Improve your cellphone's NFC antenna

Projects
1

If your phone has lackluster NFC performances with implants and you’re willing to open it, this may help.

I tested it on my Ulefone Armor 3W - the innards of which are easily accessible - but it should work with other phones provided the NFC patch antenna is separate (attached to the back cover) or not permanently glued to the battery, and you manage to open the phone’s case without breaking something. Repairability isn’t today’s electronics’ forte, sadly…

In many cellphones, the NFC antenna is right up against a large mass of metal - the battery - because there’s a limited amount of space inside the case, the battery takes up a lot of it and the antenna has to go somewhere, and it’s convenient for manufacturers to install the battery with the NFC antenna as one unit.

The problem with that arrangement is that the metal inside the battery effectively wastes a significant portion of the RF power emitted by the NFC antenna in the form of eddy currents. That’s power you want to power your implant with, that heats up the battery instead.

It’s always a compromise. But if the phone reads large passive tags easily enough and couples in HCE mode with point of sales terminals, usually that’s good enough for many manufacturers. They’re not gonna spend anymore time and money on improving NFC if it’s good enough: NFC isn’t the important function for them that it is for us.

But you can improve NFC performances by shielding the battery from the antenna. All you need is a thin RF ferrite sheet, such as this one:

image
image945×687 42.3 KB

Cut a piece as wide as will fit inside the phone’s cover and insert it between the battery and the antenna. Here in my Ulefone Armor 3W, the antenna is on the back cover, and it connects to two pins above the battery when the cover is reassembled, so it makes things easy to see:

image
image1425×996 96.2 KB

image
image1423×977 98.8 KB

Then put the phone back together. With a bit of luck, it’ll perform better - and possibly a lot better: my Armor 3W went from reading a xM1 okay at contact distance to hitting it 10 mm away easily. And it can now read my finicky BeUno somewhat reliably, while it could almost never read it before. It’s suddenly become a phone with good NFC for a few euros and 20 minutes of work!

RF sintered ferrite sheets aren’t cheap, but you only need one and if it works, it’s well worth it.

12 Likes
2

Very clever, thanks for the explanation.

However opening phones nowadays can be very difficult, as the back cover is often glued to the frame.

1 Like
3

It might be an option if you have an older phone that’s out of warranty, you don’t want to throw it away because it’s still perfectly serviceable, you don’t really care if you mess it up and you want to revive it a bit on the cheap.

4

Probably do this next time I change the back on my s10 (all glass so breaks often)

5

Cool.
I think there in not much room to insert it most phones.
Everything is really compressed inside.
I use samsung note 8.
Do you suggest any type of ferrite sheet?

6

The ferrite sheet I used is 0.8 mm thick. It should fit most cellphones even if you have to put stuff under tension a bit when it’s reassembled.

There are thinner sheets though, down to 0.1 mm. If unsure, get several kinds of various thicknesses and use whatever fits best. But that’s not really a cheap proposition.

Alternatively, you could open up the phone and try to see how pieces of paper of various thicknesses fit, then order the ferrite you need. But then you’d have to open up the phone twice.

7

Could the reduced NFC sensibility be caused by the wireless charging coil on the new phone?

I have a new Samsung Galaxy S21 FE, and a lot of problems reading NFC implants. It has wireless charging. My previous phone, a Oneplus 7T, had no problems and no wireless charging.

I watched a disassembly of the S21 but they didn’t show the NFC antenna in the back, it’s all hidden behind a black sheet.

8

I’m guessing the black thing has both the NFC antenna and the wireless charging pickup coil in it. I doubt the wireless charging coil affects the NFC antenna much, since the frequencies are so different. But who knows…

You can try to slip some ferrite behind the black thing (between it and the battery), it might help. I doubt you can do anything more than that though, but it’s worth a shot. Bear in mind that there’s no guarantee that it’ll work. It might even make it worse :slight_smile: Unlikely though. At worst, I’m guessing it won’t change anything.

9

Thanks.

I won’t do it though, I’m not really confortable opening a brand new phone and especially as the back is glued.

I wish there was a list of phones with great NFC sensitivity.

10

Cellphone sensitivity scale for me is:

  1. Terrible: has trouble picking up my Apex Mega
  2. Very poor: has trouble picking up my flexNT or my flexM1
  3. Poor: has trouble picking up my flexDF2
  4. Average: has trouble picking up my IAR Magic M1k
  5. Good: has trouble picking up my BeUno
  6. Excellent: picks up my BeUno easily.

My colleague’s Samsung S20 is 1. to 2. If your S21 is anything like that, I feel your pain.
My Nokia 4.2 is a solid 6.
My Ulefone Armor 3W went from 4. to 5. / 6. with the ferrite sheet trick.

1 Like