RFID cable extender

Thread Starter

gcaglion

Joined Jun 21, 2022
5
I am trying to build an extension cable to relay an RFID signal (125kHz) to various distances (2,5,10m). I am using 2 identical antennas at the two ends of a 75ohm coax cable, and I also put 2 identical baluns between the antennas and the cable ends. By using HackRF coupled with HamItUp, I was able to verify that the signal from the rfid emitter does seem to reach the end of the cable with seemingly identical waveform and amplitudes; however, when the passive element of the RFID is put in the proximity of the antenna (<2cm), it is not recognized.

Do I need an amplifier, even if the signal seems to be relayed with virtually no loss? If so, what kind would be most appropriate for my need?

Thanks!
 

Ya’akov

Joined Jan 27, 2019
6,575
Welcome to AAC.

You have a lot of moving parts here.

What is the characteristic impedance of the antennas?
Do you have.a datasheet?
Do you have a NanoVNA or some other VNA to check them?

Why did you choose 75Ω coax?
Why did you choose a 1:9 balun?

While the frequency is low, ther power is exceedingly low. I don’t know how you are determining there is “seemingly identical […] amplitudes” but consider the output power of the tag. It has to use the signal from the transmitter for power, it’s output is going to be quite tiny, hence the ~10cm normal range. Think of how small the output must be.

Why are you doing about things this way? What is the application?
 

Thread Starter

gcaglion

Joined Jun 21, 2022
5
Hello Ya'akov, and thank you for your reply.

First of all, I should have said that I started studying and experimenting with RF and SDR quite recently, so please bear with me if I'm not 100% clear.

I linked the antennas and baluns datasheets in my original post; however, to answer your question, antennas impedance is 50ohm.
Today I switched the 75ohm cable with a standard RG58 (50ohm) cable, but the results are the same (i.e. rfid is not detected on the remote antenna).
About the balun, when I started this project I had no clue about what kind of balun would be appropriate for this project; I would very much appreciate any guidance on this, too.
About the measurements I took: I used HackRF and HamItUp to record the signals emitted from the RFID emitter (2cm away), as well as the signals received 3m away through the antenna->balun->cable link, and they look indistinguishable.
I also took other measurements from both ends of the link (specifically, from the two wires of the antennas) using this multimeter:
specifically, I measured current (between 12 and 14 microampere) and frequency (124.8 to 125 kHz).

The aim of this project is mostly to gain a better understanding of RF and RFID systems in particular; specifically, I am trying to relay the signal from an RFID reader a few meters away.

I am attaching a picture of the experiment setup; hopefully it will make more sense.

I look forward to your feedback.
 

Attachments

Ya’akov

Joined Jan 27, 2019
6,575
OK, so if the feedpoint of the antennas is 50Ω, there is no need for the balun. Baluns are used to match impedance but it is already matched to the 50Ω coax, not the 75Ω. (I do strongly recommend you get a NanoVNA, which is cheap and incredibly useful).

Some questions:

Is the tag powering up?
Is it transmitting?
Can you modify a tag by attaching the feedline to its antenna connections instead of its own loop?

IF the tag goes on the air:

Is the tag interfering with itself by transmitting both a direct and out of phase signal from the antenna on the feedline?
 

Thread Starter

gcaglion

Joined Jun 21, 2022
5
Thank you again.

Now, something unexpected happened:
I removed the baluns and connected the rg58 cable directly to the antennae terminals: still the rfid tag won't activate.
I then tried connecting the antennae directly with plain copper wires (5m long), and suddenly the rfid tag was read!

Now, I'm glad it worked, but I am even more puzzled than before, and I'd really like to understand why...
Any idea?

By the way, I just ordered a nanoVNA, should get to learn to use it..
 

Janis59

Joined Aug 21, 2017
1,482
RE:""The aim of this project is (..) trying to relay the signal from an RFID reader a few meters away.""
In the her majesty www via the no less eminent Gooooogle may search the Haifa University students work about boosting RFID signal with aim to prove that 25 meters are not a real barrier when fising out the RFID content from walking along the peaoples wallets. They proved it is technically sound at bad hands, and explained what technicalities they used for. Article is about >5 years old.
 

Thread Starter

gcaglion

Joined Jun 21, 2022
5
RE:""The aim of this project is (..) trying to relay the signal from an RFID reader a few meters away.""
In the her majesty www via the no less eminent Gooooogle may search the Haifa University students work about boosting RFID signal with aim to prove that 25 meters are not a real barrier when fising out the RFID content from walking along the peaoples wallets. They proved it is technically sound at bad hands, and explained what technicalities they used for. Article is about >5 years old.
Thanks for the info; however, I was not able to find any paper such as those you mention. Perhaps I am not so eminent? ;)
Would you mind sharing the link here?
 

Kjeldgaard

Joined Apr 7, 2016
470
I don't think it has been discussed in the thread:

What about moving the RFID reader module out to where the reading is to take place?
It then only requires a three-wire cable for supplies and data.
 

Thread Starter

gcaglion

Joined Jun 21, 2022
5
That was phase 1 of the project, and I already did that. I tested it with cable lengths up to 10m. It worked with no problems .
 
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