Regarding the 125Khz low-frequency card, the wavelength of its electromagnetic wave is 2400 meters. Is the coil of the card reader of this kind of card an antenna or a mutual inductor?

一沙一世

Regarding the 125Khz low-frequency card, the wavelength of its electromagnetic wave is 2400 meters. Is the coil of the card reader of this kind of card an antenna or a mutual inductor? [Copy link]

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Regarding the 125Khz low-frequency card, the wavelength of its electromagnetic wave is 2400 meters. Is the coil of the card reader an antenna or a mutual inductor? If it is an antenna, then the optimal length of the antenna should be 1/4 of the wavelength, that is, the optimal length of the coil is 600 meters.

The picture above shows the card reader (left) and the card (with the coil encapsulated in it), and the picture below shows the coil on the back of the card reader

maychang

"Is the coil of the card reader of this type an antenna or a mutual inductor?"

It is both an antenna and a transformer. However, its operation is closer to that of a transformer.

maychang

The reason why it is said to work close to a mutual inductor is that a considerable portion of the magnetic flux generated by the card reader coil passes through the internal coil of the low-frequency card.

maychang

"As for the antenna, the optimal length of the antenna should be 1/4 of the wavelength, which means the optimal length of the coil is 600 meters."

I don't know who told you that "the optimal length of an antenna is 1/4 of the wavelength". Antennas come in all shapes and sizes, and not all antennas should be 1/4 of the wavelength.

一沙一世

maychang posted on 2022-7-7 14:52 "Is the coil of the card reader of this card an antenna or a mutual inductor?" It is both an antenna and a mutual inductor. However, its operation is closer to that of a mutual inductor.

Thank you very much. So when does it work in antenna state and when does it work in transformer state?

maychang

Yishayishi published on 2022-7-7 15:10 Thank you very much. When does it work in antenna state and when does it work in transformer state?

"So when does it work in antenna state and when does it work in mutual inductor state?"

If the card is far away from the card reader, more than several wavelengths, that is, more than a few thousand meters away, it is working in the antenna state. If the card is very close to the card reader, it is in the mutual inductor state. In actual applications, the distance between the card and the card reader is less than 1 meter, so it is said that "it works close to a mutual inductor."