- •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
17.5.5 Hf communications
HF antennas are used in the range of frequencies from 2 to 30MHz for mobile communications and some fixed communications. Space precludes more than a brief mention of the types of HF antennas. Surveys can be found in (Rudge, 1986; Johnson, 1984). HF antenna design is constrained by the ionospheric propagation characteristics which change both daily, seasonally and with the sun-spot cycle. Antennas can receive either the sky wave reflected from the ionosphere or the ground wave if transmitter and receiver are close together. The wavelength in the HF band is such that antennas are usually only a fraction of a wavelength in size andthis in turn means that the local environment around the antenna will have a major impact upon the performance. This is particularly true for antennas mounted on vehicles, ships and aircraft. The analysis of the antenna must take account of the environment by techniques such as wire grid modelling (Mittra, 1975). This is computer intensive and inevitably approximate which means that much HF antenna design is empirical.
Most HF antennas are based on dipoles, monopoles or wire antennas. Complementary elements such as loops or slot antennas are also used. Directionality or gain is achieved by arrays of elements. A prime requirement of most HF antennas is that they are broadband in order that the optimum propagation frequency can be used. The radiation patterns of the basic elements are wide band but the input impedance or VSWR is narrow band. To overcome this limitation a tuning unit has to be incorporated into the system. The wide band operation is achieved with automatic tuning units.
1 Write out the words and word combinations which are still unknown to you and learn them. Unit 2 (20) Frequency division multiplexing
PART 1 (20.1 – 20.3.4)
20.1 Fdm principles
Frequency Division Multiplex (FDM) is the frequency translation of a number of individual standard telephony channels so that they can be stacked side by side and form a single wide band signal. This principle is illustrated in Figure 20.1.
Two identical telephony channels Ct and C2 (0.3 to 3.4kHz) are each mixed with a carrier frequency and combined. Only the lower sideband is selected from the mixing process and thus the channel frequencies are inverted on the combined signal. The spacing of the carrier dictates the spacing between the combined channels and is normally 4kHz i.e. f2 - f l = 4kHz. The received path at B undergoes the opposite process.
As the carrier is not transmitted with the signal then t' and f2' are separately generated at B. Any frequency difference between the carriers at A or B will impose a frequency shift on the recovered channel of f - f '.
The function of filters Fal and Fa2 is to select the wanted lower sideband and suppress the unwanted carrier signal, upper sideband etc. Fbl and Fb2 filters select the required set of frequencies from the broadband line frequency spectrum for demodulation.
In the higher orders of translation it is not normally economic to design Fb filters with sufficient selectivity, such that only the required band of frequencies are selected. The signal presented to the demodulator in this case can contain some information from the adjacent channels. These unwanted signals are removed by the greater selectivity of the lower order demodulating equipments.
An additional requirement for Fb filters is to limit the total band of frequencies applied to the demodulator and thus keep the inter-modulation noise generated to a minimum. Fa and Fb filters are normally the same design.
The filters Fcl and Fc2 remove the unwanted products from the demodulation process before presentation to the user or to the next lower order of translation.
The FDM system represents the most efficient use of bandwidth. With only the lower sideband of the modulation products transmitted the system limitations of bandwidth and overload are maximised.