Short range but wide area antenna design?

In that case, please consider a detailed review matrix of them. The community could benefit from something like that, me thinks.

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Ah! The friggin’ post office just texted me that my long-range RFID readers have arrived. About time, I ordered them almost 3 weeks ago…

So I’ll toy with them this evening when I have time, see how well they read the EM4xxx in my right hand. Then I’ll know if my butt implant idea is worth pursuing.

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I’m messing around with a long range reader myself. A UHF one. It’s going really well. It can the sticker tag through my hands (sandwiched between both hands) from about a 20 cm range. Without my whole hand in the way it has several metres range.

Given it is a flat, flexible tag already I wonder if it could be encapsulated in the same way the custom payment card’s are done.

Yes, I’ve been talking with a tech from a company that makes UHF readers, and he said flesh severely impairs the read range. He said they did some tests a few years ago, to validate the concept for tracking cattle, and they ended up abandoning the idea.

The readers I ordered are 125kHz though. Transformer coupling doesn’t really care about your wet bits being in the way. Also, there ain’t no UHF implants on the market - that I know of.

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A foot of range from 20cm away through the full thickness of my hand is decent.

There is one but it’s designed for lab animals. Once I have had a bit more of a play with the tech I might see if DT can make one for me as part of there custom tag conversion service. Maybe.

Okay; I’ve done some tests with the two long-range LF readers I received yesterday. The first one is the ACM08Y, and the second one is the ACM26C - both from Goldbridge Industrial in China.

This is gonna be a mini-review of those devices, with an emphasis on performances with implants.

First of all, they’re unbelievably heavy. That’s because they’re big plastic half-clamshells with the coil and the electronics put inside, then filled up to the brim with epoxy. All that epoxy ain’t light.

Secondly, they don’t come with a plug. I can understand if you order the Wiegand versions, since those things are usually connected to an alarm system with screw connectors. But I ordered the RS232 versions. A DB9 and a power socket would have been nice. But cheap Chinese products are cheap for a reason :slight_smile:

Thirdly, they’re friggin’ loud - and I mean LOUD. They both have a tiny buzzer embedded in the epoxy, and they make a terrible racket when they self-calibrate after powering up. Then they emit a powerful beep each time they scan something. I covered up the buzzers with 5 or 6 layers of paper labels, but they’re still annoying. I’ll have to see about filling them up with epoxy or something, to shut them up permanently, as there’s no way to access the leads underneath without destroying the potting. Ironically, both have a line out labeled “buzzer”, so the manufacturer made provisions to make them even louder.

As far as I can tell, they only read EM4xxx chips. I’ve tried a few different other chips, but they don’t register. Fair enough, I didn’t ask for more.

Annoying, they’re single-shot readers. The distributor assured me they were repeating readers, but they’re not. When they read something, they send the UID once, until you remove the transponder from the EM field and present another one.

There is an RS232 RX line, and I seem to remember seeing a video from the Chinese manufacturer on Youtube in which the device was beeping continuously. So there’s a possibility that you can send them a command to put them on autorepeat, or poll them. But they don’t seem to react to anything I send them. So if they do support commands, I’ll probably need the command set from the manufacturer- that is, from a Chinese manufacturer… good luck with that…

Re range: the ACM26C has a square coil, while the ACM08Y has a round coil. But both have pretty similar performances. They both put out a torus-shaped EM field around their coil. Being a torus, there’s a hole in the middle where the field is weaker.

Transponders that have a decent antenna (full-size cards and tags) “see” a horn torus and get picked up in the hole. Not so with implants: they “see” a much slimmer ring torus. If you approach your hand dead-center above the devices and get closer, nothing happens right up to the surface of the device.

The torus an implant “sees” has a circle radius of about 4" if the implant is oriented correctly, meaning if you approach your hand anywhere around the coil within 4" of the coil, the implant will register. When I say “oriented correctly”, I mean that the implant should be perpendicular to the plane of the coil for best performances, as expected with Ampere’s law. If the implant is coplanar with the coil, the range is greatly reduced - 1" at the most.

So all in all, as far as implants are concerned, think of the two ACM devices as oversized desktop readers: they work exactly the same way with the same limitations, only with a longer range, and the added bonus that you also get a solid read if you present your implant edgewise - something desktop readers and their flat PCB coils don’t do.

Even with a limited range, an implant under the skin can be picked up reliably at close range within a 6 to 8" band strip around the coil provided it’s oriented correctly. That’s not half bad, and it’s probably suitable for my butt implant project. I’ll have to give some thought on implant placement (probably just under one of my cheeks, oriented along the leg). More importantly, I’ll have to get the devices to autorepeat for the project to make any sense. But at least as far as detecting an approximately-placed implant is concerned, the devices seem to fit the bill.

Of course, they would work even better with a FlexEM: the FlexEM-sized tag I tried is picked up anywhere over the device, and a good 20" off the surface. That would make placement even easier. But somehow I don’t see myself sitting on that thing all day :slight_smile: Maybe implanted behind my back just above one my cheeks or something, where I don’t sit… Amal, any thought?


not sure how forgiving this is, but it may help with part of your issue.

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I have one on order to test it. It would help detecting when I leave my seat. But it’s a bit overkill, when the RFID reader could tell me - if only it could be set to autorepeat.

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How long does it take to power up? It may be a real hacky idea but you could set up an arduino or pie to cycle power on the reader each time it scans a chip.

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It takes about 5 seconds to self-calibrate - which it expects to do without anything in range I would guess - and also a good 5 seconds to power down completely: apparently there’s a big cap in it that keeps it alive for a while, and causes the device to spew out a bunch of garbage on the serial line as it browns out.

But aside from the time it’d takes to power-cycle it, and the issue of filtering out the garbage, it wouldn’t solve the problem: it only sends a UID when a transponder “comes into view” so to speak. So if the transponder was already there at power-up, it won’t say anything until the transponder is taken off the EM field and brought back in it.

To say nothing of the racket it makes when it self-calibrates. I’m not going to stand that several times per minute :slight_smile:

I’ve shot an email to the manufacturer. I’m not expecting any useful replies from them - if at all - but who knows… I’ve dealt with a few Chinese companies that reacted promptly and didn’t send boilerplate marketdroid replies to my technical questions. Maybe I’ll get lucky with this one.


I’ve used readers that had automatic beeps and a BUZ pin. Usually they provide that so that the user can control the buzzer, but if it’s left floating it defaults to the automatic sounds. Try pulling the BUZ pin high or low with respect to the input voltage.


Oh right, I didn’t think about it being an input line. Silly me :slight_smile:
Well, it’s all soldered up right now and I can’t be assed to open it up again. I’ll do that tomorrow.

Thanks for the tip!

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Man I dunno here hahah I’m just all like…

giphy (1)


Is ANYBODY going to touch that???

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Hmm well, to be more specific, what I meant to ask you was: do you think the FlexEM would be too stiff in a soft and floppy skin area on which I sleep for hours every day? Would the implant risk cutting into the flesh from the inside with the pressure and movements during sleep or something?

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Yeah it’s a rigid disc. Zero flex. How it feels is totally dependant on your “floppy tissue” :wink: My hunch is that there will not be a lot of tension and that’s the biggest risk. I’d guess that the lower cheek just above the thigh and below the butt crease would be a good spot because I can’t imagine a movement that would cause a lot of tension.

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Ok thanks. I think I’m gonna give it a pass: even if it’s safe and it won’t hurt me - and that’s a pretty big if - I’m afraid it might create a nasty feeling in my backside when I’m in bed.

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Would likely have similar concerns but what about the upper back. Would depend on chair design but could work.

Small update on the long range readers: I tried them again yesterday evening at home, as far away from metal bits as possible, and it makes a world of difference in terms of reading range. At the office where I had first tried them, there was metal strips under my desk - about an inch behind the reader. That seems to be enough to kill the range.

So, placement seems to be critical. But if you get it right, it can be pretty effective.


If you could find a good spot under your desk would thigh or calf placement be an option?