HF at 13.56 MHz, and why bands use it

Short range is the point

A credential that could be read from three metres would be read constantly and by accident: two guests at a bar, a queue at a gate, a band in a pocket walking past a door. At 13.56 MHz with a wristband-sized antenna the working range is a few centimetres, which means presenting the band is a deliberate act. Every transaction is unambiguous about who intended it. Properties who ask for more range are usually asking for a problem they have not met yet.

The same band works everywhere

13.56 MHz sits in a band allocated for this purpose in essentially every country, which is why a credential specified for a property in one region behaves identically in another. That matters to hotel groups standardising across countries and it matters to any property whose readers were procured internationally. It is also why the ecosystem is so deep: readers, locks and tills at this frequency are commodity items rather than specialist ones.

It survives water better than the alternatives

Every radio frequency interacts with water, and higher frequencies interact more. At 13.56 MHz a wet band and a wet wrist detune the antenna slightly and reduce range at the margins, but the transaction still completes on contact. At the much higher frequencies used for long-range tags, water absorbs the signal severely enough that a wet body can stop a read entirely — which is the practical reason resort wristbands are not built on them.

Capability varies enormously within the frequency

Two bands can both be 13.56 MHz and be entirely different products. One may return a fixed serial number and nothing else; another may hold several kilobytes of writable memory divided into protected sections, each requiring a key. Both are HF. Specifying "13.56 MHz" therefore tells a supplier the frequency and almost nothing about compatibility, which is why the enquiry has to name the reader or the lock platform as well.