Linearly polarisedmeaning

Drive far enough out into the Sonoran desert in Arizona, and you might just stumble across strange, large crosses dotted across the dusty landscape. Spaced apart in a one-mile-grid, at a glance they seem to be the remnants of something long gone. That is not completely false. These concrete targets were built in 1967 during the Cold War era. For about a decade before their construction, satellites were launched with 77mm film tucked away. They were then sent over Russia and China to snap surveillance photographs of the landscape below, and once they were full they would eject the used film. The roll would then parachute down to be intercepted by a passing a plane.

Elliptical polarization

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When deciding which type of antenna to purchase for your RFID system, understanding the how these antennas work as well as how the RFID tags should be oriented with respect to the antennas is key.

Linearly polarisedlight

If all the tags you need to read will be on the same plane and aligned with the plane of the antenna, then you should consider a linear polarized antenna.  If your tag orientation is not something that will be reliable or consistent, then you will likely need to use a circular polarized antenna.

Due to the concentrated emission, linear polarized antennas typically have greater read range than circular polarized antennas of the same gain.

Linearlypolarized electromagnetic wave

Compared to linear polarized antennas of the same gain, circular polarized antennas will have a shorter read range because they lose about 3 dB splitting their power across two separate planes.

Linearlypolarized wave

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Each target is composed of a trapezoid rotated around a central circle. Perpendicular lines provide the framework to layout the trapezoids.

Circular polarized antennas, such as the  Laird S9028PCR Indoor RFID Antenna and the MTI MT-242043 Outdoor RFID Antenna, emit electromagnetic fields in a corkscrew-like fashion.  Technically speaking, they are broadcasting electromagnetic waves on two planes making one complete revolution in a single wavelength.

“So what is the purpose of these crosses?” You might ask. The surveillance was already happening; however, properly focusing the cameras and understanding the scale of photographed landscapes were ongoing problems. These concrete crosses were designed to combat these problems - providing a place where satellites could calibrate their cameras, and photographs that give a sense of reference for scale. Since we know that each of these crosses is sixty feet across, photographs taken halfway across the globe could be compared to them, a ruler of the sky.

Circular polarization

Linear polarization occurs when electromagnetic waves broadcast on a single plane (either vertical or horizontal).  Linear polarized antennas must have a known RFID tag orientation and the RFID tag must be fixed upon the same plane as the antenna in order to get a consistent read. Some examples of linear polarized antennas are the MTI MT-263003 Outdoor Antenna, and the Times-7 A5531 Indoor Antenna.

The choice between circular polarization antennas and linear polarization antennas can make a significant difference in an RFID system.