Author Topic: Extended distance reading of RFID pet tags  (Read 1271 times)

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Online Neomys Sapiens

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Extended distance reading of RFID pet tags
« on: December 11, 2017, 11:52:12 pm »
Has anyone tried how far the usual pet tags can be read reliably and what circuitry to use for going beyond direct contact or more than 20 cm?
 

Offline mikeselectricstuff

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Re: Extended distance reading of RFID pet tags
« Reply #1 on: December 11, 2017, 11:58:46 pm »
Inverse square law means you can't get a significant increase in range.
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Offline james_s

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Re: Extended distance reading of RFID pet tags
« Reply #2 on: December 12, 2017, 12:16:40 am »
You could probably extend the range of the excitation but even then you're going to be relying on the transmission from the tag. I suspect a clever design with a very sensitive and highly selective receiver might double the effective range but that's still not very much.
 
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Offline mikeselectricstuff

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Re: Extended distance reading of RFID pet tags
« Reply #3 on: December 12, 2017, 12:33:56 am »
Pet tags use a rod antenna which is rather orientation sensitive - you may be able to at least  improve consistency of read range using multiple antennas in different orientations.
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Offline evb149

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Re: Extended distance reading of RFID pet tags
« Reply #4 on: December 12, 2017, 02:04:46 am »
What motivates you to wish to do this?

As others have said, it is not particularly feasible to do beyond XX centimeter ranges because of the low frequency, low mutual inductance, and non-radiating signal.  The signal is really made by the mutual inductance of the two coils with one of the coils having only a ~4mm max. or so long ferrite rod inductor as the antenna.  Even with the optimum mutual orientation of the antennae your signal coming from the transceiver will fade rapidly with distance. 

It is not a propagating (in the Maxwell EM) sense RF field, just mutual dipole induction (at best), so it (mutual inductance) attenuates as the radius cubed (not squared) between the coils.

The optimum monostatic antenna configuration is considered to be dimensioned so that it achieves a suitable field and sensitivity over a distance approximately equal to the radius of the equipment's driven coil.  So you can see that beyond XX cm or 1XX cm it is not particularly practical to do except perhaps with huge antennae.

With optimized circuitry and some DSP averaging I suppose you could improve the SNR at the expense of time but still I think you will be pushing to get 1xx cm distances not much beyond that.  It does not help that the remote transceiver has to accumulate substantial actual power from the driving field so it isn't just a question of needing a sensitive receiver, you also need quite a strong field at a distance from the transmitter AND a very sensitive receiver for the low mutual inductance.

Multiple transmitting / receiving antennae (or a moving one) could be helpful if feasible.

 
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Offline Decoman

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Re: Extended distance reading of RFID pet tags
« Reply #5 on: December 12, 2017, 11:19:37 am »
I can't answer OP's question, though having spent a couple of years following computer security related news, this RFID thing had me wondering some time ago, with RFID technology even being incorporated with my new credit card (my bank told me that one can choose to have the featured disabled for my credit card, whatever that might mean), making me wonder if maybe law enforcement might one day, if not already, find a way to interrogate RFID things with some kind of directed beam of sorts from a distance.  Afaik, there is also a known risk of having your credit card read off, with use of RFID tech by passerbys, and so, you can get products that aim at preventing unauthorized access to your RFID enabled card (maybe best to check and test this efficiently instead of blindly trust a seller selling such things).

I am also reminded, though perhaps not even related technically,  of what is said to be documented spying tools by some infamous agency, where they are known for having technology where an emitter of sorts is hidden inside your monitor's power cable, and then, from a distance, the gimmick seem to be to use some kind of emission to interrogate and read off data from your monitor cable. An interesting aspect of that again, might be how such radiation could perhaps be harmful one way or another.
« Last Edit: December 12, 2017, 11:22:16 am by Decoman »
 

Offline Rerouter

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Re: Extended distance reading of RFID pet tags
« Reply #6 on: December 12, 2017, 11:41:33 am »
inverse square laws mean that for every doubling in distance you need to double the TX power, and double the sensitivity of the receiver,

You can help yourself out here by increasing the signal coupling as high as possible before you try extending the distance, e.g. a coil around a cat flap may allow that high coupling when the cat passes through it, however skin and muscle is a fairly good 125 KHz attenuator.
 
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Offline james_s

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Re: Extended distance reading of RFID pet tags
« Reply #7 on: December 12, 2017, 07:24:39 pm »
If you're referring to what I think you are about the monitor cable, it's not involving any sort of hidden device in the cable, rather it's possible under some circumstances to pick up the RFI emitted directly by equipment, particularly CRT monitors and potentially reconstruct the image. It's not too hard to see how this works, an AM radio placed near many digital devices will pick up various noises depending on what the device is doing.
 
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Online Neomys Sapiens

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Re: Extended distance reading of RFID pet tags
« Reply #8 on: December 13, 2017, 01:53:05 am »
Thanks to the posters for even replying to my very rudimentary post. I was just trying to make sure that I did not oversee a heplful thread, where someone has figured it all out. Inverse square law is obvious, and no - I do not want to cook the cat! Intended application is a individually selective cat door, of course.
A friend asked me for a beam-of-light solution and I was wondering if anyone had build a contraption based on the tags that our little beasties already carry under their skin,
 

Offline james_s

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Re: Extended distance reading of RFID pet tags
« Reply #9 on: December 13, 2017, 07:16:06 pm »
I think it would be difficult, but not necessarily impossible. If you had a small tunnel the cat had to walk through to get to the door then you could place a coil where it would likely be passed under by the microchip, usually the location of the chip is relatively consistent from animal to animal.

If the cat will wear a collar then it would give room to attach a stronger RF source. These days I wonder if it would be possible to use a camera to do image recognition to identify the cat, certainly in the not too distant future this will be feasible.
 
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Offline mtdoc

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Re: Extended distance reading of RFID pet tags
« Reply #10 on: December 13, 2017, 08:13:25 pm »
Intended application is a individually selective cat door, of course.

They already make these.  I recently bought this one and it works great.
 
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Online Neomys Sapiens

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Re: Extended distance reading of RFID pet tags
« Reply #11 on: December 13, 2017, 08:41:07 pm »
Those are known. The point was to make it work with the subcutane tags of the pet registration scheme.
 

Offline mtdoc

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Re: Extended distance reading of RFID pet tags
« Reply #12 on: December 14, 2017, 01:00:23 am »
Those are known. The point was to make it work with the subcutane tags of the pet registration scheme.

Apparently unknown to you since that is exactly what that one does. There are other brands as well.

They read the pet RFID tags placed under the pets skin, and are programmed to recognize your unique pet's RFID tag and only open in response to it.  Recognition of multiple unique tags can be programmed as well. The distance between the cats subcutaneous RFID tag and the coil in the cat door does not seem to be an issue. It works well.
 
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Online Neomys Sapiens

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Re: Extended distance reading of RFID pet tags
« Reply #13 on: December 14, 2017, 01:11:10 am »
Sorry, I obviously oversaw that. I did call up the link and saw a picture of tags, which should be worn on a collar and discarded the tab. Extra thanks for your persistence in delivering the info!
 

Offline mtdoc

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Re: Extended distance reading of RFID pet tags
« Reply #14 on: December 14, 2017, 01:19:05 am »
Yeah - I guess if your cat does not have an implanted tag you could put one of those tags on their collar.

Personally - I only use break-away collars on my cat - which eventually do their job and come off - so it's nice not to have to worry about that.
 

Online Marco

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Re: Extended distance reading of RFID pet tags
« Reply #15 on: December 14, 2017, 01:30:09 am »
There was a demonstration of a long range rfid reader at a hacker conference a few years back, blackhat I think. They used a suitcase, with a coil the full size of it. That's the way to increase range after fieldstrength/noise become limiting factors, a bigger coil.

Long range of course being relative. Still only on the order of a meter.

Ps. Google for tastic rfid thief.
« Last Edit: December 14, 2017, 01:41:24 am by Marco »
 

Offline mikeselectricstuff

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Re: Extended distance reading of RFID pet tags
« Reply #16 on: December 14, 2017, 07:47:43 am »
There was a demonstration of a long range rfid reader at a hacker conference a few years back, blackhat I think. They used a suitcase, with a coil the full size of it. That's the way to increase range after fieldstrength/noise become limiting factors, a bigger coil.

Long range of course being relative. Still only on the order of a meter.

Ps. Google for tastic rfid thief.
I suspect that was for reading much larger tags than the pet ones. Achievable range is a function of the size of both antennas
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Online Marco

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Re: Extended distance reading of RFID pet tags
« Reply #17 on: December 14, 2017, 04:07:41 pm »
There is one other trick you could do, you could read the RFID in the airgap of a solenoid. Lets you concentrate the field lines more and spend less power heating up anything conductive near the coil.
 

Offline NivagSwerdna

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Re: Extended distance reading of RFID pet tags
« Reply #18 on: December 14, 2017, 04:25:04 pm »
I have a cat flap which reads my feline's chip and opens the door accordingly.  The device (which I had an early production model and have recently replaced with a current model due to solenoid wear)... uses big coils but works very reliably.

Not exactly sure where in my cat the chip is... presume somewhere around scruff of neck but distance is probably around 30cm

I'll take  a picture of the internals if you are interested.. it's quite a simple PIC based thing.



The bit that sticks out has the coils... I actually took the coils out and remounted them above a bit of plywood tunnel due to the nature of the wall where I installed it.

 

Offline james_s

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Re: Extended distance reading of RFID pet tags
« Reply #19 on: December 14, 2017, 09:30:17 pm »
Normally the chip is placed on one side near the base of the neck, you can usually feel it if you feel around, it will be a small hard bump under the skin.
 


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