Something like this.
If it is genuinely a n EM4100 and not just a T5577 I’m not sure what the capacitance is, but you can find the datasheet somewhere.
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It didn’t cross my mind to check the datasheet 
. Thanks!
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If it is a T5577
you can find it under
T for T5577 in here
To confirm whether your fob is a t5577 or EM 4100, run
lf t5 detect
on a proxmark
but Satur9s link above for his post will be very useful for guiding you through your project
Replaced the ceramic with an smd capacitor and it honestly feels like the range has increased.
But none the less this was incredibly successful for such little invested time.
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Not that I have seen on the forum.
I have a few “Bullseye” stickers that I considered using, but went with the copperr wire because I had ceramic caps, and I wasn’t going to achieve the low profile I was after for the whole thing plus the stronger mechanical bond (solder) onto the foil would actually become the weak point; also they may be fine secured in phone case etc, but the stickers are not very robust.
I decided it was a better experiment than a “product”
Amal did look at / consider a sticker
and Satur9 at a flexible pcb version , but thats probably down the list for both of them
If someone has silicone molding skills they could make Iphone case with an integrated nfc booster 
I feel like there’s demand for that
If you can get the solder to work on that foil. IIRC, it’s aluminum and that’s not easy to solder, if that’s even possible.
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I haven’t messed with designing any stls, but I’ve got a filament and resin printer to make a mold. And I’ve got some flexy resin if anyone wants to send a design and I’ll ship.
Would be nice having a custom phone case for it.
I do have the components lying around, I can make the antennas. What I don’t have is the latest Iphone to see what the best shape is in terms of performance.
If you want to give it a go I can make a batch of generic copper ones and you bend them in whatever shape works.
Edit: I have tiny smd caps so you can fit everything in 1mm resin for sure
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Maybe. I’ll look to see if I can find some friends with iphones and ask to test out some flexy and filament prints if I can find an STL and figure out how to engrave a space for it.
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If you give me a list of models you want I can always 3d print models of the phone for you to use as templates
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Definitely a demand. NFC compatibility is my main gripe with my iPhone. If I could at least fix the coupling issue I would love my phone so much more.
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I didn’t think about printing phones to test it on, that’s a great idea! I was just going to steal phones from my friends haha
ordered some HF fobs and 22 pF capacitors and what do you know, this actually works! My previously almost unusable phone can read my apex from a few cm away now!
Version 1 POC:
Version 2 which I can put on a keychain (yes it does work inside of the plastic enclosure):
I just used hot glue to cover the capacitor, version 3 I’ll probably use some kind of epoxy.
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A keychain is a cool idea! You could make a clear one to match the field detector.
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This seems like the best thread to ask this semi derailment
Kind of a multi facet question
Is it possible to overload a READER?
I know nfckill exists, but I haven’t heard of it being used against a reader
…if yes
Would it be possible / how hard
To wire a capacitor into a fob coil, to build a charge up from a reader field, then dump it back at the reader
Purely usb kill style
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This might be possible but you’d have to sit for a while to build up a charge capable of killing any components in the reader. NFC tags themselves are really low power devices but the readers aren’t all that low power and have a lot of filtering between the antenna and circuitry.
I do wanna try though
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I opened one of those key fob thingy and plugged the stuff in the calculator and got 9pf as a cap value … Didn’t work 
Then I blew the antenna trying to pray it off the glue 
Need to get some 22pf and try it again 
I found a nice balance while making range extenders for @invalid_signal 's phone case project:
22pf smd capacitor
70cm of 0.13mm enamel wire → wound in a 35mm diameter coil (leave 5mm slack at each end to solder the cap)
The coil should end up at ~6.2uH ( between 6 an 7 really)
That will give you a pretty good tuning
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