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I have a cheap UHF RFID antenna (9dB) like this :

My antenna

How does this kind of antenna work? There is no dipole? Why does the "L" at the bottom left of the disc have finer passages than others? I can not find any information on the web about this type of antenna and the physical rules used.

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This is a circularly polarized antenna. The central disc is fed at two points at 90° from each other; furthermore, these two feed points have different lengths of transmission line from the connector. You'll find that the difference in lengths corresponds to a 90° phase shift at the operating frequency.

Two fields at 90° to each other, fed with a phase shift of 90°, combine to create a radio wave whose polarization rotates at it propagates away from the face of the disc. This is called circular polarization.

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  • \$\begingroup\$ Thank you, great answer! Just an extra question, is it possible to reverse the polarization? \$\endgroup\$ – Ludovic May 2 at 16:41
  • \$\begingroup\$ You'd have to swap the lengths of the transmission lines, which, given the fact that it's all stamped from a single piece of metal, would not be easy. \$\endgroup\$ – Dave Tweed May 2 at 17:00
  • \$\begingroup\$ There might be a "cheat" you could use -- a mirror image of a left-circular wave is right-circular. So -- I theorize -- if you put a metal plate (insulated from the antenna and everything else) at an angle across the face of the antenna like a mirror, it should both change the direction of the signal (as you'd expect) and also reverse the polarization. Can someone who knows more about antennas tell me if I'm right? \$\endgroup\$ – Glenn Willen May 2 at 19:01

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