Antenna Polarization
The following article serves to build an understanding about the antenna parameter known as Polarization. Polarization is perhaps the most fundamental characteristic of an antenna describing the plane in which it radiates its electric fields. It is an important parameter to understand as wireless communication requires transmit and receive antennas to operate on the same polarity.
Definition
The simplest way to describe polarization is the direction in which the electric field of a radio wave oscillates while it propagates through a medium. The point of reference for specifying a polarization is looking at it from the transmitter of the signal. This can be visualized by imagining standing directly behind a radio antenna and looking in the direction it is aimed.
Polarization Categories
An antenna is a device which can receive or/and transmit radio signals. As a receiving device, it can collect the radio signals from free space and convert them from electromagnetic waves (in the free space) into guided waves in transmission lines; as a transmitting device, it can transmit radio signals to free space by converting the guided waves in transmission lines into the electromagnetic waves in the free space. In some cases, an antenna can serve both functions of receive and transmit.
The electric field plane determines the polarization or orientation of the radio wave. According to the shape of the trace, three types of polarization exists for harmonic fields: linear, circular and elliptical. And within each type there are several sub categories:
Linear Polarization
Linear polarization is the most common form of antenna polarization. Linear polarization refers to an antenna system that is operating with Horizontal and Vertical polarization.
Horizontal Polarization
This form of antenna polarization has horizontal elements. It picks up and radiates horizontally polarized signals, i.e. electromagnetic waves with the electric field in the horizontal plane.
Vertical Polarization
This form of antenna is typified by the vertical elements within the antenna. It could be a single vertical element. One of the reasons for using vertical polarization is that antennas comprising of a single vertical element can radiate equally around it in the horizontal plane. Typically vertically polarized antennas have what is termed a low angle of radiation enabling a large proportion of their power to be radiated at an angle close to the earth’s surface.
Circular Polarization
Circularly polarized (CP) antennas are a type of antenna with circular polarization. Due to the features of circular polarization, CP antennas have several important advantages compared to antennas using linear polarization, and are becoming a key technology for various wireless systems. Circular polarization is a little more difficult to visualize than linear polarization. However it can be imagined by visualizing a signal propagating from an RF antenna that is rotating. The tip of the electric field vector will then be seen to trace out a helix or corkscrew as it travels away from the antenna.
A CP signal consists of two orthogonal waves that are out of phase. A single wavelength is shown in the image below.
A full wavelength is expressed as 360°, which should not be confused with the rotation of the CP signal in a three dimensional space. The phase shifting that occurs to cause a CP signal is 90°, which equals a one quarter wavelength offset. as shown in image 5 when looking at a 2D side view of phase shifted radio waves.
In three dimensional space, the effect that this will produce appears as a rotating signal, either in a left hand or right direction depending on which direction the 90° phase shift occurs, i.e. if H is ahead of V by 90° or vice versa. While the two waves remain linear in nature and orthogonal throughout the transmission, the electrical vector of the wave rotates through a full revolution in a single wavelength. This is shown in image 6.
It is possible to transmit a signal where the polarization appears to rotate while the signal travels from the transmitter to the receiver. This is referred to as a circular polarization (CP). A circularly polarized wave can rotate in one of two possible senses: right circular polarization in which the electric field vector rotates in a right-hand sense with respect to the direction of propagation, and left circular polarization in which the vector rotates in a left-hand sense.
Right Hand Circular Polarization
In this form of polarization the vector rotates in a right handed fashion. This would be considered left-handed/counter-clockwise circularly polarized if defined from the point of view of the source rather than the receiver.
Left Hand Circular Polarization
In this form of polarization the vector rotates in a left handed fashion, i.e. opposite to right handed. This would be considered right-handed/clockwise circularly polarized if defined from the point of view of the source rather than the receiver.
Note: It is possible for linearly polarized antennas to receive circularly polarized signals and vice versa. The strength will be equal whether the linearly polarized antenna is mounted vertically, horizontally or in any other plane but directed towards the arriving signal. There will be some degradation because the signal level will be 3 dB less than if a circularly polarized antenna of the same sense was used. The same situation exists when a circularly polarized antenna receives a linearly polarized signal.
Elliptical Polarization
Also known as slant polarization, this is a form of antenna polarization that is at an angle to the horizontal or vertical planes. In this way both vertical and horizontally polarized antennas are able to receive the signal. The polarization is at –45 degrees and +45 degrees from a reference plane of 0 degrees. Taking the same analogy of standing behind the radio and looking in the direction of the signal, slant polarization is equivalent to taking a linear polarization radio and rotating it 45 degrees. At 45 degrees of polarization vector offset, the maximum polarization losses would be 0.5, or 3 dB. This can be visualized as before by the tip of the electric field vector tracing out an elliptically shaped corkscrew.
Co-Polarization and Cross-Polarization
Each polarization has an orthogonal counterpart (Vertical and Horizontal, RHCP and LHCP, ± 45º slant, etc). Furthermore, each polarization can be constructed out of any two orthogonal polarization, and they are:
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When a radio signal has the same polarization as the antenna’s designated polarization, the signal and the antenna are mutually “co-polarized.”
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When the radio signal has orthogonal polarization to the antenna’s designated polarization, i.e., horizontal vs. vertical or RHCP vs. LHCP, the signal and the antenna are mutually “cross-polarized.” For instance, if the fields from an antenna are meant to be horizontally polarized, the cross-polarization in this case is vertical polarization. If the polarization is Right Hand Circularly Polarized (RHCP), the cross-polarization is Left Hand Circularly Polarized (LHCP).
While an antenna is designed with a specific polarization, the unavoidable imperfection of the antenna geometry and its mounting structures can cause the antenna to have cross-polarization characteristics in its operations. Therefore, an antenna is always associated with a co-polarization and a cross-polarization radiation pattern, and it always transmits and receives cross-polarized radio signals.
What Polarization is required
Antennas are sensitive to the polarization of electromagnetic wave and this is an important aspect of their operation. Most communication systems use either vertical, horizontal or elliptical (RHC-right hand circular or LHC-left hand circular) polarization, with vertical dominating commercial VHF/UHF applications. In some instances, the selection is determined by the installation site, with the antenna oriented to provide the best performance. If this is anticipated, your antenna should provide mounting for either polarization.
Selecting the proper polarization for the system can enhance the overall performance by minimizing the interference from adjoining systems. For example, by installing you system orthogonal to other systems in the area, you can provide up to 20 dB of isolation. This will result in up to a 99% power reduction of the interfering system! Elliptical polarization can sometimes decrease fading.
Many systems are challenged because they must interface with handheld transmitters. These units move around a room or warehouse, with the antenna often pointing many degrees off-axis. To accommodate these applications, the fixed antennas often use circular or elliptical polarization with a hemispherically shaped pattern trading off high gain for reasonable gain in all directions.
Why polarization is important in Antenna Design
Polarization is an important design consideration. The polarization of each antenna in a system should be properly aligned. Maximum signal strength between stations occurs when both stations are using identical polarization.
When choosing an antenna, it is an important consideration as to whether the polarization is linear or elliptical. If the polarization is linear, is it vertical or horizontal? If circular, is it RHC or LHC?
On line-of-sight (LOS) paths, it is most important that the polarization of the antennas at both ends of the path use the same polarization. In a linearly polarized system, a misalignment of polarization of 45 degrees will degrade the signal up to 3 dB and if misaligned, 90 degrees the attenuation can be 20 dB or more. Likewise, in a circular polarized system, both antennas must have the same sense. If not, an additional loss of 20 dB or more will be incurred.
Meraki Antenna Polarization
Different types of polarization are used in different applications to enable their advantages to be used. Accordingly different forms of polarization are used for different applications. Below is the table that shows different types of Polarization used in Meraki antennas:
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MR-PM to fill up question marks in above table and answer the following qiestion:
Why does Meraki use cross-polarized antennas?
Antenna measurements?