Strange RFID reading distance dependence on a piece of wire attached to ground - Page 1
Forum section: electronics / beginners

author: dmitrych
author_rank: Newbie
timestamp: October 23, 2022, 10:03:16 pm
content: Update: seemessage #3for the up-to-date description of the problem.Below is the original description.I'm using this RFID reader, and it's a bit finicky about the conditions required to reach the advertised reading distance:https://www.aliexpress.us/item/2251832671086419.htmlThe best reading distance I was able to get is 37 cm, when the reader is powered from batteries or a supercapacitor AND there's a piece of wire about 1m long attached to the *ground* of the reader. The wire doesn't need to be connected to anything on the other end.Without the wire, the reading distance is only about 27cm.When instead of the battery I try to power the reader from a power supply through a very low noise LDO regulator, the reading distance is also 27cm without the wire, but it increases to only about 31cm when I attach the wire.This dependence on the piece of wire doesn't make much sense to me. Could the wire be acting as an antenna of some sort?I hope that explaining this behavior might lead to a better solution.The reader creates about 9 mV of 125 kHz noise on its power input when powered from a battery. I tried adding various capacitors between ground and V+, in addition to the capacitor supplied with the reader. This reduces the noise, but doesn't seem to improve the reading distance.In my case, increased reading distance significantly improves the usability in the finished project.My ideal goal is to get the 37cm reading distance when powered from the power supply.I can "sort of" achieve this goal with supercapacitors, and with optocouplers for the TX output of the reader (to isolate the ground of the reader so that attaching the wire achieves the 37cm reading distance), but this is so hacky, I'd prefer a better solution, at least for my own understanding.What else can I try?

author: Manul
author_rank: Super Contributor
timestamp: October 23, 2022, 10:44:16 pm
content: I would say that a wire attached to ground should not significantly affect loop antenna at 125khz. But you say it does. Maybe post a picture?I think to get better range you should fine tune the loop. If you just wind a coil, big chance it is not resonant exactly where it should be.

author: Bud
author_rank: Super Contributor
timestamp: October 23, 2022, 11:18:55 pm
content: The ali page you linked has a video from which it can be seen that different rfid tags have different reading distance. What do you use as a test tag device?

author: dmitrych
author_rank: Newbie
timestamp: October 24, 2022, 02:47:32 am
content: I believe I'm using the same type of card that gives the best reading distance on the video.Also, sorry, I realized there's more layers to this problem, and it's slightly different from what I described in the first message.The simplest setup with just a battery-powered reader works perfectly with 37cm reading distance.As soon as I connect *just the ground* of a power supply (plugged into an outlet) to the ground of the reader, the reading distance drops to 27 cm. After that, the reading distance increases to 31 cm if I attach the ground to an oscilloscope probe (not connected on the other end), or to a USB cable, or to a roll of magnet wire.See the photos attached.I tried this with several different power supplies and the results are similar.So the main question is why does this happen and how can I power the reader from a power supply without losing the reading distance.Out of interest I actually built a whole circuit that isolates the ground of the reader from the ground of the power supply (with supercapacitors, reed relays and optocouplers), but somehow that didn't solve the issue at all, maybe because the ground of the power supply is close enough on the PCB to still cause interference, or maybe I'm missing something else.I hope there's a much simpler solution that I don't know about.

author: Bud
author_rank: Super Contributor
timestamp: October 24, 2022, 06:01:59 am
content: If it were me I would search for the datasheet of the IC used to check information on the output impedance of the transmitter, then measure the antenna impedance, and if required tune the matching circuit for a good match using a VNA. You may never get a sound explanation for what you are observing if the antenna is poorly matched to the transmitter. All kind of stray RF may add or substruct producing all sort of effects.To isolate the power supply you can try a common mode choke on the supply wires. Get a EMI supressing ferrite from an old computer VGA cable or a snap-on one, and wrap the supply wires coming from the board making 3-4 turns through it before the power supply connector.