Antennas transmitting and receiving parabolic. Main types of parabolic antenna

First parabolic antenna designed by Heinrich Hertz

The parabolic antenna was invented by the German physicist Heinrich Hertz in 1887. Hertz used cylindrical parabolic reflectors to spark dipole antennas during his experiments. The antenna had an aperture size of 1.2 meters wide and was used at a frequency of about 450 MHz. The reflector was made of zinc sheet steel. With two such antennas, one transmitting and one receiving, Hertz successfully demonstrated the existence of electromagnetic waves, which were predicted by Maxwell 22 years earlier.

Typically, in reflector antennas, the wider radiation pattern of the feed is converted into a narrow radiation pattern of the antenna itself.

The mirror edge and the Z plane form a surface called the mirror opening. In this case, the radius R is called the opening radius, and the angle 2ψ is called the opening angle of the mirror. The type of mirror depends on the opening angle:

if ψ< π/2 - зеркало называют мелким или длиннофокусным;
if ψ > π/2 - deep or short focus,
if ψ = π/2 - average.

The focus of the antenna feed can either be located at the focus of the mirror F or be shifted relative to it. If the focus of the irradiator is located at the focus of the antenna, then it is called direct focus. Direct focus antennas come in various sizes, while autumnally symmetrical antennas, whose feed is not in the focus of the mirror, usually do not exceed 1.5 m in diameter. Such antennas are often referred to as offset antennas. The advantage of the offset antenna is the higher gain of the antenna, which is due to the lack of shading of the mirror opening by the feed. The reflector of offset antennas is a side cutout of a paraboloid of revolution. The focus of the irradiators in such antennas is located in the focal plane of the reflector.

The reflector antenna may have an additional elliptical mirror (Gregory's two-mirror scheme) or an additional hyperbolic mirror (Cassegrain's two-mirror scheme), with foci located in the focal plane of the reflector antenna. In this case, the irradiator is located at the focus of the additional mirror.

A reflector antenna can simultaneously have several feeds located in the focal plane of the antenna. Each irradiator forms a radiation pattern directed in the desired direction. The irradiators can operate in different wavelength ranges ( , , ) or each simultaneously in several ranges.

The location of the focus and the focal plane of the antenna mirror does not depend on the operating wavelength range.

Depending on the tasks and the irradiator reflector antenna forms one narrowly directed total, sum-difference pattern (for direction finders) or several differently directed patterns at the same time - when using several irradiators.

Mirror types

In technology, the following types of mirrors are most widely used:

Design features

The mirror usually consists of a dielectric base (carbon fiber - for space antennas), which is covered with metal sheets, conductive paint, foil. At the same time, the sheets are often perforated or mesh, which is due to the desire to reduce the weight of the structure, as well as to minimize the resistance to wind and precipitation. However, such a non-continuous mirror leads to the following consequences: part of the energy penetrates through the mirror, which leads to a weakening of the directivity of the antenna, and an increase in radiation behind the reflector. The efficiency of an antenna with a non-solid mirror is calculated by the formula T = P p r P p a d (\displaystyle T=(\frac (P_(pr))(P_(pad)))), Where P p r (\displaystyle P_(pr)) is the radiation power behind the reflector, and P p a d (\displaystyle P_(pad))- reflector radiation power (incident wave) . If T< 0 , 01 {\displaystyle T<0,01} , a non-solid mirror is considered good. This condition is usually met when the hole diameter of the perforated mirror is less than 0 , 2 λ (\displaystyle 0,2\lambda ) and the total area of holes up to 0 , 5 − 0 , 6 (\displaystyle 0.5-0.6) from the entire area of the mirror. For mesh mirrors, the diameter of the holes should not exceed 0 , 1 λ (\displaystyle 0,1\lambda ) .

Irradiator

The radiation pattern of a parabolic antenna is formed by a feed. There can be one or more feeds in the antenna, respectively, one or more radiation patterns are formed in the antenna. This is done, for example, in order to receive a signal simultaneously from several space communication satellites.

The opening of the irradiators is located at the focus of the parabolic reflector or in its focal plane if several irradiators are used in one antenna. Several irradiators form several radiation patterns in one antenna, this is necessary when pointing one antenna at once to several communication satellites.

Beam Width

Parabolic antenna parameters. Beam Width, Sidelobe Level, Gain

The angular beamwidth of an antenna and its radiation pattern does not depend on whether the antenna is transmitting or receiving. The beamwidth is determined by the level of half the power of the beam, that is, by the level (-3 dB) from its maximum value. For parabolic antennas, this level is determined by the formula:

θ = k λ / d (\displaystyle \theta =k\lambda /d\,),

where K is a factor that varies slightly with the shape of the reflector, and d is the reflector diameter in meters, the width of the half power pattern θ in radians. For a 2 meter satellite dish operating in the C band (3-4 GHz receive and 5-6 GHz transmit), this formula gives a beamwidth of about 2.6°.

Antenna gain is determined by the formula:

G = (π k θ) 2 e A (\displaystyle G=\left((\frac (\pi k)(\theta ))\right)^(2)\ e_(A))

There is an inverse relationship between gain and beam width.

Parabolic antennas of large diameters form very narrow beams. Pointing such beams at a communication satellite becomes a problem, since instead of the main lobe, you can point the antenna at the side lobe.

The antenna pattern is a narrow main beam and side lobes. Circular polarization in the main beam is set in accordance with the tasks, the level of polarization in different places of the main beam is different, in the first side lobes the polarization changes to the opposite, left to right, right to left.

Characteristics of reflector antennas

The characteristics of a reflector antenna are measured in the far field.

In a single-mirror antenna with circular polarization, the feed must have a direction of rotation of the field opposite to the direction of rotation of the antenna field.
Reflector antennas with a directional pattern directed to a moving object usually have an electric drive to track the angular direction behind the object.
Measurements of the RP of large reflector antennas in the far field are associated with great difficulties associated with significant distances from the antennas to the places where their signals are measured. For RP measurements, noise signals from the Sun, communication satellites, and large collimator antennas are used.
Large reflector antennas located in different places on the planet Earth are used as elements of antenna arrays for deep space exploration.

Application

Parabolic antennas are used as high gain antennas for the following types of communications: radio relay between nearby cities, wireless WAN / LAN data links, for satellite and spacecraft communications. They are also used for radio telescopes.

Parabolic antennas are also used as radar antennas to control ships, aircraft and guided missiles. With the advent of home satellite television receivers, parabolic antennas have become a feature of the landscapes of modern cities.

6.1. Parabolic Antennas

Reception of satellite television signals is carried out by special receivers, an integral part of which is the antenna. For professional and amateur receptions of satellite transmissions, parabolic antennas are the most popular, due to the property of a paraboloid of revolution to reflect the rays incident on its aperture, parallel to the axis, to one point, called the focus. The aperture is the part of the plane bounded by the edge of the paraboloid of revolution.

A paraboloid of revolution, which is used as an antenna reflector, is formed by rotating a flat parabola around its axis. A parabola is the locus of points equidistant from a given point (focus) and a given straight line (directrix) (Fig. 6.1). Point F is the focus and line AB is the directrix. Point M with coordinates x, y is one of the points of the parabola. The distance between the focus and the directrix is called the parameter of the parabola and is denoted by the letter p. Then the coordinates of the focus F are: (p/2, 0). The origin of coordinates (point 0) is called the vertex of the parabola.

By definition of a parabola, the segments MF and PM are equal. According to the Pythagorean theorem MF^2 =FK^2+ MK^2. At the same time FK = x - p/2, KM = y and PM = x + p/2, then (x - p/2)^2 + y^2 = (x + p/2)^2.

Squaring the expressions in brackets and bringing like terms, we finally obtain the canonical equation of the parabola:

y^2 = 2px, or y = (2px)^0.5. (6.1)

According to this classical formula, millions of antennas have been made to receive satellite television signals. What is it about this antenna?

Parallel to the axis of the paraboloid, the rays (radio waves) from the satellite, reflected from the aperture to the focus, pass the same (focal length). Conventionally, two beams (1 and 2) fall on the opening area of the paraboloid at different points (Fig. 6.2). However, the reflected signals of both beams pass the same distance to the focus F. This means that distance A+B=C+D. Thus, all the rays emitted by the transmitting antenna of the satellite and to which the parabo mirror is directed

loid, are concentrated in phase at the focus F. This fact is proved mathematically (Fig. 6.3).

The choice of the parabola parameter determines the depth of the paraboloid, i.e. the distance between the vertex and the focus. With the same aperture diameter, short-focus paraboloids have a large depth, which makes it extremely inconvenient to install the irradiator in focus. In addition, in short-focus paraboloids, the distance from the feed to the top of the mirror is much less than to its edges, which leads to uneven amplitudes at the feed for waves reflected from the edge of the paraboloid and from the zone close to the top.

Long-focus paraboloids have a shallower depth, the irradiator installation is more convenient and the amplitude distribution becomes more uniform. So, with an aperture diameter of 1.2 m and a parameter of 200 mm, the depth of the paraboloid is 900 mm, and with a parameter of 750 mm - only 240 mm. If the parameter exceeds the aperture radius, the focus, in which the feed should be located, is located outside the volume bounded by the paraboloid and the aperture. The optimal option is when the parameter is slightly larger than the aperture radius.

A satellite dish is the only amplifying element of the receiving system that does not introduce its own noise and does not degrade the signal and, consequently, the image. Antennas with a mirror in the form of a paraboloid of revolution are divided into two main classes: symmetrical parabolic reflector and asymmetric (Fig. 6.4, 6.5). The first type of antennas is usually called direct focus, the second - offset.

The offset antenna is, as it were, a cut out segment of a parabola. The focus of such a segment is located below the geometric center of the antenna. This eliminates the shading of the useful area of the antenna by the feed and its supports, which increases its efficiency at the same mirror area with an axisymmetric antenna. In addition, the irradiator is installed below the center of gravity of the antenna, thereby increasing its stability in windy conditions.

It is this design of the antenna that is most common in the individual reception of satellite television, although other principles for constructing terrestrial satellite antennas are currently used.

It is advisable to use offset antennas if the antenna size up to 1.5 m is required for stable reception of the programs of the selected satellite, since with an increase in the total area of the antenna, the mirror shading effect becomes less significant.

The offset antenna is mounted almost vertically. Depending on the geographical latitude, its angle of inclination is slightly

is changing. This position excludes the collection of atmospheric precipitation in the antenna bowl, which greatly affects the quality of reception.

The principle of operation (focusing) of direct focus (axisymmetric) and offset (asymmetric) antennas is shown in fig. 6.6.

For antennas, directional characteristics are of particular importance. Thanks to the ability to use antennas with high spatial selectivity, satellite television is received. The most important characteristics of antennas are gain and radiation pattern.

The gain of a parabolic antenna depends on the diameter of the paraboloid: the larger the diameter of the mirror, the higher the gain.

The dependence of the parabolic antenna gain on the diameter is shown below.

The role of the parabolic antenna gain can be analyzed using a light bulb (Fig. 6.7, a). The light is evenly scattered into the surrounding space, and the observer's eye perceives a certain level of illumination corresponding to the power of the light bulb.

However, if a light source is placed at the focus of a paraboloid with a gain of 300 times (Fig. 6.7, b), its rays, after reflection by the surface of the paraboloid, will be parallel to its axis, and the color strength will be equivalent to a source with a power of 13,500 W. The observer's eyes cannot perceive such illumination. On this property, in particular, the principle of operation of the spotlight is based.

Thus, the antenna paraboloid, strictly speaking, is not an antenna in its understanding of the transformation of the electromagnetic field strength into a signal voltage. A paraboloid is only a reflector of radio waves, concentrating them at a focus, where the active antenna (feeder) should be placed.

The antenna pattern (Fig. 6.8) characterizes the dependence of the amplitude of the electric field strength E, created at a certain point, on the direction to this point. In this case, the distance from the antenna to this point remains constant.

An increase in the gain of the antenna entails a narrowing of the main lobe of the radiation pattern, and narrowing it to less than 1 ° leads to the need to supply the antenna with a tracking system, since geostationary satellites oscillate around their stationary position in orbit. An increase in the width of the radiation pattern leads to a decrease in the gain, and hence to a decrease in the signal power at the receiver input. Based on this, the optimal width of the main lobe of the radiation pattern is

the width is 1 ... 2 °, provided that the transmitting antenna of the satellite is kept in orbit with an accuracy of ± 0.1 °.

The presence of side lobes in the radiation pattern also reduces the gain of the antenna and increases the possibility of receiving interference. In many ways, the width and configuration of the radiation pattern depend on the shape and diameter of the receiving antenna mirror.

The most important characteristic of a parabolic antenna is shape accuracy. It should repeat the shape of a paraboloid of revolution with minimal errors. Shape accuracy determines the gain of the antenna and its radiation pattern.

It is almost impossible to make an antenna with a perfect paraboloid surface. Any deviation from the real shape of the parabolic mirror from the ideal one affects the characteristics of the antenna. Phase errors occur, which degrade the quality of the received image, and the antenna gain decreases. Shape distortion also occurs during the operation of antennas: under the influence of wind and precipitation; gravity; as a result of uneven heating of the surface by the sun's rays. Taking into account these factors, the allowable total deviation of the antenna profile is determined.

The quality of the material also affects the characteristics of the antenna. For the manufacture of satellite dishes, steel and duralumin are mainly used.

Steel antennas are cheaper than aluminum ones, but heavier and more prone to corrosion, so anti-corrosion treatment is especially important for them. The fact is that a very thin near-surface metal layer participates in the reflection of an electromagnetic signal from the surface. If it is damaged by rust, the efficiency of the antenna is significantly reduced. It is better to first cover a steel antenna with a thin protective layer of some non-ferrous metal (for example, zinc), and then paint it.

With aluminum antennas, these problems do not arise. However, they are somewhat more expensive. The industry also produces plastic antennas. Their mirrors with a thin metal coating are subject to shape distortion due to various external influences: temperature, wind loads, and a number of other factors. There are mesh antennas that are resistant to wind loads. They have good weight characteristics, but have proven themselves poorly when receiving Ki-band signals. It is advisable to use such antennas for receiving C-band signals.

A parabolic antenna at first glance seems like a rough piece of metal, but nevertheless it requires careful handling during storage, transportation and installation. Any distortion of the shape of the antenna leads to a sharp decrease in its efficiency and a deterioration in the quality of the image on the TV screen. When buying an antenna, you need to pay attention to the presence of distortion of the working surface of the antenna. Sometimes it happens that when anti-corrosion and decorative coatings are applied to the antenna mirror, it “leads” and it takes the form of a propeller. You can check this by placing the antenna on a flat floor: the edges of the antenna should touch the surface everywhere.

Squaring the expressions in brackets and bringing like terms, we finally obtain the canonical equation of the parabola:

y^2 = 2px, or y = (2px)^0.5. (6.1)

According to this classical formula, millions of antennas have been made to receive satellite television signals. What is it about this antenna?

loid, are concentrated in phase at the focus F. This fact is proved mathematically (Fig. 6.3).

It is this design of the antenna that is most common in the individual reception of satellite television, although other principles for constructing terrestrial satellite antennas are currently used.

The offset antenna is mounted almost vertically. Depending on the geographical latitude, its angle of inclination is slightly

is changing. This position excludes the collection of atmospheric precipitation in the antenna bowl, which greatly affects the quality of reception.

The principle of operation (focusing) of direct focus (axisymmetric) and offset (asymmetric) antennas is shown in fig. 6.6.

The gain of a parabolic antenna depends on the diameter of the paraboloid: the larger the diameter of the mirror, the higher the gain.

The dependence of the parabolic antenna gain on the diameter is given below.

the width is 1 ... 2 °, provided that the transmitting antenna of the satellite is kept in orbit with an accuracy of ± 0.1 °.

The quality of the material also affects the characteristics of the antenna. For the manufacture of satellite dishes, steel and duralumin are mainly used.

Squaring the expressions in brackets and bringing like terms, we finally obtain the canonical equation of the parabola:

y^2 = 2px, or y = (2px)^0.5. (6.1)

According to this classical formula, millions of antennas have been made to receive satellite television signals. What is it about this antenna?

loid, are concentrated in phase at the focus F. This fact is proved mathematically (Fig. 6.3).

It is this design of the antenna that is most common in the individual reception of satellite television, although other principles for constructing terrestrial satellite antennas are currently used.

The offset antenna is mounted almost vertically. Depending on the geographical latitude, its angle of inclination is slightly

is changing. This position excludes the collection of atmospheric precipitation in the antenna bowl, which greatly affects the quality of reception.

The principle of operation (focusing) of direct focus (axisymmetric) and offset (asymmetric) antennas is shown in fig. 6.6.

The gain of a parabolic antenna depends on the diameter of the paraboloid: the larger the diameter of the mirror, the higher the gain.

The dependence of the parabolic antenna gain on the diameter is given below.

the width is 1 ... 2 °, provided that the transmitting antenna of the satellite is kept in orbit with an accuracy of ± 0.1 °.

The quality of the material also affects the characteristics of the antenna. For the manufacture of satellite dishes, steel and duralumin are mainly used.

Or-bi-ta satellite-no-sit-name-of-geo-sta-qi-o-nar-noy, if, with the rotation of the Earth, is the satellite always visible sits over one and the same point on the earth in the upper-no-sti. Such or-bi-you for-often use in the system of communication and in-zi-qi-o-ni-ro-va-nia.

Sputnik, someone you see on the car-tin-ke, yav-la-et-sya symbol-in-scrap kos-mi-che-sky program-we are our country -us. This is SOYUZ-TM.

And so, at some point, you-look-de-la for-stav-ka program-we "Time" - basic-new-in-for-ma-chi-on- Noah te-le-pro-gram-we country.

Well, in the cartoon film, we look at how the pro-is-ho-dit process re-re-da-chi sig-na-la, for example, modern-men-but th satellite-no-to-go-tele-vi-de-niya.

We-we-we-we-we-we-we-we and we-we-we-we call her di-rek-tri-soy. Let's take a point outside of it. Geo-met-ri-che-s-one place to-check, equal-to-remote from di-rivers-tri-sy and given point-ki (fo-ku-sa), on- zy-va-et-sya pa-ra-bo-loy.

If you-whether to-right-wind on the pa-ra-bo-lu ray-chi of the light, its para-ral-lel-nye axes of symmetry, then all the ray-chi co-be-rut-sya in fo-ku-se pa-ra-bo-ly. This property is on-zy-va-et-xia op-ti-che property of pa-ra-bo-ly.

True and vice versa. If it’s possible to place lam-poch-ku in focus, then lu-chi, from-ra-ziv-shis from pa-ra-bo-ly, go pa-ral-lel-but, moreover, gra-ni-tsa light-ta will be direct-mine.

If you rotate the pa-ra-bo-lu from-no-si-tel-but of its symmetry axis, then it’s better to read the surface of the rotation niya second-ro-go in a row - pa-ra-bo-lo-id. Since in any se-che-nii the plane, containing the axis of symmetry, is one and the same pair-ra- bo-la, then the op-ti-che property is true for pa-ra-bo-lo-and-yes. If it’s possible to place lam-poch-ku in the focus of pa-ra-bo-lo-and-yes, then beam-chi, from-ra-ziv-shis from top-no-sti, go-dut pa-ral-lel-but to each other-gu. The reverse is also true.

Namely, this property is used in satellite-no-ko-vy pa-ra-bo-li-che-sky an-ten-nah. Since the satellite is on-ho-dit-sya very yes-le-ko from an-ten-na, then the rays can be considered almost par-ral-lel-us-mi, and when -em-nick sig-na-la sta-vit-sya in focus pa-ra-bo-lo-i-da.

Additional Information:

On la-ty-ni focus means "hearth, fire". Like ma-te-ma-ti-che-sky ter-min word "focus"