# Up to 25 meters range passive RFID?

In this document, we can read:

Passive:
Read range from near contact up to 25 meters

about UHF RFID. This is impressive indeed.

Which kind of passive tags can be found up to a 25 meter distance?

What kind of reader is necessary? All I've found are 300-1500\$ reader devices on eBay, so it's not really available for electronic-enthusiast DIY projects.

Is this technology nowadays available with small unexpensive modules?

(NB: I'm not asking about shopping recommendation, but only to know if this technology really exists with a 25 meters range, and if it's really available for home DIY projects, or if it's still a professional high-end technology that requires high-price investment and/or weeks of engineering to make it work)

(source: atlasrfidstore.com)

• I'm completely shocked by the claim. 25 meters out to power the RFID? And then 25 meters back, with the RFID powering a transmitter to do that? Count me on the skeptical side of this, for now.
– jonk
Nov 24 '19 at 20:28
• maybe they meant to use feet instead of meters ... here is one that looks more reasonable ... cdn.barcodesinc.com/themes/barcodesinc/pdf/Motorola/… Nov 24 '19 at 21:07
• what is the antenna power gain? a many-element YAGI might do this? Nov 24 '19 at 21:10
• @jonk, yep RFID has a lot more uses than the half-an-inch-away card readers and car key immobilisers. I used it for detecting cash boxes. It's used for tracking parts in factories over a large area, identifying lorries, all sorts. A quick delve on the internet will find you plenty of info'. Nov 24 '19 at 21:29
• Can you compare the "mobile phone", 20 years ago (size and weight) ... with GSM now ? And yet, they use "nearly" the "some power" ( ~ 2 W) Sep 28 at 7:38

Which kind of passive tags can be found up to a 25 meter distance?

Not too many. To be able to extract data from a tag (ignoring passively powering it) you would need the tag to "reply" using a proper radio technique such as UHF. To power a passive tag you can use either a radio waves (such as UHF) or by NFC (using magnetic coupling).

If you want to know about powering a passive tag (at distances of 25 metres or so) using UHF techniques, then you would need a directional transmit antenna to concentrate the radio wave power towards the tag. If you try and use an omni-directional transmit antenna (like a dipole or monopole) then you will transmit a lot of unused power into space. Think about how much light power a lamp produces and what miniscule fraction of that light energy is received by the eye.

But, if you are interested in exploring powering NFC tags (at distance) then, this is what my answer concentrates on. It's all about gleaning as much magnetic energy as possible to produce sufficient DC voltage to run a tag for a few tens of milliseconds.

What kind of reader is necessary?

Regards the tag responding to the reader, that's more problematic; at distances greater than a metre or so, it can probably only be accomplished reliably using regular UHF transmission.

Maybe also read this Q and A on UHF RFID Read Ranges - it covers the types of RF power levels and antenna needed to energize a tag at distance.

In the main...

This answer is about how you can acquire sufficient energy to power a tag using the magnetic near-field. If tag and reader are close, then remodulation of the near-field to respond to the reader is viable. But, firstly you need to power the tag.

What the blog article doesn't say is important

What is not disclosed in the article are any details about the transmit and receive coils. For instance, in the plane of a 10 cm radius coil carrying 1 amp, the B field (magnetic flux density) at the centre of a single-turn loop will be 6.28 μT (micro teslas) and this is certainly a level that can be turned into electrical energy that can power a passive tag device quite easily. This is easily established as is shown further down.

How much flux density is needed?

At 15.5 cm distant from the above loop, the B field will drop to around 1 μT and we're probably on the edge of being able to convert that field density into usable volts to power a passive tag with a small sized receive coil so, take that as a benchmark when reading the rest of this answer based on my experiences.

A bigger transmit coil and more current

If the radius of the reader's transmit coil became 1 metre and, the loop current was increased to 10 amps (easily done using capacitive resonance without significant power losses) then, the distance from the reader's transmit coil to achieve 1 μT flux density is now 1.55 metres.

if this technology really exists with a 25 meters range, and if it's really available for home DIY projects, or if it's still a professional high-end technology that requires high-price investment and/or weeks of engineering to make it work

As a comparison, a 10 metre radius transmit coil with 100 amps flowing will produce a flux density of 1 μT at a distance of 15.5 metres. Now 100 amps sounds a lot (and it is) but, with high grade capacitors and a decent driving circuit it can be done (but it ain't cheap).

A little bit about the calculations I'm doing

The following picture is an extract from a web-based calculator on HyperPhysics: -

It's definitely worth having a play around with it.

Receive coil size and number of turns

Up until now, I've regarded the passive tag as having a "small sized" receive coil but, there's nothing in the blog that suggests this is the case (other than the assumptions we might make). What I'm getting at here is that when we calculate B (magnetic flux density) it's just a number we can use for making a comparison. It's the total flux through the tag's receive coil that is important when liberating enough energy to power the device.

So, if a small sized receive coil was assumed to be 10 cm radius, we could easily convert to "area" and make a not-unreasonable assumption that B was fairly constant over the whole coil area and, calculate the voltage induced. Then, with a decent tuning capacitor that voltage could be magnified by about 30 times (experience allows me say this), rectified and used to charge a capacitor to power the tag for a short time (a few tens of milliseconds).

Above picture from this related Q and A

But, if the tag's receive coil was 1 metre radius, we would have a capture area that is 100 times bigger hence, the induced voltage would also be 100 times bigger. If the receive coil also had ten times more turns, the received voltage would be 1,000 times bigger. So now, we can "adjust" our expectations of a viable B field amplitude and assume that it can be 1,000 times lower at 1 nT. This is now the "new" flux density benchmark in this answer.

So, a 10 metre radius transmit coil powered with 100 amps produces a flux density of 1 nT at a whopping distance of 184 metres!

Sanity check on transmit coil current

But, we know we don't like to run 100 amps through the transmit coil so, if we dropped that back to 10 amps (perfectly feasible without Litz wire or hyper expensive capacitors), 1 nT is present at 85 metres distant (still a very reasonable distance): -

We might want to reduce the transmit loop size a bit to maybe 5 metres radius and the distance that we get 1 nT flux density is now 53.7 metres. With a 2 metres radius we can get 1 nT at a distance of 29 metres.

Does 1 nT and a single-turn 1 metre radius receive coil cut the mustard?

• The coil area is $$\\pi r^2 =\$$ 3.14 square metres.
• 1 nT at 29 metres from the transmitter will be constant across the coil aperture.
• Hence total flux at receive coil is 3.14 nano webers
• If we operated at (say) 1 MHz, $$\\dfrac{d\Phi}{dt} = 2\pi\times 10^6 \times 3.14 \times 10^{-9}\$$
• This is 19.7 mV per turn hence, ten receive coil turns produces about 0.2 volts RMS.
• This is a decent level for extracting energy to power a tag.

Conclusion

• So, is a 2 metre radius, single-turn transmit coil carrying 10 amps all that unreasonable - not in my book. It's a perfectly normal transmit coil in many applications.
• Is a 1 metre radius, ten-turn tag coil all that unreasonable in some applications - I don't believe so. It's a less common scenario but there will be some applications.
• Does the blog article say anything useful about coil sizes and currents - not really.

• Hmm can the down-voter explain his or her reason? Oct 4 at 11:44

Sure this technology is available today, and the 25m is nothing compared to RFID application that were implemented back in the days, but it will require a lot of power.

In the '40 RFID was used to identify friendly aircraft by radar stations, by the way the idea of "powering something" from distance is very old in electronics and we can count lots of examples that range from spy equipment to space exploration.

RFID, like many other inventions it's "old stuff". Never confuse innovation with advertisement, and in recent years lots of old technologies are selled as new (read: old with name changed), with the sole purpose to create attention and sometimes just because a technology that were unadapt is now viable for the techno-environement.

• Thank you for your answer @gino. but it will require a lot of power can you elaborate on this? If we want, for example, RFID that can be detected in a maximal 10-meter range, what kind of power is needed? More generally, what's the highest range we could achieve with a consumer-level home product (let's say less than hundred dollar equipement - note that I'm not asking for a shopping rec, but it's just to give an idea of the kind of technology)? Is 10 meters possible with consumer-grade products?
– Basj
Sep 27 at 15:11
• I think referring to "consumer level product" is not a good way to evaluate, let me explain: your problem is power and a consumer level device connected to the mains can have let's say 1000W power (es. like a power drill). You have to think at the problem with the correct point of view, imagine a transformer but you take your windings in open air (not iron core that help flux) and at distance.. you will have a very bad transformer, this is your RFID basic idea. No idea for the power needed i need to do some calculations.
– gino
Sep 27 at 15:15
• Would this could be a "good" start ? electronics.stackexchange.com/questions/82938/… Sep 28 at 13:04
• Very intriguing. Do you know any of specific passive RFID chip (IC) that can work at 25m range?
– jay
Sep 30 at 20:25

At UHF you would go close to achieving 25m with a passive SAW device. I did some trials on some of these some years ago and we were reliably reading tags in cars on freeways with modest antenna gain. I would guess we were detecting reliably at 15m. A bit more antenna gain / more power / better tag would have achieved 25m.

A SAW tag is completely passive, there are no active devices that need to be powered, so they have the potential to operate at ranges higher than systems where some semiconductor device needs to harvest power from the reading signal.