

  • Frequency modulation • FM: the carrier’s height stays the same; the audio’s height sets how far its frequency shifts • The audio’s pitch sets how often it shifts
  • Why FM sounds cleaner • Static mostly changes a signal’s height; an FM receiver ignores height • But for the same power it reaches less far
  • Why not FM for aircraft voice • FM needs a wide channel: broadcast 150 kHz (AM 9 kHz) • Even narrow FM (≈ 8 kHz) is wider than the 6 kHz aviation channel
  • Phase modulation • The carrier’s phase jumps; for data, 0° = 0 and 180° = 1 • GPS uses it (binary phase shift keying, BPSK), so does MLS
  • Pulse • Short bursts, then listen for the echo or reply • Radar, DME, SSR

What do codes like A3E mean?

Emission codes

  • Three symbols • A short code that says exactly what a signal is • 1st: how the carrier is changed · 2nd: what changes it · 3rd: what it carries • A3E = AM, one sound channel, voice: VHF R/T
  • HF voice • J3E: one sideband, no carrier, voice (that is SSB)
  • The ADF ones • NDB: N0N (plain carrier) with A1A (the carrier keyed in Morse) or A2A (a keyed tone)

How long should an aerial be?

Aerial length and feeders

  • Why length matters • An aerial is a metal rod. The radio pushes electricity up and down it, very fast • Like pushing a swing: push in time and it builds; out of time and it hardly moves • A rod is “in time” when its length suits the wave: then the signal is strong
  • Half-wave dipole • Two rods, with the cable from the radio joined in the middle: a dipole • Together they are half a wavelength long (a half-wave dipole): half of the wave drawn beside them • The signal goes out mostly sideways, at right angles to the rods
  • Quarter-wave (Marconi) • Keep one rod, half the size, and stand it on metal • The metal works like a mirror and supplies the missing half. A stick on a mirror looks twice as long • Only ¼ λ long, short and smooth: this quarter-wave or Marconi aerial is the one aircraft use
  • Work it out • Higher frequency → shorter wave → shorter aerial • Drag the frequency and watch the rods on the ruler
  • Feeders • The feeder is the cable from the radio to the aerial • The higher the frequency, the more an open wire leaks, so the feeder gets more closed in • Like water: a gutter spills, a sealed pipe keeps it all. At the very top: a hollow tube, the waveguide

Which way does an aerial send its energy?

Polar diagrams

  • What the drawing means • A polar diagram is a map of how far the signal reaches in each direction • Go out from the aerial until the signal drops to one chosen level. Mark the spot. Do it every way, and join the dots • Same as walking away from a bare bulb until you can only just read. It gives a circle
  • Not always a circle • A torch throws its light one way, so you can walk much further in front • An aerial can do the same. The long thin shape says at a glance: it sends this way
  • The dipole in 3D • An upright rod sends sideways, all round, and nothing off its ends • So in 3D its shape is a ring doughnut, with the rod through the hole • Turn it: the sparks show the energy leaving
  • From above: a circle • Diagrams usually show a flat slice. Cut the doughnut flat and look down: a circle • The same strength in every direction round the rod
  • From the side: an eight • Now cut it upright and look from the side: a figure of eight • The gap at top and bottom is the rod’s ends, where nothing goes out
  • Making a beam • A slightly longer rod behind (the reflector, 5 % longer, ¼ λ back) sends energy forward, like the mirror behind a bulb • Shorter rods in front (the directors) squeeze it into a beam • The price: small extra lobes to the sides, side lobes
  • The ILS localizer • 16 or 24 aerials in a line work together • They make two narrow lobes, one each side of the runway centreline
  • The loop • Edge-on to the station: one side meets the wave a moment before the other, so there is signal • Face-on: both sides meet it together and cancel: a null • Turn the loop. The shape is a figure of eight with two nulls

Dish or slotted plate: which is better?

Radar aerials

  • The dish • A car headlight has a small bulb in front of a curved mirror, and the light leaves straight ahead • A radar dish (a parabolic reflector) is the same. A small horn sits at the special point, the focus • So the radio waves leave parallel: a narrow beam
  • Side lobes • Most of the energy goes forward, in the main beam • A little spills past the rim and makes weak side beams: side lobes
  • The slotted plate • A flat plate with rows of slots. Each slot sends out a small wave • They all start together, so they join into one straight wave • Drop a line of stones in a pond at the same moment, and you see the same thing
  • Plate or dish? • The plate wins: narrower beam, smaller side lobes, less power, finer detail
  • Radar letters • L 1–2 GHz · S 2–4 · C 4–8 · X 8–12 • Weather radar, 9 375 MHz, is X band

Test yourself on chapter 36

Test yourself

  • Five questions • Bands and the aids that live in them