Antenna Pola Radiasi
The radiation pattern or antenna pattern is the graphical representation of the radiation properties of the antenna as a function of space. That is, the antenna’s pattern describes how the antenna radiates energy out into space (or how it receives energy). It is important to state that an antenna radiates energy in all directions, at least to some extent, so the antenna pattern is actually three-dimensional. It is common, however, to describe this 3D pattern with two planar patterns, called the principal plane patterns. These principal plane patterns can be obtained by making two slices through the 3D pattern through the maximum value of the pattern or by direct measurement. It is these principal plane patterns that are commonly referred to as the antenna patterns.
The radiation pattern of an antenna is one of its basic properties since it shows the way the antenna distributes its energy in space. It generally consists of a number of lobes and if it is measured far away from the antenna it is independent of distance. It is a function of angles and can be expressed as field or power pattern. It usually can be completely specified from the patterns in two planes, perpendicular to each other, the E and H planes, respectively. Related to the radiation pattern of an antenna, a number of important parameters exist, such as radiated power, radiation efficiency, directivity, gain, and antenna polarization.
Antenna radiation patterns are graphical representations of elements of the radiation characteristics of an antenna. The antenna pattern is usually a graphical representation of the antenna's directional characteristic. It represents the relative intensity of the energy radiation or the amount of the electric or magnetic field strength as a function of the direction to the antenna. Antenna diagrams are measured or generated by simulation programs on the computer, for example, to graphically display the directivity of a radar antenna and thus estimate its performance.
In contrast to an omni-directional antenna, which radiates uniformly in all directions of a plane, a directional antenna prefers one direction and therefore achieves a longer range in this one direction with lower transmission power. The antenna radiation pattern graphically illustrates the preference determined by measurement. Due to the reciprocity, which guarantees the same transmission and reception characteristics of the antenna, the diagram shows both the directionally distributed transmission power as field strength and the sensitivity of an antenna during the reception.
back lobes
sidelobes
main lobe
Figure 2: Horizontal cross-section of the radiation pattern in a polar coordinate system
Presentation Formats Many display formats are used. Cartesian coordinate systems, as well as polar coordinate systems, are common. The main goal is to display a radiation diagram that is representative either horizontally (in azimuth) for a complete 360° representation or vertically (in elevation) mostly only for 90 or 180 degrees. In the Cartesian coordinate system, the data of an antenna can be represented better. Since these data can also be printed out as a table, the more descriptive representation as a locus curve in a polar coordinate system is usually preferred. In contrast to the Cartesian coordinate system, this indicates the direction directly.
For easy handling, transparency and maximum versatility, radiation patterns are usually normalized to the outer edge of the coordinate system. This means that the measured maximum value is aligned to 0° and plotted on the upper edge of the diagram. Further measured values of the radiation diagram are usually displayed relative to this maximum value in dB (decibels).
Usual Scales The scale in the diagram can be varied. Three types of plotting scales are in common usage; linear, linear logarithmic and modified logarithmic. The linear scale emphasizes the main radiation beam and usually suppresses all sidelobes, as they are often in the order of less than one-hundredth of the main lobes. However, the linear logarithmic scale represents the sidelobes well and is preferred when the level of all sidelobes is important. However, it leaves the impression that the antenna is bad because the main lobe is relatively small. The modified logarithmic scale (figure 4) emphasizes the shape of the major beam while compressing very low-level (<30 dB) sidelobes towards the center of the pattern. The main lobe is thus twice as large as the strongest side lobe, which is advantageous for visual presentations. However, this form of presentation is rarely used in technology because exact data would be difficult to read from it.
Horizontal Radiation Pattern A horizontal antenna diagram is a plan view of the electromagnetic field of an antenna, represented as a two-dimensional plane with the antenna in the center.
The interest in this representation lies in the simple acquisition of the directivity of the antenna. Usually, the value -3 dB is also given on the scale as a dotted circle. The crossing points between the main lobe and this circle result in the so-called half power beam width of the antenna. Other easily readable parameters are the forward/backward ratio, i.e. the ratio between the main lobe and the back lobes, as well as the size and direction of the sidelobes.
With radar antennas, the ratio between main and sidelobes is important. This parameter directly influences the evaluation of the immunity of radar to interference.
Vertical Radiation Pattern The shape of the vertical pattern is a vertical cross cut of the three-dimensional graph. In the shown polar diagram (a quarter part of the circle) with the antenna site as the origin, the x-axis is the radar range, and the y-axis the aims height. One of the antenna measurement techniques is the Sun-Strobe-Recording using RASS-S, a measurement tool of Intersoft Electronics. The RASS-S (Radar Analysis Support System for Sites) is a radar manufacturer-independent system for evaluating the different elements of radar by connecting to signals which are already available and this under operational conditions.
Figure 3: Vertical antenna pattern with cosecant squared characteristic
In the shown Figure 3, the measurement units are nautical miles as range, and feet as height. Both measurement units are still used in Air- Traffic- Management by historical reasons. These units are of secondarily meaning only because the plotted quantity of radiation pattern is defined as a relative level. This means the boresight axis has got the value of the (theoretical) maximum range calculated with the help of the Radar Equation. The shape of the plot provides the required information only! To get absolute values you need a second plot, measured under the same conditions. These two plots you can compare and then you achieve over increasing or decreasing the antennas performance.
The radiant lines are marks of elevation angles, here in half-degree-steps. The unequal scales of the x-axis and the y-axis (many feet versus a lot of nautical miles) cause the non-linear spacing between the elevation angle marks. The height is shown as a linear grid pattern. A second (dotted) grid is orientated on the curvature of the earth.
3D Antenna Patterns Three-dimensional antenna pattern of a feedhorn, (click to enlarge: 800·600px = 49 kByte) Figure 4: Three-dimensional antenna pattern of a feed horn
Antenna diagrams in three-dimensional representation are mostly computer-generated images. Mostly they are generated by simulation programs whose values are astonishingly close to a real measured diagram. To generate a real measured diagram means an immense measuring effort since each pixel of the image represents its own measured value.
Most antenna measurement programs, therefore, choose a compromise for this representation. Only a vertical and a horizontal section through the antenna diagram are available as real measured values. All other pixels are calculated by multiplying the entire measurement curve of the vertical diagram by a single measurement value of the horizontal diagram. The computing power required is enormous. With the exception of a pleasing representation in presentations, the benefit is doubtful, since no new information can be gained from this representation compared to the two individual diagrams (horizontal and vertical antenna diagram). On the contrary: especially in the peripheral zones, a diagram generated with this compromise should deviate considerably from the reality.
Also, 3D diagrams can be represented in Cartesian as well as in polar coordinates.