Can you make it compatible with ChampionChip (now merged with MyLaps)? Many runners have their own chips, and that would be a good standard to work from. You can pick up
In response to the comments and answers about the applicability of this technology to racing, I can say that I've run at least two dozen races with RFID tags, and have never had a problem with tag collisions, misses, or accuracy/finish order.
For redundancy, use two mats, and program in the rate at which runners are crossing. Then, if the first mat has a problem, you can use the second and extrapolate the time to cross the line.
Find a race that uses disposable chips, or just keep a pair from your next race, and see what you can come up with. Alternatively, this site sells personalized tags.
Passive tags are powered from a antenna in the environment. The tag then puts a varying load on the RF field, which the transmitter detects. This varying load is a digital bit stream with the tag's ID, checksum, and sometimes additional information.
This system inherently works at close distances. To get enough power to a tag for it to run 5 meters away would take a large antenna and lots of power being sent out by the antenna. If your installation can support a coil of wire a few meters in diameter, then maybe the size of the antenna is OK in your case.
However, the amount of power it must dump into the near field will be very large. As distances go further, the volume over which the antenna has to provide enough power to run a tag goes up. In fact, it goes up with the cube of the distance. This in itself isn't a issue since this is near field and the energy returns to the antenna each cycle. The problem is that the amount of power the tag can absorb as a fraction of the total the antenna sends out gets smaller. Put another way, for the same power at the tag, the signal to noise ratio at the receiver goes down rapidly with distance. Eventually it becomes impractical, which I think will be well before 5 meters.
Another problem is the RF noise (from the rest of the world's point of view) a large antenna dumping lots of power will cause. No matter how carefully you try to arrange the antenna and objects in its near field, some of the power will escape the near field and propagate outwards. Not only are there legal limits to what you are allowed to radiate, but it can cause problems with your own equipment. There will also be unknown effects on humans in the field. There is still much study and disagreement about how much RF energy a human can safely absorb at various wavelengths without long term effects.
We may be able to suggest alternatives if you explain what you are really trying to accomplish instead of asking about a supposed solution. There are such things as semi-passive RF tags. They contain a battery, but don't transmit until they see a particular RF signature.
There are also fully active tags that contain a battery and transmit occasionally on their own. These are true propagating RF transmissions, so can be picked up a good distance away. I worked on such a system that used 434 MHz carrier. The tags transmitted every 10 seconds, a single 2032 coin cell would last 1-2 years, and they could be received up to 60 feet away in a open environment.
Best Answer
The first problem you face is that the magnetic flux density will not be powerful enough to energize the passive tag at 500 mm. Power is transferred magnetically and it is flux density that dictates power transmitted to the tag.
Here is the formula that defines flux density at a distance Z from a coil that has a radius R: -
Here is a calculator that will give you figures for B\$_Z\$.
It can be seen that the flux density reduces by over 1000 when distance increases from 0 to 500 mm. At a distance of 50 mm the flux has only fallen by about 2.9 times.
I think the only option you have is to redesign the coil to be bigger and increase the current into the coil. A 10 amp coil that is 500 mm radius produces a flux density of 0.0000044 teslas at 500 mm (same as smaller coil with tag at 50 mm) - this would be my starting point but you also have another really significant problem to overcome....
Powering the tag can be overcome with a bigger coil and more amps but think about what happens on a normal passive tag situation - the tag responds by modulating the magnetic field it receives and this is picked up by the reader. Now, with a distance of 500 mm, that miniscule modulation has to be received over that immense distance AND it has to fight the 10 amps in the reader's coil in order to register a return signal at the reader.
I'm not going to say it can't be done (because it can) but you need a really big transmit coil and some very clever electronics to discriminate a very weak signal in the presense of a very powerful signal.
One alternative is to find tags that transmit their ID using (maybe) 433MHz i.e. keep the data transmission "away" from the power transmission spectrally.