RFID Antenna for 00, H0 and P4 Track

I am trying, along with several others, to develop a method of detecting model trains by using RFID tags. One of the problems is getting the antenna of the reader close enough to the tag. This is complicated by the rails, sleepers and baseboard.

New Antenna Shape

The cheap reader/writer boards have a PCB antenna printed on them. I am trying to make a replacement antenna that will fit between the rails.

The first task was to find the inductance of the printed antenna. After some hunting around on the Internet I discovered a useful web site posted by Josip Medved that aids in the calculations required. Using Josip’s web page I determined that the inductance of the printed coil was 1.34┬ÁH.

Next I had to design a similar coil with the same inductance that would fit between the rails. Even if the new antenna was fitted under the baseboard it would still be better to have the profile fit between the rails to minimise the interference with them. I decided to 3D print a jig to hold some suitable copper wire. The dimensions of the new coil would have to be determined using another web page as I was using copper wire. I ended up using Nicola Asuni‘s page on Tecknick.net.

The gap between the rails is about 12mm allowing for safe clearances and so one dimension of the coil was fixed. The copper wire I had was 32SWG (0.27432mm) so that parameter was fixed as well. Decided that I would keep with the original design of a four turn, centre tapped coil as well. The coil windings would be 2mm apart as this was a constraint of the 3D printed jig. This means that the controlling parameter for the correct inductance would be the coil length (along the track bed).

The dimensions of the coil turned out to be 12mm x 15mm. This covered a very small linear run of track and so I re calculated using 3 turns. This suggested a coil 12mm x 25mm. Further reducing the turns to 2 suggested a coil 12mm x 56mm.

It must always be born in mind that the coil is used to power the tag. Reducing the number of turns may reduce the amount of power that can be coupled into the tag.

I constructed all three coils and tested them. They were joined to the read/write electronics by cutting the copper tracks on an RFID reader/writer and soldering on the new coil.