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