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Horn feeds for reflector antennas
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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.
17.4.4.4 Conical corrugated horns
These are the leading choice for a feed for dual reflector and medium size offset reflectors, Figure 17.11(b). They have excellent radiation pattern symmetry and radiate very low crosspolarisation over a broad range of frequencies.
A corrugated horn propagates a mixture of TEj f and TM ] f modes called a hybrid HE mode. The corrugations are approximately quarter of a wavelength deep so that the electric short circuit at the base of the slot is transformed to a magnetic short circuit at the top of the slot. The result is that the azimuthal magnetic field is forced to zero at the corrugations and the azimuthal electric field is zero due to the ridges. Consequently the boundary conditions of the ТЕ and TM modes are identical and the mutual propagating modes are linear combinations of the two parts. The design procedure for corrugated horns is well understood (Clarricoats, 1984a) and it is possible to accurately predict the radiation characteristics.
17.4.4.5 Array feeds
They are used to form multiple beam and shaped beam reflector antennas used on satellites, Figure 17.11(e). The individual elements of the array can be any type of horn, although for compactness small diameter open ended waveguides are preferred. The radiation patterns of the array are mainly determined by the element spacing and the amplitudes and phases of the signals sent to the individual elements. In addition to being able to form a wide range of multiple or shaped beams, the array has the advantage that the cross-polarisation of the total array is lower than that of an individual element. However the closeness of the array elements gives rise to mutual coupling between the aperture fields which can distort the radiation patterns (Clarricoats, 1984b; Clarricoats, 1984c). A significant disadvantage of an array is that a beam forming network of waveguide components must be used behind the array elements to produce the correct amplitudes and phases to the array. For large arrays this can be heavy, expensive and a significant part of the design of the complete antenna system.
1 Learn the words & word combinations:
| sub reflector | вспомогательное зеркало |
| convex hyperboloid | выпуклый гиперболоид |
| concave | вогнутый |
| subtend | стягивать, противолежать |
| noise temperature | шумовая температура |
| revolution | круглое вращение, обращение, оборот |
| prime focus | передний фокус |
| blockage loss | потери затенения |
| offset reflector | облучатель с вынесенным облучателем |
| primary feed | первичный облучатель |
| VSWR (voltage standing wave ratio) | КСВН (коэффициент стоячей волны по напряжению) |
| with the advent | с приходом |
| feed pattern | диаграмма направленности облучателя |
| scattering | рассеяние |
| Cassegrain antenna | антенна Касегрена |
| Gregorian antenna | антенна Грегори |
| circular polarisation | круговая поляризация |
| guided wave | ведомая волна (направляемая, канализированная) |
| flange | выступ буртик |
| corrugated horn | гофрированная (ребристая) рупорная антенна |
| array feed | облучатель антенной решетки |
2 Read & translate the text (orally) 17.4.2 – 17.4.4:
3 Find Russian equivalents:
| ü flexible | ü adequate |
| ü by shaping the surfaces … | ü alter |
| ü create ü essentially | ü tightening in the radiation pattern specifications |
| ü removal | ü multiple |
| ü feasible | ü horn feeds |
| ü rarely | ü straightforward |
| ü appropriate | ü shaped beams |
4 Find English equivalents:
| ü Преимущества | ü удобное расположение |
| ü направлен | ü сокращает |
| ü увеличить эффективность | ü может быть полностью устранен |
| ü для достижения | ü соотношение |
5 Answer the questions:
1 What are the advantages of the dual reflectors?
2 What does the analysis of the radiation patterns depend on?
3 What is the limiting factor to obtaining high efficiency in a standard parabola?
4 How is it possible to increase the efficiency and produce a more uniform illumination across the aperture?
5 What has the growth in communication systems led to?
6 What other advantages has the offset reflector?
7 What is of particular interest in horn feed design?
PART 4 (17.5.1 – 17.5.2)
Дата публикования: 2015-03-29; Прочитано: 317 | Нарушение авторского права страницы | Мы поможем в написании вашей работы!
