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I've been using a 134.2 KHz (FDX-B, ISO 11784/85) transponder/reader for use with implanted and pendant tags. However, I've been frustrated as to the range. Is anyone aware of a chip/circuit level solution incorporating that or a similar standard (like 13.56 MHz, ISO/IEC 14443, etc) that can be designed to work reliably at a range of up to 12 inches? For the tag, I only need the basic requirement of transmitting a unique identifier and can use a pendant size tag.

As I understand RFID transponders, I should be able to build an RF emitter with a power output capable of powering a suitable tag at that distance, which when sufficiently energized will automatically broadcast its unique code. And then the remaining challenge is just to build a suitable reader. Is it that simple? Anyone have experience in that area?

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The easiest way to achieve higher ranges is to increase the size of the antenna. But I don't think that such a large distance is possible with 134.2 kHz and a reasonable sized antenna.

Another possibility is to increase the frequency but still remaining inductive coupling. UHF-RFID (860-960 MHz) would be great for range but terrible for energy harvesting because there is electromagnetic coupling and no inductive.

HF (13.56 MHz) seems to be a good solution. You already mentioned the possibility to move towards another standard like ISO/IEC 14443. But if you want to switch to 13.56 MHz, you should use ISO/IEC 15693 because the minimum required field strength is much lower. The maximum range is up to 1 m, which is suitable for your application.

One more note. The communication from tag to reader is done by load modulation. A relatively new chip from NXP (NTAG 5 boost) uses active load modulation. NXP claims to achieve the same read range like passive load modulation but with a 40 times smaller antenna. Conversely, with the same antenna size, the range with active load modulation should be significantly greater than with passive. There may also be solutions from other manufacturers that I am not aware of.

In summary, I would recommend using ISO/IEC 15693 and maybe consider a chip with active load modulation or similar.

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  • \$\begingroup\$ Unfortunately, active load modulation doesn't appear to be a possibility, since the tag can't have an associated battery or other external power and must be powered by the carrier field. But I like the 15693 idea and have pretty much come to the same conclusion myself. Does 'load modulation' always involve the modulated shunting of a weakly coupled secondary transformer coil to gnd? \$\endgroup\$ Aug 17, 2020 at 18:19
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    \$\begingroup\$ I dont know if its shunting to ground but I’m pretty sure it’s binary. The RFID Handbook from Finkenzeller states that the SNR of the antenna voltage is around 80 dB. Even if load modulation can be done with more than two states, I think it is unlikely to work with RFID because the circuitry would become complex. That’s why the data transfer is done by a subcarrier. \$\endgroup\$
    – mais
    Aug 18, 2020 at 20:50
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If you use a large diameter coil, perhaps solenoidal, the field may reach 12 inches in adequate strength.

The inductance will be large, and resonance may be delicate; you may need to trim the resonance in real time.

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I decided to focus on an ST25R3911B-AQFT reader chip (supporting the ISO/IEC 15693 standard and with 1.4W power to the antenna) with an ST25TV (NFC Type 5) tag. It will work with 3.3V. The 15693 (13.56MHz) standard has longer range than the 14443 and still better than 125kHz and 134.2kHz and is well established. ST includes some detailed instructions on designing, building, tuning and matching the antenna for its $5.75 chip. Calibration requires a Vector Network Analyzer, and fortunately I was able to find a little $30 NanoVNA that HAM radio guys on YouTube seem to think a worthwhile tool. ST has a user forum with reasonably good participation. One customer achieved a 24 cm read range with a 14 cm x 10 cm antenna, which should be adequate. The problem with ST is wading through tons and tons of technical data.

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