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I am very new to this. I'm a dentist trying to improve client flow in a busy office.

I would like to develop a cheap proof of concept for an NFC enabled client tracking system using a hacked mobile phone. My idea is to develop a system where I place passive NFC tags on the door frames of my office. As a client walks into the room, the cell phone reads the NFC tag and sends a message via Wi-Fi to a computer which collects the data.

Since NFC only works at short distances, I would like to make an external antenna that would extend the range of the cell phone (Samsung Galaxy 4) and allow it to read tags at a distance of about 30-60 cm. The size/shape of the antenna is not a concern at this point, but practically speaking the largest dipole antenna I would consider is 200 cm. (I'm not completely sure what a dipole antenna is but I read this and it sounded important :)

I know RFID is better at these distances but it's also more expensive and I'd like to keep the cost to the absolute minimum. The same goes for the mobile device. This may be possible with more specialized equipment but the cost and ubiquity of mobile phones make it the preferred reader (also makes programming the tags easier).

My questions are:

  • Is this feasible?
  • How would the antennas be designed? Shape - planar circular vs cylindrical vs dipole; Size - 22.11 meters long
  • What is the best way to connect to the native NFC antenna pins?
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  • \$\begingroup\$ Clabacchio posted a great link. Just to summarize it, the math boils down to this: The result indicates that the optimum loop radius, a, is 1.414 times the demanded read range r. So the effective range of NFC is limited by the size of loop antenna that can be put around the rim of a cellphone. My iPhone 5C has a rim perimeter of about 14 inches, so then the optimal read distance for an NFC antenna placed around it would be about 10 inches. Mind you, all of this is for optimal. There's nothing stopping you from going overboard on a powerful NFC transmitter \$\endgroup\$
    – krb686
    Apr 24, 2014 at 16:02
  • \$\begingroup\$ Thank you for the summary, you mention adding a "powerful NFC transmitter" would this item replace the cell phone as the reader or augment it? Do you have an example of such a device? Thanks \$\endgroup\$
    – user40828
    Apr 24, 2014 at 20:10
  • \$\begingroup\$ Well as stated in clabacchios source in the original question, the Reactive Near field is commonly considered [to drop off at] λ/2π. For NFC, this would be 3.51 m. So the reactive region can be used further than 20cm, but it will lose signal strength since 20cm is approx. the optimal distance. To compensate, you can make the transmitter more powerful, ie. switch more current. I = V/R, or V/Z, and if you're keeping the coil the same, Z won't change, so you need to increase V. I have no examples of this \$\endgroup\$
    – krb686
    Apr 24, 2014 at 22:04
  • \$\begingroup\$ @krb686 just to clarify, 20 cm is not at all the "optimal" distance, but the maximum. Sometimes tags don't work properly at that distance, that's why it's commonly referred as "tapping", where the devices get in contact \$\endgroup\$
    – clabacchio
    Apr 24, 2014 at 22:22
  • \$\begingroup\$ If you want to track the client, why not doing it the other way, give them a tag and ask for it on each office and then you read it with your phone (or whatever other NFC reader device). In that way the client don't need to do anything but carry the NFC tag \$\endgroup\$
    – ghm1014
    Sep 17, 2015 at 2:52

6 Answers 6

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You want a magnetic loop antenna. A wire loop around the doorframe with a tuning capacitor on the left and cellphone on the right. Borrow a grid dip meter from your local ham radio club and adjust the capacitor to resonate at 13.56 MHz. The cellphone must be slightly within the loop. This will boost your signal to a range of a few meters if set up correctly. Again a ham radio expert can set this up in half hour including coffee. You can make a similar booster for the tag. Use the biggest loop you can and tune it with a capacitor. Aluminium duct tape works well and carefully overlap the ends just the right amount to form the tuned capacitor.

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The main problem is not concerned with the size or type of antenna, but with the physical operation of NFC. Being Near Field Communication, in order to achieve proper coupling requires that the devices are at most 10 cm apart [source].

As you can read in the link (spoiler: lots of math!), the limitations are due to the wavelength of the signal, and the antenna size is only a consequence of that. Can you request the patients to actively "scan" the tags when entering?

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  • \$\begingroup\$ I think your link is good, but I do have to nitpick a little. You say the limitation is not the size of the antenna. Rather, the limitations are due to the wavelength of the signal, and the antenna size is only a consequence.... This seems incorrect (maybe just semantics). Inherently, there is no limitation due to the wavelength. According to the math presented, optimal antenna size is 1.4 * read distance. Thus any distance could be achieved with enough power, and a large enough antenna. The antenna size becomes the limitation because of the space available in a cell phone. Right? \$\endgroup\$
    – krb686
    Apr 24, 2014 at 16:11
  • \$\begingroup\$ Indeed I might have been superficial in the interpretation of the sources, due to time limitations. I think that you're right, and the near field for 13.56 MHz tops at 3.5 m (same source). \$\endgroup\$
    – clabacchio
    Apr 24, 2014 at 19:41
  • \$\begingroup\$ This makes sense. I'm using the Samsung S4, would that have the power to operate a larger antenna? I assume I will have to create an antenna separate from the device to get within the 30-60 cm range. Would something like this be appropriate? (see page 4 Antenna Designs) \$\endgroup\$
    – user40828
    Apr 24, 2014 at 20:16
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As others have pointed out, the physics of NFC are against you. It is specifically designed as a very short range protocol.

Your use case sounds like it might work with Bluetooth Low-Energy, though. For example, Apple's iBeacon uses BLE tags as "breadcrumbs" to mark specific locations. You should be able to implement something similar, even if you don't want to buy into the iDevice ecosystem.

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When I first read your question I assumed the following:

  • You are trying to track your clients' movements using your clients' smartphones
  • Your clients would have to install an app on their smartphones in order to enable this

Then I saw the discussion about hacking the smartphones to change their antennas. This would imply that you would be providing the phones and giving them to the clients.

Hacking a cellphone to make it read NFC at a greater distance, would IMHO be quite a difficult task. As has been pointed out, NFC is specifically designed to limit distance (for privacy and security considerations, I believe).

Unless I'm missing something... if your scheme requires giving them something to carry with them in the clinic, you might just as well give them RFID tags, and place RFID readers in the doorways.

On the other hand, if you want to use the clients' cellphones with a downloaded app, as another poster suggested, Bluetooth low energy beacons might be a better approach, as BLE is designed to be readable at a greater distance.

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As the other people say, the NFC is designed for short distances in order to keep the user safe of other device not desired.

If I were you, I use a Bluetooth Low Energy, that will filter devices depending of the RSSI. You can use any kind of Beacon, (for example iBeacon)

Pros of BLE: -They can be programmable for example TI CC2540/41 for your need. -The devices are small and cheap. -The batteries give you enough battery for more of 6 months. -I Suggest you any ble or beacon from aliexpress. Aliexpress ibeacons/ble -More difficult of programing but you can filter with the distance to customer's phone.

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Most antennas noways are geared more for aesthetics than functionality. I should point out that I have had a conversation with a person in California while I was in Colorado around 14.250 MHz. This was done at 5 watts, a 5/8 wave dipole, and excellent atmospheric conditions. Now, I don't think you want to walk around the office caring a car battery and a large whip around all day while trying to ionize your ceiling. When dealing with antennas, the formula 984/F can be used to get an idea of the physical wave length of the signal. F is frequency, and the result for NFC (13.56 MHz) will give you a length of 68.8 Feet, or 68' 9.6". A dipole antenna is just a wire with a ground plane. Think of TV bunny ears. One wire to balance out the other.

My suggestion would be this, Since communication will be one way, let's enhance the passive tags. First hurtle is induction, or rather powering the passive device. Let's face it, we're dumping very little power in the first place ( I think around 15mA), and using inefficient antennas. One idea would be to change the induction coil in the passive tag to be responsive on another frequency; ether generated by you or a close by source. I remember building a radio when I was a kid that was powered purely by the incoming radio signal. It wasn't going to blow any windows out, but it was loud enough to hear everything. Next you could try and make a directional yaggi antenna, but at that size I am not sure if it could be accomplished easily. Maybe just make sure the antenna has as much surface as possible, like maybe a metal door jam? I don't know just some ideas to help. I like the NFC idea, but maybe since your not tracking specific clients you could just use a motion sensor with a WiFi module, fairly cheap these days. Anyway, good luck!

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