- •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:
- •3 Find Russian equivalents:
- •4 Find English equivalents:
- •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:
- •3 Find Russian equivalents:
- •4 Find English equivalents:
- •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:
- •17.5.2 Earth station antennas
- •1 Learn the words & word combinations:
- •2 Read & translate the text (orally) 17.5.1 – 17.5.2:
- •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
- •2 Read & translate the text (orally) 20.1 – 20.3.4:
- •3 Find Russian equivalents:
- •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
- •20.6.2 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
- •3 Find Russian equivalents:
- •4. Find English equivalents:
- •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:
- •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:
- •20.11.2 Multichannel load increase
- •20.11.3 Compandor noise advantage
- •20.11.4 Attack and decay time
- •20.11.5 Usage of companders
- •20.12 Through connections
- •20.12.1 Through connection filter
- •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
- •«Чтение и перевод технических текстов по специальности ткс»
- •Часть I
17.3 Generic antenna types
17.3.1 Radiation from apertures
The radiation from apertures illustrates most of the significant properties of pencil beam antennas. The radiation characteristics can be determined by simple mathematical relationships. If the electric fields across an aperture, Figure 17.4, is Ea(x,y) then the radiated fields is given by Equation 17.3, whereis given by Equation 17.4. (Oliver, 1986; Milligan, 1985).
For high or medium gain antennas the pencil beam radiation is largely focused to a small range of angles around 0 = 0. In this case it can be seen from Equation 17.3 that the distant radiated fields, and the aperture fields are the Fourier transformation of each other. Fourier transforms have been widely studied and their properties can be used to understand the radiation characteristics of aperture antennas. Simple aperture distributions have analytic Fourier transforms, whilst more complex distributions can be solved numerically on a computer.
The simplest aperture is a one dimensional line source distribution of length This serves to illustrate many of the features of aperture antennas. If the field in the aperture is constant, the radiated field is given from Equation 17.3 as in Equations 17.5 and 17.6.
width and is .The first sidelobe level is at -13.2dB which is a disadvantage of a uniform aperture distribution. The level can be reduced considerably by a tapered aperture distribution where the field is greatest at the centre of the aperture and tapers to a lower level at the edge of the aperture. For example if Equation 17.7 holds, then the first sidelobe level is at -23dB.
The energy which was in the sidelobes moves to the main beam with the result that the beamwidth broadens to . In practicealmost all antennas have natural tapers across the aperture which result from boundary conditions and waveguide modes. Rectangular apertures are formed from two line source distributions in orthogonal planes.
Circular apertures form the largest single class of aperture antennas. The parabolic reflector is widely used in communications and is often fed by a conical horn. Both the reflector and the horn are circular apertures. For an aperture distribution which is independent of azimuthal angle the simplest case is uniform illumination which gives a radiated field as in Equation 17.8, where J1 (x) is a Bessel function of zero order.
The first sidelobe level is at-17.6dB. Table 17.2 lists a number of circular aperture distributions and corresponding radiation pattern properties. The pedestal distribution is representative of many reflector antennas which have an edge tapers of about -10 dB corresponding to .The Gaussian distribution is also important because high performance teed horns ideally have Gaussian aperture distributions. The Fourier transform of a Gaussian taper which decreases to zero at the edge of the aperture gives a Gaussian radiation pattern which has no sidelobes.