A horn antenna is a structure consisting of a radio waveguide and a metal horn. They have a wide range of applications, are used in measuring devices and as an independent device.

What is this

A horn antenna is a device that consists of an open-ended waveguide and a radiator. In shape, such antennas are H-sectoral, E-sectoral, conical and pyramidal. Antennas - broadband, they are characterized by a small level of lobes. The horn design with effort is simple. The amplifier allows it to be small in size. For example, or a lens aligns the phase of the wave and positively affects the dimensions of the device.

The antenna looks like a bell with a waveguide attached to it. The main disadvantage of the horn is its impressive parameters. In order to bring such an antenna into working condition, it must be located at a certain angle. That is why the horn is longer in length than in cross section. If you try to build such an antenna with a diameter of one meter, it would be several times longer in length. Most often, such devices are used as a mirror irradiator or for servicing radio relay lines.

Peculiarities

The radiation pattern of a horn antenna is the angular distribution of power or energy flux density per unit angle. The definition means that the device is broadband, has a feed line and a small level of the rear lobes of the diagram. In order to obtain highly directional radiation, it is necessary to make the horn long. This is not very practical and is considered a disadvantage of this device.

One of the most modernized types of antennas is horn-parabolic. Their main feature and advantage is low sidelobes, which are combined with a narrow radiation pattern. On the other hand, horn-parabolic devices are bulky and heavy. One example of this type is the antenna installed on the Mir space station.

According to their properties and technical characteristics, horn devices are no different from installed receivers in mobile phones. The only difference is that the latter antennas are compact and hidden inside. However, miniature horn antennas can be damaged inside a mobile device, so it is recommended to protect the phone case with a case.

Types

There are several types of horn antennas:

pyramidal (made in the form of a pyramid of a tetrahedron with a rectangular section, it is used most often);
sectoral (has a horn with an H or E extension);
conical (made in the form of a cone with a circular cross section, emits waves of circular polarization);
corrugated (horn with a wide bandwidth, a small level of side lobes, used for radio telescopes, parabolic and satellite antennas);
horn-parabolic (combines a horn and a parabola, has a narrow radiation pattern, a low level of side lobes, operates at radio relay and space stations).

The study of horn antennas allows you to study their principle of operation, calculate the radiation patterns and antenna gain at a certain frequency.

How does it work

Horn measuring antennas rotate around their own axis, which is perpendicular to the plane. A special detector with amplification is connected to the output of the device. If the signals are weak, a quadratic current-voltage characteristic is formed in the detector. A stationary antenna creates electromagnetic waves, the main task of which is the transmission of horn waves. In order to remove the directional characteristic, it is deployed. Then readings are taken from the device. The antenna is rotated around its axis and all the changed data is recorded. It is used to receive radio waves and radiation of microwave frequencies. The device has huge advantages over wire assemblies, as it is able to receive a large amount of signal.

Where is used

The horn antenna is used as a separate device and as an antenna for measuring devices, satellites and other equipment. The degree of radiation depends on the opening of the antenna horn. It is determined by the size of its surfaces. This device is used as an irradiator. If the design of the device is combined with a reflector, it is called a horn-parabalic. Gained units are often used for measurements. The antenna is used as a mirror or beam irradiator.

The inner surface of the horn can be smooth, corrugated, and the generatrix can have a smooth or curved line. Various modifications of these emitting devices are used to improve their characteristics and functionality, for example, in order to obtain an axisymmetric diagram. If it is necessary to correct the directional properties of the antenna, accelerating or decelerating lenses are installed in the aperture.

Settings

The horn-parabolic antenna is tuned in the waveguide part using diagrams or pins. If necessary, such a device can be made independently. The antenna belongs to the aperture class. This means that the device, unlike the wire model, receives the signal through the aperture. The larger the horn of the antenna, the more waves it will receive. Strengthening is easy to achieve by increasing the size of the unit. Its advantages include broadband, simple design, excellent repeatability. To the disadvantages - when creating one antenna, a large amount of consumables is required.

To make a pyramidal antenna with your own hands, it is recommended to use inexpensive materials, such as galvanization, durable cardboard, plywood in combination with metal foil. It is permissible to calculate the parameters of a future device using a special online calculator. The energy received by the horn enters the waveguide. If you change the position of the pin, the antenna will operate in a wide range. When creating a device, keep in mind that the inner walls of the horn and waveguide must be smooth, and the bell must be rigid on the outside.

Radiation comes from the open end of the waveguide. Waveguides of a rectangular or round type are used to channel electromagnetic energy.

However, waveguides can be used not only to channel electromagnetic energy, but also to radiate it.

The open end of the waveguide can be considered as the simplest microwave antenna.

The open end of the waveguide is a platform with an electromagnetic field.1

Features of the electromagnetic field at the open end of the waveguide.

1. The wave is not a transverse TEM type. (has a more complex structure).

2. In addition to the incident wave, there is a reflected wave.

3. Along with the main type of wave, higher types of waves are present at the end of the waveguide.

In addition, the field is present not only in the waveguide opening, but also on the outer surface due to the flow of currents to this surface from the end of the waveguide.

Accounting for these factors greatly complicates the problem of determining the radiation field from the open end of the waveguide, and its rigorous mathematical solution encounters great difficulties. For this reason, approximate solution methods are usually used. For this solution, the problem is divided into two problems: internal and external.

1) The internal task is to find the field in the opening of the waveguide.

2) The external task is to find the radiation field from the known field in the aperture.

Consider a rectangular waveguide.

Basic wave type.

Rice. 45. Rectangular waveguide (a) and the structure of the field in it for a wave of the type: in the xOy plane (b); in the xOz plane (c); in the yOz (d) plane.

;

The intensity of the incident electromagnetic field in the middle of the waveguide opening.

Wavelength in a waveguide.

Wavelength in free space.

Complex reflection coefficient.

Field in the far zone:

Characteristic impedance of the wave front at the open end of the waveguide.

The wave resistance of the medium is .

Taking into account the found field relations in the main planes

Waveguide opening area.

Radiation pattern of the open end of a rectangular waveguide.

Rice. 46. Directional pattern of radiation from the open end of a rectangular waveguide at

As can be seen from the figures, the width of the radiation pattern is large. To obtain a sharper radiation pattern, the cross section of the waveguide can be gradually increased, turning the waveguide into a horn. In this case, the field structure in the waveguide is largely preserved.

A smooth increase in the cross section of the waveguide improves its matching with free space.

Rice. 47. Basic types of electromagnetic horns.

Sectorial and pyramidal horns are the most widespread.

Consider a longitudinal section of a rectangular horn by plane E or H.

Rice. 48. Longitudinal section of a rectangular horn.

Horn opening

Horn opening width.

Horn length.

The top of the horn.

The study of the horn is usually carried out by approximate methods due to mathematical difficulties.

Initially, the field in the disclosure is defined. When solving this problem, the horn is assumed to be infinitely long, and its walls are ideally conductive.

After solving the internal problem, the external problem is solved by the usual method, i.e. is the radiation field.

H - planar sectorial horn.

To find the structure of the field in the horn, we use a cylindrical coordinate system.

The wave will have components .

Rice. 49. Cylindrical coordinate system for the analysis of sectorial horns.

Solving the system of Maxwell equations and using asymptotic expressions for the Hankel functions for large values of the argument , we obtain the following values for the field components

(1)

Here, the electric field strength at the horn point with coordinates and .

Formulas (1) show that at large components and the field in the horn is a transverse electromagnetic cylindrical wave. Due to the fact that most of the used horns have a flat opening, and the wave in the horn is cylindrical, the field in the opening will not be in phase.

To determine the phase distortions in the opening, consider the longitudinal section of the horn. The arc of a circle centered at the top of the horn passes along the wave front and, therefore, is a line of equal phases. At an arbitrary point with coordinate , the phase of the field lags behind the phase in the middle of the opening (at point ) by an angle

Rice. 50. To the definition of phase distortions in the opening of the horn.

Since usually in horns, we can restrict ourselves to the first term of the expansion

Formula (2) and are approximate. They can be used when or. In the horns used, these conditions are usually met.

Sometimes it is convenient to determine the maximum phase errors in the opening of the horn through its length and half the opening angle.

The formula is true for any and .

It can be seen from the formula that for a given field in the opening, the less it will differ from the in-phase one, the greater the length of the horn. Dimensional restrictions require finding a compromise solution, i.e. determining such a horn length at which the maximum phase shift in its opening will not exceed a certain allowable value. This value is usually determined by the largest directivity value that can be obtained from a horn of a given length. For a sectorial horn, the maximum allowable phase shift is , which corresponds to the following relationship between the optimal horn length, opening size, and wavelength :

To determine the distribution of field amplitudes in the opening of the horn, we take

Thus, the field in the aperture of a sectorial horn can be finally represented by the expressions

In-plane radiation pattern

Typical dependences of the directional coefficient on the relative opening of the horn for various horn lengths are given below.

Rice. 51. Dependence KND H - sectorial horn on the relative opening width

with different horn lengths.

In order to exclude the dependence of the directional coefficient on the y-axis, the product is plotted. It can be seen from the graphs that for each horn length there is a certain horn opening at which the directional coefficient is maximum. Its decrease with a further increase is explained by a sharp increase in phase errors in the aperture.

The horn, which, for a given length, has the maximum coefficient of directional action, is called optimal. It can be seen from the curves shown in Fig. 3 that at , the maximum points of the curves correspond to the equality

If the length of the horn is taken longer, then with the same opening area, the directional coefficient increases, but not very much. The maximum points of the coefficient of directional action correspond to the coefficient of utilization of the opening area.

If the length of the horn is continuously increased, then in the limit at we will get an in-phase field in the opening of the horn. The utilization factor of an in-phase area with a cosine distribution of the field amplitude is equal to . Thus, increasing the length of the horn in comparison with its optimal length cannot increase the directivity by more than

The efficiency of horn antennas due to low losses can practically be taken as unity.

E-planar sectorial horn.

Field in the opening of a planar sectorial horn

(1)

Here ; distance from the throat of the horn.

It can be seen from formula (1) that the main difference between the field in a planar horn and the field in a waveguide is the cylindrical waveform. As a result, there will be phase distortions in the opening of the horn, similar to distortions in a planar horn.

If the opening angle of the horn is small, then you can put . In this case, the electric field strength in the opening can be represented as:

The radiation field of the sectorial horn in the plane

(2)

It follows from this formula that the radiation pattern in the plane of the planar horn is the same as that of the open end of the waveguide.

Field in plane:

(3) . . mouthpieces.

In this case, the formula can be conveniently represented as:

the values in parentheses are directly plotted along the ordinates in the indicated graphs.

Calculation of a single horn

Let's calculate the wavelength? and wave number k:

where c \u003d 3 * 10 8 m / s is the speed of light.

The choice of the dimensions of the cross section of a rectangular waveguide is made from the condition of propagation in the waveguide of only the main type of the H10 wave:

According to the received value? choose a waveguide brand R100 with dimensions a*b=22.86*10.16 mm.

Calculate the directional coefficient of the horn:

Let us find the values of the optimal lengths of the horn in the planes E and H:

We use the equation for docking a horn with a waveguide:

h 1 (1-a / a 1) = h 2 (1-b / a 2).

To ensure that the phase distortions in the aperture do not exceed the allowable limits, we take the larger value of the length h as a constant number and express the smaller value through the larger one:

Calculate the opening angles of the horn antenna:

Calculate and construct the horn DN.

a) In plane E

Rice. 3.

DN width at the level of 0.5: ? 0.5 \u003d 5.4 about.

b) In the plane H

Rice. 4. Horn pattern in the H plane

DN width at the level of 0.5: ? 0.5 \u003d 4.9 about

Antenna Beam Calculation

1. Common mode operation.

Directional pattern of a line of horn antennas:

The lattice factor is determined by the formula:

where d is the distance between the emitters.

There will be several diffraction maxima in the multiplier pattern. Since the opening dimensions of one horn are equal to 20 * 30 cm, the condition ensuring the existence of one maximum is not satisfied. But as long as the diffraction maxima are outside the main lobe of the RP of one emitter, they will not be in the RP of the array, since they are destroyed when the diagrams are multiplied. Based on this, we determine the distance between the emitters d opt , at which diffraction lobes begin to appear in the pattern of the emitter line:

d opt = ?/sin(? 0 ex) .

According to the DN of a single horn, we find that in both planes (H- and E-planes) ? 0 izl \u003d 9 o, then

d opt = 3.1/sin9 o = 19.8 cm.

The obtained value d opt is close in terms of the size of the horn opening in the plane E a 2 = 20 cm, so we take the distance between the radiators d = 20 cm. Then the location of the horns in the antenna will be as shown in Fig. 5

Considering that for an in-phase line of emitters?? = 0, we find the radiation pattern of the entire antenna in the E plane using the following formula:

Rice. 6.

The width of the antenna radiation pattern at the zero level and at the level of 0.5 is determined as follows:

Sidelobe level:

The position of the first diffraction maximum is determined by the formula:

Diff = ± arcsin(p?? / d),

where p is the number of the diffraction lobe.

Diff \u003d ± arcsin (3.1 / 20) \u003d ± 8.9 °.

The radiation pattern of the line of emitters in the H - plane will be the same as that of one emitter in the H - plane.

2. Out-of-phase operation.

Let us calculate the maximum deviation of the antenna pattern from the normal to its surface:

max=? 0.7 izv.

According to the DN chart of a single horn in the E plane (Fig. 3), we determine what? max = 4 o.

The distance between the radiators of the grating with electric beam oscillation should be less than the optimal one. In our case, the size of the opening of the horn in the plane in which the beam is deflected is equal to the optimal value. Thus, it is impossible to reduce the distance between the emitters, which means that the diffraction lobes of the grating multiplier will be included in the main lobe of the emitter pattern. This will lead to an increase in the side lobes of the antenna pattern.

The phase difference of the currents of the emitters?? we find from the formula that determines the direction of maximum radiation.

We find the radiation pattern of the antenna in the out-of-phase mode by multiplying the pattern of one emitter in the E-plane F 2 (? 2) by the grating factor F n (? 2) at ?? = 2.8 rad.

Rice. 7.

Calculate the directivity and gain of the antenna.

where S a \u003d S? n is the area of \u200b\u200bthe radiating surface of the antenna.