It is important to realise that this book is not Very high frequency VHF radio has long designed to replace aircraft maintenance manuals. Chapter 4 required by those engaged in the maintenance of describes the principles of VHF communications specific aircraft types. Instead it has been both voice and data. The chapter also provides designed to convey the essential underpinning an introduction to the aircraft communication knowledge required by all aircraft maintenance addressing and reporting system ACARS.
High frequency HF radio provides aircraft Chapter 1 sets the scene by providing an with an effective means of communicating over explanation of electromagnetic wave propagation long distance oceanic and trans-polar routes. In and the radio frequency spectrum. Chapter 5 aviation world that an accurate and reliable short- describes the principles of HF radio range navigation system was needed.
Since radio communication as well as the equipment and communication systems based on very high technology used. Chapter 6 describes flight-deck audio system, and is described in Chapter This systems including the interphone system and all- system is in widespread use throughout the world important cockpit voice recorder CVR which today.
The The detection and location of the site of an air advent of radar in the s led to the crash is vitally important to the search and rescue development of a number of navigation aids SAR teams and also to potential survivors. The provide accurate navigation fixes. The system is chapter also provides a brief introduction to based on secondary radar principles.
ADF, VOR and DME navigation aids are Chapter 8 introduces the subject of aircraft installed at airfields to assist with approaches to navigation; this sets the scene for the remaining those airfields. These navigation aids cannot chapters of the book. This landings. Chapter 12 introduces some basic aircraft navigation describes how the ILS can be used for approach terminology, e.
The ILS uses a combination reckoning etc. The chapter concludes by of VHF and UHF radio waves and has been in reviewing a range of navigation systems used on operation since Many Chapter 13 continues with the theme of guided aircraft navigation systems utilise radio frequency approaches to an airfield.
There are a number of methods to determine a position fix; this links shortcomings with ILS; in the microwave very well into the previous chapters of the book landing system MLS was adopted as the long- describing fundamental principles of radio term replacement.
The system is based on the transmitters, receivers and antennas. Ground waves have two basic components; a direct wave and a ground reflected wave as shown in Figure 1. Four different effects can occur see Figure 1. An example of the use of a direct path is that which is used by terrestrial microwave repeater stations which are typically spaced 20 to 30 km apart on a line-of-sight basis.
Another example of the direct path is that used for satellite TV reception. In this case, and since the wave travels largely undeviated through the atmosphere, the direct wave is often referred to as a space wave. As shown in Figure 1. Ground reflection depends very much on the quality of the ground with sandy soils being a poor reflector of radio signals and flat marshy ground being an excellent reflecting surface.
Note that a proportion of the incident radio signal is absorbed into the ground and not all of it is usefully reflected. An example of the use of a mixture of direct path and ground or building reflected radio signals is the reception of FM broadcast signals in a car.
It is also worth mentioning that, in many cases, the reflected signals can be stronger than the direct path or the direct path may not exist at all if the car happens to be in a heavily built-up area.
Such waves are predominant at frequencies below VHF and we shall examine this phenomenon in greater detail a little later but before we do it is worth describing what can happen when waves meet certain types of discontinuity in the atmosphere or when they encounter a physical obstruction. Reflection occurs when a plane wave meets a plane object that is large relative to the wavelength of the signal. In such cases the wave is reflected back with minimal distortion and without any change in velocity.
The effect is similar to the reflection of a beam of light when it arrives at a mirrored surface. Refraction occurs when a wave moves from one medium into another in which it travels at a different speed. For example, when moving from a more dense to a less dense medium the wave is bent away from the normal i. Conversely, when moving from a less dense to a more dense medium, a wave will bend towards the normal. The effect is similar to that experienced by a beam of light when it encounters a glass prism.
Diffraction occurs when a wave meets an edge i. In such cases the wave is bent so that it follows the profile of the discontinuity. Diffraction occurs more readily at lower frequencies typically VHF and below. An example of diffraction is the bending experienced by VHF broadcast signals when they encounter a sharply defined mountain ridge.
Scattering occurs when a wave encounters one or more objects in its path having a size that is a fraction of the wavelength of the signal. When a wave encounters an obstruction of this type it will become fragmented and re-radiated over a wide angle. In the former case, signals can be become scattered i.
Tropospheric scatter requires high power transmitting equipment and high gain antennas but is regularly used for transmission beyond the horizon particularly where conditions in the troposphere i. Tropospheric scatter of radio waves is analogous to the scattering of a light beam e. In addition to tropospheric scatter there is also tropospheric ducting not shown in Figure 1.
Ducting usually occurs when a large mass of cold air is overrun by warm air this is referred to as a temperature inversion. Although this condition may occur frequently in certain parts of the world, this mode of propagation is not very predictable and is therefore not used for any practical applications.
This was soon followed by the discovery of another layer at around km now called the F-layer. This was achieved by broadcasting a continuous signal from one site and receiving the signal at a second site several miles away. By measuring the time difference between the signal received along the ground and the signal reflected from the atmosphere and knowing the velocity at which the radio wave propagates it was possible to calculate the height of the atmospheric reflecting layer.
Today, the standard technique for detecting the presence of ionised layers and determining their height above the surface of the earth is to transmit a very short pulse directed upwards into space and accurately measuring the amplitude and time delay before the arrival back on earth of the reflected pulses.
This ionospheric sounding is carried out over a range of frequencies. The ionosphere provides us with a reasonably predictable means of communicating over long Aircraft communications and navigation systems 8 distances using HF radio signals. The useful regions of ionisation are the H-layer at about 70 miles in height for maximum ionisation and the F-layer lying at about miles in height at night. During the daylight hours, the F-layer splits into two distinguishable parts: F1 lying at a height of about miles and F2 lying at a height of about miles.
After sunset the Fr and F2-layers recombine into a single F-layer see Figures 1. This lower layer primarily acts to absorb energy in the low end of the high frequency HF band.
The F-layer ionisation regions are primarily responsible for long distance communication using sky waves at distances of up to several thousand km greatly in excess of those distances that can be achieved using VHF direct wave communication, see Figure 1. The characteristics of the ionised layers are summarised in Table 1. Table 1. Reaches maximum ionisation when the sun is at its highest point in the sky Responsible for the absorption of radio waves at lower frequencies e. The maximum ionisation of this layer occurs at around midday Reflects waves having frequencies less than 5 MHz but tends to absorb radio signals above this frequency Es 80 to km An intense region of ionisation that sometimes appears in the summer months peaking in June and July.
Usually lasts for only a few hours often in the late morning and recurring in the early evening of the same day Highly reflective at frequencies above 30 MHz and up to MHz on some occasions. Of no practical use other than as a means of long distance VHF communication for radio amateurs F to km Appears a few hours after sunset, when the Fr and Frlayers see below merge to form a single layer Reflects radio waves up to 20 MHz and occasionally up to 25 MHz F1 to km Occurs during daylight hours with maximum ionisation reached at around midday.
The F,-layer merges with the F2-layer shortly after sunset Reflects radio waves in the low HF spectrum up to about 10 MHz F2 to km Develops just before sunrise as the F-layer begins to divide. Maximum ionisation of the F2-layer is usually reached one hour after sunrise and it typically remains at this level until shortly after sunset.
The intensity of ionisation varies greatly according to the time of day and season and is also greatly affected by solar activity capable of reflecting radio waves in the upper HF spectrum with frequencies of up to 30 MHz and beyond during periods of intense solar activity i.
MUF varies considerably with the amount of solar activity and is basically a flinction of the height and intensity of the F-layer. A similar plot for the summer months would be flatter with a more gradual increase in MUF at dawn and a more gradual decline at dusk.
The reason for the significant variation of MUF over any hour period is that the intensity of ionisation in the upper atmosphere is significantly reduced at night and, as a consequence, lower frequencies have to be used to produce the same amount of refractive bending and also to give the same critical angle and skip distance as by day.
Fortunately, the attenuation experienced by lower frequencies travelling in the ionosphere is much reduced at night and this makes it possible to use the lower frequencies required for effective communication.
The important fact to remember from this is simply that, for a given path, the frequency used at night is about half that used for daytime communication. The lowest usable frequency LUF is the will support frequency that lowest communication over a given path at a particular time and on a particular date. LUF is dependent on the amount of absorption experienced by a radio wave. This absorption is worse when the D layer is most intense i.
This diagram assumes a critical frequency of 5 MHz. This is the lowest frequency that would be returned from the ionosphere using a path of vertical incidence see ionospheric sounding on page 7. The relationship between the critical frequency,.
In order to explain in simple terms how the Yagi antenna works we shall use a simple light analogy. An ordinary filament lamp radiates light in all directions.
Just like an antenna, the lamp converts electrical energy into electromagnetic energy. The only real difference is that we can see the energy that it produces! The action of the filament lamp is comparable with our thndamental dipole antenna. In the case of the dipole, electromagnetic radiation will occur all around the dipole elements in three dimensions the radiation pattern will take on a doughnut shape.
In the plane that we have shown in Figure 2. In order to concentrate the radiation into just one of the radiation lobes we could simply place a reflecting mirror on one side of the filament lamp.
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. The reflector needs to be cut slightly longer than the driven dipole element. The resulting directional pattern will now only have one major lobe because the energy radiated will be concentrated into just one half of the figure-ofeight pattern that we started with. Continuing with our optical analogy, in order to further concentrate the light energy into a narrow beam we can add a lens in front of the lamp.
This will have the effect of bending the light emerging from the lamp towards the normal line see Figure 2. In order to achieve the same effect in our antenna system we need to place a conducting element, known as a director, on the other side of the dipole and about one quarter of a wavelength from it.
Once again, this element is parasitic but in this case it needs to be cut slightly shorter than the driven dipole element. The resulting directional pattern will now have a narrower major lobe as the energy becomes concentrated in the normal direction i.
The resulting antenna is known as a three-element Yagi aerial, see Figure 2. Sonic comparative gain and beamwidth figures are shown on page Such an arrangement will usually provide a 3 dB gain over a single antenna but will have the same beamwidth. A disadvantage of stacked arrangements is that they require accurate phasing and matching arrangements.
As a rule of thumb, an increase in gain of 3 dB can be produced each time the number of elements is doubled. Thus a two-element antenna will offer a gain of about 3 dBd, a four-element antenna will produce 6 dBd, an eight-element Yagi will realise 9 dBd, and so on. In other words, as the gain of an antenna increases its radiation pattern becomes more confined.
In many cases this is a desirable effect e. In other cases e. The directional characteristics of an antenna are usually presented in the form of a polar response graph.
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. The polar diagram for a horizontal dipole is shown in Figure 2. Note that there are two major lobes in the response and two deep nulls. The antenna is thus said to be bi-directional. Figure 2.
The radiation from this antenna is concentrated into a single major lobe and there is a single null in the response at to the direction of maximum radiation. Sketch a typical horizontal radiation pattern for this antenna. Test your understanding 2. The following are some of the most common types several other antennas will be introduced in later chapters.
Such antennas produce an omnidirectional radiation pattern in the horizontal plane and radiate vertically polarised signals. In order to produce a reasonably flat radiation pattern and prevent maximum radiation being directed upwards into space it is essential to incorporate an effective ground plane.
All four radials are grounded at the feed-point to the outer screen of the coaxial feeder cable. A slight improvement on the arrangement in Figure 2.
This arrangement produces a flatter radiation pattem. However, to reduce the earth resistance and increase the efficiency of the antenna, it is usually necessary to incorporate some buried earth radials see Figure 2.
These radial wires simply consist of quarter-wave lengths of insulated stranded copper wire grounded to the outer screen of the coaxial feeder at the antenna feed point.
This type of antenna must be voltage fed rather than current fed as is the case with the quarterwave antenna. J4 Radiate 4 off 50 a coaxial feeder Figure 2,22 Quarter wave vertical antenna with sloping radials between the low-impedance coaxial feeder and the end of the antenna.
Such an arrangement is prone to losses since it requires high-quality, lowloss components. It may also require careful adjustment for optimum results and thus a quarter-wave or three-quarter wave antenna is usually preferred. Antennas 25 2. The two reflecting surfaces which may be solid or perforated to reduce wind resistance are inclined at an angle of about This type of aerial is compact in comparison with a Yagi and also relatively unobtrusive.
Buried earth radials Reflecting surface Figure 2. In order to match the antenna, an inductive loading coil is incorporated at the feed-point. The need for very high gain coupled with directional response at UHF or microwave frequencies is often satisfied by the use of a parabolic reflector in conjunction with a radiating element positioned at the feed-point of the dish see Figure 2.
In order to be efficient, the diameter of a parabolic reflecting surface must be large in comparison with the wavelength of the signal. The gain of such an antenna depends on various factors but is directly proportional to the ratio of diameter to wavelength. The principle of the parabolic reflector antenna is shown in Figure 2. Signals arriving from a distant transmitter will be reflected so that they pass through the focal point of the parabolic surface as shown.
With a conventional parabolic surface, the focal point lies directly on the axis directly in front of the reflecting surface. Placing a radiating element together with its supporting structure at the focal point may thus have the Aircraft communications and navigation systems 26 Parabolic reflecting surface Feed arrangement dish. This feed arrangement is often used for focal plane reflector antennas where the outer edge of the dish is in the same plane as the halfwave dipole plus reflector feed.
An alternative arrangement using a waveguide and small horn radiator see page 27 is shown in Figure 2. The horn aerial offers some modest gain usually 6 to 10 dB, or so and this can be instrumental in increasing the overall gain of the arrangement. Such antennas are generally not focal plane types and the horn feed will usuall31 require support above the parabolic surface. Reflecting surface Figure 2. In order to overcome this problem the surface may be modified so that the focus is offset from the central axis.
It is important to realise that the reflecting surface of a parabolic reflector antenna is only part of the story. Equally important and crucial to the effectiveness of the antenna is the method of feeding the parabolic surface.
The dipole and reflector has a beamwidth of around and this is ideal for illuminating the parabolic surface. The dipole and reflector is placed at the focal point of the Horn radiator Waveguide feed Figure 2. Horn aerials may be used alone or as a means of illuminating a parabolic or other reflecting surface. Horn antennas are ideal for use with waveguide feeds; the transition from waveguide see page 38 to the free space aperture being accomplished over several wavelengths as the waveguide is gradually flared out in both planes.
During the transition from waveguide to free space, the impedance changes gradually. The gain of a horn aerial is directly related to the ratio of its aperture i. However, as the gain increases, the bearnwidth becomes reduced. Give reasons for your answers: a an SHF satellite earth station b a low-frequency non-directional beacon c an airfield communication system d a long-range HF communication system e a microwave link between two fixed points.
Small horn antenna for use 10 at 10 GHz 20 3 m diameter parabolic antenna for tracking space vehicles at UHF 4 40 Test your understanding 2.
In the case of a receiver, the source is the receiving antenna whilst the load is the input impedance of the first RF amplifier stage. In the case of a transmitting system, the source is the output stage of the transmitter and the load is the impedance of the transmitting antenna. Ideally, a feeder would have no losses i. In practice, this is seldom the case. This section explains the basic principles and describes the construction of most common types of feeder.
The characteristic impedance, Z0, is a fUnction of the inductance, L, and capacitance, C, of the feeder and may be approximately represented by: Identify the antenna shown in Figure 2. L and C are referred to as the primary constants of a feeder.
In this respect, L is the loop inductance per unit length whilst C is the shunt capacitance per unit length see Figure 2. In practice, a small amount of DC resistance will be present in the feeder but this is usually negligible.
For the twin open wire shown in Figure 2. For the coaxial cable shown in Figure 2. Example 2. Determine the characteristic impedance of the cable. Antennas 29 U L U U 1. The coaxial cable shown in Figure 2. The two conductors are concentric and separated by an insulating dielectric that is usually air or some form of polythene.
The impedance of such a cable is given by: b coaxial cable Figure 2. Flat twin ribbon cable is a close relative of the two-wire open line the difference between these two being simply that the former is insulated and the two conductors are separated by a rib of the same insulating material.
When determining the characteristic impedance of ribbon feeder, the formula given above must be modified to allow for the dielectric constant of the insulating material.
In practice, however, the difference may be quite small. The open wire feeder used with a high-power land-based HF radio transmitter uses wire having a diameter of 2. Determine the characteristic impedance of the feeder. High impedance b coaxial cable Figure 2. Obviously, the lower the resistance of the feeder, the smaller will be the power losses. Whilst the attenuation of a feeder remains reasonably constant throughout its specified frequency range, it is usually subject to a progressive increase beyond the upper frequency limit see Figure 2.
It is important when choosing a feeder or cable for a particular application to ensure that the operating frequency is within that specified by the manufacturer. In order to filly understand the behaviour of a feeder, whether balanced or unbalanced, it is necessary to consider its equivalent circuit in terms of four conventional component values; resistance, inductance, capacitance and conductance, as shown in Figure 2. These four parameters are known as primary constants and they are summarised in Table 2.
The ratio of the two velocity in the feeder compared with the velocity in free space is known as the velocity factor. Obviously, velocity factor must always be less than 1, and in typical feeders it varies from 0. Table 2. Connectors should be reliable, easy to mate, and sealed to prevent the ingress of moisture and other fluids.
Coaxial connectors are available in various format see Figure 2. Of these, the BNC- and N-type connectors are low-loss constant impedance types. The need for constant impedance connectors e. BNC and N-type connectors rather than cheaper non-constant impedance connectors e.
PL becomes increasingly critical as the frequency increases. Below this frequency, the loss associated with using non-constant impedance connectors is not usually significant. Fitting requires careftil preparation of the coaxial cable. The outer braided screen is fanned out, as shown in Figure 2. Antennas 33 2. Ideally, a feeder should present a perfect match between the impedance of the source and the impedance of the load.
Unfortunately this is seldom the case and all too often there is some degree of mismatch present. 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. Where the impedance of the transmission line or feeder perfectly matches that of the aerial, all of the energy delivered by the line will be transferred to the load i.
Under these conditions, no energy will be reflected back to the source. If the match between source and load is imperfect, a proportion of the energy arriving at the load will be reflected back to the source. The result of this is that a standing wave pattern of voltage and current will appear along the feeder see Fig. The standing wave shown in Figure 2. It is important to note that both the forward and reflected waves are moving but in opposite directions. The standing wave, on the other hand, is stationary.
As indicated in Figure 2. The current distribution along the feeder will have a similar pattern note, however, that the voltage maxima will coincide with the current minima, and vice versa. Four possible scenarios are shown in Figure 2. In Figure 2. Only the forward wave is present and there is no standing wave. This is the ideal case in which all of the energy generated by the source is absorbed by the load. This represents one of the two worst-case scenarios as the voltage varies from zero to a very high positive value.
In this condition, all of the generated power is reflected back to the source. This represents the other worst-case scenario. Here again, the voltage varies from zero to a high positive value and, once again, all of the generated power is reflected back to the source.
This condition lies somewhere between the extreme and perfectly matched cases. The standing wave ratio SWR of a feeder or transmission line is an indicator of the effectiveness of the impedance match between the transmission line and the antenna.
The SWR is the ratio of the maximum to the minimum current along the length of the transmission line, or the ratio of the maximum to the minimum voltage. When the line is absolutely matched the SWR is unity. In other words, we get unity SWR when there is no variation in voltage or current along the transmission line. Aircraft communications and navigation aystems 34 Voltage I — o 4 I. Also, I 21? This example further underlines the importance of SWR and the need to have an accurate means of measuring it.
Assume that we are dealing with a simple halfwave dipole aerial that is designed with the following parameters: Centre frequency: Feed-line impedance: Dipole length: Element diameter: Bandwidth: Q-factor: MI-Iz 75 ohm 0. As predicted, zero reactance at the feed point occurs at a frequency of MHz for the dipole length in question.
This relationship is shown in Figure 2. Measurements of SWR show a minimum value of about 1. Clearly this could be a problem in an application where a transmitter is to be operated with a maximum SWR of2:l The bandwidth limitation of a system comprising transmitter, feeder and aerial is —40 Frequency MHz Figure 2.
Most aerials will radiate happily at frequencies that are some distance away from their resonant frequency—the problem is more one of actually getting the power that the transmitter is capable of delivering into them!
Aircraft communications and navigation systems 38 2. However, at microwave frequencies above 3 GHz, or so , this type of feeder can have significant losses and is also restricted in terms of the peak RF power voltage and current that it can handle. Because of this, waveguide feeders are used to replace coaxial cables for SHF and EHF applications, such as weather radar. A waveguide consists of a rigid or flexible metal tube usually of rectangular cross-section 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. A simple waveguide system is shown in Figure 2. The SHF signal is applied to a quarter wavelength coaxial probe. The wave launched in the guide is reflected from the plane blanked-off end of the waveguide and travels through sections of waveguide to the load in this case a horn antenna, see page An example of the use of a waveguide is shown in Figure 2.
In this application a flexible waveguide is used to feed the weather radar antenna mounted in the nose of a large passenger aircraft. Citations Publications citing this paper.
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