Radar Antenna

Найдите в словаре и запишите в тетрадь транскрипцию данных слов. Прочитайте их вслух несколько раз.

Azimuth, accuracy, equate, weigh, ratio, rectangular, desired, aperture, key, major, vary.

Words to be learnt

to perform- выполнять, исполнять

essential- важнейший, основной, неотъемлемый

distribution- распределение, распространение

to apply- применять, прикладывать

accurate- точный

in the case of…- в случае…

revolution rate- скорость вращения

in terms of…- с точки зрения…, в пересчете на…

to take into account- брать в расчет, принимать во внимание

gain- коэффициент усиления

to compare- сравнивать

field- поле, область

radiation pattern- диаграмма излучения

axis- ось

intersection- перекрещивание, пересечение

beam- луч, пучок света

lobe- лепесток диаграммы направленности антенны

to eliminate- устранять, аннулировать

appropriate- подходящий, соответствующий

(un) desirable- (не) желательный

Text

The antenna is one of the most critical parts of a radar system. It performs the following essential functions:

• It transfers the transmitter energy to signals in space with the required distribution and efficiency. This process is applied in an identical way on reception.

• It ensures that the signal has the required pattern in space. Generally this has to be sufficiently narrow in azimuth to provide the required azimuth resolution and accuracy.

• It has to provide the required frequency of target position updates. In the case of a mechanically scanned antenna this equates to the revolution rate. A high revolution rate can be a significant mechanical problem given that a radar antenna in certain frequency bands can have a reflector with immense dimensions and can weigh several tons.

• It must measure the pointing direction with a high degree of accuracy.

The antenna structure must maintain the operating characteristics under all environmental conditions. The basic performance of radar can be shown to be proportional to the product of the antenna area or aperture and the mean transmitted power. Investment in the antenna therefore brings direct results in terms of system performance. Taking into account these functions and the required efficiency of a radar antenna, two arrangements are generally applied:

• the parabolic dish antenna

• the array antenna.

Independently of the use of a given antenna for transmitting or receiving, an important characteristic of this antenna is the gain. Some antennas are highly directional; that is, more energy is propagated in certain directions than in others. The ratio between the amount of energy propagated in these directions compared to the energy that would be propagated if the antenna were not directional (Isotropic Radiation) is known as its gain. When a transmitting antenna with a certain gain is used as a receiving antenna, it will also have the same gain for receiving.

Most radiators emit (radiate) stronger radiation in one direction than in another. A radiator such as this is referred to as anisotropic. However, a standard method allows the positions around a source to be marked so that one radiation

pattern can easily be compared with another. The energy radiated from an antenna forms a field having a definite radiation pattern. A radiation pattern is a way of plotting the radiated energy from an antenna. This energy is measured at various angles at a constant distance from the antenna. The shape of this pattern depends on the type of antenna used. To plot this pattern, two different types of graphs, rectangular-and polar-coordinate graphs are used. The polar-coordinated graph has proved to be of great use in studying radiation patterns. In the polar-coordinate graph, points are located by projection along a rotating axis (radius) to an intersection with one of several concentric, equally-spaced circles.

The main beam (or main lobe) is the region around the direction of maximum radiation (usually the region that is within 3 dB of the peak of the main beam). The sidelobes are smaller beams that are away from the main beam. These sidelobes are usually radiation in undesired directions which can never be completely eliminated. The sidelobe level (or sidelobe ratio) is an important parameter used to characterize radiation patterns. It is the maximum value of the sidelobes away from the main beam and is expressed in Decibels. One sidelobe is called backlobe. This is the portion of radiation pattern that is directed opposing the main beam direction.

For the analysis of an antenna pattern the following simplifications are used:

Beam Width

The angular range of the antenna pattern in which at least half of the maximum power is still emitted is described as a „Beam With”. Bordering points of this major lobe are therefore the points at which the field strength has fallen in the room around 3 dB regarding the maximum field strength. This angle is then described as beam width or aperture angle or half power (- 3 dB) angle.

Aperture

The effective aperture of an antenna Ae is the area presented to the radiated or received signal. It is a key parameter, which governs the performance of the antenna.The aperture efficiency depends on the distribution of the illumination across the aperture.

Major and Side Lobes (Minor Lobes)

The radiation intensity in one lobe is considerably stronger than in the other. The strongest lobe is called major lobe; the others are (minor) side lobes. Since the complex radiation patterns associated with arrays frequently contain several lobes of

varying intensity, you should learn to use appropriate terminology. In general, major lobes are those in which the greatest amount of radiation occurs. Side or minor lobes are those in which the radiation intensity is least.

Front-to-Back Ratio

The front-to-back ratio of an antenna is the proportion of energy radiated in the principal direction of radiation to the energy radiated in the opposite direction. A high front-to-back ratio is desirable because this means that a minimum amount of energy is radiated in the undesired direction.

Задание I. Найдите в тексте ответы на вопросы. Отработайте вопросы и ответы в парах. Старайтесь проговаривать их, не глядя в текст.

1. What are the essential functions of a radar antenna?

2. What are the main types of a radar antenna?

3. Why is the antenna structure very important?

4. What are the main characteristics of an antenna?

5. What is antenna gain?

6. What is an anisotropic radiator?

7. What is the main beam of the antenna?

8. What is beam width?

9. What is aperture?

10. What is front-to-back ratio of the antenna?

Задание II. Приготовьтесь побеседовать по изученной теме в группе и с преподавателем.