Senin, 04 Juli 2011

Antenna Resonance

Antenna Resonance

RF antenna comprises a tuned circuit inductance and capacitance, and as a result he has a resonant frequency. This is the frequency where the capacitive and inductive reactances cancel each other. At this point the RF antenna appears purely resistive, the resistance to be a combination of loss resistance and radiation resistance.

It is important to match the polarization of the RF antenna to that of the incoming signal. In this way the maximum signal obtained. If the polarization of the RF antenna is not matched with the signal corresponding decrease in signal level. This was reduced by a factor of cosine angle between the polarization of the RF antenna and signal.

By the polarization of the antennas located in free space is very important, and obviously they must be in exactly the same plane to provide optimum signal. If they are at right angles to one another (ie cross-polarized) then in theory there is no signal will be received.

For terrestrial radio communication applications has been found that once the signal has been transmitted then the polarization will remain the same area. However, reflections from objects on the road can change polarization. As the received signal is the sum of the direct signal plus a signal reflecting the overall polarization signal can be changed a bit though still broadly the same.

Polarization category

Vertical and horizontal are the simplest form of antenna polarization and they both fall into the category known as linear polarization. But it is also possible to use circular polarization. It has a number of benefits for areas such as satellite applications where it helps overcome the effects of anomalous propagation, ground reflection and spin effects that occur in many satellites. Circular polarization bit more difficult to visualize from a linear polarization. However it can be imagined by visualizing the spread signal from the RF antenna that rotates. End of the vector electric field it will be seen to trace a helical or corkscrew as it travels away from the antenna. Circular polarization can be viewed either right hand or left depending on the direction of rotation as seen from the transmitter.

Another form of polarization is known as the polarization ellipse. This occurs when there is a mixture of linear and circular polarization. This can be visualized as before by the end of the electric field vector tracing out an elliptical-shaped bottle opener.

However it is possible for linear polarization antenna for receiving circularly polarized signals and vice versa. The power will be the same whether the antenna is linearly polarized mounted vertically, horizontally or in another plane, but directed the signal arrives. There will be some degradation because the signal level will be 3 dB lower than if a circularly polarized antenna of the same meaning is used. The same situation occurs when a circularly polarized antenna receiving the signal is linearly polarized.

Application of antenna polarization

Various types of polarization are used in different applications to allow their profits to be used. Linear polarization is the most widely used for radio communications applications most. Vertical polarization is often used for mobile radio communications. This is because many antenna designs have a vertical polarization omni-directional radiation pattern and that means that the antenna should not be re-oriented as a position like that always happen to the mobile radio communication as the vehicle moves. For the other radio communication applications the polarization is often determined by considerations of RF antennas. Several multi-element antenna array can be mounted in a large horizontal field more easily than in the vertical plane. This is because the RF antenna elements at right angles to the vertical mast towers they are mounted and therefore by using an antenna with horizontal elements there is little physical and electrical interference between the two. This determines the polarization of the standard in many cases.

In some applications there is a difference between horizontal and vertical polarization performance. Eg medium wave broadcast stations generally use vertical polarization because the ground wave propagation over the earth is a far better use of vertical polarization, while the horizontal polarization shows marginal improvement for long-distance communication using the ionosphere. Circular polarization is sometimes used for communication satellite radio because there are some advantages in terms of propagation and in overcoming the fading caused if a satellite changes orientation.

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