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The Early Years of RFID – LF RFID

As race timers, we are always aware of the limitations of our systems. In the early years of chip timing, low frequency RFID made it a slam dunk for picking up runners. That’s because there was nothing outside of the timer’s control that would affect read rates. The only thing that would cause a missed read was if the runner wasn’t wearing their chip on their shoe or ankle strap. And any external interferences (magnetic fields from power wires, for example) would be accounted for by a good timer. A standard practice by a good timer is measuring the EM field of the start/finish lines to ensure there’s no interference. The only way a runner would be missed was outside of the timer’s control. So, in essence, the timer could get 100% accuracy if you account for chip mishandling or misplacement by the runner.

UHF RFID

The introduction of UHF RFID was tremendously well received by race directors and race timers. You didn’t need to distribute or recollect hard chips. It lowered operational costs all the way around. So it was a tremendous advancement in race timing technology. There was only one drawback: It was often not possible to get 100% accuracy anymore. While it is very possible to get 100% read rates at the finish line , the start line read rates are often less than 100%. Why?

UHF RFID Technology

UHF RFID operates in the UHF frequency range (~850-925MHz). In this frequency range, there are two major sources of interference for the propagation of the UHF signal. These are water and metal. Whereas LF RFID signals could travel THROUGH water up to 2 feet deep, UHF RFID signals are completely absorbed by water. Metal (and carbon fiber) do the same thing. So if a passive UHF RFID tag is placed too close to a body of water (i.e. the human body), then the tag cannot be read.

The Start Line

The presence of hundreds of bodies crossing the start line simultaneously presents a large body of water in the UHF field all at the same time. As previously explained, this reduces the ability of the RFID reader to read the tags on those bodies. One way to improve read rates is to place the tag on the shoe. This moves the tag away from the larger water mass in the upper part of the body. And it moves the tag closer to the antennae lying on the ground. However, the popularity of the big tag has virtually eliminated the shoe tag in race timing applications.

Numbers

Start line read rates using best known practices and top-of-the-line equipment will get about 99.3-99.7% start line read rates. This means that 3 to 6 runners out of every 1000 will not have a start read. Instead they will receive the gun time as their start time. If a race timer using UHF RFID technology says they get 100% read rates, then they are not being truthful. The fact is, those 3 to 6 runners for every 1000 likely will not notice or simply don’t care if their time is a few seconds off. And it almost never matters from an awards perspective because the runners at the front are rarely missed. And they are the runners receiving awards.

The Short End Of It

UHF RFID technology has significantly simplified race timing all the way around. The tags simply need to be placed on bibs. And the bibs are given to the runners. Virtually every runner knows you need to wear the bib on the front of the body, so it’s difficult to misplace the tag. Sure, some runners will wear their bib on their backs (which hurts read sensitivity), but most wear it right. The race director doesn’t have to do anything special except send their bibs to the race timer beforehand. And the race timer doesn’t have to spend hours dealing with chip allocation and return.

Most people in the industry have decided that the small misses at the start line are worth the significant cost and time savings. If someone does notice their missed start time, it’s simple to add it in if they have a watch time. And if they are an award winner, the race timer always performs a manual backup anyway, so it’s known that they are an award winner.

All in all, the benefits outweigh the cost of a few missed start reads.


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