Education
How an RFID Chip Works in Running: Passive UHF Tags, Antennas, Read Zones and Placement
· 7 min read
In short
- A passive UHF chip has no battery: it draws the energy needed to respond from the reader’s signal and sends back its unique code.
- A reader and antenna create a read zone. Software determines the crossing time from multiple reads of the same chip and merges repeated reads into a single passage.
- The body and water weaken the signal, while metal detunes the chip: the chip needs 5 to 15 mm of clearance from the body, and use on metal requires an on-metal chip or a pad approximately 5 to 10 mm thick.
- UHF chips operate in the 865 to 868 MHz band in Europe, 902 to 928 MHz in the USA and 920 to 925 MHz in China.
An RFID chip on a bib number may look like a sticker, but behind it is a small radio system. Once you understand how it works, you can position it correctly and avoid common mistakes.
What Is Inside the Chip
A passive UHF chip is a small circuit with an antenna. It has no battery. It draws the energy needed to respond from the signal transmitted by the reader. The chip replies with its unique code, and the reader records that code together with the time. This is why the chips can be inexpensive and disposable. An active transponder has a battery and transmits its own signal. It offers a longer range but is larger and more expensive, and is used mainly in motorsport.
Reader and Antenna: The Read Zone
The antenna creates a field in which chips can respond. This read zone may extend for several metres. When a runner enters the zone, the reader detects the chip and records multiple reads until the runner leaves. Software uses those reads to determine the crossing time and merges duplicates into a single passage. Because the zone has a physical size, standard UHF timing is suitable for whole-second accuracy rather than hundredths of a second, as explained in Race Timing Accuracy.
Frequencies Around the World
A passive UHF chip has no battery, is inexpensive and operates in the 865 to 868 MHz band in Europe. The 902 to 928 MHz band is used in the USA and Canada, 920 to 925 MHz in China and approximately 916 to 921 MHz in Japan. The reader must support the frequency band used in the country where the event takes place.
Where to Place the Chip
The body and water weaken the signal, so the chip must not sit directly against the body. A typical clearance is 5 to 15 mm, although the exact figure depends on the chip’s design, and can be provided by the bib number or a foam spacer. Metal detunes the chip: use on metal requires either an on-metal chip or a pad 5 to 10 mm thick. For running, a proven solution is a bib worn on the chest with the chip inside it or attached to the back. In cycling, the chip can be mounted on the fork or helmet; read more in Timing for Cycling and MTB Races. Further details are available in Which Events Use RFID Chips.
Why a Crossing Can Be Missed
The most common causes are poor chip placement, such as being covered by a backpack, positioned too close to the body or attached to metal; insufficient antenna range; or a loss of power or connection to the reader. A list of potential issues is available in The Most Common Race Timing Mistakes.
From ChampionChip to the Phone
The first chips used for mass-participation running events, ChampionChip tags introduced in 1993, were worn on the shoe. The system was first used on a large scale at the 1994 Berlin Marathon. Today, inexpensive disposable chips attached to bib numbers are common; read more in How Time Was Measured 100 and 1,000 Years Ago.
RFID in TAQIO
TAQIO supports RFID readers and inexpensive UHF chips: the app filters duplicate reads and records each passage. Accessories, including UHF chips and a portable reader, are being prepared for the upcoming online store.
Frequently asked questions
How does a passive RFID chip work?
The chip contains a small circuit and an antenna. When it enters the reader’s field, it draws energy from the signal and responds with its unique code. No battery is required, which makes the chip inexpensive and durable.
Why does chip placement matter?
The body and water weaken radio waves, while metal detunes the chip. The chip therefore needs some clearance from the body, and use on metal requires either an on-metal chip or a suitable pad.
How does the signal become a recorded time?
The reader records the time of every read of the chip’s code. Software selects the crossing time from multiple reads of the same chip and merges repeat reads made within a short period into one passage.
Which frequencies do UHF chips use?
The band varies by region: 865 to 868 MHz in Europe, 902 to 928 MHz in the USA and Canada, 920 to 925 MHz in China and approximately 916 to 921 MHz in Japan. The reader must support the frequency band used in the country where the event takes place.