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Are Circular-Polarization Antennas Available for LF RFID Readers?
If so, who makes them?
No. Low-frequency (LF) and high-frequency (HF) RFID systems work differently than ultrahigh-frequency (UHF) RFID systems.
With a UHF system, the tag is in the far field (meaning more than one wavelength away). The antenna on the transponder receives energy from the reader and reflects back a signal using a technique called backscatter. A circular-polarized antenna is used to ensure that the transponder can receive energy regardless of its orientation. The energy is emitted in a spiral so that it will reach a dipole tag whether that tag is oriented vertically or horizontally.
A UHF system is like one person holding a bright flashlight on a dark night, with another person located 25 feet away and holding a long, thin mirror. The latter holds up the mirror or puts it down to communicate in a kind of Morse code. If the mirror is long and thin and is being held vertically, and if the light source is also long and thin and is being held horizontally, then only a small fraction of the light would be reflected back. (In the case of an RFID system, there is not enough energy reaching the antenna to power the tag.) If the light is circular, it doesn't matter how the person holds the mirror.
With LF and HF systems, the transponders have antennas in the shape of a coil and the reader antenna is also in the shape of a coil. The two coils form an electromagnetic field when a transponder is in the read field. As the transponder modulates and demodulates its antenna, it creates changes in this field that the reader antenna can sense. Imagine two people holding the same balloon. One squeezes the balloon in long and short sequences, sending Morse code to the other. The transponder's orientation doesn't matter, as it can be on any edge of the "balloon."
—Mark Roberti, Founder and Editor, RFID Journal
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