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 E_a(x,y) then the radiated fields is given by Equation 17.3, where is 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 practice almost 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.

This can be compared to and is also plotted i n Figure 17.5.

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.

1 Learn the words & word combinations:

radiation	излучение, радиация
to confine	ограничивать
radio relay	радиорелейная связь
satellite communications	спутниковая связь
pencil beam antenna	антенна с игольчатой диаграммой направленности
omnidirectional pattern	диаграмма всенаправленности
toroidal pattern	торроидальная диаграмма
reflector	отражатель
collimate	фазировать, коллимировать
feed horn	рупорный облучатель
aperture antenna	апертурная антенна
stringent	строгий, обязательный
gain antenna	направленная антенна
radiated field	поле излучения
Fourier transformations	преобразование Фурье
aperture distribution	апертурное распределение поля в раскрыв (антенны)
line source	линейный источник (излучения)
sidelobe level	уровень боковых лепестков
taper	ослабление (спад) поля к краям раскрыва антенны
waveguide mode	1. световодная мода 2. волноводный режим
horn	рупор
azimuth(al) angle	1. полярный угол, фаза, амплитуда 2. угол перехода
Bessel function	функция Бесселя
Gaussian distribution	нормальное (Гауссово) распределение

2 Read & translate the text (orally) 17.1 – 17.3.2:

3 Find Russian equivalents:

ü the link	ü guided parts
ü free-space parts	ü efficiently purpose processing
ü environmental characteristics	ü to meet stringent specifications
ü basic properties	ü reciprocity
ü the main beam	ü sidelobes
ü a conventional antenna test range	ü inherent
ü dissipative losses	ü along the boresight direction
ü mutual coupling self impedance	ü a spatially selective filter
ü a frequency selective filter	ü the pedestal distribution

4 Find English equivalents:

ü в цепи	ü принимать
ü преобразовывать	ü включает
ü в зависимости	ü требовать
ü в вертикальной плоскости	ü определить
ü оказываться под влиянием	ü окружающие объекты
ü предыдущий опыт	ü играют равную роль
ü разница	ü усиление мощности
ü в определенном направлении	ü подобно (чему-то)
ü заменяется	ü ближайших объектов

5 Answer the questions:

1. What is the purpose of a transmitting antenna?

2. What is the purpose of a receiving antenna?

3. What two broad categories do antennas for communications systems fall into?

4. What is the design of a pencil beam antenna?

5. What antennas are used in communications?

6. What are the basic properties of an antenna?

7. When will the maximum power transfer occur?

8. What does a communication link consist of?

PART 2 (17.3.2 – 17.4.1)