Understanding Antenna Polarization
Optimizing signal propagation over wireless transmission paths has never been easy, hindered as it is by obstructions, fading, multipath propagation and various other impediments between the transmitted signal and its intended recipient. Fortunately, there are ways to mitigate some of these factors, ranging from antenna designs to polarization schemes, as well as multiple-input multiple-output (MIMO) communications technology. To understand how these schemes deliver their benefits, it’s important to first cover the basics.
There are three general types of antenna polarization: linear, circular and elliptical. An antenna is linearly polarized when it radiates RF energy on a single plane, either horizontal or vertical, in relation to the Earth’s surface (Figure 1) or some angle between both. Radiation from horizontally polarized antennas parallels the Earth’s surface; vertically polarized antennas radiate energy on a plane perpendicular to it.
Ideally, the transmitting and receiving antennas should have identical polarization because signal strength decreases in direct proportion to how far they stray from that relationship. This is termed polarization mismatch, and the loss in signal strength is calculated in dB as, in an ideal scenario, is the angle between the receive and transmit antennas.
Circular polarization is mathematically defined as a linear combination of equal magnitude horizontally and vertically polarized waves that are 90 degrees out of phase. This equates to a wave rotating in time at a steady rate that is either left-hand or right-hand polarized (i.e., spinning in opposite directions) and includes the horizontal and vertical planes and all planes in between.
Compared with two linearly polarized antennas of the same orientation and forward gain, having one circularly polarized antenna and one linearly polarized antenna will reduce the link’s range because the circularly polarized antenna splits its power equally across two planes, reducing the system gain by 3 dB. Although this scenario does reduce the link budget, circularly polarized antennas are beneficial when the opposite antenna’s linear polarization is not known, or fixed.
The third common type of polarization is the generalization of circular polarization, known as elliptical polarization. It occurs when the electric field’s two linear perpendicular components are 90 degrees out of phase and have unequal magnitude. Like circular polarization, an elliptically polarized antenna can be either right- or left-hand polarized. Circular and elliptical polarizations are shown in Figure 2.
All this being said, once an antenna launches a radio wave, the wave’s characteristics continuously change. So, once the wave reaches the receiving antenna, the result is typically the initial polarization modified by fading, reflections, multipath interference, changes in phase and many other factors specific to the operating environment (urban or rural, for example) that affect the received signal strength. These factors can significantly degrade the signal in both strength and quality, and this is where the challenges begin for any type of system.
The following information will concentrate on the regions in which cellular and other services operate, currently from about 600 MHz to 3700 MHz. Propagation at these frequencies (Figure 3) is best accomplished over an unobstructed visual line-of-sight (LOS) path between transmitter and receiver, as attenuation and changes in signal characteristics are minimal. LOS is an ideal condition for wireless transmission because the propagation challenge comes only from weather or atmospheric parameters and the characteristics of its operating frequency. Consequently, the transmission path can be longer and the signal strength higher, resulting in greater throughput.
The non-line-of-sight (NLOS) scenario is far more common and presents challenges for all types of wireless systems, especially those in which one end of the link is mobile. When there is no clear line of sight, degradation will result from reflections, refraction, diffraction, scattering and atmospheric absorption. The multiple signals created by these factors will then arrive at the receiving antenna at different times, from different paths and with different strengths. The result will be a reduced link margin and decreased throughput, and in a worst-case scenario, will make communications impossible.
Antennas can mitigate some of these problems using various techniques, the most common being polarization diversity. It is used in all types of wireless applications, including cellular and the fixed wireless access (FWA) systems used in rural areas to deliver residential broadband service. Polarization diversity is basically the use of antenna systems that radiate signals in more than one polarization, such as horizontal and vertical.
Horizontal and vertical dual polarization was used for many years in wireless systems but has mostly been replaced by slant polarization (Figure 4) in which two linearly polarized antennas radiate at 45-degree angles (+45 degrees and −45 degrees) from horizontal and vertical — that is, midway between the two fundamental polarization angles. Polarization slants don’t have to be 45 degrees, and in some applications, including satellite communications systems, they’re not, for reasons specific to their operating environments. Of course, the wireless industry could have chosen a variant other than 45 degrees, but having done so, manufacturers increasingly supported it, ensuring its longevity.
Slant Configuration Benefits
Various studies have determined that this ±45-degree slant configuration can provide benefits that H and V configurations do not. Dual-slant polarization is midway between horizontal and vertical, and signals from the two antennas combine into a linearly polarized transmitted wave. Therefore, reception can be improved over pure H or V. Slant polarization has also proven its ability to provide signal improvement through foliage as well as in NLOS conditions.
In addition, slant polarization can minimize some of the effects of signal variability, reduce interference between antennas and increase the signal-to-noise ratio (SNR). These benefits apply to any operating scenario and especially to urban and other environments where signals are scattered, reducing their strength in a given location.
In antenna design, the horizontal and vertical polarizations often have unequal patterns and gain due to the physical asymmetries of the antenna’s construction. This can be readily observed in each polarization’s pattern, in which the beam width of the vertical polarization is narrower than the horizontal beamwidth. As a result, the gain of the vertical signal is weaker near the sector edges, which causes a chain imbalance. In a ±45-degree slant configuration, there are no physical asymmetries in the antenna, and each polarization has nearly identical patterns that equalize the signal strength of both polarizations.
Slant polarization appears to be able to withstand the effects of fading caused by reflections better than horizontal/vertical polarization, and some sources cite its ability to reduce interference where there are many simultaneous emitters. Finally, received signals typically appear at the receiving end more vertically than horizontally polarized, creating an unequal relationship, as vertical polarization often delivers a stronger signal than its horizontal counterpart at the receive location. Slant polarization can minimize this issue by equalizing the signal levels from both orientations.
Although slant polarization should theoretically cause a 3 dB (half-power) reduction in link budget caused by polarization mismatch, multipath propagation has the effect of restoring it because polarization is no longer purely horizontal/vertical and at a ±45-degree slant. The result is typically only about a 1 dB reduction in the link budget.
Dual Polarization Benefits
Dual polarization offers other benefits not related to signal propagation but nevertheless potentially beneficial when attempting to receive local approval for antenna installations. It results from having the two antennas together in a single housing, eliminating one enclosure. In addition to reduced visual effect, this approach also has little effect on wind loading and adds minimal additional weight.
The model KP-900-DPOMA-45 (Figure 5) is an example of an omnidirectional ±45-degree slant-polarized antenna for operation between 824 MHz and 928 MHz. It provides 360-degree coverage with minimal azimuth ripple and 10 dBi signal gain. The antenna supports any 900 MHz radio, including the popular Cambium® model PMP450i™. KP Performance Antennas also makes omnidirectional antennas for other bands, including the four-port model KP-25DOMNI-HV that covers 2300 MHz to 2700 MHz and the band from 5150 MHz to 5850 MHz with a gain of 12 dBi and supports 2×2 MIMO on both bands.
Another technique for reducing the losses associated with polarization mismatch is MIMO communications, whose best known benefit is dramatically increasing link performance and capacity by simultaneously sending and receiving multiple data streams. It also exploits the normally detrimental effects of multipath propagation. Even a minimal 2×2 MIMO approach can effectively double the maximum data rate of a communications channel.
MIMO communication requires polarization diversity (the use of antennas with different polarizations), one of which is ±45-degree slant polarization. One approach combines spatial multiplexing and polarization diversity to allow two data streams to maintain their separation in a way that allows them to arrive with high isolation between them.
Hurdling propagation problems has become more and more important as wireless services employ new modulation techniques, operate at higher frequencies and deploy large numbers of small cells to provide extremely high data rates and low latency. Polarization diversity employing slant polarization along with the innovative use of MIMO-enabled radios is playing a central role in making these possible — and further advances are sure to come.