What RFID tags are, and what they do
RFID for model railways need to be small - on the picture you can see various tags, in comparison with N-gauge track.
The StaRFIshrail tag is available from the store on this website, and has been designed to be a good size for model railways. The Murata 3mm is also available from the store. The Murata 8mm tag is available from Digikey, #LMXSAPHA08-136 - though this has no advantages over the StaRFIshrail tag, and is considerably more expensive.
All the tags are large compared with the chip that is the active component - these are only 1.5mm by 1mm - hard to find if you drop one on the floor!.
The 3mm tags will give a read distance of 15mm - more than enough on N or Z gauge if the aerials are beneath the track. The StaRFIshrail tag gives a read distance of 30mm or more with a small aerial, and 40mm with a large aerial.
The tag comprises the tag chip, which just has two connections, and a loop aerial, and a matching capacitor. The loop aerial is an inductance, and spirals round on each side of a very thin printed circuit board (0.1mm thick). Each end of this spiral is connected to one terminal of the tag chip, and a capacitor is added to across the chip to help with matching. Matching is required to make sure that the inductance and capacitance (L/C) form a resonant circuit at 13.56MHz, as this maximises the read distance. After all, we have to transfer power to the tag in order to make it work.
The tag chip is tiny, 1mm by 1.5mm, but packs in a lot of functionality. There is a RF receiver for the power - this is needed to power up the chip as there is no battery or any other power source in the chip. There is RF decoder for data coming from the aerial attached to the reader, in order to command the tag chip. There is a RF transmitter for transmitting data back to the reader. There is digital coding and decoding, and 1k bit of memory. The tag chip takes only about 1mS to power up, and then it is ready to receive commands. In the case of StaRFIshrail, the reader asks the tag chip for it's UID (Unique Identification Number) - this is a 48 bit number, and no other tag has this number, or ever will, it is truly unique. The tag then replies by transmitting the UID, and this is what the reader reports back to the hub. The total transaction time of data flow is around 10mS, hence the ability of the hub to read the tags in 11mS. It's a bit mind boggling, but it works!
Although the original reasoning behind using 13.56MHz ISO15693 as a communication standard was the availability of 3mm and 8mm square tags, the tag chips were also available from normal electronic distributor channels. When fitting these to rolling stock, I realised that square was not necessarily the best format, a rectangular tag might be better, and I may be able to get a larger read distance. I was looking for around 25mm for N-gauge, and more like 30mm for HO/OO gauge, and possibly more for O gauge. This would allow the aerials to be mounted under a 9mm (3/8") baseboard. The StaRFIshrail tag is 6mm by 9mm, allowing it to be used on rolling stock easily, from N-gauge to O-gauge. The StaRFIshrail tag has a read distance of 30mm+. The tags can be quite invisible when viewed on a working layout. This image shows a StaRFIshrail tag on the bottom of a N-gauge Graham Farish 0-6-0 shunter. More images are below.
The tag is mounted centrally on the truck, and spaced away from the chassis.
This shows that you really cannot see the tags.
Tags for N-gauge can be mounted across (as here) or in-line with the direction of travel.
Almost impossible to see the tag.
The tag has been mounted a long way off the chassis, about 2mm above the axle.
Almost impossible to see the tag (only given away by a slight reflection from the LED).
Tags can be placed quite close together. Here are two on a OO shunter, front and back.
On rolling stock with bogeys, tags should be placed on the bogies, rather than in the centre of the carriage.
The tag is spaced a long distance from the chassis, approximately in line with the axle, which makes the tag to aerial distance less than 30mm with a 9mm baseboard.
Despite the tag being set off from the chassis to the level of the axle, it is still almost impossible to see.
Due to the small size, it is not possible to offset the tag vertically on this N-gauge loco. Because of metal in the chassis and axles, the read distance is reduced to 22mm, but it still works with a 12mm baseboard.