Gah - it only works if I put my hand near it (13.56Mhz / NFC) - Page 1
Forum section: electronics / rf-microwave

author: jpyeron
author_rank: Regular Contributor
timestamp: February 07, 2025, 01:09:32 pm
content: So everything work perfectly on the bench.It gets installed and the digital communications fails to handshake. But if I put my hand in front (~3-5 CM) of the device it works. The device uses near field 13.56MHz to communicate.I am way out of my league on this.Going to need to hire an RF engineer to help on the redesign, but need a bandage for monday (that is not me standing at the device all day long).Not sure what other information I can provide.

author: CaptDon
author_rank: Super Contributor
timestamp: February 07, 2025, 01:37:55 pm
content: You haven't really provided any yet. Pictures? model / make?

author: jpyeron
author_rank: Regular Contributor
timestamp: February 07, 2025, 02:10:37 pm
content: Ansel Adams says...

author: jpyeron
author_rank: Regular Contributor
timestamp: February 07, 2025, 02:11:49 pm
content: attachments keep dropping off....

author: jpyeron
author_rank: Regular Contributor
timestamp: February 07, 2025, 02:31:18 pm
content: Quote from: CaptDon on February 07, 2025, 01:37:55 pmI was having trouble with attachments.The device is a sandwich of a NFC reader and NFC device, no specific make / model.Quote from: coppercone2 on February 07, 2025, 02:17:12 pmYes, exactly - thats me.

author: benseno
author_rank: Regular Contributor
timestamp: February 07, 2025, 02:52:27 pm
content: Either shielding happens upon installation or card's resonance frequency change. Both will reduce NFC reader's sensitivity.What distance is expected between card and reader to work? I recall for passive RFID tags 10 cm was acceptable.

author: jpyeron
author_rank: Regular Contributor
timestamp: February 07, 2025, 03:44:08 pm
content: 2-3 mm (yes, 0.2-0.3 CM)

author: benseno
author_rank: Regular Contributor
timestamp: February 07, 2025, 04:24:23 pm
content: Search returns "The NFCIP-1 standard stipulates that the communication distance of NFC is within 10 cm".13MHz isn't that hard to deal with but you need equipment to measure things.What it was meant when said - everything work perfectly on the bench? Were these 2 sandwiched boards laying apart from each other?Is it possible to measure resonance frequency of NFC's LC coil antenna using means available around? Function generator should suffice along with low capacitance probe or home made diode based detector.

author: jpyeron
author_rank: Regular Contributor
timestamp: February 07, 2025, 04:42:11 pm
content: When I put the equipment here, it works.When the device is installed it does not work.When I put my hand 2-3 inches away from it where it is installed it starts working again.I suspect interference, as I am a meat popsicle I absorb RF.

author: jpyeron
author_rank: Regular Contributor
timestamp: February 07, 2025, 04:59:37 pm
content: A videohttps://youtube.com/shorts/uTIy--_eZNA?feature=shareNote 1: the Janky wago was a hasty attempt of adding some grounds "near" the balanced circuits.Note 2: In all the repair / mitigation attempts, we have a) taken the device off the DIN rail in the grounded closed metal box; b) replaced the hardware with known good hardware from the bench; c) replaced the wires with known good working on the bench; d) added a grounded traveler wire; e) tried wrapping in a grounded shield - made it worse??; f) let it dangle outside the metal enclosure (better??)

author: Kean
author_rank: Unknown
timestamp: February 07, 2025, 05:37:15 pm
content: I don't really understand what you are trying to achieve. It looks like you are simulating an NFC card remotely?Maybe the reader is tuned to work best when there is human hand holding the NFC card - as would be normal for access control.Not sure why it works differently on the bench, but presumably there is some difference in nearby materials that change the tuning once mounted in the wall.

author: jpyeron
author_rank: Regular Contributor
timestamp: February 07, 2025, 05:57:36 pm
content: I am trying to achieve putting on a spare tire until I can get to the service center. I am hoping someone here has a clue, because I do not.RF is black magic to me.But after playing Mr. Bean it works, so it MUST be solvable. There are lots of power and computers in and around the device now. I suspect interference.I am going to look to hire an engineer to redesign it, but until then I need to get it "working".

author: jpyeron
author_rank: Regular Contributor
timestamp: February 07, 2025, 06:17:48 pm
content: Minor progress: turning off all LED lighting on the floor makes it almost reliable.

author: benseno
author_rank: Regular Contributor
timestamp: February 07, 2025, 06:36:31 pm
content: LED lighting has builtin AC-DC conversions running internally for led powering. May be switching frequency harmonics of led power modules were interfering with NFC frequency.

author: Kean
author_rank: Unknown
timestamp: February 07, 2025, 06:49:45 pm
content: Maybe replace the twisted pair with some shielded cable or coax.

author: jpyeron
author_rank: Regular Contributor
timestamp: February 07, 2025, 07:16:29 pm
content: Quote from: Kean on February 07, 2025, 06:49:45 pmAre you thinking the line, not the antenna, is where the interference is happening?It is a balanced antenna, so it would then need a balun to use coax. I guess something like:https://www.newark.com/murata/dxw21bn2511nl/balun-transformer-13-56mhz-0805/dp/57AC6705Might be able to wind one this weekend - but the original designer said "this was better" design.

author: jpyeron
author_rank: Regular Contributor
timestamp: February 07, 2025, 08:23:33 pm
content: Figuring we are essentially bags of water, I tried a bag of water. It attenuates.https://www.youtube.com/embed/8Kd-XHXQlscNext, 3D print a box to hold the water.

author: Kean
author_rank: Unknown
timestamp: February 08, 2025, 02:13:08 am
content: Quote from: jpyeron on February 07, 2025, 07:16:29 pmWe are just guessing as you haven't really explained what it is doing other than being an NFC reader, apparently modified somehow per your earlier post.https://www.eevblog.com/forum/blog/sponsored-videos/msg5042662/#msg5042662It could be affected by interference picked up on the cable or by the antenna, or just that the antenna is tuned to read when in the presence of a human hand.The bag of water could be attenuating the interference, but it could also be "improving" the antenna tuning.I'm also no RF/antenna expert, just picked up a thing or two in my "travels" and know I am lacking a bunch of theory.

author: benseno
author_rank: Regular Contributor
timestamp: February 08, 2025, 07:03:10 am
content: Thanks to Kean's post the picture is more clear.The schematic posted shows R17, R18 2.7k resistors where the DS for Philips RFID reader (which is a compatible to the reader ic used in the design) recommends few ohms for those dumping resistors.Hand wound twisted pair is used to extend antenna. Not sure how your are going to control the line uniformity and how big the impact could be due to that variable. Changing to coaxial could help (could require more effort than plainly putting Murata's baluns on both sides).Good luck.

author: jpyeron
author_rank: Regular Contributor
timestamp: February 09, 2025, 06:54:59 am
content: Thanks everyone - I think the water attenuators will get me through the week until I can find a local RF EE to resign. The previous RF EE that designed the system was not local. He made the system, design, and test procedures.We just did not do the test procedures in a noisy environment.