chapter 01 - basic radio theory

8/13/2019 Chapter 01 - Basic Radio Theory

http://slidepdf.com/reader/full/chapter-01-basic-radio-theory 1/28



WILJAM FLIGHT TRAINING

Period The duration of one cycle (T). In the figure above T =1/100 seconds

Velocity The speed with which a wave travels through a given medium. For aradio wave this is 300 000 000 metres per second better expressedas 300 X 10

6m/s

Frequency The number of complete waves passing a fixed point in one second,denoted by the symbol “f”. Usually expressed as Hertz (Hz). It isobvious that f =

1 /T

Wavelength The distance between similar points on successive waves or thedistance occupied by one complete cycle when travelling in freespace (AB), denoted by the symbol λ (Lambda)

Amplitude The maximum height of the wave. This can be positive or negative.The positive amplitude is represented by “b”

Electro-Magnetic Waves The atmosphere carries light, heat and radio waves. Thesewaves differ only in their frequency and wavelength and the effects they have on differentmaterials. Termed “Electro-magnetic” because of their electrical and magnetic nature. Allthese waves travel at the same velocity, denoted by the letter c. For the Radio Navigationsyllabus this velocity is:

300 000 000 metres per second

for simplicity of calculation this is usually written as

300 X 106 metres per second

Properties of Radio Waves A radio wave that leaves a transmitter has the followingproperties:

They consist of oscillating electric and magnetic fields that are at right angles toeach other and at right angles to the direction of propagation

They require no supporting medium They can be reflected, refracted and diffracted

They are subject to interference and Doppler effect

They can pass through an opaque object such as a building although they dosuffer attenuation in doing so

Relationship Between Frequency, Wavelength and Velocity The frequency of asinusoidal wave is the number of cycles occurring in one second. Conversion of frequency towavelength and vice versa is needed for the JAR exam. Frequency is given the symbol f andthe unit is the Hertz, wavelength is λ the unit used is metres.

1-2




Examples Convert the following wavelengths into the corresponding frequencies (Theanswers are given below):

1. 1500 m

2. 20 cm

3. 3500 m

Convert the following frequencies into the corresponding wavelength:

1. 300 KHz

2. 75 MHz

3. 600 MHz

4. 8800 MHz

5. How many wavelengths, to the nearest whole number, of frequency 200 MHz areequivalent to 35 feet?

Answers

Wavelength to Frequency

1. 200 KHz

2. 1500 MHz

3. 85.7 KHz

Frequency to Wavelength

1000 m

4 m

50 cm

4. 3.41 cm

5. Number of wavelengths in 35 feet for a frequency of 200 MHz

λ = c = 300 x 106

= 1.50 metres

f 200 x 106

1 m is equivalent to 3.28 feet

1.5m = 4.92feet

The number of wavelengths in 35 feet = 35

4.92

1-4




Which is approximately 7

Phase Difference In the diagram below the two waves are said to be in phase. The wavespass the same point of their cycle at the same time.

In the diagram below the waves are said to 90° out of phase:

Wave B leads wave A by 90°, or

Wave A lags wave B by 90°

A B

1-5




Where two waves have a phase difference of 180°, then they are said to be in anti-phase.

Radio Spectrum The electromagnetic spectrum is shown in the diagram below. Thedifferent effects brought about by electro-magnetic waves are determined by their frequency.The lower limit is determined by the size and efficiency of the aerials required and the upperlimit by the attenuation and absorption of the radio waves by the atmosphere.

Wavelength

CosmicRays

Gamma

RaysX-RaysUltraViolet

Li

ght

100 km 1 mm

3 KHz 300 GHz

Infra RedRadioWaves

The part of the frequency spectrum which is of interest to the pilot is further sub-dividedbelow.

VLF LF MF HF VHF UHF SHF EHF

3 30 300 3 30 300 3 30 300

KHz MHz GHz

Radio Spectrum

1-6




Wave Propagation There are three principle paths which radio waves may follow overthe earth between the transmitter and the receiver:

Surface Wave A wave which follows the contours of the earth’s surface

Sky Wave A wave that is refracted by the Ionosphere and returned to earth

Space Wave A wave which is line of sight

A combination of the surface and space waves is called a ground wave.

The radio energy reaching a receiver may be made up of components due to any one or more

of these mechanisms but, depending on the part of the radio spectrum concerned one of thethree will predominate. In general:

Surface

Wave

Ionosphere

SkyWave

Space

Wave

Low frequencies are propagated by surface wave

Middle range frequencies by sky wave, and

Upper range frequencies by space wave.

1-7




Surface Wave The surface wave follows the curvature of the Earth, a processknown as diffraction. The process is helped by the Earth’s attenuation of the radio energy.The wave is slowed as it touches the Earth’s surface. Therefore, the wave front in the

direction of motion will lag at the surface.

Wavefront falls towards the Earth as it

progresses

The wave front is tilted and diffractive bending occurs. The stability of this type of propagationmakes the low frequency surface wave suited to systems requiring consistency of signal overlong distances. The propagation does require large aerials and the cost of transmission canbe considerable.

Type of Surface High conductivity favours the passage of a radio signal. So passageover the sea is better than over rock or desert.

Transmitter Power Surface absorption and free space loss reduce the signalstrength of a radio wave. If there is no restriction in the available transmitter powerthen global ranges can be achieved by VLF radio waves.

Noise and Interference Noise affects the lower frequencies so affecting thesignal/noise ratio. This can limit the usable range.

For maximum ground wave range:

Use low frequency — for maximum diffraction and least attenuation

1-8




Use vertical polarization (see polarization)

Sky Wave The sky wave ascends into the upper atmosphere and encounters a regioncontaining electrically charged particles (the Ionosphere) where it is refracted sufficiently toreturn to Earth.

β Is the Critical Angle. Note that it is measured from the vertical down.

Dead Space

Ground Wave

Skip Distance

β

When the wave enters the Ionosphere it changes direction due to a change in velocity. If thewave penetrates halfway through the layer before being bent parallel to the layer it will bendback in the opposite direction to emerge from the top of the layer as an escape ray. This islikely if:

The Ionization is insufficiently intense.

The frequency is too high

The angle of entry is too acute

Critical Angle For a particular frequency and degree of ionization, it is possible to define acritical angle below which total refraction will not take place. Defining the critical angle alsoestablishes the minimum range - the skip distance. Any ray travelling away from the aerial atgreater than the critical angle will be freely refracted down to about 5° above the horizon.

Dead Space Because of the high frequencies used in sky wave transmission thegroundwave travel is not as far as the returning sky wave. The distance between the limit ofthe groundwave and the first returning sky wave is called the dead space.

The Ionosphere The Ionosphere consists of a series of conducting layers betweenheights of 50 to 400 kilometres. It exists because of the transmission of ultra-violet radiation

1-9




1-10

from the sun. Because of this dependence upon radiation from the sun the heights anddensities of the layers vary according to the:

Time of day

Season of the year

There is also a connection between the 11 year sunspot cycle. Short term effects occur in arandom fashion and these result in the ionized layers being in a state of constant turbulence.Three main layers have been identified and are designated D, E and F. The F layer splits intotwo separate layers during the day, the time of highest ionization. The D layer is a region oflow ionization that only persists during the day. The E layer is more marked and remainsweakly ionized by night with little change in height. The F layer is the most strongly ionizedand has the greatest diurnal change in height.

Day Night

F2

F1

E

D

400 km

200 km

100 km




Frequency and Skip Distance At a fixed level of ionization an increase in frequency willcause the ray, previously the critical ray, to become an escape ray. This will cause anincrease in skip distance.

Ionization and Skip Distance At a fixed frequency if ionization decreases the effect willbe the same as above. The critical ray becomes an escape ray. This will cause an increase inskip distance.

Space Wave Transmissions at VHF and above cannot propagate by either surface or skywave. Attenuation is so severe that the surface wave is virtually non-existent. Thesefrequencies are too high to be refracted by the ionized layers aloft. Transmission is thereforeby straight line - the direct wave. In addition to the direct wave there can also be a reflectedwave. The two components make up the space wave.

Because of the different emission paths the direct and reflected wave will sometimes be in

phase and sometimes out of phase. This will produce lobes and nulls particularly when thereceiver is close to the station.

The range of a space wave appears to be line of sight. In practice it is termed quasi-optical:

The lower atmosphere causes some refraction of the wave which bends it beyondthe optical horizon, and

A further small increase is gained from diffraction when the wave becomestangential to the earth’s surface

1-11




This range can be approximated by the following formula:

R = 1.25(√HT + √HR)

Where: R Range in nautical miles

HT Height of the transmitter in feet

HR Height of the receiver in feet

Example An aircraft flying at 10 000 ft receives a transmission from a station at400 ft. What is the maximum distance communications can be madebetween the two stations?

R = 1.25(√10 000 + √400)R = 1.25(100 + 20)

R = 150 nm

Duct Propagation Under certain abnormal climatic conditions transmissions on afrequency greater than 50 MHz can be received at ranges in excess of the quasi-opticalexpected.

VERY DRY AND RELATIVELY WARM

COOL AND MOIST

-40°C

-3°C+7°C

LOW CLOUD TRAPPEDBELOW INVERSION

INVERSION BASE

SUBSIDINGAIR

AIR TEMPERATURE

DEW POINT

TEMPERATURE

25 JANUARY 1989 - 0001 GMT. ST.HUBERT, BELGIUM

PRESSURE TEMPERATURE DEW POINT MODIFIED REFRACTIVE INDEX

900mb

920mb

945mb

+ 7.1

+ 7.5

- 3.5

- 40.0

- 28.0

- 3.9

250.8

257.3

295.8

The conditions that cause this abnormal propagation are:

A temperature inversion

A rapid decrease in humidity with height

This forms a duct between the earth and a few hundred feet above the surface.

1-12




1-13

Radio waves have a wavelength that is small compared with the duct height. This allows theduct to refract the wave back to earth. The wave is then reflected by the earth’s surface backto the duct ceiling. A series of these refraction/reflection hops occur and thus the wave can be

received well in excess of the quasi-optical range. The same conditions can occur when thereis an inversion aloft.

TRANSMITTER

TRANSMITTER

ELEVATED DUCT

SURFACE DUCT

EARTH

LAYER OF HIGHDIELECTRICCONSTANT

LAYER OF HIGHHUMIDITY

LAPSERATE ANDTEMPERATUREINVERSION

These conditions are normally associated with large high pressure systems; a condition whichis a regular feature in the tropics.

Aerials A transmitter/receiver is only as good as the aerial. An aerial can be definedas a device used for the efficient transmission and reception of electromagnetic energy.Generally we look at aerials that radiate, however, the properties of a transmitting aerial applyequally to the receiving aerial.

Aerial Characteristics When an aerial radiates an electro-magnetic wave two radiofrequency fields are transmitted;

E Field Electric or electrostatic field

H Field The magnetic field.

These fields are transmitted at right angles to each other.




Note that the H field is always at right angles to the aerial and the E field is always parallel tothe plane of the aerial.

By convention the wave is said to be vertically polarized if the E Field Is perpendicular to theearth and horizontally polarized if parallel to it.

A vertically polarized wave is produced by a vertical aerial: a horizontally polarized wave by ahorizontal aerial.

Note This is not true for slot aerials where a vertical slot aerial produces ahorizontally polarized signal and a horizontal slot aerial produces avertically polarized signal.

Aerial Length The aerial is manufactured to a specific length dependent on the frequency tobe used.

Polar Diagrams The effective radiation or reception of an aerial is shown by a polardiagram. These can be shown as:

Horizontal Looking down on the aerial from above

Vertical Looking at the aerial from the side

Omni-Directional Aerials

Simple Half-Wave Dipole In its simplest form a dipole consists of a metal rod or a wire cut toa specified length. The aerial is cut to a half wave-length.

Example For a frequency of 30 MHzλ = 10 m

The aerial for this wavelength will be λ /2 or 5 m

This is called the Electrical Length.

In an ideal world the Electrical Length would be the length of aerial required for a givenfrequency. The speed of electro-magnetic radiation through a vacuum is constant. When an“aerial feeder” is used the speed of the radiation is slower. This slower speed is approximately5% less than the in-vacuo speed and we must take this into account by factoring the ElectricalLength to 95% of its value. This is the Physical Length of the aerial for a given frequency.

Example For a frequency of 100 MHzλ = 3 m

Electrical Length of the aerial =λ

/2 = 150cm

1-14




Physical Length = 95% of λ /2 = 142.5cm

Marconi Quarter Wave Aerial Most practical aerials are cut to

λ

/4. By using the reflectiveproperties of electro-magnetic waves the aerial compensates for the missing half of thedipole.

The Marconi aerial is particularly suitable for fitting into aircraft structures. To ensure that theaerial can be used over a range of frequencies an aerial loading unit (ALU) is fitted. This unitelectronically matches the aerial to the frequency selected.

Aerial Feeders There needs to be a connection between the transmitter/receiver and theaerial, this is known as a feeder. The type of feeder used depends upon the frequency to beused. The most common feeder in use in aircraft communications is the co-axial cable (betterknown to us as the TV aerial wire). Higher frequencies need a more sophisticated feeder,such as radar where a wave guide is required.

Aerial Directivity The dipole radiates power evenly in all directions or omni-directionally.The plan and side views show the radiating pattern.

Note that the radiating polar diagram is from the centre of the aerial not the tip.

1-15




To modify the omni-directional properties and give the aerial directivity parasitic elementshave to be added. The most common directional aerial in everyday use is the TV antenna -The Yagi. The directional properties are derived by adding parasitic elements in front and

behind the dipole.

To change the omni-directional properties a parasitic reflector, 5% larger than the dipole, isplaced at a distance of λ

/4 from the dipole. The normally circular polar diagram is nowchanged into an elongated heart shape. The reflector reflecting the power back towards theaerial. Note that the dipole is the only part of the aerial that has any power.

To enhance the directional properties parasitic directors are added on the opposite side to theparasitic reflector. These elements are 5% shorter than the dipole.

1-16




The resulting polar diagram is narrow in beam width and gives excellent directionalproperties. One disadvantage with the directivity achieved is that unwanted side lobes areproduced. The side lobes are approximately 50% of the power of the main beam and can givespurious indications if not dealt with. Methods of suppression or removal of the side lobes are

discussed in individual chapters on equipment.

Different polar diagrams can be achieved for different aerial combinations. An example of thisbeing the figure of eight’ produced by two dipoles.

The significance of changing the polar diagram will become apparent as each piece ofequipment is discussed in detail.

Modulation Modulation is the superimposing of intelligence, such as speech or Morseidentification, onto a carrier wave. Varying a parameter of the carrier, such as its amplitude orfrequency does this. When electro-magnetic energy is radiated as a sinusoidal wave nointelligence is transmitted. The frequency is beyond the scope of human hearing and thewave itself would be meaningless.

Keying By interrupting the wave, a process known as keying, Morse Code can betransmitted.

1-17




1-18

The frequency may be identifiable as Morse code, but is still outside the audible range. Tohelp with audible reception the carrier frequency has to be converted into a signal within the

audio range.

This is achieved by mixing the received frequency with a known frequency; this produces asignal in the audio range.

Example Received frequency 500 KHz

Known frequency 501 KHz

Four frequencies are produced

500 KHz}

501 KHz} outside the audible range

1001 KHz}

1 kHz an audible frequency

This mixing process is known as heterodyning. The process is carried out by a receiver unit,the detector and an oscillator called the Beat Frequency Oscillator.




Note that the only piece of equipment that uses a BFO in the aircraft is the ADF. Normallyelectromagnetic radiation is modulated by one of the three methods listed below:

Amplitude Modulation (AM) AM is where the modulating frequency alters the amplitude ofthe wave.

Where a carrier is amplitude modulated by a single tone the resultant waveform consists ofthree components:

The carrier wave f c

The upper sideband (f c + f s)

The lower sideband (f c - f s), where f s is the modulating signal

1-19




The AM signal will consist of:

1 500 KHz the carrier2. 501 KHz the upper sideband3. 499 KHz the lower sideband

Intelligence is carried by both sidebands. The spread of the side frequencies is known as thebandwidth. For an amplitude modulated signal the bandwidth is 2f s.

Both sidebands carry the same information, if one of the bands is suppressed (eg the uppersideband) then the only frequencies that need transmitting are 500 KHz and 499 KHz. Thistype of transmission will have two main advantages:

Less power is required to transmit one sideband and the carrier

The signal occupies less of the radio spectrum. This means that a more efficientuse can be made of the frequency band the signal is in.

HF transmissions make use of the single sideband transmission.

Frequency Modulation (FM) FM is where the modulating frequency alters the frequencyof the wave.

The frequency of the carrier varies by an amount proportional to the instantaneous amplitudeof the modulating signal. The rate of change of the carrier frequency is proportional to thefrequency of the modulating signal, the amplitude of the modulated carrier remaining constantthroughout.

FM signals are relatively noise free. Unfortunately this type of broadcast uses a much widerbandwidth than AM and so FM has limited application in commercial aviation but is used in:

VOR

1-20




Radio Altimeters

Doppler

Pulse Modulation (PM) PM is where the carrier is transmitted in short pulses. Thesepulses can be coded to carry information. Two types of PM need consideration:

Pulse Amplitude Modulation (PAM) In a similar way to AM it is possible for anaudio waveform to modify the amplitude of a fixed train of pulses.

Pulse Code Modulation (PCM) A system where each pulse amplitude is assigned abinary number.

Classification of Emissions Radio regulatory agencies have designed a coding system

that fully describes the form that a radio transmission may take. The table below details thecoding system.

First Character

Type of Modulation

Second Character

Nature of the ModulatingSignal

Third Character

Type of Information BeingTransmitted

N – Unmodulated carrier 0 – No modulation N – No informationtransmitted

A - Amplitude 1 – Interrupted carrier A – Telegraphy – for auralreception

J – Single sideband (nocarrier) 2 – Keyed or digital audiomodulation E – Telephony – includingsound broadcasting

F – Frequency 3 – Telephony (voice ormusic)

W – Combination of theabove

P – Unmodulated pulses 8 – Two or more channels ofanalogue information

X – Cases not otherwisecovered

9 – Composite systemscomprising of 1 & 2 abovewith 3 or 8

X – Cases not otherwisecovered

The emission characteristics for civil aviation use that you need to know are:

ADF N0N A1AN0N A2A

HF J3EVHF A3EVDF A3EILS A8WVOR A9WDME P0N

1-21




Basic Radar Theory

Introduction Radar is derived from the expression radio detection and ranging. It may bedefined as any system employing radio to detect the presence of objects and to determinetheir position and movement.

Radar Frequencies Radar occupies the frequency bands from VHF upwards. Higherfrequencies are used because:

They are free from external noise

Narrow beams operate more efficiently with a short wavelength

Primary radar use pulses, high frequencies produce short pulses

The efficiency of reflection depends upon the size of the target in relation to

wavelength. High frequencies are reflected more efficiently

Principles A transmitter sends out, via the aerial, a brief pulse of radio energy. Every 6.2microseconds (µs) this pulse will travel 1 nautical mile. If this pulse strikes a target, a smallproportion of the radio energy will be reflected back to the aerial. The aerial picks up thisreflected energy and passes it to the receiver. If the time of travel is known then the range canbe calculated.

Pulse Radars Pulse radars are employed as:

Primary radars - ATC surveillance radars, Airborne weather radars

Secondary radars - DME and SSR

Doppler

The radar transmits energy in very short bursts of high energy. Timing the pulse yields adirect measurement of the range and requires a sensitive receiver. The transmission, travel

1-22




and reception of the pulse must be achieved before the next pulse is transmitted. This willthen ensure that we have an unambiguous target.

Primary Radar A primary radar relies on the weak reflections from a passivetarget. The effectiveness of the radar depends upon the transmitter power and thereceiver sensitivity.

Secondary Radar Relies on the target co-operating with the transmitter. Thetarget transmits a reply signal to an interrogatory signal such as in SSR and DME.The interrogation and reply are usually on different frequencies.

Secondary radar has both advantages and disadvantages over a primary radar:

Advantages

Primary radars require much more power to achieve the same range

Target size and aspect are irrelevant because the target transmits the response

Responses on the secondary radar are much more reliable

Information can be encoded to give the transmitter and receiver information

Clutter on the radar screen can be eliminated

Disadvantages

The radar requires the co-operation of the target

Bearing resolution can be inferior

Side lobes can be a problem at short range

Beacon saturation can be a problem

Radar Direction Finding There are two principle means:

Lobe Comparison

Beam Direction Finding

Lobe Comparison Mainly used by secondary radars two aerials are used to definedirection. The aerial is rotated till an equal strength signal is received.

1-23




Beam Direction Finding By using a parabolic aerial a near parallel beam can beachieved. Because the direction of the aerial is known and the pulse is transmitted andreceived before a second pulse is transmitted the azimuth of the target can be calculated.

The beamwidth of a parabolic aerial can be calculated by the formula:

Beamwidth = 70λ /d

Where: λ = wavelength of the radar

d = diameter of the parabolic aerial

Remember with this calculation that λ and d must be in the same units.

Radar Terminology Certain terms are used in radar and these need to be understood.

Pulse Recurrence Frequency (PRF) This is the rate at which pulses aretransmitted by the radar. The units used are pulses per second (pps). The maximumPRF is determined by the fact that each pulse must be able to reach the most distanttarget and return before the next pulse is transmitted. Otherwise there is a possibilityof ambiguous range measurement.

1-24




Pulse Recurrence Interval (PRI) The time interval between pulses. The unitsare normally microseconds. The PRI is used to determine the maximum range of theradar. The relationship between PRI and PRF is simple.

PRI = 1 ÷ PRF

Example For a radar with a PRF of 250 pps find the maximum range

PRI = 1 / PRF = 1 / 250 = .004 seconds

= 4000 µseconds

= 4000 X 10-6

seconds

(to convert seconds into microseconds multiply by 1 000 000)

Distance = speed X time

The total time of travel out and back for the pulse is 4000 µseconds

The time of travel one way, so that the range can be calculated =2000 µseconds or 2000 X 10

-6 seconds

Distance = (2000 X 10-6

) seconds X (300 X 106) metres per second

Distance = 600 000 metres = 600 kilometres

This is the maximum unambiguous range of the radar

Pulse Width (PW) The duration of the pulse. This determines the minimum rangeof a radar. The pulse must travel half its distance before it hits a target and returns tothe radar. Otherwise the radar will still be transmitting the same pulse.

Example A radar has a PW of 2 µseconds, what is its minimum range

The minimum range must be half the time of travel, which is 1µsecond

Distance = speed X time = (1 X 10-6

) seconds X (300 X 106) metres

per second

Distance = 300 metres

1-25




Choice of Frequency To produce a narrow beam a high frequency must be used. Theadvantages of using a narrow beam are obvious:

Bearing accuracy will be greater

There is an increase in effective power

The radar will be able to resolve closely spaced targets

High frequencies also generate a squarer pulse shape

Wavelength has to be shorter than the target size

All the above have to be taken into consideration

The basic radar has seven elements:

Master Timer This is the trigger unit and has two functions:

It generates the basic PRF

Synchronizes the firing of the transmitter

Modulator The output from the modulator switches the transmitter on and offand so controls the pulse length of the transmitter output

Transmitter Delivers the pulse to the aerial

1-26




Receiver A sensitive superhet that can amplify the very weak returning echoes.These are then processed for display.

T/R switch The same aerial is used for both transmission and reception. Thereceiver must be protected from the high power transmitter. This isachieved by electronically isolating the waveguides for both. Aduplexer which in real terms is the brains of the radar does thisisolation.

Indicator Radar information is usually displayed on a Cathode Ray Tube

Aerial A parabolic dish on older aerials. Now a flat bed array whichelectronically simulates a parabolic dish.

1-27




Intentionally Left Blank

chapter 01 - basic radio theory

Documents