Butterworth-Heinemann is an imprint of Elsevier. Aircraft Communications and. Navigation Systems: Principles, Operation and Maintenance. Mike Tooley and. thousands of aircraft aloft at any one time, communication and navigation systems are essential to safe, successful flight. Continuing development is occurring. Chapter 5 - [Free] Aircraft Communications And Navigation Systems Maintenance Chapter 5 [PDF] [EPUB] Contents vii Chapter 10 VHF.
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Aircraft Communication and Navigation - Download as PDF File .pdf), Text File . txt) or read Aircraft Communications and Navigation Systems: Principles. Aircraft Communication and Navigation Mike Tuley - Free ebook download as PDF File .pdf), Text File .txt) or Aviation Radio Navigation. Download as PDF, TXT or read online from Scribd . Chapter 17 Inertial navigation systems Aircraft Communication and Navigation Systems, Principles Maintenance and Operation. Armando Valiente lesforgesdessalles.info
ADF is a short—medium range nm navigation system providing directional information. This characteristic shows how the be stronger than the wanted signal. BNC and N-type connectors rather than in Figure 2. Increasing traffic density. This additional constraint may increase the received by an LUT.
Acknowledgements The authors would like to thank the following persons and organisations for permission to reproduce photographs and data in this book: Further examination practice can be gained from the four revision papers given in Appendix 2. The system is functionally book are available at www.
Othei factors that need to be taken into account The military applications of radio were first include the efficiency of piactical aerial systems exploited during the First World War to in the range concerned and the bandwidth and. It was also Marconi that made radio a 3 MHz and 30 MHz ionospheric propagation reality by pioneering the development of regularly peimits intercontinental broadcasting telegraphy without wires i.
Marconi demonstrated the commercial At other frequencies signals may propagate by potential of the phenomenon that Maxwell various means including reflection from ionised predicted and Hertz actually used in his layers in the ionosphere At frequencies between apparatus. The energy radiated considerable bandwidths are available sufficient by the transmitting loop was in the form of an to transmit many television channels using point- electromagnetic wave—a wave that has both to-point links or to pennit very high definition electric and magnetic field components and that radar systems and signals tend to propagate travels at the speed of light.
Heinrich Rudolf Hertz used an arrangement of rudimentary Radio frequency signals are generally understood resonators to demonstrate the existence of to occupy a frequency range that extends fiom a electromagnetic waves. Some of the curvature of the earth over very long distances energy radiated by the transmitting loop was At the other extreme. Chapter i Introduction I Maxwell first suggested the existence of 1.
The receiving aerial more correctly as the near field. In this far field region the angular Note that in Figure 1. Introduction 3 1. The transmitter aerial a simple dipole.
Figure 1. Radio waves are said to be polarised in the plane of the electric B field. These two components. This transmitter and the receiver and so the wave that voltage and current is similar but of smaller reaches the receiving antenna will have a plane amplitude to that produced by the transmitter.
This gives rise to an alternating electric field region between a transmitter and a field between the ends of the aerial and an receiver the magnetic field has not been alternating magnetic field around and at right shown but is perpendicular to the electric angles to it. In practice there intercepts the moving field and voltage and will be some considerable distance between the current is induced in it as a consequence. The resulting wave travels away from the source with the B and H lines mutually at right angles to the direction of propagation.
An average value for the height of the tropopause is around 11 km or The velocity of propagation. Determine the frequency at Example 1. Determine the wavelength of the at a frequency of 10 MHz will have a wavelength signal.
These layers. The thermosphere and the upper parts of the Puttingfr MHz gives: The boundary between the troposphere and the Example 1. Ground waves have two basic components. As their name suggests. It is in various ways. These the stratosphere that incoming solar radiation include: For example.
The Depending on a number of complex factors. Reflection occurs when a plane wave meets a line-of-sight LOS basis between the transmitter plane object that is large relative to the and receiver. Such waves travel over LOS constructed at right angles to the boundary.
Ionospheric waves or sky waves can travel Scattering occurs when a wave encounters one for long distances at MF. Note that a which has dimensions that are large relative to the proportion of the incident radio signal is absorbed wavelength of the signal. In this case. The effect is similar to that means of reflection from the ground. An radio signals is the reception of FM broadcast example of diffraction is the bending experienced signals in a car. An example of the use of a mixture of discontinuity.
Scattering occurs more readily at higher describing what can happen when waves meet frequencies typically VHF and above and certain types of discontinuity in the atmosphere or regularly occurs in the troposphere at UHF and when they encounter a physical obstruction. In EHF. In such cases the wave path is that which is used by terrestrial is reflected back with minimal distortion and microwave repeater stations which are typically without any change in velocity.
It is also worth mentioning that. Such waves fraction of the wavelength of the signal. UHF and beyond. When a are predominant at frequencies below VHF and wave encounters an obstruction of this type it will we shall examine this phenomenon in greater become fragmented and re-radiated over a wide detail a little later but before we do it is worth angle.
An example of the use of a direct wavelength of the signal. HF and exceptionally or more objects in its path having a size that is a also at VHF under certain conditions. Diffraction occurs more readily at direct path and ground or building reflected lower frequencies typically VHF and below. In such cases the wave into the ground and not all of it is usefully is bent so that it follows the profile of the reflected. Ground experienced by a beam of light when it reflection depends very much on the quality of encounters a glass prism.
The effect is spaced 20 to 30 km apart on a line-of-sight basis. Tropospheric scatter of radio measuring the time difference between the signal waves is analogous to the scattering of a light received along the ground and the signal reflected beam e. Sir Edward Appleton was one of the first proportion arrives back at the ground. This was soon followed by the discovery but is regularly used for transmission beyond the of another layer at around km now called the horizon particularly where conditions in the F-layer.
By communication. In the former case. This was achieved by broadcasting a troposphere i. This ionospheric Although this condition may occur frequently in sounding is carried out over a range of certain parts of the world. Ducting usually occurs upwards into space and accurately measuring the when a large mass of cold air is overrun by warm amplitude and time delay before the arrival back air this is referred to as a temperature inversion.
Reaches waves at lower frequencies e. Maximum ionisation of the upper HF spectrum with frequencies of F2-layer is usually reached one hour after up to 30 MHz and beyond during sunrise and it typically remains at this level periods of intense solar activity i. Usually lasts occasions. During the daylight excess of those distances that can be achieved hours.
F1 lying at a height of about miles Figure 1. Of no practical use other for only a few hours often in the late than as a means of long distance VHF morning and recurring in the early evening communication for radio amateurs of the same day F to km Appears a few hours after sunset. The F. Table 1. The F-layer The useful regions of ionisation are the H-layer ionisation regions are primarily responsible for at about 70 miles in height for maximum long distance communication using sky waves at ionisation and the F-layer lying at about distances of up to several thousand km greatly in miles in height at night.
Much of the a single F-layer see Figures 1. The characteristics of the ionised and F2 lying at a height of about miles.
The than 5 MHz but tends to absorb radio maximum ionisation of this layer occurs at signals above this frequency around midday Es 80 to km An intense region of ionisation that Highly reflective at frequencies above sometimes appears in the summer months 30 MHz and up to MHz on some peaking in June and July. The intensity of the peak of each year sunspot cycle ionisation varies greatly according to the time of day and season and is also greatly affected by solar activity -.
Introduction 9 Frlayer F—layer F. A typical value of LUF is 4 decline at dusk. This absorption is worse when the D Figure 1. During a period of intense solar activity the MUF The lowest usable frequency LUF is the can exceed 30 MHz during daylight hours but is lowest frequency that will support often around 16 to 20 MHz by day and around 8 communication over a given path at a particular to 10 MHz by night.
A similar plot for the summer layer is most intense i. MUF varies considerably with the from this is simply that.
A frequencies have to be used to produce the same typical example might be a working frequency of amount of refractive bending and also to give the 5 MHz at a time when the MUF is 10 MHz and same critical angle and skip distance as by day. The important fact to remember particular date. This diagram assumes a critical frequency of 5 MHz. This is the lowest. Note also that. The condition is known as multi- hop propagation.
The onward reflected frequency and the MUF for an angle of attack of signal will suffer attenuation but in sonic The MF range extends from: Explain the following terms in relation to HF radio 6. The height of the E-layer is approximately: A transmitted radio wave will have a plane to: Plot a graph showing the variation of MUF with time and 5.
A radio wave has a frequency of 15 MHz. Lisbon on 28th August Ionospheric sounding is used to determine: Introduction 13 Table 1. When a large mass of cold air is overrun by explain the shape of the graph. A radio wave at kHz is most likely to Which one of the following gives the propagate as: The critical frequency is directly proportional 1.
This phenomenon a solar radiation is known as: Radio waves at HF can be subject to b lower at the equator than at the poles reflections in ionised regions of the upper c the same height at the equator as at the atmosphere. Radio waves at VHF and UHF can sometimes a mid-day propagate for long distances in the lower b mid-night atmosphere due to the presence of a c dawn and dusk.
This with distance phenomenon is known as: Radio waves tend to propagate mainly as line. For a given HF radio path. Which one of the following gives the velocity The Fr and F2-layers combine: The free-space path loss experienced by a c atmospheric ducting. The layer in the atmosphere that is mainly responsible for the absorption of MF radio In the HF band radio waves tend to propagate waves during the day is: The F2-layer is: This phenomenon is known as: The maximum distance that can be achieved responsible for the reflection of HF radio from a single-hop reflection from the F-layer waves during the day is: The main cause of ionisation in the upper bent by a sharply defined obstruction such as a atmosphere is: The layer in the atmosphere that is mainly l7.
Radio waves at UHF can sometimes be distance subject to dispersion over a wide angle in b decreases with frequency but increases regions of humid air in the atmosphere. Chapter Antennas 2 It may not be apparent from an inspection of the 2. All practical A receiving antenna captures the electromagnetic antennas have directional characteristics as energy in the surrounding space and converts this illustrated in Figure 2.
Figure A transmitting antenna converts the directions. What should be apparent from this is that used for comparison purposes and as a reference many of the antennas are of the low profile when calculating the gain and directional variety which is essential to reduce drag.
In other words. Antennas are used both for transmission and Isotropic antennas radiate uniformly in all reception. This theoretical type of antenna is often HF comms.
TCAS upper. VHF comma. We shall of reciprocity indicates that an antenna will have look at antenna gain and directivity in more detail the same gain and directional properties when later on but before we do that we shall introduce used for transmission as it does when used for you to some common types of antenna.
To illustrate this point. The conductor is then split in the centre to enable V connection to the feeder. The half-wave dipole is one of the most 2 ffindamental types of antenna. End effects.
In practice. Figure 2. The voltage is zero at pattern. Sidelote 0. The dipole antenna has directional properties This is an important point as we shall see the centre and maximum at the ends. The 3D plot shown in Figure 2. The current is maximum at the dipole will have a hi-directional radiation centre and zero at the ends.
Slice Animulh Elevation Angle 0. This later. Otter Ring 2. Of these two quantities. This is an resistance. Test your understanding 2. The equivalent circuit of an antenna is shown I in Figure 2. Rr contains no reactive component i. Ar 0ff-tune reactance. The length of a half-wave dipole for MHz can be determined from: Rr see Figure 2.
Ignoring resistance. It is also worth noting that the current in the case of a transmitting antenna an DC resistance or ohmic resistance of an aerial is said to have impedance. The three series-connected You might infer from Figure 2. X varies with of an antenna may be regarded as its radiation frequency whilst R remains constant. In this case X is negligible receiving antenna and a much larger voltage and compared with 1?. If a current of 0.
If the antenna the same current flows in the DC resistance. This power gain is unimportant in the case of a receiving antenna. The efficiency of an antenna is given by the relationship: A half-wave dipole is operated at its centre From the equivalent circuit shown in Figure 2. At this point it is worth stating produce different values of field strength for the that whilst efficiency is vitally important in the same applied RF power level.
Antennas 19 Note that when the antenna is operated at a Example 2. In the case of a transmitting antenna. Hence I. Example 2. In most output is actually wasted! It is also worth bearing in mind that the resistance of 12 The 70 Q to 75 12 impedance normally associated with a Now: An ordinary filament lamp radiates light in all directions.
Putting this b Antenna configuration another way. Yagi and Uda. The only real difference is that we can see the energy that it produces! This element is referred to as a reflector and it is said Figure 2. In the plane that we have shown N N in Figure 2.
In the case of the dipole. In order to achieve the same effect in our antenna system we need to place a conducting element about one quarter of a wavelength behind the dipole element. As an example. Some comparative values of antenna gain are shown on page In N N order to concentrate the radiation into just one of N the radiation lobes we could simply place a reflecting mirror on one side of the filament lamp. Just like an antenna. In order to [ explain in simple terms how the Yagi antenna works we shall use a simple light analogy.
This will have the effect of bending the light emerging from the lamp towards the normal line see Figure 2. The resulting directional pattern will farther increase the gain and reduce the now have a narrower major lobe as the energy beamwidth i. Sonic comparative resulting antenna is known as a three-element gain and beamwidth figures are shown on Yagi aerial. The characteristic of Yagi aerials. The L resulting directional pattern will now only have one major lobe because the energy radiated will be concentrated into just one half of the figure-of- eight pattern that we started with.
Antennas 21 Dipole the feeder. Continuing with our optical analogy. The reflector needs to be cut slightly Reflector driven element Director longer than the driven dipole element.
Once again. Cursor Ax Gah 0. Thus a two-element antenna will offer a gain of about 3 dBd. Such an arrangement will usually provide a 3 dB gain over a single antenna but will have the same beamwidth. Yag i.
Oosmwielh A disadvantage of stacked arrangements is that they require accurate phasing and matching arrangements. Elevalion Anglo 0. As a rule of thumb. In other cases e. This diagram allows users to determine directions in which maximum and minimum gain can be achieved and allows the antenna to be positioned for optimum effect. Note that there are two Osdor Ring 6. Antenna gain is achieved at the expense of Seelobe Gain 2.
The radiation from this antenna is concentrated into a single major lobe and there is a single null in the response at Figure 2. Sloe Mae Gain 8. In many cases this is a Figure 2. Sketch a typical horizontal radiation pattern for this antenna.
Use the polar plot to determine: Antennas 23 Test your understanding 2. J4 A slight improvement on the arrangement in Figure 2. It may also require careful adjustment for optimum results and thus a 2.
This arrangement produces a flatter radiation pattem. All four radials are grounded at the feed-point to the outer screen of the coaxial feeder cable.
At VHF. Such an arrangement is at the antenna feed point. This type of antenna must be voltage fed rather 2. An alternative to the use of a quarter-wave radiating element is that of a half-wave element. Practical quarter-wave antennas can be produced Figure 2. These radial wires simply consist of quarter-wave lengths of insulated stranded copper wire between the low-impedance coaxial feeder and grounded to the outer screen of the coaxial feeder the end of the antenna.
In order to produce a reasonably flat radiation pattern and prevent maximum radiation being Radiating element directed upwards into space it is essential to incorporate an effective ground plane. The X14 following are some of the most common types several other antennas will be introduced in later chapters. The gain of such an antenna depends on various factors but is directly proportional to the ratio of diameter to wavelength.
With a conventional parabolic top. In order to be efficient. UHF antenna offering some gain over a basic The principle of the parabolic reflector antenna quarter-wave antenna. Signals arriving from a vertical antenna behaves electrically as a three. In order to match the antenna. This type of aerial is compact in 50 0 coaxial feeder comparison with a Yagi and also relatively unobtrusive. A directly in front of the reflecting surface. The two reflecting surfaces which may be solid or perforated to reduce wind resistance are inclined at an angle of about Buried earth radials Reflecting surface Figure 2.
In fact. Antennas 25 2. The dipole and reflector Waveguide feed has a beamwidth of around and this is ideal for illuminating the parabolic surface. Such antennas are generally not focal plane types and the horn feed will usuall31 require support above the parabolic surface.
The dipole Figure 2. An alternative arrangement using a waveguide and small horn radiator see page 27 is shown in Figure 2. Equally important and crucial to the effectiveness of the antenna is the method of feeding the parabolic surface. In order to overcome this problem the surface may be modified so that the Horn focus is offset from the central axis.
The horn aerial offers some modest Feed gain usually 6 to 10 dB. This feed arrangement is often used for Parabolic reflecting surface focal plane reflector antennas where the outer edge of the dish is in the same plane as the half- wave dipole plus reflector feed. Reflecting Figure 2. The gain Figure 2. Antennas 27 2. Horn aerials may be used alone or as a means of illuminating a parabolic or other reflecting surface. Give reasons for your answers: Horn antennas are ideal for use with waveguide feeds.
During the transition from waveguide to free space. L and C are referred to as the primary constants of a feeder. Identify the antenna shown in Figure 2. For the coaxial cable shown in Figure 2.
In the case of a receiver. Determine the Figure 2. In the case of a transmitting system. In this respect. The characteristic impedance. For the twin open wire shown in Figure 2. This section explains the basic wave dipoles principles and describes the construction of most common types of feeder. L is the loop inductance per unit length whilst C is the shunt capacitance per unit length see Figure 2.
The impedance of such a cable is given by: D is the inside diameter of the outside. The two conductors are concentric and separated by an insulating dielectric that is usually air or some form of polythene.. The coaxial cable shown in Figure 2.
The open wire feeder used with a high-power choosing a feeder or cable for a particular land-based HF radio transmitter uses wire application to ensure that the operating frequency having a diameter of 2. Determine the characteristic an example. As 15 mm.
The attenuation is given by: When determining the characteristic impedance of ribbon feeder. It is important when 2. Whilst the attenuation of a feeder remains Test your understanding 2. The attenuation of a feeder is directly Flat twin ribbon cable is a close relative of the proportional to the DC resistance of the feeder two-wire open line the difference between these and inversely proportional to the impedance of two being simply that the former is insulated and the line.
Determine the characteristic impedance of a Open wire feeder the cable. Determine the progressive increase beyond the upper frequency characteristic impedance of the cable. C G whether balanced or unbalanced. Antennas 31 1. These four parameters are known as primary constants and they are summarised in Table 2. R and 0 are both MHz.
In order to filly understand the behaviour of a feeder. The ratio of the two velocity in the feeder compared with the velocity in free space is known as the velocity factor. As a general rule. Connectors should be reliable.
The need for constant impedance connectors The outer braided screen is fanned out. Fitting impedance types. They should also be designed to minimise contact resistance and. BNC and N-type connectors rather than in Figure 2. Of these. Antennas 33 2. Here interacts with the wave travelling from the load again. The SWR is versa. This is unity. This section explains the consequences of mismatching a source to a load and describes how the effect of a mismatch can be quantified in terms of standing wave ratio SWR.
This condition lies wave is present. Under these conditions. Only the forward wave is When the line is absolutely matched the SWR is present and there is no standing wave. In this condition. Where the impedance of the transmission line Distance or feeder perfectly matches that of the aerial.
The current The standing wave ratio SWR of a feeder or distribution along the feeder will have a similar transmission line is an indicator of the pattern note. This source to the load i.
Reflected wave a feeder should present a perfect match between the impedance of the source and the impedance a Forward and reflected waves travelling along the line of the load. This The result of this is that a standing wave pattern represents one of the two worst-case scenarios as of voltage and current will appear along the the voltage varies from zero to a very high feeder see Fig. In Figure 2. Unfortunately this is seldom the case and all too often there is some degree of Voltage mismatch present.
The standing wave. It is positive value and. L2 and associated components. For values of SWR of between 1 and 2 this additional feeder loss is not usually significant Figure 2. To obtain the most. The pointers simultaneously indicate forward and instrument comprises a short length of reflected power and the point at which they transmission line with two inductively and intersect read from a third scale gives the value capacitively coupled secondary lines.
Each of of SWR present. More complex instruments Figure 2. D2 and R2 is Standing wave ratio is easily measured using an connected so that it senses the reflected wave. Despite the different forms of this and VRI is adjusted for full-scale deflection. Since SWR cannot be less:: I than 1. For a purely resistive load: Dl and RI is arranged so that it 2.
Secondary line. Note that. It is unimportant as to which of these terms r is in the numerator. Antennas 35 The greater the number representing SWR. SWR is optimum i. LI and associated components. SWR In use. RF power is applied to the system. The measured SWR will electrical failure of the feed-point connection. Gradual deterioration. The more lossy the feeder the arrangements is liable to some considerable better the SWR!
The reason for this apparent corrosion or ingress of fluids into the anteima anomaly is simply that the loss present in the structure. For this reason. This graph shows that the transmitting usually attributable to the inability of a bandwidth is actually around 33 MHz extending transmitter to operate into a load that has any from MHz to around MHz for an SWR appreciable amount of reactance present rather of 2: Antennas 37 advisable to make measurements at the extreme 40 limits of the frequency range as well as at the S -c centre frequency.
This relationship is shown in Figure 2. In the case of a typical C transmitting aerial. This example further underlines the importance of SWR and the need to have an Figure 2. As predicted. Clearly than to an inability of the aerial to radiate this could be a problem in an application where a effectively. Most aerials will radiate happily at transmitter is to be operated with a maximum frequencies that are some distance away from SWR of2: Wideband S C 0 -c aerials.
The antenna comprises A waveguide consists of a rigid or flexible a flat steerable plate with a large number of metal tube usually of rectangular cross-section radiating slots each equivalent to a half-wave in which an electromagnetic wave is launched.
The wave travels with very low loss inside the waveguide with its magnetic field component the H-field aligning with the broad dimension of the waveguide and the electric field component the E-field aligning with the narrow dimension of the waveguide see Figure 2.
In this of this. An example of the use of a voltage and current that it can handle. Because waveguide is shown in Figure 2. HF and VHF. The SHF signal is applied to a quarter Conventional coaxial cables are ideal for coupling wavelength coaxial probe.
The wave launched in RF equipment at LF. The radiation efficiency of an antenna: An isotropic radiator will radiate: Explain what is meant by standing wave ratio 4. A full-wave dipole fed at the centre must be: R Reactance.
Use the graph to determine the b in two main directions following: Antennas 39 Resistance. Another name for a quarter-wave vertical c The reactance of the antenna at MHz antenna is: The attenuation of an RF signal in a coaxial would be most suitable for a fixed long cable: When two antennas are vertically stacked the Which one of the following antenna types A standing wave ratio of 1: Which one of the following gives the Cu 1.
What type of antenna is shown in Figure 2. A vertical quarter-wave antenna will have a 3. Which one of the following gives the 2: The characteristic impedance of an RF coaxial combination will have: The characteristic impedance of a coaxial If a transmission line is perfectly matched to The beamwidth of an antenna is measured: If a usually results in impaired frequency stability.
This will then be amplified within the off. On beat frequency of 1. Keying the oscillator stage A radio wave has a frequency of These stage mixes a locally generated radio frequency themes are fUrther developed in Chapters 4 and 5. A signal at for use with continuous wave CW signals. This chapter provides a which provides gain and selectivity followed by general introduction to the basic principles and a detector and an audio amplifier.
The detector operation of transmitters and receivers. Keying can be achieved by interrupting the audio stage before being fed to the loudspeaker.
This makes them particularly modulation are amplitude modulation AM and useful for disaster and emergency communication frequency modulation FM In the former case. Signals are transmitted according to the voltage.
What is the input signal frequency to the detector?
This unit matches the antenna to the RF power amplifier and also helps to reduce the level of any Modulating unwanted harmonic components that may be signal input present. The output of this stage is then amplified and passed to a modulated RF power amplifier stage.
We shall see how this works a little later. The low-level signal from the microphone is Modulating amplified using an AF amplifier before it is signal input passed to an AF power amplifier. The output of a Amplitude modulation the power amplifier is then fed as the supply to the modulated RF power amplifier stage.
The AM transmitter shown in Figure 3. The modulated RF signal is then passed carrier wave Modulated carrier input wave output through an antenna coupling unit.. The inclusion of an amplifier between the carrier wave Modulated carrier RF oscillator and the modulated stage also helps input wave output to improve frequency stability. An accurate and stable RF oscillator generates the radio frequency carrier signal.
Figure 3. In this arrangement the modulation is applied to a low. In order to carrier signal. As with the AM transmitter. Here again. Low-level modulation avoids oscillator and the RF power stage helps to the need for an AF power amplifier. Transmitters and receivers 45 Ground Microphone Figure 3. This stage provides a moderate passed to a variable reactance element see amount of gain at the signal frequency.
This stage recovers the audio from the power amplifier is passed through an frequency signal from the modulated RF signal. It also Chapter 4 within the RF oscillator tuned circuit. This stage increases the level of the audio signal from the demodulator so that it is sufficient to drive a 3.
TRF receivers have a number of limitations Tuned radio frequency TRF receivers provide a with regard to sensitivity and selectivity and this means of receiving local signals using fairly makes this type of radio receiver generally minimal circuitry.
Further information on The output of the demodulator stage is fed to transmitters will be found in Chapters 4 and 5. This helps reactance element causes the frequency of the RF the receiver to reject signals that may be present oscillator to increase and decrease in sympathy on adjacent channels. Antenna Figure 3. The simplified block schematic unsuitable for use in commercial radio of a TRF receiver is shown in Figure 3.
The converted to a fixed intermediate frequency IF desired local oscillator frequency can be at which the majority of the gain and selectivity is calculated from the relationship: Example 3.
IF tuned to receive a signal at 5. As the signal level increases. Over what frequency range should the local stage increases the level of the audio signal from oscillator be tuned? The signal from the antenna is applied to an for example. The impedance-frequency characteristics Impedance of these circuits are shown in Figure 3. Radio receivers use tuned circuits in order to Parallel tuned circuits. It is important to note that the impedance of the series tuned circuit falls to a very low value at the resonant frequency whilst that for a parallel tuned circuit increases to a very high value at L C fo Frequency a Series tuned circuit a Series tuned circuit Impedance L C fo Frequency b Parallel tuned circuit b Parallel tuned circuit Figure 3.
Transmitters and receivers 47 Antenna Figure 3. Optimum results are obtained with a critical value of coupling see Figure 3. Band-pass filters are often found in the IF stages of superhet receivers where they are used Figure 3. The frequency response of this type of filter depends 0. Its bandwidth will be: This characteristic shows how the be stronger than the wanted signal. These points correspond to additional tuned circuits at the signal frequency.
A band-pass filter can be constructed using Vmax two parallel tuned circuits coupled inductively or capacitively. A typical kHz crystal filter for receivers that we met earlier is the lack of use with an HF receiver is shown in Figure 3.
The receiver will simply be unable to differentiate range of frequencies accepted by the circuit is between the wanted and unwanted signals. An RF tuned circuit will very high degree of attenuation at the two normally exhibit a quality factor Q-factor of adjacent channels on either side of the pass- about C2 As an example.
With only a signal developed across the circuit reaches a single tuned circuit at the signal frequency. The relationship between bandwidth. What frequency must the local oscillator operate at when the receiver is tuned to 5. Transmitters and receivers 49 vout Critical coupling Over-coupled Under- coupled fo Frequency Figure 3. Explain why this is. What Q-factor is required? From Figure possible so that the image channel lies well 3. An FM receiver tuned to This can be achieved by the frequency scale has been changed for the two making the RF tuned circuits as selective as different intermediate frequencies.
One of these is the wanted signal i. Being able to reject any signals that may just happen to be present on the image channel of a superimposed onto both of the graphs the same superhet receiver is an important requirement of response curve has been used in both cases but any superhet receiver. Determine the frequency of the image kHz IF and b a 1. A typical response channel given that the local oscillator operates curve for the RF tuned circuits of the receiver above the signal frequency. In relation to your answer.
We also derived the response following formula for determining the frequency of the local oscillator signal: In more sophisticated equipment. Beyond this. In simple receivers. Transmitters and receivers 51 3. The second local oscillator is almost achieved. Such receivers are said to use dual conversion. A typical double superhet receiver is shown in Figure 3. In conventional tuned circuits or is synthesised receivers that feature delayed AGC there is no using digital phase-locked loop techniques see gain reduction until a certain threshold voltage is page This produces the second IF at directly to the bias circuitry of the IF stages see kHz note that There is.
It is worth noting that the bulk of applied to the IF and RF stages. Second oscillator injection Typical IF bandwidths in the receiver shown in Figure 3. Frequency MHZ There are. If this image is present at the input of the second mixer. The first IF filter not shown in Figure 3. The requirements of the filter are not stringent since the ultimate selectivity of the receiver is defined by the second IF filter which 9. Phase detector Figure 3. Complex as they were. The phase detector senses any error employed involving large numbers of discrete between the VCO and reference frequencies.
The VCO is designed so that its free- application cost was not a primary consideration running frequency is at. DC amplifier and voltage and stable frequencies in a multi-channel controlled oscillator VCO. Transmitters and receivers 53 3. With the advent of large scale frequency or a limited number of channels. LSI devices frequencies must be covered.
In this particular 3. The cost-effectiveness of this which a single quartz crystal oscillator is used in approach is now beyond question and it is conjunction with LSI circuitry to generate a range unlikely that. The components and integrated circuits. In the area have a constant channel spacing typically 3 kHz. The frequency appearing at very far apart.
The bandwidth of the system is determined A similar divider arrangement can also be used at by the time constants of the loop filter. The basic form of PLL shown in Figure 3. The frequency presented to the incorporates a programmable divider driven from phase detector will thus be fgn.
The VCO thus remains locked to the reference frequency. Figure divider is incorporated in the VCO feedback path 3. Output fofl4er Phase detector Figure 3. In the reference input to the phase detector.
The RF performance is greatly receiver is simple and straightforward to align enhanced by the use of dual gate MOSFET and does not suffer from the limitations. The circuit caters for the reception The design uses conventional discrete of AM. Used in conjunction programmable divider. This receiver was a low-cost kHz filter. The receiver is based on the single superhet 3.
In gain with excellent strong—signal handling such cases it will be necessary to mix the high— capability. This approach ensures that the resonator. To aid stability. They also permit simple and effective frequency VCO output with a stable locally coupling between stages without the need for generated signal derived from a crystal oscillator.
Adequate image developed by the author for monitoring trans rejection is provided by two high-Q ganged RF Atlantic HF communications in the 5. These devices offer high counter. The mixer output a relatively low difference The receiver is tunable over the frequency frequency will then be within the range of the range 5. This frequency is low We shall bring this chapter to a conclusion by enough to ensure reasonable selectivity with just providing a design example of a complete HF two stages of IF amplification and with the aid of communications receiver.
These two stages provide The receiver incorporates two detector stages. TR3 and TR4. The kHz carrier image channel rejection. The local oscillator Figure 3.
Transmitters and receivers 57 associated with several of the popular integrated The local oscillator stage TR7 provides the circuit IF stages. The AM circuits and filter are instrumental in reducing the detector makes use of a simple diode envelope IF bandwidth to about 3. Delayed AGC: Diode switching is used to provide c has no effect on receiver sensitivity.
Amplified AGC is provided by c superhet receiver. A receiver in which selected signals of any frequency are converted to a single frequency Figure 3. Suite Oxford OX2 8DP. Butterworth-Heinemann is an imprint of Elsevier Linacre House. Contents Preface Acknowledgements Online resources Chapter 1 1. Contents Chapter 10 Contents Chapter 22 Nor does it attempt to provide the level of detail required by those engaged in the maintenance of specific aircraft types.
The series also provides a useful source of reference for those taking ab initio training programmes in EASA Part and FAR approved organisations as well as those following related programmes in further and higher education institutions. Chapter 4 describes the principles of VHF communications both voice and data. The aim has been to make the subject material accessible and presented in a form that can be readily assimilated. The chapter concludes with a brief introduction to waveguide systems.
High frequency HF radio provides aircraft with an effective means of communicating over long distance oceanic and trans-polar routes.
Very high frequency VHF radio has long been the primary means of communication between aircraft and the ground. In addition. The chapter also provides an introduction to the aircraft communication addressing and reporting system ACARS. The book provides an introduction to the principles.
Chapter 2 also provides an introduction to feeders including coaxial cable and open-wire types. This chapter provides readers with an introduction to the operating principles of AM and FM transmitters as well as tuned radio frequency TRF and supersonic-heterodyne superhet receivers.
Radio transmitters and receivers are the subject of Chapter 3. The book assumes a basic understanding of aircraft flight controls as well as an appreciation of electricity and electronics broadly equivalent to Modules 3 and 4 of the EASA Part syllabus. This chapter explains the principles of isotropic and directional radiating elements and introduces a number of important concepts including radiation resistance. Yagi beam antennas. Preface The books in this series have been designed for both independent and tutor assisted studies.
It is important to realise that this book is not designed to replace aircraft maintenance manuals. Antennas are introduced in Chapter 2. Modern aircraft radio equipment is increasingly based on the use of digital frequency synthesis and the basic principles of phase-locked loops and digital synthesisers are described and explained.
Several practical forms of antenna are described including dipoles. Instead it has been designed to convey the essential underpinning knowledge required by all aircraft maintenance engineers.
Chapter 1 sets the scene by providing an explanation of electromagnetic wave propagation and the radio frequency spectrum. The chapter also describes the various mechanisms by which radio waves propagate together with a detailed description of the behaviour of the ionosphere and its effect on radio signals.
This book is designed to cover the essential knowledge base required by certifying mechanics. Military operators of MLS often use mobile equipment that can be deployed within hours. These systems are based. As well as communication with ground stations.
ADF is a short—medium range nm navigation system providing directional information. The system is based on secondary radar principles.
These navigation aids cannot however be used for precision approaches and landings. Radio waves have directional characteristics as described in the early chapters of the book.
The system is based on the principle of time referenced scanning beams and provides precision navigation guidance for approach and landing. Chapter 13 continues with the theme of guided approaches to an airfield. Many aircraft navigation systems utilise radio frequency methods to determine a position fix.
The advent of radar in the s led to the development of a number of navigation aids including distance measuring equipment DME. Since radio communication systems based on very high frequency VHF were being successfully deployed.
Chapter 7 describes the construction and operation of emergency locator transmitters ELT fitted to modern passenger aircraft. The system provides multiple approach angles for both azimuth and elevation guidance. Chapter 9 looks at the historical background to radio navigation. The chapter also provides a brief introduction to satellite-based location techniques. This is the basis of the automatic direction finder ADF. Chapter 5 describes the principles of HF radio communication as well as the equipment and technology used.
Long-range radio navigation systems are described in Chapter Chapter 6 describes flight-deck audio systems including the interphone system and allimportant cockpit voice recorder CVR which captures audio signals so that they can be later analysed in the event of a serious malfunction of the aircraft or of any of its systems.
Despite the advantages of MLS. Chapter 12 describes how the ILS can be used for approach through to autoland. MLS provides threedimensional approach guidance. The standard approach and landing system installed at airfields around the world is the instrument landing system ILS. Chapter 8 introduces the subject of aircraft navigation.
Preface During the late s. The detection and location of the site of an air crash is vitally important to the search and rescue SAR teams and also to potential survivors. This system is in widespread use throughout the world today. There are a number of shortcomings with ILS. Chapter 11 develops this theme with a system for measuring distance to a navigation aid. The chapter concludes by reviewing a range of navigation systems used on modern transport and military aircraft.
Being self-contained. Doppler navigation systems were developed in the mids and introduced in the mids as a primary navigation system. Increasing traffic density. The advent of computers. This system requires no external inputs or references from ground stations. Omega and Loran. The system is able to compute navigation data such as present position. The system does not need radio navigation inputs and it does not transmit radio frequencies.
An artificial constellation of navigation aids was initiated in and referred to as Navstar navigation system with timing and ranging. Loran systems are still available for use today as stand-alone systems.