- •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
17.4.4 Horn feeds for reflector antennas
A reflector antenna consists of the reflector plus the horn feed at the geometric focus of the reflector. Thus the correct choice and design of the feed is an important part of the design of the total reflector antenna. High performance feeds are necessary to achieve high performance antennas. The diameter of the feed in wavelengths will be determined by the angle subtended by the reflector at the feed. A prime focus reflector with an F/D between 0.25 and 0.5 will have a subtended half angle of between 90 degrees and 53 degrees. Application of the general rule that beamwidth is approximately equal to the inverse of the normalised aperture diameter shows that this means a feed with an aperture diameter of between about one and three wavelengths. Dual reflectors (Cassegrain or Gregorian) and offset reflectors have subtended angles between 30 degrees and 7 degrees, leading to feed diameters of between three and ten wavelengths.
Of particular interest in horn feed design is the polarisation performance and the quality of a feed is usually expressed by the level of the peak cross-polarisation. The radiation characteristics of horns are predicted by a two part process. Firstly the fields in the aperture are computed from a knowledge of the guided wave behaviour inside the horn. Secondly the aperture fields are used to compute the radiated fields. The Fourier transform method has been found to work very well for the case of horns. The main types will now be briefly described. For more details see (Love, 1976; Love, 1986).
17.4.4.1 Rectangular or square horns
These are the simplest type of horn, Figure 17.1 l(a) but they are rarely used as feeds for reflectors because they have very high cross-polarisation unless the aperture size is large.
17.4.4.2 Small conical horns
These can have reasonably good cross polarisation performance, Figure 17.1 l(b). They are widely used as prime focus feeds in small symmetric and offset reflectors. The basic design will have an aperture diameter of about one wavelength and is essentially an open-ended circular waveguide propagating a TEn mode. The radiation pattern can be improved by adding one or more rings or chokes around the aperture, Figure 17.1 l(c). These have the effect of changing the distribution of current on the flange and creating a more symmetric radiation pattern. The theoretical design of the open-ended waveguide is straightforward, but the analysis of the choked version is much more complicated. As a consequence most small feeds are designed empirically with measured data.
17.4.4.3 Multi-mode conical horns
These improve the performance of conical horns by generating a second mode inside the horn in such a manner that the aperture fields are linearised. The second, TM11 mode, is generated by a step change in the conical horn diameter and the length of the horn is determined by the need to have the modes in the correct phase relationship at the aperture. The dual mode horn gives low crosspolarisation over a narrow band of frequencies. Although narrow band it is simple to make and of low weight.
The concept of adding higher order modes in a horn can be extended for other purposes. In tracking feeds a higher order mode is used to provide tracking information. The inherent crosspolarisation which occurs in offset reflectors can be cancelled by the appropriate addition of higher order modes (Love, 1986). Finally the main beam can be shaped to provide higher efficiency in prime focus reflectors although only over a narrow frequency band.