“Electronic engineers know that antennas send and receive signals in the form of waves of electromagnetic (EM) energy described by Maxwell’s equations. As with many topics, these equations and the propagation, properties of electromagnetism can be studied at different levels, from relatively qualitative terms to complex equations.
By Bill Schweber
Electronic engineers know that antennas send and receive signals in the form of waves of electromagnetic (EM) energy described by Maxwell’s equations. As with many topics, these equations and the propagation, properties of electromagnetism can be studied at different levels, from relatively qualitative terms to complex equations.
Electromagnetic energy propagation includes many aspects, polarization is one of them, and can have different degrees of influence or attention in the application and its antenna design. The fundamental principles of polarization apply to all electromagnetic radiation, including radio frequency/wireless, light energy, and are often used in optical applications as well. This article focuses on RF only.
What is antenna polarization?
Before understanding polarization, we must first understand the basic principles of electromagnetic waves. These waves consist of an electric field (E field) and a magnetic field (H field) and move in one direction. The E field and the H field are perpendicular to each other and to the direction of plane wave propagation.
Polarization refers to the view of the E-field plane from the signal transmitter’s perspective: for horizontal polarization, the electric field will move laterally inside the horizontal plane, while for vertical polarization, the electric field will oscillate up and down in the vertical plane (Figure 1).
Figure 1: Electromagnetic energy waves consist of mutually perpendicular E-field and H-field components. (Image credit: Electronics-Notes)
A pair of transmit and receive antennas work best when the polarizations of the transmit and receive antennas are in the same plane. Of course, as in “In space, no one can hear you scream” (apologies and nods to the 1979 classic Alien). In space, there really is no horizontal or vertical. Nonetheless, the concepts of polarization and antenna alignment remain valid for maximum signal energy transfer and capture.
Linear and circular polarization
Polarization modes include the following:
• In basic linear polarization, the two possible polarizations are orthogonal (perpendicular) to each other (Figure 2). In theory, a horizontally polarized receive antenna would not “see” a signal from a vertically polarized antenna, and vice versa, even if both operate at the same frequency. The better they are aligned, the more signal is captured, and energy transfer is maximized when the polarizations are matched.
Figure 2: Linear polarization provides two polarization options at right angles to each other. (Image credit: Mimosa Networks, Inc.)
• The oblique polarization of the antenna is a type of linear polarization. Like basic horizontal and vertical polarization, this polarization is only meaningful in a terrestrial environment. The oblique polarization is at an angle of ±45 degrees from the horizontal reference plane. While this is really just another form of linear polarization, the term “linear” usually refers only to antennas that are either horizontally or vertically polarized.
Despite some losses, the signal sent (or received) by a slanted antenna is feasible for antennas polarized only horizontally or vertically. Slanted polarized antennas are useful when the polarization of one or both antennas is unknown or changes during use.
• Circular polarization (CP) is more complex than linear polarization. In this mode, the polarization represented by the E-field vector rotates as the signal propagates. When rotated to the right (looking out of the transmitter), the circular polarization is called right-handed circular polarization (RHCP); when rotated to the left, it is left-handed circular polarization (LHCP) (Figure 3).
Figure 3: In circular polarization, the E-field vector of an electromagnetic wave rotates; this rotation can be right-handed or left-handed. (Image credit: JEM Engineering)
A CP signal consists of two quadrature waves that are out of phase. Three conditions are required to generate the CP signal. The E-field must consist of two quadrature components; the two components must be 90 degrees out of phase and equal in amplitude. A simple way to generate CP is to use a helical antenna.
• Elliptical Polarization (EP) is a type of CP. An elliptically polarized wave is the gain produced by two linearly polarized waves, just like a CP wave. When two mutually perpendicular linearly polarized waves of unequal amplitude are combined, an elliptically polarized wave is produced.
The polarization mismatch between antennas is described by the polarization loss factor (PLF). This parameter is expressed in decibels (dB) and is a function of the difference in polarization angle between the transmit and receive antennas. Theoretically, the PLF can range from 0 dB (no loss) for a perfectly aligned antenna to infinite dB (infinite loss) for a perfectly orthogonal antenna.
In reality, however, the alignment (or misalignment) of the polarization is not perfect because the mechanical position of the antenna, user behavior, channel distortion, multipath reflections, and other phenomena can cause some angular distortion of the transmitted electromagnetic field. Initially, there will be 10 – 30 dB or more of signal cross-polarization “leakage” from orthogonal polarizations, which in some cases may be enough to interfere with the recovery of the desired signal.
Conversely, for two perfectly polarized and aligned antennas, the actual PLF may be 10 dB, 20 dB, or more, depending on the situation, and may hinder signal recovery. In other words, unintended cross-polarization and PLF act in both directions by interfering with the desired signal or reducing the desired signal strength.
Why focus on polarization?
Polarization works in two ways: the better the alignment of the two antennas and the same polarization, the better the strength of the received signal. Conversely, poor polarization alignment can make it more difficult for an intended or unsatisfactory receiver to capture enough of the desired signal. In many cases, the “channel” distorts the transmitted polarization, or one or both antennas are not in a fixed static orientation.
The choice of which polarization to use is usually determined by installation or atmospheric conditions. For example, a horizontally polarized antenna will perform better and maintain its polarization when mounted near the ceiling; conversely, a vertically polarized antenna will perform closer to the nominal polarization performance when mounted near a sidewall.
Widely used dipole antennas (plain or folded) are horizontally polarized in their “normal” mounting orientation (Figure 4) and are often rotated 90 degrees to exhibit vertical polarization when needed or to support the preferred polarization mode (Figure 4). Figure 5).
Figure 4: A dipole antenna is typically mounted horizontally on its mast to provide horizontal polarization. (Image source: KAC Radio)
Figure 5: For applications requiring vertical polarization, a dipole antenna can be mounted accordingly to catch up with the antenna. (Image credit: Progressive Concepts)
Vertical polarization is commonly used in handheld mobile radios, such as those used by first responders, because many vertically polarized radio antenna designs also provide an omnidirectional radiation pattern. Therefore, such antennas do not have to be reoriented even if the orientation of the radio and antenna has changed.
3 – 30 MHz high frequency (HF) frequency antennas are usually simple long wire structures strung together horizontally between brackets. Its length is determined by the wavelength (10 – 100 m). Such antennas are naturally horizontally polarized.
It’s worth noting that calling this band “high frequency” started decades ago, when 30 MHz was really a high frequency. Although this description now seems outdated, it is officially designated by the International Telecommunication Union and is still widely used.
The preferred polarization can be determined in two ways: either ground waves for stronger short-range signaling by broadcast equipment using the 300 kHz – 3 MHz medium wave (MW) band, or sky waves for longer distances via the ionosphere Link. In general, vertically polarized antennas have better ground wave propagation, while horizontally polarized sky waves perform better.
Circular polarization is widely used in satellites because the orientation of satellites relative to ground stations and other satellites is constantly changing. Efficiency between the transmit and receive antennas is highest when both the transmit and receive antennas are circularly polarized, but linearly polarized antennas can be used with CP antennas, although there is a polarization loss factor.
Polarization is also important for 5G systems. Some 5G multiple-input/multiple-output (MIMO) antenna arrays achieve increased throughput by polarizing more efficiently using the available spectrum. This is achieved using a combination of different signal polarizations and spatial multiplexing of the antennas (spatial diversity).
The system can transmit two data streams, which can be recovered individually because they are connected by separate orthogonally polarized antennas. Even with some cross-polarization due to path and channel distortion, reflections, multipath, and other imperfections, sophisticated algorithms employed by the receiver recover each original signal, resulting in a low bit error rate (BER) and ultimately improved spectrum utilization.
Standard antennas provide polarization options
It is natural to think that only highly visible, large, pole-mounted antennas have polarization problems, which is not the case. For example, the PCTel BOAH515905NM is a horizontally polarized Wi-Fi antenna for the 5.1GHz – 5.9GHz frequency band, primarily for outdoor 802.11ac access points (Figure 6).
Figure 6: The PCTel BOAH515905NM horizontally polarized Wi-Fi antenna is designed for 5.1 GHz to
A 5.9 GHz (802.11 ac) Wi-Fi connection provides an outdoor access point. (Image credit: PCTel)
This fully sealed IP67-rated antenna features a white plastic radome. The radome is rugged and UV resistant and includes an integral N-type panel mount connector (male and female versions available). The radome measures 1.26 in outer diameter x 6.32 in long (3.20 x 16.1 cm), provides a nominal 5 dBi gain, and has a voltage standing wave ratio (VSWR) of less than 2 : 1 across the specified frequency band.
In smaller antennas it can also be designed to introduce polarization. Taoglas’ PC140.07.0100A is a circularly polarized 2.45 GHz (nominal) antenna for Industrial, Scientific and Medical (ISM), Bluetooth and Wi-Fi applications (Figure 7).
Figure 7: Taoglas’ PC140.07.0100A miniature antenna is designed to be embedded in the housing along with the system circuit board. (Image credit: Taoglas)
This tiny 50 (Ω) antenna, measuring only 57 x 57 mm square and 0.97 mm thick, features a 1.13 mm diameter, 100 mm long coaxial cable with IPEX connectors (a standard 50 Ω surface mount connector, soldered directly on the printed circuit board). This antenna is made of FR-4 circuit board material and comes with stickers for easy installation.
As can be seen from its XY and XZ radiation patterns (Figure 8), the radiation pattern of this antenna is highly omnidirectional. Its VSWR is less than 2:1 and its efficiency is about 60% in the 2.4 – 2.5GHz operating frequency band.
Figure 8: The radiation pattern of the Taoglas PC140.07.0100A antenna shows comparable omnidirectionality in both the X – Y (left) and X – Z (right) planes. (Image credit: Taoglas)
Polarization is an important antenna property that is often overlooked. Linear (both horizontal and vertical) polarization, oblique polarization, circular polarization and elliptical polarization are used for different applications. The range of end-to-end RF performance that an antenna can achieve depends on its relative orientation and alignment. Standard antennas have different polarizations and are suitable for different parts of the spectrum, providing the preferred polarization for the target application.
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