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April 30, 2020
The radiation field of antenna consists of electric field and magnetic field. These fields are always at right angles. The electric field determines the polarization direction of the wave. When a wire antenna extracts energy from the passing radio waves, the maximum electric field will be generated when the antenna direction is the same as the electric field direction.
The oscillation of electric field can be unidirectional (linear polarization), or the oscillation direction of electric field can rotate with wave propagation (circular polarization or elliptical polarization).
The receiving antennas installed vertically and horizontally receive vertical and horizontal polarization waves respectively. Because the antenna cannot receive signals with different polarization, the change of polarization will cause the change of received signal level. There are mainly two kinds of polarization surfaces:
In the vertical polarization wave, the electric field direction is vertical.
In the horizontally polarized wave, the electric field direction is horizontal.
Linear polarization can receive signals from all planes except for two orthogonal polarizations. When a single wire antenna is used to receive radio waves, the energy received by the receiving antenna is the largest when the electric field direction is the same, so the vertical antenna is used to receive the vertical polarization wave efficiently, and the horizontal antenna is used to receive the horizontal polarization wave.
Circular polarization refers to the 360 degree rotation of electric field in every RF energy cycle. Circular polarization is caused by two 90 ° phase-shifting receivers and two 90 ° plane polarized antennas. Since the intensity of the wave is usually measured by the electric field intensity (volts, millivolts or microvolts per meter), the electric field is chosen as the reference field.
In some cases, the direction of the electric field is not constant. Therefore, as the wave propagates in space, the magnetic field rotates. Under these conditions, the horizontal and vertical components of the field exist, and the wave has elliptical polarization.
Circular polarization includes right-handed circular polarization and left-handed circular polarization. The circularly polarized wave is reflected by a spherical raindrop opposite to the transmitted wave. When receiving, the antenna will reject the wave in the opposite direction of circular polarization, so as to minimize the detection of raindrops.
Because the aircraft target is different from rain, it is not spherical, so the reflection of the target has an important component in the sense of original polarization. Therefore, the intensity of the target signal will be enhanced relative to the raindrop target.
In order to absorb the maximum energy from electromagnetic field, the receiving antenna must be on the same polarization plane. If the antenna with different polarization direction is used, considerable loss will be generated, and the actual loss is between 20 and 30 dB.
When strong air clutter appears, air traffic controllers tend to turn on the circularly polarized antenna. In this case, the hiding effect of air clutter on the target will be reduced.