There is sometimes confusion over the performance of RFID tags placed on wet objects. This is not a problem for low frequency passive tags used by our HDD System, but for the newer UHF passive tags this can present challenges. Unless race timers use Battery Assisted Passive (BAP) tags, they will need to make allowances for proximity of a passive UHF tag directly next to the liquid-filled, moist human body.
There is no ‘black magic’ here. Even when high performing, industry leading Monza 4 or Higgs 3 ICs are used with the very best dipole designs, passive UHF tags suffer performance problems when directly placed on the human skin. Simply put, it is just physics and the result of detuning and signal loss due to water.
The solution is to space the tag away from the skin with the cheapest electrically inert substance: closed cell foam. Our testing has shown that the spacer needs to be only 2-3 mm thick to guarantee excellent read rates, even when the runner’s race bib is stretched tight against the body – and the singlet dripping wet with sweat.
Some of our competitors claim they don’t need a spacer, yet they have to employ two tags per bib. Even a 1 mm thin PET spacer will help things, but we believe that to get near 100% read rates, some type of thin foam spacer is required to guarantee performance in all racing conditions. Using two tags is also a good insurance policy given that each tag costs less than $0.15 each*. Yes, that’s 15 US pennies, not the exorbitant $1.00 per tag that our competitors are charging!
* Approximate cost of the passive tags that we recommend: Alien Squiggle and UPM Raflatac Dogbone. Adding a spacer adds about $0.04 per tag.