Help designing a 125khz RFID antenna

Hello, Sorry I think this might be the wrong forum to be posting this on but I am trying to design a PCB antenna for a 125khz RFID tag that will go within a watch (the project). I have a PCB built for a 13.56mhz antenna (not designed by me) that works well but I am really looking for a 125khz version. I was wondering how I would start to replicate this PCB for 125khz. Or even if is possible to do a 125khz PCB antenna? Thanks

A PCB antenna for 125 khz is going to be very difficult because it will need many more turns and a lot more copper involved. Have you ever seen a PCB antenna for 125khz? They’re typically very beefy and larger than a watch. You might have better luck getting a wire coil inductor made, but either way it’s going to be pretty big… either in z height or diameter or both.

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Also, a passive inductor coil antenna is only half the problem. The other half is the capacitance of the chip you’re using. The exact inductance required of the coil is a direct function of the capacitance of the chip and the frequency you want to resonate at. It’s an LC tank circuit basically. Do you know the capacitance of the chip you’re using?

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thank you for the reply. I was thinking that a 125khz might be too much for a smaller PCB antenna but was hoping not. The chip is was a t5577 that was taken out of a blue key fob. I did this due to not being able to find any other (hand) solderable IC online. I found this IC for the 13.56mhz NFC version of this and it worked well. But could not find anything like it for a t5577, so just took the chip out of the blue key fob. thanks

My guess is that the key fob wire coil antenna is probably the best you are going to find for a watch sized inductor… can you extract the coil and chip from the keyfob and use that?


I did extract the coil and chip but they are unfortunately too thick. About 1mm Thick.

The coil in the xEM chip seams really small. Is that different due to it being a spiral instead of flat?

I think you’re going to have a tough time finding anything thinner than that and also a reasonable diameter. The xEM chip has a cylindrical winding around a ferrite rod which drastically changes the requirements… but also the range suffers greatly because of this.

are you actually wanting to put this into a watch body? if so it will need all the performance it can get.

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Ahh I was wondering if there was a ferret rod in the xEM. The watch project I am trying to make is this:

Also does the LF side of the DT diagnostic card work due to its large size? Because it seems like a pcb antena for 125khz. Thanks

the DT diag card does work, but it is not very good and only works with most readers, not all readers… readers that are low power or battery operated usually have problems lighting the LF side of the DT diag card.

Yeah so I watched that video and the dude is a shithead. I call him a shithead because he is making a false statement about his antenna… so either he is just lying or he is super ignorant about how NFC antennas actually work… specifically he says “… the two contacts on the back of the antenna allow you to add any kind of NFC chip that you can solder and fit inside the watch.” … this is absolutely false. As I said before there are tons of different types of NFC chips with different capacitances which means they each need a unique antenna design of the proper inductance to properly resonate at 13.56MHz … if you stick a 17pF P60 chip to an antenna designed for a 70pF P71 chip (both “NFC chips”) then you’re gonna have a bad time. This is just lazy or stupid.

Next he says “One very cool option is to use the chip from a tap to pay card. There are tutorials on the net showing you how to dissolve the card plastic to reveal the chip and antenna that can be soldered into the watch. I’m pretty sure you can also buy chips with their serial numbers unlocked, allowing you to clone your card. Either way, it would be pretty cool to be able to make direct payments” … meaning he is saying you can clone a credit card to a sector 0 writable magic mifare chip to make payments… also just flat out wrong. For the real deal on that read this;

I had to stop watching the video at that point… it was starting to anger me. I’m pretty sure this is misinformation put into the video just to make people give up and buy his modified watches. If you’re going to share how-to videos, share the information… don’t purposely confuse people into buying your products.

I am not agreeing with all of his information but I don’t think is videos are just meant for people to buy his watches ( he has many many other videos not about the watches which he is not selling items) . I do understand that a cloning a pay chip is not possible and also understand that different NFC chips work differently with different antennas. But what I do know is his provided antenna PCB design works very well with this chip. I got a PCB fab to make it and soldered the NFC chip and am getting about 1.5in of scanning range with my iPhone XR. I have been able to store a lot of data on the chip as it is 8kb.

So I don’t comply agree that his video is made with misinformation in mind but instead the information was just not thoroughly vetted.

Ok that’s fair… it’s just… wrong. The ST chip is what he designed the pcb for but by no means will it “work with any nfc chip”… anyways… I’m irritated haha can you tell?

For 125khz you would probably have to make a flex pcb or semi-rigid pcb with 4 or 6 layers to be honest… then it should be ok thickness wise… but you’d have to design it for the required inductance for your desired chip. The T5577 comes in a few different capacitances if I remember right.

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