Antenna diagram
An antenna diagram is the graphic representation of the radiation characteristics of an antenna (intensity, field strength, polarization, phase, time differences) in a spatial coordinate system. Antenna diagrams are recorded using measurement technology or generated by computer simulation programs in order to graphically represent the directional effect of an antenna and thus assess its performance. They can be displayed as a surface in three-dimensional spherical coordinates or as a line diagram in Cartesian or polar coordinates for a two -dimensional section . An antenna diagram that shows the directional characteristics of an antenna is also called a directional diagram . It represents the relative intensity of the energy radiation or the electric or magnetic field strength in the far field depending on the direction towards the antenna.
While an omnidirectional antenna radiates uniformly in all directions of a plane, a directional antenna prefers one direction and therefore achieves a greater range in this direction with a lower transmission power. The antenna diagram graphically shows the preference determined by measurement or calculation. Due to the reciprocity - which ensures the same transmission and reception properties of the antenna - the diagram shows both the direction-dependent transmission power and the reception sensitivity of an antenna. In the case of a transmitting antenna, the curve, which is usually drawn by a measurement program, shows the locations with the same power density around the transmitting antenna, true to scale . In the case of receiving antennas, the same curve means the measured sensitivity for a constant high-frequency field. A small measuring transmitter is thus moved around the receiving antenna at a constant distance and the power received by the antenna is entered as a value in the diagram.
Horizontal antenna diagram
Horizontal antenna diagrams only show the directional characteristics for the horizontal directions, mostly in polar coordinates, i.e. with the antenna in the center. It is a horizontal section through the three-dimensional diagram. Parts of the antenna diagram that are limited by relative minima are called lobes according to their appearance in polar coordinates :
- The main lobe is the global maximum and contains the main direction of radiation ;
- Side lobes are pronounced local maxima contain the mostly unwanted radiation in a direction other than the main direction;
- a back lobe is a side-lobe or directly in a wide range opposite to the main lobe;
- Grating lobes are periodically occurring strong sidelobes.
Antenna diagrams 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 recognizable if plotted linearly.
Since the directivity of antennas is frequency-dependent, an enveloping antenna diagram can be created as a special form of the antenna diagram . Such special forms of antenna diagrams are required when assessing radiation exposure due to field strengths. The measured diagrams of the highest and lowest frequency are superimposed here and a new diagram is formed from the highest individual values for each side angle. In the case of an antenna diagram expanded by mounting tolerances , the measured diagram or the diagram that has already been created as an enveloping antenna diagram is superimposed three times: once in the original orientation and once each rotated by the mounting tolerance clockwise and counterclockwise. A new diagram is in turn formed from the highest individual values for each side angle.
Many parameters can be read from an antenna diagram, which determine the quality and directivity of the antenna shown:
- the front-to-back ratio (VRV), also called back attenuation ,
- the front-to-side ratio (VSV) and
- the sidelobe attenuation .
Vertical antenna diagram
A vertical antenna diagram is a side view of the electromagnetic field of the antenna. The dimensions of the antenna diagram are therefore the distance to the antenna in the x-axis and the height above the location of the antenna in the y-axis.
In the diagram, the vertical axis (y-axis) shows the height in feet ( feet , ft), the horizontal axis (x-axis) shows the distance in nautical miles (Nm), both of which are units of measurement Air traffic control radar can be used. The units of measurement play a role in the scale ratio of the axes in the diagram. The measured values, however, are relative levels that have nothing to do with the distance values of the axes.
The radial straight lines from the origin are the elevation markers, drawn in “one-degree steps”. Because of the superelevation , that is, the vertical axis has a different scale than the horizontal axis, the distances between the elevation angle marks are unequal.
The heights plotted on the vertical axis are projected into the diagram not only as a grid, but also as dotted lines, whereby these indicate the real height above the ground and are therefore not straight lines, but lines that slope down slightly according to the curvature of the earth.
The diagram is a real Cosecans² diagram of an Airport Surveillance Radar . The "frayed" flank of the diagram, which is far away from the origin, shows the influence of the earth's surface on the diagram (see Fresnel zone ), as this antenna was unfortunately built a little too deep.
Spatial directional characteristic
If 2D antenna diagrams of many cutting planes are combined to form a spatial structure, a three-dimensional directional characteristic is created. The distance from the center of the antenna to any point on the surface of this body indicates the intensity measured in this direction at equal distances. The recording of such spatial characteristics would, however, represent a great expense in terms of measurement technology. That is why in practice such antenna diagrams are only created in exceptional cases and then only in sections. With the help of computers, however, each antenna and its spatial characteristics can be simulated in a model.
Elements of an antenna diagram
Main lobe
The main lobe in an antenna diagram shows the maximum amount of energy sent in one direction for transmitting antennas or the maximum sensitivity for receiving antennas. A directional antenna bundles this radiation in one direction and thereby increases the range of the antenna. This increase in range is called gain and is given as the ratio of the measured antenna to the values of an omnidirectional antenna . This antenna gain is not used in an antenna diagram ! All values measured and shown graphically in the antenna diagram are related to the maximum value of the main lobe. This is entered in the diagram with 0 dB and all other measured values must therefore be negative levels ( attenuation ). They are therefore only relative values. The antenna gain, on the other hand, is an absolute value since it is related to a calibrated measure .
In the very simple antenna diagram of a dipole antenna (antenna diagram see there) there are only two distinct maxima that are directed in opposite directions. In this case, a main lobe is not yet spoken of, since both maxima are approximately the same size.
Sidelobes
The part of the electromagnetic radiation from an antenna that is not radiated in the desired direction is referred to as the side lobe . Side lobes are usually undesirable because they withhold part of the transmission power of the main lobe, thereby weakening it and impairing the unambiguous directional effect of an antenna. In the case of a receiving antenna, interference received via side lobes can degrade the reception quality; they are not masked by the antenna . In the case of transmitting antennas, the transmitting power is radiated unused in an undesired direction via the side lobes.
The intensity of the side lobes can be reduced by cleverly designing an antenna. However, if the receiver dynamics are greater than the sidelobe attenuation specified by the antenna, signals are also received via the sidelobes. In order to reduce the effects of this unwanted reception, additional measures for sidelobe suppression are taken in the radar location technology .
Lattice clubs
Grating lobes (engl .: grating lobes ) are side lobes reach approximately the size of the main lobe and are distributed in a grid-like diagram. They sometimes arise with phased array antennas (and also with ultrasound probes used in sonography ) and are the result of a too large and even distance between the individual radiator elements in relation to the wavelength. With a good design of a phased array with the best possible spacing between the radiator elements, they should not occur, but this requirement is difficult to meet with very large bandwidths ( UWB ).
Back club
As a back lobe of an antenna pattern sidelobe is referred to in the direction pointing in the opposite direction of the main lobe. It is usually much smaller than the main lobe. If the exact opposite direction in the antenna diagram shows a radiation minimum, the side lobes that are located within an angle of ± 15 ° from this exact direction are often referred to as back lobes.
zeropoint
The points in the antenna diagram at which the radiant energy is practically zero are called zeros. Their position in the coordinate system can be referred to as the zero value angle , which is measured between the maximum of the main lobe and the first zero point. A zero width is measured between the first zero positions to the left and right of the main lobe.
Parameters readable from the antenna diagram
Half width
According to general convention, the critical angles of a club are defined by the drop in the received power or the emitted intensity to half the maximum value (factor 0.5 ≈ −3 dB ). The beam width ( half width , opening angle ) is the span between these angles and is usually denoted by the Greek letter Θ ( theta ). The main lobe of the parabolic antenna characterized in the adjacent diagrams has a beam width of 1.67 °, a very good value for a radar antenna.
Side lobe attenuation
The side lobe attenuation is one of the essential parameters of an antenna and represents the ratio of the gain of the main lobe in 0 ° to the level of the largest side lobe (here in the diagram at around 20 °). This ratio is given as a relative level and should be as large as possible.
Front-to-back ratio
The front to back ratio ( VRV , engl. Front-to-back ratio ), also called return damping is an important parameter of the antenna and represents the ratio of the measured level of the main lobe in the 0 ° to the level of the back lobe in 180 °. This ratio is given as a relative level and should be as large as possible. In addition to the side lobe attenuation, the front-to-back ratio is a measure of the bundling of a directional antenna: the greater the front-to-back ratio, the better the antenna.
In some publications, the VRV is not only related to this one back lobe , but all side lobes between 90 ° and 270 ° are considered under the term front-to-back ratio and only the strongest side lobe from this angle range is used to determine the VRV. This is useful, for example, if an antenna has a pronounced minimum at an angle of 180 ° and the back lobes are, for example, at around 175 ° and 185 °.
Front-to-side ratio
A front-to-side ratio is sometimes given in place of the front-to-back ratio. This is useful for antennas in which the size of the back lobe is in the order of magnitude of the main lobe, or because two diametrical radiation maxima are formed, for example in the vertical antenna diagram of a dipole antenna . Radiation in two opposite directions is also desired with certain directional antennas. For example, with the radar Kabina 66 and the air traffic control radar SRE-LL , in which two parabolic mirrors are mounted back to back to form a so-called Janus head antenna.
Measurement methods
Due to the reciprocity of antennas, it is possible to measure a receiving antenna as a transmitting antenna (and vice versa) and to infer properties as a transmitting antenna from the measured receiving data (and vice versa). A second variation consists in either moving a mobile measuring device (which can be configured as a transmitter or receiver) around the rigidly constructed antenna to be measured in the far field , or unfolding this measuring tool in a fixed place and rotating the antenna to be measured.
Field measurements
Which case is chosen when measuring an antenna depends on the influences of the environment and to what extent these can falsify the antenna diagram. Often, however, the geometric dimensions of the antenna prevent it from rotating in one plane. In principle, the radiation source should be moved if the antenna to be measured has a strong directional effect. If the antenna to be measured can be rotated, it can be rotated in a direction from which the lowest possible external interference power can be received. So that this interference power level does not falsify the antenna diagram, the measuring transmitter should then be moved around the antenna. This should work in the far field of the antenna but still under optical conditions so that the current angle can be determined by suitable optical measuring tools ( directional circle or theodolite ).
A measurement at the final location with a fixed measuring transmitter and rotating receiving antenna is very complex, since the measurement result can be falsified by environmental influences. The reception of reflections and interference must be kept as low as possible by suitable measures. From a great distance, a directional radiator with constant power shines precisely in the direction of the antenna. By rotating and tilting the antenna to be measured, the received power is measured at different angles and then displayed graphically. A parallel interference field measurement with a calibrated measuring antenna with a very wide aperture angle or even omnidirectional characteristics can be used to correct the measurement result.
If the antenna diagram of the antenna to be measured is also composed of components of power reflected on the ground, it is not always possible to rotate the antenna to be measured. Often the geometrical dimensions of the antenna also prevent rotation. In this case, a radiation source must be moved at a sufficient distance (i.e. not in the near field , but in the far field of the antenna) from the antenna to be measured. With antennas that are already equipped with a rotating device, such as radar antennas , this measurement can be determined quite easily with a special measuring tool. The radar field analyzer (RFA) shown in the picture is part of such a measurement tool.
The horizontal antenna diagram is recorded by the radar antenna from a sufficiently distant location with this measuring tool. Here the radar antenna works as a transmitting antenna and the measuring tool receives a series of measuring pulses. A small logarithmic periodic dipole antenna ( LPDA ) receives the pulses emitted by the radar device. The RFA is configured here as a radar receiver and demodulates the received high-frequency pulses. The video signal data is transferred to the laptop via a USB cable (previously via SCSI interface) .
The measuring program must know the approximate speed of rotation of the antenna and the pulse repetition frequency of the radar. (These values can, however, be determined with the program itself.) The amplitudes of all received transmission pulses of a complete antenna revolution are saved with a time stamp. The strongest impulse is taken as a reference and interpreted as the center of the main lobe and thus represented in 0 °. In the evaluation, all other measured values are assigned to a side angle relative to the angle of the main lobe.
The vertical antenna diagram can be determined by statistical measurements, for example the electromagnetic spectrum of solar radiation ( sunstrobe recording ). Here the measuring tool is set up within the radar station and connected to the video outputs of the receivers. In this measurement process, the radar antenna works as a receiving antenna. During sunrise or sunset, all video amplitudes of the sun noise are recorded and assigned to an elevation angle in a later evaluation routine.
Since unpredictable factors are always included in the measurements, only values from a single series of measurements can be compared as relative levels, in which all individual measured values must be measured under conditions that are as identical as possible. This means that when comparing differently dated series of measurements, only the diagram form can be meaningful.
Measurements under laboratory conditions
In order to be able to measure antenna systems under laboratory conditions with the aim of creating an antenna diagram, the antenna must be mounted on a moveable rotating and swiveling table. The entire measuring apparatus and the test subject are in a massive Faraday cage to protect them from external interference . B. from soldered copper sheets. The ceilings, walls and floors of the measuring room as well as the measuring equipment must be clad with damping material to avoid reflections. The mostly pyramid-shaped structural elements consist of a hard foam with a high content of graphite in order to absorb electromagnetic radiation energy and convert it into heat during a lossy multiple reflection between the individual wall elements. Very large measurement laboratories are also EMC - anechoic chamber called.
Here the antenna is mostly used as a receiving antenna. A test transmitter sends with a very narrow directional diagram in the direction of the antenna to be measured. This is rotated or swiveled in one plane with motors.
This method brings usable measurement results, but is more of theoretical value, since the vertical antenna diagram in particular can change due to the influence of the earth's surface at the final location of the antenna. It is mainly used in antenna construction and repair. With very low frequencies and thus geometrically large antennas, the antenna can be reduced in size with sufficient accuracy and the measurement frequency used can be increased to scale.
Examples
There are a variety of antennas, often named after the geometric shape of the antenna diagram:
- Omnidirectional antennas with as uniform radiation as possible in all directions within a plane,
- Directional antennas with a pronounced main lobe,
- Pencil beam antennas with an extremely thin main lobe and very high antenna gain ,
- Fan antennas and
- Cosecans² antennas , both specially designed for radar applications .
literature
- Edgar Voges : high frequency technology . Dr. Alfred Hüthig Verlag, Heidelberg 1987, ISBN 3-778-51270-6 .
- Karl Rothammel : Antennenbuch . 10th edition. Military publishing house of the German Democratic Republic, Berlin 1984.
Web links
- Antennen ( Memento from August 27, 2010 in the Internet Archive ), lecture script (PDF file; 1300 kB)
- Basics of antenna theory (PDF file; 411 kB)
- Antennentechnik - Radar antennas (PDF file; 662 kB)
Individual evidence
- ↑ Radio and radio paging systems ( Memento from June 19, 2008 in the Internet Archive ) (PDF, 985 kB) - Implementation recommendation for the NISV (draft of July 6, 2005)
