Antenna polarity can be one of the most misunderstood aspects of wireless network design. It also happens to be one of the most important factors in a properly installed wireless network. This article is intended to provide an introduction to antenna polarity as it applies to both point to point, and client WiFi.
Antenna polarity refers to the orientation of the standing wave as it egresses the transmitting antenna. Think of any RF transmission as a wave form. Tie a rope to the wall and rapidly move the opposite end up and down to create a standing wave. This is an example of a vertically oriented wave. All of the following examples of antenna polarity have this single wave in common, regardless of directionality.
Horizontal Polarization
The radio wave is transmitted horizontally with relation to the base of the antenna.
Dual Polarization
This antenna contains both vertically and horizontally polarized antennas, This design is used to orthogonally separate signal on the same antenna and frequency to increase throughput. The polarity must be matched to the antenna on the other end of the link. Polarity is usually indicated by arrows on the back of the antenna.
Cross Polarization
Cross polarized antennas are a dual polarized antenna manufactured or mounted 45 degrees off of vertical. This allows further separation from nearby emitters transmitting in either vertically or horizontally polarized orientation.
Circular Polarization
A circular antenna pattern can be thought of as a series of angularly defined vertical waves separated by time. Consider a clock face where the first RF signal is transmitted oriented at 12:00 and the next transmission at 1:00. Continue transmitting around the clock face, this is an example of a LHCP antenna (Left Hand Circularly Polarized). This method is generally used by satellites to allow for the fact that the orientation of receiving antennas is unknown.
EMANIM: This tool will allow you to dynamically manipulate visualizations of radio wave. This application is a .exe and will only work on a windows OS.
An AP with vertically installed antennas will provide an optimal signal to a client device that also has its antennas vertically oriented. However when the client device moves from portrait to landscape mode the ability to receive this signal is greatly reduced.
Some AP vendors compensate for this by having antennas mounted in many orientations. The AP will monitor the orientation of signals received from client devices and use antennas similarly oriented to transmit back to the client.
Depending on the type of antennas involved, there can be 30 dB of loss when there is an antenna polarity mismatch. As we know from our dB math, each 10 dB loss is equal to 1/10th . If the EIRP of an antenna is 100 mW and the polarity between the client and AP is mismatched, the client would receive (pre FSPL) .03 mW. The math... 30/10=3, 3/10=.3, .3/10=.03. That is a huge amount of loss that can be avoided by using solid design practices.
This should show the importance of matching antenna polarity in all wireless installations, but particularly in fixed installations such as point to point links where the antennas at both ends are out in the open and permanently fixed. Mobile client devices can take advantage of reflection and inherent mobility to catch some percentage of properly polarized RF energy.
Go forth and design.
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