An antenna diagram is the graphical representation of the radiation pattern of an antenna (intensity, field strength, polarization, phase, delay time differences ) in a spatial coordinate system. Antenna patterns are measured technically recorded or generated using simulation software on the computer to plot the directivity of an antenna and so assess their performance. They can be represented as a surface in three-dimensional spherical coordinates, or for a planar interface as a line graph in Cartesian or polar coordinates. An antenna diagram illustrating the directivity characteristic of an antenna is also called directivity pattern. It represents the relative intensity of the energy radiation or electric or magnetic field strength in the far-field, depending on the direction of the antenna

While an omnidirectional antenna radiates equally in all directions of a plane, preferably a directional antenna one direction and therefore achieves this with less transmit power greater range. The antenna diagram represents the metrological or mathematically determined preference graphical form the basis of reciprocity - ensures the same transmitting and receiving properties of the antenna - the diagram shows to both the direction-dependent transmission power and reception sensitivity of an antenna. Thus, the mostly drawn by a measurement program curve shows at a transmitting antenna to scale to the places with the same power density around the transmitting antenna. Upon receiving antennas, the same curve is the measured sensitivity for a constant high-frequency field. A small test transmitter is thus moved at a constant distance to the receiving antenna around and the power that is received by the antenna, is entered as a value in the chart.

• 5.1 half-width
• 5.2 sidelobe attenuation
• 5.3 front-back ratio
• 5.4 Prior to pages ratio
• 6.1 Field measurements
• 6.2 Measurements under laboratory conditions

## Horizontal antenna pattern

Horizontal antenna diagrams represent the directional characteristic only for the horizontal directions is usually in polar coordinates, ie with the antenna at the center. There is a horizontal section through the three-dimensional diagram. Parts of the antenna pattern, which are limited by relative minima are referred to according to their appearance in polar coordinates as lobes:

• The main lobe is the global maximum, and contains the main radiation direction;
• Sidelobes are pronounced local maxima contain the most unwanted radiation in a direction other than the main direction;
• A back lobe is a side lobe directly or in a wide range opposite the main lobe;
• Grating lobes are intermittent strong sidelobes.

Antenna patterns are often plotted logarithmically in decibels, as the side lobes can be several orders of magnitude smaller than the main lobe and would not be seen with linear plot.

Since the directivity of antennas is frequency dependent, can be created as a special form of the antenna pattern an enveloping antenna pattern. Such special forms of antenna patterns are required by field strengths in the assessment of radiation exposure. Here the measured diagrams are of the highest and the lowest frequency superimposed and made ​​from the highest individual values ​​for each side angle a new chart. With an expanded by mounting tolerances the measured antenna pattern or chart created as an enveloping antenna pattern is superimposed three times: once in the original orientation, and once each rotated about the mounting tolerance in the clockwise and counterclockwise. From highest individual values ​​for each side angle turn over a new diagram is formed.

From a radiation pattern can be read many parameters which determine the quality and directionality of the antenna as shown:

• The front-back ratio ( VRV ), also known as return loss,
• The on- side ratio ( VSV) and
• The side lobe attenuation.

## Vertical antenna pattern

A vertical antenna pattern is a side view of the electromagnetic field of the antenna. The dimension of the antenna pattern is therefore the distance from the antenna in the x- axis and height above the site of the antenna in the y- axis.

In the diagram, the vertical axis (y axis) is the height in feet ( Sheet feet, ft) plotted in the horizontal axis ( x-axis), the distance in nautical miles (Nm ), both units operating at a used air traffic control radar. The units play a role in the scale ratio of the axes in the chart. However, the measured values ​​are relative levels that have nothing to do with the distance of the axes.

The radial lines from the origin, the angle of elevation marks drawn in "one- degree increments ." By the camber, that is, the vertical axis has a different scale than the horizontal axis, the distances between the elevation angle marks are not equal.

The is transferred to the vertical axis heights are projected into the diagram not only as a grid, but also as dotted lines, which are indicating the real height above ground and thus not straight but slightly sloping downward according to the curvature of the earth lines.

The diagram is a real cosecant square diagram of an airport surveillance radar. The " frayed ", far away from the origin edge of the diagram shows the influence of the earth's surface to the diagram (see Fresnel zone ), since this antenna was built, unfortunately a little too deep.

## Spatial directivity

Are 2D antenna patterns of many section planes together to form a three-dimensional structure, creates a three-dimensional directivity. The distance from the center of the antenna to each point of the surface of this body is to the measured in this direction at equal intervals intensity. The inclusion of such spatial characteristics, however, would represent a major metrological effort. Therefore, such an antenna diagrams are created only in exceptional cases and then only in excerpts in practice. With the help of computers, however, each antenna are simulated with their spatial characteristics in a model.

## Elements of an antenna pattern

### Main lobe

The main lobe of an antenna pattern has the maximum amount of energy transmitted in one direction or receive antennas, the maximum sensitivity at transmitting antennas. A directional antenna focuses such radiation in one direction and thereby increases the range of the antenna. This increase in range is called profit and is expressed as the ratio of the measured antenna to the values ​​of an omnidirectional antenna. An antenna pattern of the antenna gain is not used! All measured and graphically represented in the antenna pattern values ​​are relative to the maximum value of the main lobe. This is registered with 0 dB in the diagram and all other measured values ​​must therefore be negative level ( attenuation ). Therefore, there are only relative values. The antenna gain is, however, an absolute value because it is based on a calibrated measure.

In the very simple antenna patterns of a dipole antenna ( antenna pattern qv) there are only two distinct maxima, which are in opposite directions. In this case, it is not yet speak of a main lobe, as both are maxima approximately equal.

### Sidelobes

As a side lobe of the part of the electromagnetic radiation of an antenna is known, which is not radiated in the desired direction. Side lobes are usually undesirable because they withheld a portion of the transmit power of the main lobe, thereby weaken and impair the unique straightening effect of an antenna. At a receiving antenna interference received on the side lobes can degrade the reception quality; they are not hidden from the antenna. In transmitting antennas, the transmission power of the side lobes is unused radiated in an unwanted direction.

By clever design of an antenna, the intensity of the side lobes can be reduced. If the receiver dynamics is, however, greater than the predetermined by the construction of the antenna side lobe attenuation, signals are received via the side lobes. To reduce the effects of this unwanted reception, additional measures of the side lobe suppression measures when radar detection technology.

#### Grating lobes

Grating lobes (English: Grating lobes ) are side lobes, which reach approximately the size of the main lobe and are distributed in a grid -like in the diagram. They occur sometimes in phased array antennas (and also used in ultrasonography ultrasound probes) and are a result of a large and uniform spacing of the individual antenna elements to one another in relation to the wavelength. With a good design of a phased array with optimal possible distance of the radiating elements, they should not occur, but this requirement is for very large bandwidth (UWB ) difficult to maintain.

#### Back lobe

As a back lobe side lobe in the direction is called an antenna pattern that shows in the opposite direction of the main lobe. They are usually a lot smaller than the main lobe. If the exact opposite direction of the antenna pattern comprises a radiation minimum, are often referred to as a back lobe and side lobes, which are located within an angle of ± 15 ° from the exact direction.

### Zeros

The points of the antenna pattern, in which the radiant energy is substantially zero is referred to as zero points. The position in the coordinate system can be designated as zero angle that is measured between the maximum of the main lobe and the first zero point. A zero -width is measured between the first zeros of the left and right of the main lobe.

## Read from the antenna pattern parameters

### Half-width

The critical angle of a leg after general convention by the drop in received power or the radiated intensity to half of the maximum value defined (factor 0.5 ≈ -3 dB). The beam width ( FWHM opening angle) is the span between these angles and is most often the Greek letter Θ (theta ) means. The main lobe of the characterized found in the graphs parabolic antenna has a beam width of 1.67 °, a very good value for a radar antenna.

### Sidelobe attenuation

The side-lobe rejection is one of the essential parameters of an antenna and represents the ratio of the gain of the main lobe in the 0 ° to the level of the largest side lobe ( here in the chart at about 20 °) dar. This ratio is expressed as a relative level, and should be as large as possible.

### Front-back ratio

The front-back ratio ( VRV, Eng. Front -to-back ratio), also known as return loss, is an important parameter of an antenna and represents the ratio of the measured level of the main lobe in the 0 ° to the level of the back lobe at 180 ° dar. this ratio is indicated as relative levels, and should be as large as possible. The front-back ratio is next to the side-lobe rejection is a measure of the concentration of a directional antenna, the larger the front-back ratio, the better the antenna.

In some publications, the VRV is based not only on this one back lobe, but are under the term front-back ratio all side lobes between 90 ° and 270 ° considered and for the determination of the VRV only the strongest side lobe is used in this range of angles. This is useful, for example, if an antenna at an angle of 180 ° has a pronounced minimum and are the rear legs for example at about 175 ° and 185 °.

### Prior to pages ratio

The specification of an on -page ratio is only useful for such antennas, where no pronounced back lobe can be seen or because, as in the antenna pattern of a dipole antenna two diametrical radiation maxima are formed. Therefore, this antenna pattern parameters are rarely used unless the antenna is a so-called Janus diagram with two diametrically opposite main lobes.

## Measurement methods

Due to the reciprocity of antennas, it is possible to provide a receiving antenna and transmitting antenna are measured ( and vice versa) and from the measured reception characteristics include data on as a transmitting antenna (and vice versa). A second variation is either a mobile measuring apparatus (which may be configured as a transmitter or receiver) to the rigidly structured to move to measured antenna in the far field around, or to unfold this measuring tool on a firm place and rotate the antenna to be measured.

### Field measurements

What case is selected in the measurement of an antenna depends on and how far these can distort the antenna pattern of influences of the environment. Often, however, prevent even the geometrical dimensions of the antenna, such rotating in a plane. In principle, the radiation source should be moved when the measured antenna has strong directivity. Is the antenna to be measured rotationally, it can be rotated in one direction, from the smallest possible external noise power is to be received. For this interference power does not distort the antenna pattern, then the measuring station should be moved around the antenna. This should work even under optical visibility conditions in the far field of the antenna, however, so that the current azimuth angle can be determined by suitable optical measuring tools ( aiming circle or theodolite ).

A measurement at the final location with a fixed measuring stations and rotating the receiving antenna is very complex, since the measurement result may be distorted by environmental influences. The reception of reflections and interference powers must be kept as low as possible by appropriate measures. From a great distance, a directional spotlight with constant power is radiating in the direction of the antenna. The services received are measured at various angles and then tabulated by rotating and tilting the antenna to be measured. A parallel Störfeldmessung with a calibrated measuring antenna with a very wide opening angle or even omnidirectional pattern can be used for the correction of the measurement result.

When the antenna pattern of the antenna to be measured is composed of components of the ground and reflected power, a rotation of the antenna to be measured is not always possible. Often, the geometric dimensions of the antenna prevent rotation. In this case, a radiation source at a sufficient distance needs (ie not in the near field, but in the far field of the antenna) are moved from the antenna to be measured. For antennas that are already equipped with a rotating device such as radar antennas, this measurement, however, can quite easily be determined using a special measuring tool. The analyzer shown in the image ( engl. Radar Field Analyzer, XRF) is part of such a measuring tool.

The measurement program must approximate the rotational speed of the antenna, and the pulse repetition frequency of the radar know. ( These values ​​can also be determined with the program itself.) The amplitudes of all the received transmit pulses of a complete revolution of the antenna are stored with a time stamp. The strongest pulse is taken as a reference, and interpreted as a center of the main lobe, and thus shown in the 0 °. All other readings are allocated in the evaluation of a side angle relative to the angle of the main lobe.

The vertical antenna pattern can be obtained by statistical measurements, for example the electromagnetic spectrum of solar radiation, determined ( Sunstrobe - Recording). Here, the measurement tool will be established within the radar station and connected to the video outputs of the receiver. The radar antenna operates at this measuring method, as a receiving antenna. During the sunrise or under ganges all video amplitudes of the solar noise are recorded and assigned an elevation angle in a later evaluation routine.

As more imponderable factors included in the measurements, only values ​​of a single series of measurements can be compared with each other as a relative level, in which all individual measured values ​​have to be measured under identical conditions as possible. This means that compared various datings measurement series only the diagram form may have a significance.

### Measurements under laboratory conditions

In order to measure antenna systems under laboratory conditions with the aim of creating an antenna pattern, the antenna must be mounted on a movable rotating and tilting table. The entire measuring apparatus and the subject are for protection against external interference power in a massive Faraday cage, eg from soldered copper sheets. Ceilings, walls and floors of the measuring chamber and the measuring equipment must be covered with damping material to prevent the emergence of reflections. The generally pyramid-shaped structure elements consist of a graphite-containing strong foam to absorb electromagnetic radiation and convert energy during a lossy multiple reflection between the individual wall elements to heat. Very large measuring laboratories are also called EMC anechoic chamber.

Here the antenna is usually used as a receiving antenna. A measuring station transmits with a very narrow radiation pattern in the direction of the antenna to be measured. This is rotated or pivoted by motors in a plane.

This method brings useful measurement results, but is more of theoretical value, since especially the vertical antenna pattern may change due to the influence of the earth's surface at the final location where the antenna. It is mainly used in the antenna construction and repair. At very low frequencies, and thus geometrically large antennas, the antenna can be reduced with sufficient accuracy and the measurement frequency are used to scale increases.

## Examples

There are a number of antennas that are often named according to the geometric shape of the antenna pattern:

• Omnidirectional antennas with a highly uniform radiation in all directions within a plane
• Directional antennas with strongly pronounced main lobe,
• Pencil-beam antennas with an extremely thin main lobe and very high antenna gain,
• Subjects antennas and
• Cosecant ² antennas, both specifically designed for radar applications.
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