I advise against it.
The repeaters introduce a bit of latency in the signal transfer. Adding additional repeaters increases that latency and the signal may be too slow to effectively couple
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I donât know, I would try it. I donât think itâs latency per se because the physics is the speed of light there but I think whatâs happening is that there is some negotiation interference happening. Many chips can negotiate a lower bandwidth connection to overcome interference and I think that might be what weâre seeing. If thatâs the case, then I donât think stacking would introduce any additional delay⌠though it may degrade the quality of the connection overall and eventually you will see the situation where it doesnât work at all.
Iâm basing this off of the initial discovery that the RDC could be laid over a reader and then a tag read at the far end, I then placed two RDCs over top of each other and got the tag to read even further away from the reader. I was able to place a third card out and get periodic reads but it wasnât reliable. I didnât notice any particular gain in latency though, so I think itâs just a matter of signal quality over noise generated by the eddy currents in the secondary coils as the field collapses through the inductors.
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Stacking works. 2x MFGC 2.1L.
Reader is under the table facing down. 1 MFGC, unable to read thru my hand to the Apex Flex. One on table, and one on Apex, able to read with my hand about 1/2" off table. Flipped over, can read about 3-4" above table.
Edit: But note, repeaters are spaced out a bit. With them âstackedâ stacked, it doesnât really help much. If I had more, Iâd try making an âNFC Repeater Cannonâ.
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If repeater cannon works at the DT meet up you should see if you can get a Guinness book world record for longest read of an NFC implant
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Pretty sure the NSA already has it.
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So according to the rectangle, i put 2 stickers on the phone and on the case like this:
Is it correct placement?
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Do they work?
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I dont see any difference with or without them
The UG4 is much more difficult to read than the Apex
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Hey I designed these and they tested great, but Amal is getting some different tuning results than I did with his VNA. Can somebody else who has a VNA test these and let me know what the resonant peak is? @Hamspiced @Az_F ?
I donât have any of them in my possession and shipping is as much as the stickers themselves 
I know i spoke with you about it before but i get pretty crazy variances with my repeaters in each batch i get made.
As a result I have to QC each batch and throw every single repeater under the meter because it can be off peak by +/- 1mhz. I end up tossing around 15% of my stock because of these variances. even though they are identical, there are some factory freaks that just are coasters.
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Thereâs also the splitting the peak behavior I detailed in this post when you use resonant repeaters to extend the range. Try it with an actual NFC tag in the field as well like you would in the intended use case.
This was really difficult to get.
Marker is at 13.56mhz
Repeater is the dip to the left at the same 12.8mhz
Fob is to the right at 14.59mhz
I like to challenge you 
Did the peaks move around much as you set it up and for example as you approached the repeater with the tag. Which directions did the peaks move?
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Peaks never moved. They just got deeper as I moved closer and away.
Truest statement of the year
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Eh I guess weâre fine then. Itâs not perfect but itâs pretty damn good, and the manufacturing tolerances arenât really something we can get around because I already ordered them to a tight specification.
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Yeah, me too.
I started writing this up a while ago, its not quite finished, I held off because I know how much, time, money and effort you put in to your projects.
but since you asked, I might as well share it now.
I wasnât getting great results, infact, I was getting reduction in range.
50% drop.
and the peaks were off my starting scale (13MHz to 14MHz ) so I had to expand just to see them.
Knowing your usual quality of work, I triple checked my results.
But I also tested yours along side Hammys, and his stickers were 25% range increase.
And they were very close to 13.56MHz.
I tested 4 of each
My âreviewâ below, was a full test across the gamut of repeaters, including the stickers; this is what I gotâŚ
ACTUALLY, It is almost done, but needs a couple of format / layout tweaks.*
Let me do those, and Iâll post it tonight (my tonight)
*easier on my laptop than phone I am currently on
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Not perfect, not quite finished, but close enough.
Thanks to a few dedicated community members
Notably @Hamspiced and @Satur9
There are now a number of resonant repeaters for us to choose from.
I have carried out independent testing on a variety of repeaters, and the below serves to document my findings for you to make your own decision as to what will suit your own scenario.
For the purposes of clarity, I have broken the testing down into 3 Categories
Large Form Factor - Small Form Factor - Sticker Form Factor
Previous similar tests, I carried out with:
My Phone, A PM3 Easy, A PM3 RDV4 and a Flipper.
Due to [considerations, blah blah decisions blah blah, explained, deleted, boring] this round of testing I have chosen to simply use:
EQUIPMENT
- Vector Network Analyzer
- Flipper Zero
TARGET
The target was an OG copper antenna FlexM1 (chinese knock off)
PROCESS
-
The average was taken over 5 tests on each repeater.
*The repeater placed on reader and âimplantâ presented to reader. using my test rig.
-
I used hammys Diagnostic Discs to find the sweet spot.
BASELINE MEASUREMENT
(No Repeater)
FLIPPER 
FlexM1
20mm
LARGE FORM FACTOR
V2.1 - Larger sized repeater
42mm
13.959MHz
NFC Amplifier V3L
42mm
13.56Mhz
SMALL FORM FACTOR
25mm
13.5MHz
RFID Antenna Adapter for Tuning Antennas
Flex model is the thinnest possible for tight spaces and needs.
PCBâs come bare, you will need to supply your own Antenna coil and capacitor
25mm
13.56MHz
STICKER FORM FACTOR
(testing was carried out on 4 stickers of each offering)
Flex NFC Repeaters 13.56mhz Range Extenders
25mm
13.35MHz
DT Sticker pack
10mm
12.78MHz
My personal picks/ recommendations
-
Large Form Factor
If you have the space, go for a Large Form Factor
To me there was no discernable performance factor between the V2.1L or the V3.0.
But the V3.0 is Sleek having no protruding components.
GREAT for a desk reader mounted under a desk, you could even place the V3.0 on the top surface of the desk as functional eye candy.
and it is tuned BANG ON!!! -
Small Form Factor
They all provide an improvement, so you canât go wrong.
The decision will depend on where you want to use it -
Sticker Form Factor
Hammys stickers are what I am using and I am super happy with them
BONUS CONTENT
Curiosity test - Field detector = 23mm
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This is great. Are you going to add this to the review section?
My biggest shock here is the lack of efficiency loss from FR4 to flex and then from the factory coil in the antenna adapter to PCB repeater.
I always knew the big boy would outperform the others but seeing the consistency in performance for the small sizes makes me happy.
Thank you for this Pilgrim. Great work and I appreciate the time you put into this testing
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