- •Учреждение образования «высший государственный колледж связи» «чтение и перевод технических текстов по специальности ткс»
- •Часть I
- •Введение
- •Unit 1 (17) Antennas
- •17.1 Types of antennas
- •17.1.1 Antennas used in communications
- •17.2 Basic properties
- •17.3 Generic antenna types
- •17.3.1 Radiation from apertures
- •1 Learn the words & word combinations:
- •2 Read & translate the text (orally) 17.1 – 17.3.2:
- •5 Answer the questions:
- •17.3.2 Radiation from small antennas
- •17.3.3 Radiation from arrays
- •17.4 Specific antenna types
- •17.4.1 Prime focus symmetric reflector antennas
- •17.4.1.1 Parabolic reflectors
- •17.4.1.2 Aperture fields and radiation patterns
- •17.4.1.3 Gain of reflector antennas
- •1Learn the words & word combinations:
- •2 Read & translate the text (orally) 17.3.2 – 17.4.1:
- •5 Answer the questions:
- •17.4.2 Dual symmetric reflector antennas
- •17.4.3 Offset reflectors
- •17.4.4 Horn feeds for reflector antennas
- •17.4.4.1 Rectangular or square horns
- •17.4.4.2 Small conical horns
- •17.4.4.3 Multi-mode conical horns
- •17.4.4.4 Conical corrugated horns
- •17.4.4.5 Array feeds
- •1 Learn the words & word combinations:
- •2 Read & translate the text (orally) 17.4.2 – 17.4.4:
- •5 Answer the questions:
- •17.5 Antennas used in communication systems
- •17.5.1 Microwave line of sight radio
- •17.5.2 Earth station antennas
- •1 Learn the words & word combinations:
- •2 Read & translate the text (orally) 17.5.1 – 17.5.2:
- •5Answer the questions:
- •17.5.3 Satellite antennas
- •17.5.3.1 Telemetry, tracking and command (tt&c)
- •17.5.3.2 Spot beams
- •17.5.3.3 Multiple beams
- •17.5.3.4 Shaped beams
- •17.5.4 Vhf and uhf communications
- •17.5.5 Hf communications
- •1 Write out the words and word combinations which are still unknown to you and learn them. Unit 2 (20) Frequency division multiplexing
- •20.1 Fdm principles
- •20.2 History
- •20.3 Fdm hierarchy
- •20.3.1 General considerations
- •20.3.2 Channel bandwidth
- •20.3.3 Group and supergroup
- •20.3.4 Higher order translation
- •20.3.4.1 15 Supergroup assemblies
- •20.3.4.2 Mastergroup
- •20.3.4.3 Bell system
- •1 Learn the words & word combinations:
- •2 Read & translate the text (orally) 20.1 – 20.3.4:
- •4Find English equivalents:
- •5 Answer the questions:
- •20.4 Frequency translation
- •20.4.1 Ring bridge modulator/demodulator design considerations
- •20.4.1.1 Carrier compression.
- •20.4.1.2 Carrier and signal suppression
- •20.5 Carriers
- •20.5.1 Carrier frequency accuracy
- •20.5.2 Carrier purity
- •20.5.3 Carrier level
- •20.6 Pilots
- •20.6.1 Translation equipment pilots
- •20.6.1.1 Use of reference pilots for automatic gain control
- •20.6.2 Line equipment pilots
- •20.6.2.1 Regulation pilots
- •20.6.2.2 Frequency comparison pilots
- •1 Learn the words & word combinations:
- •2 Read & translate the text (orally) 20.4 – 20.6
- •5. Answer the questions:
- •20.7 Noise contributions
- •20.7.1 Definitions
- •20.7.2 Psophometric weighting
- •20.7.3 Thermal noise
- •20.7.4 Noise due to unlinearity
- •20.7.4.1 Single channel load
- •20.7.4.2 Multichannel load
- •20.7.4.3 Unlinearily characterisation
- •20.7.4.4 Determination ofunlinearity noise from a multichannel load
- •20.7.4.5 Approximate value for the weighted intermodulation noise contribution
- •20.7.4.6 Weighted noise power in pWOp
- •20.7.4.7 Determination of unlinearity noise using spectral densities
- •1 Learn the words & word combinations:
- •2 Read & translation the text (orally) 20.7:
- •5 Answer the questions:
- •20.8 Measurement of noise contributions
- •20.9 Overload
- •20.9.1 Overload measurement.
- •20.9.1.1 Harmonic/intermodulation products
- •20.9.1.2 Gain change
- •20.10 Hypothetical reference system
- •20.10.1 Noise contributions
- •20.10.2 Line sections
- •1 Learn the words & word combinations:
- •2 Read & translate the text (orally) 20.8 -20.10:
- •5 Answer the questions:
- •20.11 Companding
- •20.11.1 Compander characteristics
- •20.11.2 Multichannel load increase
- •20.11.3 Compandor noise advantage
- •20.13 Transmultiplexers
- •20.13.1 Synchronisation
- •20.13.2 Pcm alarms
- •20.14 Repeatered cable line equipment
- •20.14.1 Pre-Emphasis
- •20.14.2 Thermal noise
- •20.14.3 Regulation
- •20.14.3.1 Regulation range
- •20.14.4 Power feeding
- •Translate the text 20.11 – 20.14.4
- •Translate in written form point 20.13 “Transmultiplexers”
- •«Чтение и перевод технических текстов по специальности ткс»
- •Часть I
17.1.1 Antennas used in communications
Table 17.1 lists the principal types of antennas which are used in communications. Under each category of communication system, the specific type of antenna which is usually used is given. The last column gives the generic type of either an aperture antenna, an array antenna or a small element antenna. The generic type describes the general radiation characteristics which can be obtained from the antenna and is useful because it makes the explanation of the performance easier.
The following sections describe first the generic antenna characteristics, then the specific antenna types and then brief discussion of the practical implementation of antennas in communication systems.
17.2 Basic properties
The principle of reciprocity is one of the most important properties of an antenna. It means that the properties of an antenna when acting as a transmitter are identical to the properties of the same antenna when acting as a receiver. For this to apply, the medium in between the two antennas must be linear, passive and isotropic, which is always the case for communication systems.
The directional selectivity of an antenna is represented by the radiation pattern. It is a plot of the relative strength of the radiated field as a function of the angle. A pattern taken along the principal direction of the electric field is called an E- plane cut, the orthogonal plane is called an H-plane cut. The most common plot is the rectangular decibel plot, Figure 17.3 which can have scales of relative power and angle chosen to suit the antenna being characterised. Other types of plots such as polar plots (used for small antennas and two dimensional), contour plots (or three dimensional), and isometric plots are also used. A radiation pattern is characterised by the main beam and sidelobes. The quality is specified by the beamwidth between the -3dB points on the main beam and the sidelobe level.
Communication antennas radiate in either linear polarisation or circular polarisation. In modern communications cross- polarisation is important. This is the difference between the two principal plane patterns and is specified relative to a reference polarisation, called the co-polar pattern. There are three definitions of the cross-polarisation. The one in normal use with reflector antennas and feed systems is Ludwig's third definition (Ludwig, 1973) which assumes that the reference polarisation is that due to a Huygens source. It most closely corresponds to what is measured with a conventional antenna test range.
The power gain in a specified direction is defined by the ratio of the power radiated per unit solid angle in direction Θ, φ to the total power accepted from the source, as in Equation 17.1.
This is an inherent property of an antenna and includes dissipative losses in the antenna. The dissipative losses cannot easily be predicted so a related parameter, the directivity, is used in calculations. The definition of the directivity is similar to that of the gain except that the denominator is replaced by the total power radiated. The terms gain and directivity are often used interchangeably in the literature. Normally only the peak gain along the boresight direction is specified. If the direction of the gain is not specified, the peak value is assumed. The value is normally quoted in dB's. The definitions given above are in effect specifying the gain relative to a loss-less isotropic source. This is sometimes stated explicitly by using the symbol dBi.
The efficiency of an aperture antenna is given by the ratio of the effective area of an aperture divided by the physical area. Normal aperture antennas have efficiencies in the range 50-80%.
As far as circuit designers are concerned the antenna is an impedance. Maximum power transfer will occur when the antenna is matched to the transmission line. The impedance consists of the self impedance and the mutual impedance. The mutual impedance accounts for the influence of nearby objects and of mutual coupling to other antennas. The self impedance consists of the radiation resistance, the loss resistance and the self reactance. Loss resistance is the ohmic losses in the antenna structure. Radiation resistance measures the power absorbed by the antenna from the incoming plane waves. It is one of the most significant parameters for small antennas where the problem is often to match very dissimilar impedances.
A receiving antenna is both a spatially selective filter (measured by the radiation pattern) and a frequency selective filter. The bandwidth measures the frequency range over which the antenna operates. The upper and lower frequencies can be specified in terms of a number of possible parameters: gain, polarisation, beamwidth and impedance.
A communication link consists of a transmitting antenna and a receiving antenna. If the transmitter radiates P( watts, then the received power, Pf at a distance r is given by Equation 17.2 where G( and G^ are the transmitter and receiver antenna gains respectively.
This formula is known as the Friis transmission equation. It assumes that the antennas are impedance and polarisation matched. If this is not the case then extra factors must be multiplied to the equation to account for the mismatches.