[This thread was created from an older thread, here. —Moderator]

I was hoping to find something like the impossible as described. But I think the issue here is NOT impossible. The original questions was limited by seaking a specific way to solve the problem. Basically it looks like the post started because the guy wanted to use radio waves to find a lost ball (or similar object) when he was near it. I have the same issue with my constantly lost TV remote. The answers hear make it clear that having it make audible beeps is impractial to the point that impossible might be the correct word. BUT somewhere you mentioned RFID tags. As I understand them, they make a 'beep' in radio frequency. Ages ago in the early Spider Man TV show, the special effects used let Spidy drop a 'bug' on people to track them. In the TV show, Spidy had a hand held gizmo that showed him the direction of the 'bug', but not the distance. My next step is to learn more about RFIDs to see if the TV show 'bug' really works that way. I have seen dog trackers for sale that CLAIM to do exactly this over outdoor distances. I think they will work for my application because I think the lost remote will always be with a 3x3 meter sized room. And it should be easy to just go in the direction the RFID return signal is strongest. (OK easy is probablly not correct as I have no idea how sensitive the reciever for the returned signal is but....)

If you have a system that will make the ball like object 'beep' in the radio spectrom, and have 'bionic ears' to give sterioscop signals to earphones. I think the guy might be able to buy all the parts for SYSTEM 1) rfid 2) radio transmitter that causes the rdif to 'beep', 3) a network of repeaters for that beep covering the area to be searched and 4) something that converts the info from the network into sound or picture or other human interface to let the guy find the ball. There is no real requirement for the 'beep' to be audubly generated at the ball is there?

 

Welcome to AAC.

You have a good point concerning the nature of the original question. This is a common problem: a naïve solution is settled on for some problem and it quickly becomes the problem itself. Starting from a clear description of the problem to be solved is the only way to a quick and well crafted solution using expert help. As I often say to clients: you are the expert on your problem; if you were an expert on the solution I wouldn’t be here—so let’s talk about the problem you had before you set out to solve it.

That said, your limited understanding of the issues with your proposed solution is leading you astray. In fact, you‘ve made things worse. While the original poster underestimated the power requirements to achieve something useful concerning locating a device, you‘ve jumped all the way to making the device passive.

RFID is a wildly useful technology and at least some variants allow for a decent range when reading tags. But this doesn’t solve the problem of location. Take a look at one of the smallest, most practical location tags available: Apple‘s AirTag. The AirTag works very well in terms of the sort of location you would like to do—but it’s much bigger than an RFID tag, with a large Lithium coin cell as a power source.

While compared to a device like a smartphone, the AirTag uses very little power, it does rely on its CR2032 cell with a capacity of about 240mAh. Contrasting this with an iPhone 15‘s LiPo cell with its ~3,300mAh capacity makes the low power operation of the AirTag clear. Nonetheless, the CR2032, with its ⌀20mm and 3.2mm thickness is already twice the volume of the largest passive RFID tags (about ~1000mm³ for the CR2032 vs. ~450mm³ for the passive tag).

So, if we need to use a powered tag we’ve already got a size requirement approaching or exceeding the maximum acceptable for the application. The problem lies in the nature of precision location via RF, and the specific technologies available* and applied. *regulatory limits and industry standard will always limit the sorts of practical technologies that rely on radio emissions

Let’s take a high level look at how an AirTag actually works. This will provide some insight into the problems facing a passive solution as you are suggesting. The AirTag is primarily based on two RF technologies: BLE (Bluetooth Low Energy) and UWB (Ultra Wide Band).

BLE is designed—as its name suggests—to provide very low power operation. BLE devices powered by very small Lithium cells have astonishingly long life. BLE operates with exceedingly low RF output averaging about 10µW! To really understand this, 10µW is .01mW, or 0.00001W! It is truly tiny.

In standby mode a BLE device will consume ~2.5µA (0.000025A). To put this in perspective, the self discharge rate or a typical AA cell* is about ~2%/year, this translates to ~6µA or about twice the consumption of the BLE device. *the AA cell will be an alkaline chemistry while the Lithium-based CR2032 has a much lower self discharge—about 1/10 of the BLE standby current at ~.25µA

On the other hand, when actively transmitting (either in advertising or interacting with another device according to the Bluetoorh protocol, the BLE device will consume as much as 15mA! Again, for perspective, 1mA is 1000µA making 15mA 6000 times more than the standby current of the device. BLE transmits with power levels between -20dBm and +10dBm which works out to 10µW to 10mW.

So despite its tiny power requirements the zero on-board power needs of an RFID tag is infinitely smaller. Te other technology, UWB, requires a lot more power—from an idling low of about 100µA to an actively high of about 100mA. This is difference of three orders of magnitude, or 1000 times. It is important to keep a perspective on how different µ-, m-, and base units are. It is easy to miss the impact of these prefixes.

On the other hand UWB uses a truly astonishingly low RF power output. The exact power is dependent of the total bandwidth in use. To understand this we need to talk about how UWB works. The “Wide” in UWB refers to its spread spectrum nature. UWB spreads the transmissions over an enormous bandwidth. FCC regulations require a minimum of 500MHz.

But UWB devices can use much more that that. UWB operation is in the 3.1 GHz to 10.6 GHz band and it is possible for a UWB device to use all 7.5GHz of this. In practice, UWB devices will genreally operate with between 500MHz and 1GHz of bandwidth. Aa a cogent example, Apple’s U1 chip—the silicon responsible for the precision location of AirTags—operates from 6.24GHz to 8.24 GHz so it has a 2 GHz bandwidth.

Takin this example we can now figure out the maximum transmit power. The FCC restricts UWB to -41.3 dBm/MHz (75nW or 0.000075 mW per MHz). The reason for this is that since the power is spread over the bandwidth no one frequency will see more than the 75nW but the overall power will depend on the actual bandwidth.

To use our AirTag example, if we assume maximum power and use the 2GHz (2000MHz) bandwidth, the the total power will be 150µW which is 15 times the output power of BLE. The reason that UWB uses much more power is tied to its precision location ability. UWB uses a technique called time of flight (ToF) to determine the distance to a tagged object.

This is not hard to understand in principle: the locating device calculates how long it takes for a transmission to reach a tag by examining the timestamps in the response. So, the locator sends out a poll message and the tag responds—after a known, fixed interval with a timestamp representing the time of reception. In a full exchange, a third message adds more certainty but it isn’t required. AirTags do use the full protocol.

The locator takes the information in exchanged messages and calculates the ToF using \[ \text{ToF} = \frac{(T₄ - T₁) - (T₃ - T₂)}{2} \]ToF is converted to distance using \[ \text{distance in m} = \text{ToF} \times \text{C} \text{ where C is the speed of light (~299,792, 458m/s)} \]

If you are paying attention, you will notice the intense critically of precision timing to the process. Everything depends on it. Anything that would interfere would reduce accuracy or simply make the scheme unworkable. This means ToF is not feasible for a passive device like an RFID tag. Immediately, the startup time for a tag depending on being powered by an RF transmission is indeterminate and so it will not be possible to create a set of packets that can reliably provide timing information.

And, even if you worked out scheme to overcome this, the precision and resolution of the position information is directly proportional to the accuracy of timing which will necessarily be very poor compared to the speed of light and so far exceeding the dimensions of the small room that seemed at first to be an advantage.

ToF works very well for distance a single ToF datum isn’t useful for determining bearing. To get this, UWB uses AoA (Angle of Arrival). AoA uses mulitple antennas and calculates the differences among the arrival times to get an angle. In practice, this is accomplished by comparing the phase angle of the received transmissions. This takes advantage of knowing the precise frequency of the waveforms involved and comparing how far off they are from each other when overlaid.

Combining ToF and AoA produces a nice vector which can be displayed as a pointing arrow for the user to follow. This vector is a combination of magnitude and direction. Something that will come up a little later so keep it in mind.

RFID technology just can’t match UWB. Though there are active RFID tags specifically designed for long range use, as soon as with throw in a power source all the potential value of an RFID tag is gone, just use UWB which is designed for the application. Now you mentioned “repeaters” and there could be something to that.

Not as repeaters, per se, but as receivers. In traditional RDF (Radio Direction Finding) a version of AoA is used. This can be done with a goniometer, single array of antennas that are switched quickly so phase angle measurements can be made by comparing the signal with each antenna or it can be done by comparing the RSSI (Received Signal Strength Indication) from three or more receiver sites—called triangulation.

One receiver will get you a magnitude, and if you can somehow* calibrate this, you could get a distance that lies on a circle around the antenna. Two receivers can place the target on a line, and if calibrated at two points on the line 180° apart. *this is not trivial and anything changes to the environment would throw it out the window

With three receivers it is possible to triangulate and, simply by observing the relative signal strengths isolate the location to a triangle of uncertainty with a size depending on a number of factors but generally a useful fix. So, in principle you could possibly put three receiving nodes in the room and use them to triangulate the location of a responding RFID tag.

But there’s another problem—the range of RFID tags. Because passive tags rely on the received radio transmission for power, and radio follows the inverse square law. This means the power of a radio signal loses 75% every time you double the distance.

As a result, while common RFID tags operate with relatively low power and short distances, long range tags require a much larger signal with the maximum power being about 4W. Make no mistake, 4W is a lot of power to be squirting around the media room and adverse outcomes are likely.

But, even if the power is cut down to 1W (still a lot) you have the problem of needing infrastructure—the three receivers—and their attendant cost and complexity. At that point finding your remote is looking like it’s going to cost more than buying several extras so there is always one in sight.

So where did you go wrong? You made a flawed analogy. The utility of “beeping” is entirely dependent on the free direction finding gear we have in the form of our ears. The ToF is replaced with simple magnitude (how loud is it?) to determine relative distance as you move around. The AoA is the result of our two ears and their pinnas. The two ears make use of ITD (Interneural Time Difference) and ILD (Interneural Level Difference) along with the pinna’s direction-dependent influence on spectral content to work out an angle of arrival.

So, “beeping” has inherent utility to a human. To conflate beeping at 1KHz to “beeping” a 1GHz is to misunderstand the nature of RDF. You have to build something that can “hear” the beeping and then somehow use what it hears to determine where it is coming from.

As you can see, this is not trivial. Consider: if this was possible in the simple way you envision, what wouldn’t it be a product you can buy? It’s not as if this isn’t something that people haven’t considered. If you were willing to stick something the size of an AirTag on your device—something the original poster was not interested in—then get an AirTag!

Or, what could be made relatively easily given the space is something that would beep on demand—it’s just not going to be battery free. In an imaginary world that has no regulatory demands, you could make a passive RFID tag that beeped when you pinged it, but the power levels required at the transmitter makes it practically impossible.

It was a good thought—not crazy or stupid or anything like that. It was a naïve solution which is certainly no sin. It’s just not practical.

 

The "High Tech" scheme will be to purchase an "airtag" device and attach it to the remote device.
The cheap trick scheme would be to purchase a foot of bright orange ribbon and attach it to the lost TV remote .

 

The "High Tech" scheme will be to purchase an "airtag" device and attach it to the remote device.
The cheap trick scheme would be to purchase a foot of bright orange ribbon and attach it to the lost TV remote .

The “Mil Spec” solution would be to chain the remote to the chair.