Senin, 04 Juli 2011

RF Antenna

RF Antenna or antenna does not radiate the same in all directions. It is found that any realization of RF antenna design will radiate more in some directions than others. The actual pattern depends on the type of antenna design, size, environment and other factors. The pattern directions can be used to ensure that the power of radiation is focused on the desired direction.

It is normal to refer to the pattern in terms of direction and get the transmitted signal. It is often easier to visualize the RF antenna that radiates the power, but the antenna is doing exactly the same way for acceptance, have identical numbers and specifications.

In order to visualize the ways in which the antenna radiates a diagram known as a polar diagram is used. This is usually two-dimensional plot shows the intensity of radiation around the antenna at any point to a particular field. Usually the scale used is logarithmic so a difference can be easily seen in the plot. Although the antenna radiation pattern varies in three dimensions, it is normal to make the plot in a particular plane, usually either horizontal or vertical because these are the two most frequently used, and simplifies the measurement and presentation. An example for a simple dipole antenna is shown below.

Antenna design is often categorized by their type of polar diagram shows. As an example of omni-directional antenna design is one that radiates equally (or approximately equal) in all directions in the field of interest. Design an antenna that radiates equally in all directions in all planes is called an isotropic antenna. As already mentioned it is not possible to produce this one in reality, but useful as a theoretical reference for multiple measurements. Other RF antenna shows a highly directional pattern and can be used in a number of applications. Yagi antenna is an example of directive antenna and probably the most widely used for television reception.

RF antenna beamwidth

There are a number of key features that can be seen from the polar diagram. The first is that there is a main beam or lobe and a small lobe. It is often useful to determine the RF antenna beam-width. It is considered as the angle between two points where the power falls to half its maximum level, and as a result are sometimes called half-power beam-width.

Antenna gain

An RF antenna emits a given amount of power. This is the power dissipated in the resistance of the antenna RF radiation. Isotropic radiator will distribute is the same in all directions. For an antenna with a directional pattern, less power will be radiated in several directions and more in others. The fact that more power is radiated in the directions given implies that it can be considered to have an advantage.

Gain can be defined as the ratio of the signal transmitted in the direction of "maximum" with a standard or reference antenna. This can sometimes be called "profit in the future". Figures obtained and is usually expressed in decibels (dB). In theory the standard antenna can be almost anything but two types are generally used. The most common type is a simple dipole as it is easily available and are the basis of many types of antennas. In this case the gain is often expressed as the advantage of being expressed in decibels dBd over a dipole. However, the dipole radiates not the same in all directions in all planes and isotropic sources are sometimes used. In this case the gain can be specified in dBi gain in decibels is more than an isotropic source. The main drawback to using isotropic source (dBi antenna) as a reference is that it is impossible to realize in practice and that the figures using only a theory. However it is possible to connect two advantages as a dipole has a gain of 2.1 dB over an isotropic source is 2.1 dBi. In other words, the figure is expressed as an advantage over an isotropic source will be higher than 2.1 dB relative to dipole. When choosing an antenna and see profits specifications, be sure to check if the gain relative to a dipole or isotropic source, namely the figure of the figure dBd dBi antenna antennas.

Apart from the advantage ahead of an antenna is another important parameter is the ratio of front to back. It is expressed in decibels and as the name suggests it is the ratio of the maximum signal in the forward direction for the signal in the opposite direction. This figure is usually expressed in decibels. It is found that the antenna design can be customized to provide maximum benefit both advanced from front to back ratio is optimal as the two usually do not coincide exactly. For VHF and UHF operation most designs are usually optimized for optimum benefit to the front as this gives a maximum signal radiation in the required direction.

RF antenna gain / beamwidth balance

It may seem that maximize the gain of the antenna will optimize the performance of the system. This may not always be the case. By the very nature of gains and beamwidth, increasing the gain will result in reduction of the beamwidth. This will make setting the direction of the antenna is more critical. This may be quite acceptable in many applications, but not in others. This balance should be considered when designing and setting up radio links.

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