I made a simple repeater by repurposing the antenna of a comemrcial NFC NDEF keyfob. Cut away the MOB chip and replaced it with a 22pF capacitor. I measured the resonant frequency using a NanoVNA, and I got close enough (peak at 13.9 MHz). Taped it to the back of my phone (the case hides it), and it improves the read performance of my Flex quite a bit.
I mean….If you maybe want to maybe make another….lol
Sorry I didn’t mean to ignore this, I just forgot to respond. I’m pretty sure a single coil formed into a figure 8 with a direction change in the middle would cancel itself out because of the right hand rule.
Was there some specific advantage you thought it would add?
The dashed lines are the section of a larger coil, like a large coil design for a larger badge reader.
Well fuck that didn’t work as expected at all. Guess I’ll have to waste another $170 and two weeks on another revision
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This is my diy -not a sticker- nfc extender similar to the repeaters discussed here but much simpler and probably less efficient. Unless you have some tiny capacitors and magnet wire handy I wouldn’t bother. The gain was significant for me but you might as well wait for the stickers instead of making it yourself with no guarantees.
The other is a chinese usb-c wireless charging adaptor thingy which I made permanent by stripping as much thickness and sticking it iside the phone’s casing.
so since I have some magnet wire and capacitors on hand, if I wanted to make a repeater sticker that would couple better with an x series and I could stick on the back of my phone, would I want to make one in the same circular shape but smaller or maybe in an ovular shape? since the gauge of my wire is fairly small I could do a cylindrical one around some empty wire coating but im not sure that would help much unless it was physically wrapped around the x series
I would go for a flat 2-3cm wide circle. You are going to have to know the properties of your wire to determine the number of turns based on the diameter and then try maych that with an appropriate capacitor.
Okay so using this and an inductance calculator, since I have 22pf capacitors, and 0.0034" diameter wire, I’m gonna be looking at around 283.7cm of wire or close to ten feet, if I get a larger diameter wire I could do a smaller length but I don’t have that on hand so we’ll see how clunky it actually gets 
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Yes, that sound about right. As long as it fits in a phone case 
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I figured it out. The flux must flow!
Ordered the final revision. Even if it’s a tiny bit off I’m going with it. Can’t endlessly stress over perfection or you’ll never get anything done. I’ll report back once Amal encapsulates mine and I’m ready to get it installed. Then I will probably open up these as an option for conversion.
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Worked out okay. Gonna go test it with a couple transactions on verifone readers. I’ll post video. Then I’ll be sending it to Amal for encapsulation so I can implant.
Apex for comparison
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Got it working very well on most readers.
Unfortunately @Eriequiet couldn’t get it working on the big Verifone readers. Haven’t tried the smaller MX915 but after I implant I’ll let you know if it works on those. It’s better than the Purewrist conversion but only 20-40% better because I didn’t nail the tuning (unfortunately this is self funded so that’s the best it’s going to get for now)
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Speaking of the veriphone,
Came across a new reader at one of the gas stations near my work that get to act as a bit of a test location for terminals and pumps for they chain
This new thing looks to have a nice compact and potent field near the top…
You’re frequency master, all I can do is stick a xFD on it, but looks promising 
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The problem we’ve seen with these terminals, specifically from verifone, is that they tend to overpower the field. It’s like every aspect of their design was meant to communicate with active devices like phones for payment. This means they work great for Apple pay and Google wallet, even if the terminal is shrouded by a metal mount, or wedged between some other point of sale equipment and effectively does not have very good “clear air” around the antenna. The downside of course is that these terminals are pretty shit at actually communicating with passive transponders. They might power the transponder at pretty good distances, but the quality of the connection is so bad that data cannot flow across the shared magnetic field. At least, this has been my experience.
This one seems better, there is a very strong and concentrated sweet spot on the top left, the teller Has been advising customers to put their credit cards there for a better read, teller says it seems to perform better than the previous bastard
I only had a minute or 2 to poke at it, lest the teller loose amusement with implant shenanigans
But it seems like the antenna is smaller, starts in top left and only travels half to 3/4 the length and depth… screen feels much thinner and body feels like there’s more plastic and less metal Inside it, along with no metal clamp
I was stupid and forgot there’s a chance it could have picked it a x series… should have tried
I’ll dig out my show and tell rfid bag, and take that failed encapsulated flexnt2 you let me have back and see how it responds next week
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Still waiting on the encapsulation from Amal, but here’s some data on another CoM module I tested. Unfortunately it was much bigger and had a slightly lower peak, but neither of those are deal breakers. Maybe if I get mine back and it works okay I can see how many people are interested and make one more pass at the design to account for these inconsistencies.
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Is funding still an issue for this project?
Hey Jirvin
Yup, you want to help out?
Hey Satur9,
Happy to help out.
What stage are you at with this project and how much additional funding would be need to complete/advance it?
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