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I'm 2 months into my RF and SDR learning, and I'm stuck in the middle of a project, looking for some clues:

First, as detailed here, I connected two 125kHz RFID antennas through a 5m, 2-wire copper cable (no coax), and I was able to relay the RFID signal through that link (my tag was recognized by the reader).

As a second step, I want to replace the copper cable link with a radio link:

[RFID transmitter antenna] - [RFID receiver antenna 1] - [HamItUp 1] - [HF antenna 1] - [HF antenna 2] - [HamItUp 2] - [RFID receiver antenna 2] - [RFID tag]

so I connected the input port of a HamItUp nano to one of the RFID antennas, and the output port to a 60cm antenna (300000000/125125000/4); using a remote HackRF with its own antenna, I was able to receive the signal from the RFID emitter -> HamItUp -> antenna over the air at 125.125MHz.

I then connected the input port of a second HamItUp nano to the second RFID antenna, and its input port to a second 60cm antenna, identical to the first; this way I was expecting the radio link to work, and being able to read my RFID tag at the second; however, despite having verified with HackRF that the signal is reaching the second RFID antenna, this time the tag is not recognized.

I already tried switching the input and output ports of the first and second HamItUp, with no luck; I feel I'm missing something, and would appreciate some guidance.

Thanks.

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The system you describe would certainly not work. 125 kHz RFID (and all others) are two-way links. In some cases the tag transmits while the reader is transmitting, in others the tag power capacitor is first charged up and then the transmitter is turned off while the tag talks. The turnaround is extremely fast, just a few milliseconds, so this is very different to anything done in amateur radio.

First I must ask why you're trying to do this at the physical layer? Why not just extend the data connection between reader and the rest of the system? Move the reader to the place you want to read the tag, and transmit the tag information by bluetooth or whatever to the place that needs to know about the tag.
If this is simply impossible, for example the reader is installed and cannot be modified, then devise and mount a tag emulator at the reader. Read the tag elsewhere, transmit its ID to the emulator, and have the original reader capture it there. (The emulator might be a regular tag, programmed on the fly by a microcontroller). This is all more difficult if you need to write to the tag as well as reading it.

To get in to this you first need to learn and master the subtle details of the physical layer protocol, probably specified by ISO 18000-2. The standard itself is expensive, (Though you can often buy an identical but less expensive standard from other countries). More useful and cheaper, you will find lots of ancilliary commentary in manufacturers datasheets. For example the TI TMS3705 has some good detail about the protocol. Search for 125 kHz or "LF" reader ICs from the usual big names (Texas, Microchip, Dallas/Maxim, NXP, Nordic, STmicro, Atmel and others).

For HF tags (13.56 MHz) there is also quite a bit of literature from security researchers / "ethical hackers" trying to find ways to break the contactless payment systems, clone cards, read them from a long distance, etc., which partially overaps with what you're trying to do. Search this community for papers and presentations about RFID hacking and you'll find some low level technical detail of how things work.

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  • $\begingroup$ Thank you very much for your extensive answer; I will definitely look closely into the standards you mentioned. To answer your question, I went about doing this at the physical layer because I thought I could skip the nitty-gritty details of the RFID exchange protocol: once I saw that the copper link was working bidirectionally (the tag was successfully read from the reader 5m away), I naively thought I could replace it with a 125.125MHz RF carrier. "Read the tag elsewhere, transmit its ID to the emulator, and have the original reader capture it there": that is precisely what I had in mind. $\endgroup$
    – gcaglion
    Sep 19, 2022 at 17:29
  • $\begingroup$ I suppose a carefully designed full duplex bidirectional RF link might work, with well separated frequencies for each way and careful filtering of everything. But it would be a pretty complex arrangement, you now have two links with uncontrolled path loss which will need AGC.... Tricky. $\endgroup$
    – tomnexus
    Sep 19, 2022 at 19:19
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This doesn't work for reasons that are related to the reasons given in answer to your previous question. LF RFID is not really a "radio link" in the ordinary sense and you can't treat it as one. The tag is in the transmitter's near field, and the two of them are exchanging, not only signals, but a noticeable amount of power, to the point where the reader can "feel" the load of the tag on the transmitter. If you replace that load with a transverter or an SDR or whatever, then the whole system becomes decoupled. The tag might hear and try to respond to the query, but the reader will never hear the response.

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  • $\begingroup$ Thank you very much for your explanation; I have been studying LF RFID standards for the last couple of days, and my understanding is growing, However, In my research process, I came across this paper, which seems to describe a working implementation of exactly what I have in mind, albeit for a different application. What would you say is the difference between this paper and the radio link I am proposing? $\endgroup$
    – gcaglion
    Sep 21, 2022 at 17:32

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